The Fundamentals of Spruing, Investing, and Casting

The Fundamentals
of Spruing, Investing,
and Casting
Chapter
5
84
Success in the fabrication of metal-ceramic crowns and
fixed partial dentures depends to a large extent on the abil
ity to obtain high-quality castings that are not only properly
designed but also fit well. This chapter provides an over
view of important topics related to the fundamental princi
ples of spruing, investing, and casting. In addition, empha
sis is placed on the laboratory steps for the use of the
buttonless casting technique.
Terminology
While investing and casting wax patterns are procedures
that have been part of dentistry for quite some time, it is not
uncommon for several key terms involved in these pro
cesses to be used interchangeably but not always cor
rectly. Therefore, the following descriptions and explana
tions are presented to clarify the intended meanings and
use of the terminology found in this chapter. Instead of an
alphabetic list, the terms appear in a sequence that is
intended to promote a better understanding of their mean
ing and recommended use.
Sprue former In its simplest
application, a sprue former is a
piece of dental wax or plastic,
generally round in shape, that is
attached to the thickest portion of
a wax pattern at one end and a
crucible former at the other.
1
Sprue formers are used
for direct spruing (ie, extended from the top of a cruci
ble former straight to the wax pattern) and indirect
spruing (ie, oriented circuitously from the crucible for
mer to the pattern area). Today, prefabricated wax
and plastic sprue former patterns are available for pur
chase in different shapes and sizes (ie, gauges) for
both the direct and indirect spruing methods.
Pattern sprue former The sprue
former that is attached directly to
a wax pattern can be described
as a pattern sprue former to dif
ferentiate it from other compo
nents of the sprue former net
work, such as the runner bar (ie,
connector bar) and the ingate
sprue formers that are part of indirect spruing. The
length and gauge of a pattern sprue former are deter
mined by the size (ie, volume) and thickness of the
wax pattern(s) to be cast.
Sprue way The channel (ie, void)
in the set investment (ie, mold)
created after the elimination of
the wax or plastic sprue former
network (ie, wax elimination or
“burnout” procedure), is called
the sprue way. A sprue way is
formed with both direct or indi
rect spruing, but the sprue way network tends to be
more complex with indirect spruing than with direct
spruing, as the name alone would suggest.
Ingate sprue former This is a
term preferred by the author to
describe the large-diameter
sprue formers that are attached
to the crucible former at one end
and the runner bar at the other
end in the indirect spruing pro
cess. Ingate (ie, feeder) sprue
Naylor_Chap_05.indd 84 9/6/17 12:07 PM
85
Terminology
formers must be of sufficient diameter to permit
needed volume of molten alloy to fill spaces created
by the runner bar and the attached wax patterns. So
ingate sprue formers would be a smaller gauge (ie,
larger diameter) than the gauge of the pattern sprue
formers. This makes more sense when you consider
that ingate sprue formers are attached to the under
side of a runner bar and pattern sprue formers are
attached to the superior side of the runner bar. And
because molten alloy should flow from areas of large
volume to areas of small volume, the sprue way net
work must be designed properly for the casting pro
cess to be successful.
Sprue Technically, a sprue is
cast metal. More specifically, it is
the portion of a metal casting that
reproduced the sprue former.
(Note: Some incorrectly use the
word sprue when referring to the
wax sprue former). In other
words, a sprue is a part of a cast
ing while the sprue former is the wax or plastic form
that creates the channel in the set investment after the
wax (or in some cases, plastic) has been eliminated in
a burnout furnace.
Direct spruing This is the name
given to the technique in which a
sprue former system creates a
channel that extends from the
opening in a mold (created by
the crucible former) directly to a
wax pattern area.
Indirect spruing The term indi
rect spruing describes a sprue
former system that forms multiple
channels in the mold that run
indirectly from the open end of
the mold to the patterns, with a
runner bar strategically posi
tioned between the mold open
ing and the pattern area.
Runner bar The part of the wax
indirect spruing system that
is between the mold opening
and the wax pattern area (see
arrow). The role of a runner bar
is to support multiple single wax
patterns, a fixed partial denture,
or a combination of both. The
function of the runner bar is to create a large sprue way
in the mold that can retain a reservoir of molten alloy
during the casting process. When the thermodynam
ics of casting are controlled, as with buttonless casting
(described later), molten metal in this large area serves
as a reservoir of alloy to ensure the pattern areas are
able to solidify completely and solidify before the run
ner bar. A runner bar also is referred to as a reservoir
bar or a connector bar, and the term may be used to
describe the wax component as well as the metal com
ponent once it has been reproduced in metal.
Reservoir That portion of a sprue former (in wax) or
sprue system (in cast metal) that retains a large vol
ume of wax or alloy, respectively. A straight sprue for
mer with a round ball is an example of direct spruing
with the ball portion serving as a reservoir. With indi
rect spruing, the runner bar creates a large space in
the mold, after wax elimination, to hold molten metal.
An effective reservoir is expected to be the largest vol
ume of metal cast and the area of the casting that
solidifies last. A reservoir ball or runner bar can be a
component in a spruing system but not actually func
tion effectively as a reservoir if the thermodynamics
allow these areas to solidify early and a button (or
other mass of alloy) to solidify last. The goal is to have
molten alloy reproduce the patterns areas first, fol
lowed by the ball reservoir in direct spruing and the
runner bar with indirect spruing, and not to produce a
button with either method.
Crucible former The round or
oval base to which a sprue for
mer or prefabricated sprue for
mer pattern (wax or plastic and
direct or indirect) is attached is
referred to as a crucible former.
Wax gauge This term is used to identify the diameter
(ie, gauge) of the different wax sprue formers avail
able for use in the dental laboratory. The gauge num
ber is inversely related to the diameter of the wax, so
the larger the gauge number, the smaller the diameter
of the wax sprue former. The gauge sizes used in den
tistry generally range from 18 gauge (1.02 mm/0.04
inch) to 4 gauge (5.20 mm/0.21 inch).
2
Popular sizes
include 4 gauge, 6 gauge (4.12 mm/0.16 inch), 8
gauge (3.26 mm/0.13 inch), 10 gauge (2.59 mm/0.10
inch), and 12 gauge (2.05 mm/0.08 inch).
Porosity The term refers to the
presence of voids (ie, spaces) in
an otherwise solid structure. In
a casting, porosity forms in the
area of metal that solidifies last.
Solidification shrinkage At the
end of the casting process, a molten alloy transitions
from the liquid state to the solid state, and alloy shrink
age is inherent in that transition. In other words, solid
dification shrinkage (or casting shrinkage) is a normal
phenomenon, one that technicians counterbalance
through planned investment expansion.
Casting porosity Porosity occurs
with every casting in the area
that solidifies last, and that
porosity (see arrow) most often
is referred to as casting poros
ity or localized shrinkage porosity. When casting, the
challenge is to understand and control the thermody
namics of the process, so the porosity that is going
to occur resides in a noncritical portion of the sprue
network rather than somewhere in the pattern area.
Controlling porosity is best achieved by choosing an
appropriate spruing method, creating smooth tran
sitions from the opening in the mold to the pattern
abc
minus equals
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86
The Fundamentals of Spruing, Investing, and Casting
5
area(s), establishing an appropriate sprue design,
avoiding sharp internal angles in the mold, and cast
ing with the correct volume of alloy. Too little or too
much alloy can spell disaster for a casting regardless
of the spruing design.
Buttonless casting technique This term refers to a
casting technique for both direct and indirect spruing
in which the wax pattern to be reproduced is weighed
and the amount of alloy needed is calculated based
on that weight and the density of the alloy. No button
is cast, and the largest mass of metal is the runner bar
(indirect) or ball reservoir (direct).
Spruing Techniques
A wax spruing system creates a channel or series of chan
nels in the set investment through which molten alloy can
flow to reach the wax pattern areas. Consequently, one
of the first decisions to make before preparing a wax
pattern for investing is which type of spruing system to
employ. For best results, this decision should be made on
a case-by-case basis to ensure that sufficient molten alloy
will be made available to reproduce all of the invested
units.
No single spruing method is universally accepted as the
technique of choice. On the contrary, opinions among den
tal technicians and recommendations by alloy manufac
turers differ so widely that at times they may conflict with
one another. For example, one manufacturer may suggest
direct spruing, whereas others may insist that only indirect
spruing should be used with their alloys. To complicate
matters, reports in the dental literature offer differing views
on the subject.
Consequently, it is extremely important to understand
the general principles of spruing, including spruing meth
ods (direct versus indirect), sprue placement, sprue gauge
selection, sprue length, reservoir location, constricted spru
ing, sprue composition (wax versus plastic), and the value
of prefabricated wax sprue formers. Such knowledge,
coupled with hands-on practical experience, will greatly
improve your chances for consistent, successful casting
results.
As explained in the terminology section, wax patterns
can be sprued in one of two ways—directly or indirectly.
Each method has its advantages and disadvantages, so
it is important to understand the philosophy behind both
techniques to ensure each is employed correctly when
used.
Direct spruing
The basic concept with direct spruing is to permit mol
ten metal to flow directly to the pattern area in the heated
investment mold. This method is less complex than the indi
rect technique and usually requires less time and effort to
complete.
With the direct spruing technique, one end of a
straight sprue former is luted (ie, attached) to the thick
est cross-sectional part of the wax pattern (ie, the wax pat
tern sprue former), while the other end is luted to the top of
the crucible former.
Having a ball reservoir between the pattern and the cru
cible former is one way to modify the sprue former, and pre
fabricated direct wax sprue formers with a reservoir ball are
available commericially.
3–5
The purpose of any reservoir is
to supply the molten metal needed to fill the pattern areas
completely. Even with the added presence of a reservoir
ball, the spruing method is still considered to be direct.
Direct spruing is used most frequently to cast single units
(Fig 5-1) and small multi-unit patterns. However, a basic
weakness of this technique is the potential for suck-back
porosity, which is evidenced by the presence of a void at
the junction of the restoration and the sprue
3
(Fig 5-2). This
type of porosity is more likely to occur when the casting
includes a button and no reservoir (see Fig 5 -1).
Fig 5-1
Casting produced using direct
spruing. The straight sprue former can
be modified with a ball reservoir, but the
spruing method is still direct.
Fig 5-2 (a) One problem associated with direct spruing is the increased likelihood of suck-
back porosity (arrow) at the pattern-sprue junction. Suck-back porosity cannot always be
detected externally but may be obvious on examination of the intaglio surface of the casting. (b)
Suck-back porosity is apparent (arrow) in this cross-sectional view of a maxillary central incisor
coping that was cast directly.
a b
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87
Spruing Techniques
Indirect spruing
Once the casting process has been initiated, molten alloy
does not flow straight from the casting crucible into the pat
tern area in the heated mold.
4–6
Instead, the alloy takes a
circuitous (ie, indirect) route to reach the pattern areas—
hence the name indirect spruing (Fig 5-3).
Pattern sprue formers are used to attach wax patterns to
the superior surface of a round wax runner bar (typically 4
or 6 gauge). The void in the mold formed by the bar eventu
ally will be filled with molten alloy via the channels created
by two large ingate sprue formers. The bar’s large volume
houses molten metal for a sufficient length of time to per
mit the pattern areas to fill with metal first and draw on this
additional molten alloy as needed to complete the solidifi
cation process.
7
(This is why the runner bar is referred to
as a reservoir bar.)
If no reservoir is provided and a button is cast, alloy
can be drawn from the pattern sprue, and metal in the
pattern areas acts as a source (ie, reservoir) of molten
alloy for the runner bar or even the button. This behav
ior is directly opposite of the intended casting dynamics.
Again, because the molten metal cannot flow directly to
the wax pattern areas, this method is referred to as indi
rect spruing.
There is evidence that alloy composition influences the
manner in which molten metal fills a mold during casting.
For example, it has been shown that a palladium-silver alloy
flows unidirectionally, whereas type III gold fills in a random
or scattered fashion.
7
Opinions differ as to the value of indirect spruing for a
single crown or multiple single units when direct spruing
can produce acceptable results.
8,9
Users of the indirect
technique might cite the advantages of enhanced predict
ability and reliability in casting, better management of the
volume of metal used to cast, and greater control of cast
ing porosity.
Prefabricated sprue formers
Prefabricated sprue formers of different lengths and diam
eters are available for both direct and indirect spruing
(Fig 5-4). These products not only make it possible to stan
dardize a technique for greater consistency, they also help
eliminate errors in sprue design and permit the standard
ization of reservoir location as compared to direct spruing.
Users of high noble and noble alloys will find that the cast
indirect sprue network can be sectioned into smaller com
ponents of a size that can be reused far more readily than
can accumulated buttons (Fig 5-5).
Fig 5-3 Indirect spruing. These general guidelines should be mod
ified to meet the requirements of each case.
Fig 5-4 Prefabricated direct sprue formers with a reservoir
ball of different gauges (bottom) and indirect sprue formers
(top) with runner bars of different gauges (Whip Mix).
¼ in / 6 mm
(maximum)
Wax patterns
with attached
sprue formers
Casting ring
Investment
Ring liner (if used)
Runner bar
Ingate sprue
former
Crucible
former
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The Fundamentals of Spruing, Investing, and Casting
5
Alloys shrinkage versus alloy porosity
Proper spruing and casting methods are key to managing
two facts of the casting process: (1) investment expansion
is needed to compensate for alloy shrinkage, and (2) all
castings contain porosity. As mentioned in the terminology
section, there is an inherent shrinkage of a dental casting
alloy as it transitions from a molten liquid to a solid state,
and this change of state is referred as solidification shrink
age, and the region that solidifies last contains the casting
porosity. The ever-present challenge in the dental labora
tory is to control the location of that porosity, but that can be
achieved using proper spruing and casting methods, such
as the buttonless casting techniques.
Understanding the Spruing
Process
A multitude of factors must be considered before deciding
if wax patterns are to be sprued directly or indirectly. The
following section presents several key variables that lay the
foundation for understanding the rationale for the button
less casting technique and the laws of casting that follow.
Sprue former placement
Ideally, the sprue former attached to the wax pattern (ie, the
pattern sprue former as opposed to an ingate sprue former)
should be luted to the thickest cross-sectional area of the pat
tern to allow the molten alloy to flow from regions of large vol
ume (ie, thick areas) to regions of lesser volume (ie, thin sec
tions).
10
Placing the sprue former elsewhere on a wax pattern
might result in an incomplete casting if a thin section under
goes solidification before the mold has filled completely.
Of course, there are exceptions to every rule. With
many anterior wax copings, this option is not available or
advisable because the patterns may be small and thin. In
such instances, the most practical sprue former location is
the midincisal region. The same logic should be applied
to locate an appropriate site on a molar coping that is to
receive complete porcelain coverage. In both these exam
ples, make certain the attachment of the sprue former to
wax pattern is smooth and uniform with no sharp edges,
10
and flare this transition from sprue former to wax pattern
11
(Fig 5-6a). Avoid forming ridges that later may become
irregularities in the investment, which potentially could
break off during casting and contaminate the molten alloy.
Sprue former gauge
A pattern sprue former of sufficient diameter (ie, gauge)
should be selected to supply the volume of alloy required
by the patterns to be cast. It is important to remember that
the larger the gauge number, the smaller the diameter of
the sprue former. Manufacturers invariably include sprue
gauge recommendations for their alloys, but these sugges
tions are made without any direct knowledge of the size,
geometry, thickness, and number of patterns to be cast.
So it is the laboratory technician’s responsibility to carefully
assess the wax patterns, evaluate the particular require
ments of each case, and determine the appropriate gauge
that is needed. With direct spruing, it is advisable to use
a sprue former with a reservoir ball. With indirect spru
ing, select a prefabricated sprue former with a runner bar
that is greater in diameter than the thickest cross-sectional
area of the largest wax pattern to be cast. This requirement
is especially critical when metal pontics and large molar
retainers are to be cast (Fig 5-6b).
Pattern sprue former length
With the direct method, the pattern sprue formers should
be long enough to position the wax patterns outside the
heat center of the investment and into a cold zone.
5,12,13
The
length of this sprue former will vary depending on the size
of the wax pattern(s), the type and size of the crucible for
mer, and the length of the casting ring.
With the indirect method, it is also recommended that
the wax patterns be placed off the runner bar in a loca
tion just outside the heat center of the investment. Experi
ence has shown that a 5.0-mm-long pattern sprue former is
often sufficient to provide the needed separation between
the wax patterns and the runner bar (Fig 5-6c).
11
Fig 5-5a Once cast, the indirect spruing network
can be sectioned easily with a carborundum disk. The
sections of metal can then be combined with 50% (by
weight) of new alloy ingots for another casting.
Fig 5-5b Buttons of cast high noble and noble alloys often remain unused and have to be
sent to a refiner rather than recast with new alloy because of their large size. Base metal buttons
(shown here) have no commercial value, are unusable, and can be discarded.
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89
Understanding the Spruing Process
Fig 5-6 (a) A smooth transition from sprue former to wax pattern prevents the forma
tion of irregularities in the set investment that could break off during casting. (b) In this
type III gold casting, the large runner bar and the complete metal crowns (sectioned)
were cast using a prefabricated Tri-Wax indirect sprue former (Ivoclar Vivadent). (c) This
casting demonstrates that a sprue former with a runner bar of this size (4 gauge) would
easily be able to support large metal-ceramic pontics and retainers. Note that the wax
pattern was located off the runner bar by approximately 5 mm, as evidenced by the three
pattern sprues attached to the runner bar. The pattern sprues are also offset from the two
large ingate sprues as they should be with indirect spruing.
Pattern position in relation to a runner bar is quite import
ant. Incomplete castings may result if the wax patterns are
so far away from a runner bar that molten alloy solidifies
in the sprue channel before the vacant pattern areas have
filled completely. Conversely, wax patterns placed on or
very close to the runner bar may be denied the opportu
nity to undergo orderly solidification. Either way, the pro
cess may be dominated by the premature solidification of
the runner bar itself. In other words, you could wind up
with a dense, completely cast runner bar with incomplete
restorations attached to it. Recognizing the need for bal
ance in pattern placement between being too far from and
too close to the runner bar comes with experience and
thoughtful analysis of sprue former design, measured alloy
use, and analysis of casting outcomes. But begin with a
5.0-mm-long pattern sprue former, see how that dimension
works, and make adjustments from that starting point for
future castings.
Chill set versus handle
The term chill set
11
has been used to describe what essen
tially is an auxiliary sprue former or venting sprue former
attached to the thin portion of a wax pattern. Some believe
these extra attachments are beneficial and enable the mold
to fill completely with molten alloy, thereby allowing the cast
restorations to cool quickly. According to the theory behind
chill set use, molten alloy flows to fill the chill set area (which
is closer to the external portion of the mold), and solidifica
tion takes place here first. Then the wax pattern void fills
and solidifies before the sprue or any reservoir area.
4
Certainly, placement of a handle
14
or knob, as described
by Yamamoto,
15
on a wax pattern will make it easier for the
ceramist to hold the metal castings during porcelain appli
cation. Recommended length for such a handle ranges
from 3.0 mm
14
to 4.0 mm, depending on the preference of
the technician. The handles may have to be shortened and
rounded for a subsequent intraoral try-in of either the metal
substructure or the bisque-baked porcelain to avoid poten
tial patient injury.
15
If a technician chooses not to place an external han
dle or knob on the wax pattern, some other way of grasp
ing the casting must be used. The two most common alter
nate methods are: (1) to clamp the casting with a hemostat
modified to contact the metal above the marginal area and
(2) to grasp the work with a special instrument with two
tips that fit inside the casting against the walls of the res
toration. With the first method, care must be taken to avoid
damage to the metal margin if clamping is not performed
correctly. For the second method, the challenge is finding
a way to position the work just to gain access to the inta
glio surfaces.
As illustrated in chapter 4, the recommendation to
add 18-gauge round wax to single wax patterns (see
Figs 4-13c and 4-16d) and retainers of fixed partial den
tures (see Fig 4-11) probably does more to facilitate the
handling of a cast coping or fixed partial denture frame
work during porcelain application than to serve as a chill
a b
c
a b
c
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90
The Fundamentals of Spruing, Investing, and Casting
5
set in the casting process, provided the proper spruing
technique is followed.
Historically, textbooks included drawings and photo
graphs of castings produced using direct spruing with
pattern sprue formers of different lengths (both long and
short), large cast metal buttons, and chill sets of various
geometries. However, one explanation for why chill sets
are not essential to achieve a complete casting is the suc
cessful spruing and casting accomplished with the button
less casting technique. Dental laboratories would be better
served by developing and employing standardized tech
niques that ensure consistent high-quality castings with
spent alloy in a reusable form.
Orientation of a wax pattern
The casting of an otherwise properly sprued wax pattern
can be jeopardized if that invested pattern is not oriented
correctly in the casting ring. Recall the earlier recommen
dation to attach a pattern sprue former to the thickest por
tion of the wax pattern whenever possible.
To avoid creating sharp 90-degree angles between the
sprue former and the wax pattern in the sprue former sys
tem, flare the attachment of the pattern sprue former to the
wax pattern. Any right angles in wax will be reproduced
in set casting investment as sharp ridges. These delicate
investment ridges can break off (entirely or in part) as the
heavy molten alloy forcefully enters the sprue way, pushing
contaminants forward to the critical thin areas formed by
the wax pattern margins.
Use of an orientation reference
It is essential to avoid orienting wax patterns in such a
way that molten alloy has to flow backwards, toward the
mold entrance. You actually want to take advantage of
centrifugal, rotational, and gravitational forces when using
a broken-arm centrifugal casting machine, so the molten
alloy is directed by the sprue way toward thinner sections
of the pattern areas, such as the margins. This outcome
can be achieved provided the invested waxed restorations
have been placed in the casting machine so the pattern
margins face the trailing edge of the casting arm (which is
down and to the right). See if the crucible former you are
using has any distinguishing features to help you orient the
direction of the wax patterns. If not, simply place a wax dot
on its base to create a reference mark and then invest your
patterns (Fig 5-7). The resulting indentation in the invest
ment will be visible after wax elimination when the heated
ring is placed in the casting cradle.
Location of the reservoir
The reservoir portion of a spruing system—whether it is
in the form of a 4- or 6-gauge large runner bar (ie, indi
rect sprue former) or an 8-, 10-, or 12-gauge round ball
(ie, direct sprue former)—should be oriented in what is to
become the heat center of the ring
5,12,16,17
(Fig 5-8). Such
positioning permits the reservoir to remain molten long
enough to furnish metal to the patterns until each solidi
fies completely. Aside from being in the heat center of the
Fig 5-7
(a) If your crucible former has no reference features, a wax orientation dot can
be placed directly on the base. (b) The wax orientation dot created a depression (arrow),
and the technician knows to orient the heated ring in the cradle of the casting machine
with that dot visible in the lower right quadrant. At this orientation, the pattern margins
face down and to the right. (c) The wax orientation dot will transfer to the investment.
After wax elimination, a dimple will be visible in the investment. This can be used to
identify the location of the trailing edge of the invested wax patterns for casting.
a
b
c
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91
Buttonless Casting Technique
heated mold, the reservoir should have the largest mass
of any part in the spruing system (see Fig 5-8e). In other
words, you do not want to cast a button if it can be avoided
because the button would then compete with the reservoir
to be the true heat center (Fig 5-9). This is an essential con
cept in the buttonless casting technique.
Buttonless Casting Technique
A button is solidified excess alloy that fills the mold in the
area previously occupied by the upper portion of the cruci
ble former (see Figs 5-9a and 5-9b). The goal of the button
less casting technique is to cast everything except a button
(see Figs 5-8e and 5-9c). One of the bestways to ensure
that no button is cast is to weigh the entire assembly of wax
patterns and supporting sprue system (Fig 5 -10). If a button
is present or the invested patterns are larger than the run
ner bar, these components compete with any reservoir as
the largest mass of metal
18
and influence the location of the
heat center in the casting ring
5
(see Fig 5 11).
In contrast with direct spruing where no reservoir ball is
used, a button serves as the source of molten metal for the
pattern during solidification. In such a situation, the length of
the pattern sprue former connecting the wax pattern to the
button area is critical if the button is to serve as an effective
reservoir. Should the pattern sprue former be too long, too
short, or too narrow, there is greater likelihood of a miscast.
Fig 5-9 As one is able to better estimate the amount of alloy needed in the buttonless casting technique, the size of the cast button (a) will get smaller and smaller (b) until it
is completely eliminated (c). Note the presence of the small concavity (arrow) formed in the investment by the wax orientation dot.
Geometric center = heat center
Fig 5-8 (a) It may be incorrect to conclude that
the geometric center (GC) and the heat center of the
investment are one and the same. (b) The investment
surrounding the crucible former (arrows) is less likely to
contribute significantly to the heat center of the ring. (c)
An area other than the GC of the ring may be the location
of the ring’s true heat center in an area referred to as the
center of mass of the investment. (d and e) The center of
mass of the investment can be the heat center (H) of the
ring (d) provided it contains the largest mass (ie, runner
bar) of molten alloy (e). The location of H is influenced
more by where the largest mass of alloys is than the
position of the investment center of mass.
a
a
b
b
c
c
d e
G C
Center of mass
Center of mass = Heat centerCenter of mass = Heat center
H
H
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The Fundamentals of Spruing, Investing, and Casting
5
The buttonless casting technique can be accomplished
simply by adhering to the following four simple steps:
Step 1: Place a prefabricated “button” insert in the cru
cible former opening, or dome the crucible former by
manually filling in the opening with wax, then weigh
the crucible former on a digital scale (preferred) (see
Fig 5-10). Record the weight on the base of the cruci
ble former using an indelible pen so the crucible former
does not have to be weighed again (see Fig 5-10c).
Step 2: Attach the wax patterns to a prefabricated
sprue former (preferred) or hand-fashioned spruing
network. Insert the spruing system in the crucible for
mer and secure it in place.
Step 3: Weigh the entire assembly on the scale (see
Fig 5-10d).
Step 4: Calculate the difference between the two
weight measurements. That difference is the weight of
wax above the top of the crucible former (ie, the sprue
system) (see Fig 5-10e), which is the only part that
should be cast in the buttonless casting technique.
Too much alloy may produce a button that is larger
than the runner bar. Should this occur, the heat center
shifts from the runner bar to the button itself (Fig 5 11).
Avoid casting buttons
As mentioned earlier, it is ill-advised to cast in such a way
that you end up with a collection of spent alloy in the form
of buttons of different sizes and weights (see Fig 5-5b).
In practical terms, cast buttons, even small ones, are diffi
cult to recycle because they invariably weigh more than the
amount of alloy needed for your next casting. This is espe
cially true when trying to comply with the requirement to
include at least 50% new alloy (ie, ingots) with spent metal
to replenish lost minor alloying elements and stay within the
calculated total volume of alloy needed for the next casting.
Runner bar to wax pattern relationship
If the wax patterns collectively have a larger volume than
that of the wax runner bar you selected, or if the volume of
the runner bar is too small relative to the weight of the waxed
restorations, the heat center in the actual casting may be
shifted upward toward the wax pattern area (Fig 5 -12).
Controlling the heat center location
You now know to expect to find porosity in some portion
of the casting. So it is important to remember that correct
placement of the runner bar in the mold, use of a calcu
lated volume of alloy, and not casting a button are critical
to the successful use of the buttonless casting technique
and controlling porosity location. Adherence to these rec
ommendations will help to ensure that the largest volume
of alloy (ie, the reservoir) will remain molten longer than the
other components. In this way you control the heat center
location, and the reservoir is able to furnish needed molten
Fig 5-10
(a) Wax buttons can be purchased to fill the opening in the top of the crucible former. (b) Fill the opening in the crucible former with wax (or add a wax button) and place
a large wax dot in the base to aid in identifying the orientation of the wax patterns once they have been invested. Then weigh the crucible former. (c) After the crucible former (with wax
in the opening and the wax orientation dot) has been weighed, record that weight on the base with an indelible marker. (d) A prefabricated wax sprue former (yellow wax), with attached
wax patterns, is placed in the crucible former and weighed. (e) Technique used to arrive at the weight of the wax spruing system to be cast without a button according to the buttonless
casting technique. The difference in weight between the entire spruing assembly (A) and the weight of the domed crucible former (B) is the weight of the wax pattern to be cast (C).
a b c
d e
minus
A B C
equals
Naylor_Chap_05.indd 92 9/6/17 12:07 PM
93
Spruing Considerations
metal to the patterns to complete the pattern solidification
process as designed.
When the buttonless casting process is executed prop
erly that heat center location will be the area with the larg
est mass of metal to solidify last.
If too much alloy is cast, as in Fig 5-11, the heat cen
ter will shift downward, and porosity is likely to occur in the
button, leaving a densely cast runner bar, and the wax pat
terns could be left incomplete (Figs 5-13 and 5-14). Another
scenario that might occur is where there is a large pon
tic and a smaller-diameter runner bar. In this example, the
pontic probably will be the heat center and solidify last,
making it the location of the casting porosity. When assess
ing the castings depicted in Figs 5-11 and 5-12, the runner
bars will be dense, complete, and free of porosity, and the
cast patterns will be incomplete or contain porosity—nei
ther of which are favorable outcomes (see Fig 5-13).
Spruing Considerations
Constricted spruing
Tapering, rather than flaring, the sprue former at its point of
attachment to the wax pattern is a practice referred to as
constricted spruing.
13
This taper was proposed to permit
the sprue former to serve as a reservoir, thereby decreas
ing the likelihood of suck-back porosity.
4,6,12
In practical
terms, the constriction may be helpful in the mold-filling
process, but this technique was originally advocated
for low-density base metal alloys. It has been reported that
the use of a narrow-diameter (1.0 mm) sprue former for
patterns cast in a nickel-chromium alloy had more defects
than sprue formers of 2.0- and 3.0-mm diameters, even
when vents were added.
19
Other research has shown that,
as the density of a casting alloy increases, constricted
Fig 5-11
The large button competed with the runner bar in functioning as the
reservoir and caused the heat center (H) to shift downward toward the largest
mass of metal (in this case, the button rather than the bar). This change in
thermodynamics put the pattern area at risk of not filling completely.
Fig 5-12 Likewise, the heat center (H) can shift upward to the area of the
large patterns when no button is cast and the cast restorations weigh more than
the runner bar. In this example, the runner bar is of the proper length but not the
proper gauge to support restorations this large in size and volume.
Fig 5-13 The buttonless casting technique was not used
to produce this casting. In fact, too much metal was cast, as
evidenced by the presence of a large button, and the heat center
shifted from the reservoir bar to the button. Note the large button,
a dense runner bar, and how the metal failed to fill all three crown
patterns using the indirect technique.
Fig 5-14 This casting exemplifies what can happen
with direct spruing when a button is cast, even though ball
reservoir sprue formers were used. Too much metal was
cast, and a button formed that shifted the thermodynamics
much like is illustrated in Fig 5-29b.
Center
of
mass
H
may
shift
Center
of
mass
H
may
shift
H
H
Naylor_Chap_05.indd 93 9/6/17 12:07 PM
94
The Fundamentals of Spruing, Investing, and Casting
5
spruing is more apt to interfere with mold filling and lead
to increased porosity
3,20,21
as noted in the laws of casting
(described later in this chapter).
5,17
This behavior might
be explained by the fact that alloys with a high density
tend to be sluggish when filling a heat mold compared
to lower-density metals.
Therefore, it is recommended that you follow this gen
eral rule: The greater the alloy density, the larger the sprue-
pattern access. Keep in mind that some individuals will
report success with constricted spruing for many alloy sys
tems, regardless of density levels. This is not to say that
both techniques will not work. This recommendation to limit
the use of constricted spruing is intended to promote a
spruing methodology (ie, indirect spruing and the button
less casting technique) that is more likely to offer consistent
and highly reproducible results.
Sprue former composition: Wax versus
plastic
It is not often emphasized, but the wax elimination process
for the spruing network should be different for wax versus
plastic sprue formers. Typically, casting wax melts readily
in the normal course of heating invested patterns in a burn
out furnace, leaving little concern for the presence of car
bon residues after heat soaking at the recommended max
imum furnace temperature. In fact, casting waxes certified
Fig 5-15
(a) Prefabricated wax sprue formers in blue (Ringless Casting, Whip Mix) and red (Tri-Wax, Ivoclar Vivadent). Note the variations in size for both the direct and indirect
sprue formers. Refer to Table 5-1 for comparative product information on size, color, gauge, and diameter. (b) Prefabricated indirect Ringless Casting (left) and Tri-Wax (right) wax
patterns may differ in gauge size, but they both position the runner bar in approximately the same vertical position in the crucible former and ring. (c) The red Tri-Wax sprue former
has a 4-gauge runner bar and is a better choice to support large wax patterns than the smaller yellow Ready Sprue (KerrLab) with its 6-gauge runner bar. (d and e) Schematics of the
Tri-Wax runner bar (d) and the Ready Sprue runner bar (e) show both positioned in the same general area in the mold. This illustrates how the spruing process can be standardized.
a b
c d e
Direct and indirect prefabricated wax sprue formers
Direct Indirect
Product (mfr) Size Color Gauge
Diameter
(mm) Size Color Gauge
Diameter
(mm)
Length
(cm)
Ready Sprues Yellow ~6 4.0 4.60
(KerrLab) Green ~6 4.0 4.60
Orange ~4 5.0 4.60
Red ~4 5.0 5.50
Ringless Casting Small Blue 10 2.6 Small Blue 6 4.1 4.30
System (Whip Mix) Large Red 8 3.3 Large Red 4 5.2 4.30
Tri-Wax Mini Red 12 2.1
(Ivoclar Vivadent) Small Red 10 2.6 Small Red 6 4.1 3.75
Large Red 8 3.3 Large Red 4 5.2 4.30
–, not applicable.
Table 5-1
Naylor_Chap_05.indd 94 9/6/17 12:07 PM
95
High-Heat Casting Investments
by the American Dental Association will not leave a residue
of more than 0.1% of the specimen’s original weight. How
ever, there is no assurance that all wax sprue formers per
form equally well.
On the other hand, plastic sprue formers and wax pat
terns with any plastic or acrylic resin component have to be
handled differently. Plastic materials may not burn out com
pletely through the lower temperature range of the heating
process. In fact, plastic actually has a greater potential to
leave carbon residue in the mold. Moreover, plastic tends
to undergo greater expansion before softening than does
wax. Should this expansion occur when the investment
is in the green state, it can result in mold cracking. More
importantly, if the pathway for the escape of molten wax is
blocked in any way by unmelted plastic or its residue, that
wax may overheat (ie, boil) and erode the inner surface of
the mold. The resulting castings may have a higher degree
of surface roughness due to this damage to the mold’s inte
rior surface; this is sometimes referred to as mold wash.
To remedy this problem, it has been suggested to apply
a thin layer of waxover the entire surface of a plastic sprue
former to create potential space and an escape mecha
nism for the melting wax patterns as the mold temperature
increases. But adding wax in this manner is not practical
because it not only requires more time, it can also increase
the number of potential irregularities in the investment if the
wax is not applied evenly and smoothly over the plastic
components.
Twostage wax elimination
With or without the application of a wax coating, a two-stage
wax elimination process is recommended when plastic
sprue formers
16
are used or any time resin is a part of an
invested wax pattern. As recommended by Tombasco and
Reilly,
22
begin the first stage of this two-stage wax elim
ination process with a 30-minute heat soaking at 427°C
(801°F). At the end of this 30-minute heat soaking, reset
the oven to the desired high temperature for the alloy
being cast, and continue with wax elimination as if it were
a single-stage process.
Prefabricated sprue formers
Experience has shown that when performed properly, spru
ing and investing can be made even easier and yield more
consistent results using prefabricated wax sprue formers
5
(Fig 5 -15). In particular, these ready-made wax formers are
available with runner bars or a reservoir ball for both direct
and indirect spruing to meet the needs of a wide variety of
individual cases. Ready-made wax patterns offer a predict
able and timesaving method of spruing and standardize
that methodology at the same time. The sprue formers with
runner bars are available in different gauges and lengths
and provide an element of stability to fixed partial denture
wax patterns, thus helping to avoid pattern distortion while
investing (see Figs 5-15a and 5-15c).
Perhaps of greater importance, these prefabricated
patterns enable technicians to avoid having to construct
a personal interpretation of an appropriate indirect sprue
design. Table 5-1 includes information about several dif
ferent brands of prefabricated wax sprue formers for both
direct and indirect spruing.
High-Heat Casting Investments
The elevated melting ranges of metal-ceramic alloys (see
Table 3-4) exceed the recommended upper limits for
heating gypsum-bonded investments (704°C/1,299°F)
and require high-heat, phosphate-bonded investments
or the silica-bonded investments that can be used with
alloys heated up to 1,315°C (2,399°F). In addition, alloys
with a high melting range usually undergo more con
traction during solidification (ie, solidification shrinkage)
than alloys with a lower melting range.
2
The two variet
ies of phosphate-bonded investment—carbon-containing
and carbon-free—are more widely used than the silica-
bonded materials.
In lieu of distilled water, most phosphate-bonded invest
ments require a mixture of a special liquid (ie, colloidal sil
ica) and distilled water. Concentrations of the special liquid
may range from as little as 10% (for high palladium alloys)
to as much as 100% (for cobalt-chromium alloys). Used
full strength, this liquid provides added silica that thickens
the mix, resulting in greater thermal expansion on heating.
Diluting the special liquid with distilled water has the oppo
site effect; that is, less colloidal silica reduces the amount
of potential investment expansion.
Typically, manufacturer’s instructions include a sug
gested concentration of special liquid and distilled water
as a starting point. It should be understood that the special
liquid–to-water ratio will have to be adjusted for the condi
tions of each laboratory until the desired level of investment
expansion required of each alloy type has been achieved.
Carbon-containing phosphatebonded
investments
A variety of carbon-containing phosphate-bonded cast
ing investments are available. These high-heat refractory
materials are readily identifiable by their gray-black color,
which is due to the presence of carbon even after wax elim
ination (Fig 5 -16). Divestment is easier and castings typi
cally are “cleaner,” meaning they have little or no surface
oxidation, when retrieved from a carbon-containing invest
ment.
2
Once mixed, these investments may appear coarse
in texture compared to their gypsum-bonded investment
counterparts.
Generally, carbon-containing phosphate-bonded invest
ments are recommended for gold-based metal-ceramic
alloys. They should not be used with palladium-, nickel-,
or cobalt-based alloys; these alloy systems have the poten
tial to absorb available carbon, leading to the formation of
carbides and/or porosity (due to physical carbon inclu
sions). Some manufacturers believe that with the use of an
appropriate wax elimination technique, no carbon should
remain after heat soaking at the high temperature setting.
They contend that there is more risk of carbon contamina
Naylor_Chap_05.indd 95 9/6/17 12:07 PM
96
The Fundamentals of Spruing, Investing, and Casting
5
tion when a torch is improperly adjusted than there is in
a carbon-containing investment. However, a simple labora
tory test will show that even after a 90-minute heat soaking
at 816°C (1,501°F), a substantial amount of carbon appears
to remain in the critical interior area of a carbon-containing
investment
21
(see Fig 5-16).
Carbon-free phosphatebonded
investments
The carbon-free phosphate-bonded investments are readily
identifiable by their white color, both before and after they
are mixed (see Fig 5-9). These products were developed
to address concerns for the potential interaction of car
bon with the nickel- and cobalt-containing casting alloys,
as well as the various palladium-based metals. Proponents
of these investments prefer the added security of using a
carbonless investment system and the avoidance of pro
longed high-temperature wax elimination (871°C/1,600°F)
required to remove all residual carbon.
23
Some carbon-free investments have a grainy tex
ture like their carbon-containing counterparts, but there
are also a number of investments with a fine texture. For
example, Cera-Fina (Whip Mix) is a fine-grained carbon-
free phosphate-bonded investment that produces a
smooth, creamy consistency and provides users with
ample working time (8 to 10 minutes) to permit investing
of multiple units and rings from the same mix (see Fig 5-9).
Other similar products are available from leading invest
ment manufacturers.
Investmentcasting alloy interaction
Variations can be seen in the performance of alloys with
different investments.
11,24,25
It is possible that the problems
attributed to a given metal are, in fact, linked to the invest
ment. Excessive nodule formation on the casting surface
(Fig 5 -17) and castings with fins may occur more frequently
with one brand of investment than another. A number of
manufacturers indicate that their alloys may be used with
virtually any commercially available phosphate-bonded
investment. That may not always be true, and there are
studies to support the view that alloy-investment pairing can
influence results.
5,23–26
Therefore, it may be prudent to con
duct laboratory tests for potential adverse alloy-investment
interactions before making a large purchase of any new
investment product.
23
Casting ring liners
Asbestos had been the material traditionally used for lin
ing casting rings until its potential risk to the health of
dental laboratory technicians was discovered.
27,28
Evi
Fig 5-16a Carbon is still present in this carbon-containing investment even after
wax elimination at 816°C (1,501°F) for 90 minutes. The reducing atmosphere it cre
ates is helpful to gold-based alloys (ie, reduces oxidation), but the residual carbon is a
potential contaminant for palladium-, nickel-, and cobalt-based metal-ceramic alloys.
Fig 5-16b Even after holding this carbon-containing phosphate-bonded invest
ment for 90 minutes at 871°C (1,600°F), carbon was clearly evident in the critical
pattern areas. However, the white appearance of the top and bottom of the heated
investment would suggest the carbon had been eliminated in those areas.
Fig 5-17 A rough casting with multiple nodules may be caused by several factors,
including a less-than-ideal alloy-investment pairing or a failure to evacuate the invest
ment’s gaseous by-products during mixing. To diagnose the problem, try another
investment to determine if what occurred is due to a lapse in technique, an issue with
the investment, or an adverse alloy-investment interaction.
Naylor_Chap_05.indd 96 9/6/17 12:07 PM
97
Melting and Casting Techniques
dently, the asbestos fiber bundles were found to pro
duce hazardous-sized respirable particles capable of
causing lung disease (Fig 5 -18a).
Alternative nonasbestos ring liner materials fall into three
categories: ceramic (aluminum silicate), cellulose (paper),
and a ceramic-cellulose combination.
29
The microstruc
ture of these alternatives to asbestos varies (Figs 5 -18b
to 5-18d). Furthermore, the relative safety of ceramic ring
liners remains uncertain because aluminum silicate also
appears capable of producing hazardous-sized respirable
particles.
29
These health and safety concerns can be elimi
nated by switching to a ringless casting system.
Investing Technique
In metal-ceramic technology, the dental laboratory techni
cian relies on the same basic technique used to produce
castings with crown-and-bridge alloys with the exception
that high-heat phosphate-bonded investments are required
in place of gypsum-bonded investments. When casting a
new alloy, ask the manufacturer for the name of recom
mended investments, suggested liquid-to-powder ratios,
and an initial special liquid concentration.
Wax elimination technique
The wax elimination process, commonly known as the
“burnout technique,” refers to the protocol followed to heat
invested wax patterns in preparation for casting. The low-
and high-temperature settings and rate of rise vary for alloys
of different compositions. For example, high-temperature
settings range up to 871°C (1,600°F) for base metal alloys
and between 760°C (1,400°F) and 871°C (1,600°F) for
noble and high noble alloys. It is always advisable to refer
to the instructions on an alloy package for the recom
mended temperature settings (low and high) and wax elim-
ination time for each specific metal. Also, check the invest
ment manufacturer’s suggested temperature rate of rise for
the burnout furnace, and adhere to the rate established for
each type of investment.
Melting and Casting Techniques
Assuming that a case has been properly sprued and
invested and that the wax elimination process has been
completed, the next crucial steps involve melting and cast
ing the metal-ceramic alloy. A poorly adjusted torch or
flawed casting technique can ruin all of your efforts to this
point. Given the importance of these procedures, the fol
lowing additional topics are included to help you better
understand the casting process.
Casting torch selection
There are two types of torch tips to choose from when select
ing casting equipment: a multiorifice tip and a single-orifice
tip
5
(Fig 5 -19).
Multiorice tip
Of the two torch tips, the multiorifice design is generally
preferred for melting metal-ceramic alloys. Its principal
advantage is that heat can be distributed over a wide area,
which allows for more uniform heating of an alloy. This fea
ture is especially helpful for melting multiple ingots when
casting a high-fusing base metal alloy.
Single-orice tip
The single-orifice tip, on the other hand, allows the oper
ator to concentrate more heat in one area, but that area
of heat is smaller than what is produced by the multiori
Fig 5-18 (a) Asbestos liner (original magnification 500). (b) Ceramic liner (original magnification 150). (c) Cellulose liner (original magnification500). (d) Ceramic-
cellulose combination liner (original magnification 500).
a b c d
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98
The Fundamentals of Spruing, Investing, and Casting
5
fice tip (see Fig-19). Controlled, limited heating provided
by a single-orifice torch tip is desirable for torch soldering,
especially presoldering procedures (see Fig 7-29a).
Choice of fuels
There are three fuel sources that potentially could be used
to melt metal-ceramic alloys
30
:
1. Acetylene. This colorless gas has a distinc
tive garlic-like odor. It will burn in air and can generate
a flame approaching 1,649°C (3,000°F). Unfortunately,
acetylene is usually contaminated with carbon and
other elements, so it should not be used to melt
ceramic alloys.
2. Natural gas. This fuel is the byproduct of the natural
decomposition of organic matter in the ground. When
mixed with air, the natural gas flame approaches a
temperature of 1,204°C (2,199°F). But if oxygen is used
to replace air, a natural gas flame can reach the tem
peratures required to melt the high-fusing noble and
base metal alloys. So technically, natural gas is an
acceptable fuel source when combined with oxygen,
although it is not ideal. It is not regarded as an ideal
heat source because natural gas has its own set of
challenges. Inadequate gas line pressure, fluctuations
in pressure levels, water contamination, and variations
in composition among gas supply companies are just
some of the potential problems encountered by natu
ral gas users. Despite these issues, natural gas is a
widely used fuel.
3. Propane. The problems associated with natural gas
are avoided entirely by using bottled propane gas
with oxygen. The constant, regulated mixture of pure
uncontaminated propane—when combined with oxy
gen—provides a clean, consistent burn that allows the
operator to obtain a more ideal melt, free of concerns
for pressure fluctuations and alloy contamination.
Casting equipment
The high noble, noble, and base metal alloys can be cast
either with a torch in a centrifugal casting machine or torch
less in an induction casting machine. Irrespective of the
type of casting equipment selected, the low-density base
metal alloys typically require an additional winding of the
casting arm in a centrifugal casting machine to ensure ade
quate casting pressure.
Casting crucibles
Three types of casting crucibles are available for dental
casting alloys of different composition: (1) high-heat, (2)
clay, and (3) carbon crucibles.
Fig 5-19 (a) The proper flame pattern for a multi
orifice (left) and a single-orifice (right) torch tip differ
in appearance. (b) Base metal alloys require a different
flame configuration from that of noble alloys. Some alloy
manufacturers may recommend increasing the inner blue
cone length by up to 0.75 in.
a
b
Multiorifice tip Single-orifice tip
Oxidizing zone Reducing zone
(about 2.5 in)
Sharp blue tips
(about 0.25 in)
Melt the alloy with the reducing flame only
10.0 in
0.50.75 in
Oxidizing zone Reducing zone
(1.52 in)
Sharp
blue
cone
(11.25 in)
Naylor_Chap_05.indd 98 9/6/17 12:07 PM
99
Melting and Casting Techniques
High-heat crucibles. Crucibles made of zircon are cap able
of withstanding the temperatures required to melt metal-
ceramic as well as crown-and-bridge alloys. Both zircon
and quartz crucibles are recommended for casting noble
and base metal-ceramic alloys, but the zircon type gener
ally is the more durable of the two. Additionally, zircon cru
cibles are available in a variety of colors (yellow, pink, blue,
and white), so a particular color can be dedicated to a spe
cific alloy to avoid potential contamination during the melt.
After selecting the appropriate type of casting crucible
(ie, zircon or quartz), it is advisable to heat the crucible in
the burnout oven along with the casting ring or invested
patterns, when using a ringless casting technique. Preheat
ing the crucible prevents spalling (ie, cracking) and helps
prolong a crucible’s working life. Never cast different alloys
in the same crucible because this also will increase the like
lihood of alloy contamination and may alter the properties
of the resulting casting. Instead, use color-coded crucibles
(Fig 5-20) or, in the case of white crucible formers, simply
carve the alloy name in a visible portion of the crucible side
or base to permit ready identification.
The following two types of casting crucibles should not
be used when melting metal-ceramic alloys.
Clay crucibles. Clay crucibles are not recommended when
melting any type of metal-ceramic alloy. They are accept
able for melting gold-based crown-and-bridge alloys, but
they have a greater potential to deteriorate when subjected
to the high temperatures required to cast ceramic alloys,
which can possibly contaminate the melt.
31
Carbon crucibles. Carbon crucibles should also not be
used with metal-ceramic alloys. Although carbon crucibles
are quite suitable when melting gold-based alloys, expo
sure to such a ready source of carbon can lead to significant
contamination of palladium-, nickel-, and cobalt-based met
als. As mentioned previously, that contamination can result
in carbide formation that can embrittle these alloys and/or
cause porosity from carbon inclusions.
Electric casting machines
Electric casting machines with carbon crucibles are appro
priate for gold-based alloys, but they are not recommended
for palladium-, nickel-, and cobalt-based alloys because of
the potential for carbon contamination.
Compounds for airborne-particle
abrasion
Commercially available abrasive compounds such as alumi
num oxide (Al
2
O
3
), a general-purpose blasting compound,
and glass beads are recommended to remove investment
and surface oxides from a metal casting. A 50-μm grit, non
recycled (white) aluminum oxide abrasive is the material
of choice for air-abrading porcelain-bearing surfaces and
dental porcelain
32
(Fig 521). Aluminum oxides in colors
other than white may contain impurities such as iron. To
check, you can run a magnet through the material if you
suspect it is contaminated.
The term sandblasting is very much engrained in the
vernacular of dental laboratory technology, but the com
pounds used are not actually some form of sand (ie, sili
con dioxide). More than likely, technicians and clinicians
are using an aluminum oxide (recycled or nonrecycled) or
glass beads. So the phrase airborne-particle abrasion is
Fig 5-20a Zircon Alumina Crucibles (KerrLab) are more durable than the quartz
type. Color-coded crucibles permit assignment of different alloys to specific colors.
The name of a particular alloy or alloy type can be scribed on the back of a crucible
as well.
Fig 5-20b A white quartz casting crucible in a centrifugal casting machine.
Fig 5-21 Pure aluminum oxide (left) is white and less likely to contain contami
nants than is a comparable gray all-purpose abrasive (right).
Naylor_Chap_05.indd 99 9/6/17 12:07 PM
100
The Fundamentals of Spruing, Investing, and Casting
5
preferred and considered more accurate when describing
this procedure.
The Laws of Casting
Casting dental alloys is both an art and a science as well
as a process governed by numerous rules, or “laws.” Per
sonal interpretation of these laws is demonstrated in a tech
nician’s approach to the procedure—that is, the art of cast
ing. Integrating the theoretical principles supporting this art
enables the user to blend science with art in dental technol
ogy. At times, casting problems can be attributed to poor
technique, omitting or altering a step, and/or a failure to
adhere to one or more of the basic concepts inherent to
the casting process.
Three key concepts of casting
The foundation of these laws is based on three key con
cepts: (1) alloy shrinkage is inherent in the casting process
as the molten alloy transitions from a liquid state to the solid
state (ie, solidification shrinkage), (2) every casting con
tains porosity attributed to the solidification shrinkage, and
(3) porosity invariably occurs in the area of the casting that
is last to solidify.
The challenge faced by the technician is how best to
use that knowledge of the science of casting to plan the
location of that porosity in an area of the spruing system
away from the actual restoration(s).
Building on the historical work of Ingersoll and Wan
dling
17
and their 14 laws governing the principles from spru
ing to casting, the author created an expanded set of 17
recommendations dealing with each phase of the casting
process (spruing, investing, wax elimination, melting, and
casting procedures) in 1992. Collectively, these guidelines
are referred to as the laws of casting. Included in these
laws are the principles behind the recommended button
less casting technique described in this chapter. As with
any set of regulations, there are penalties when these laws
are not followed, so technicians should acquaint them
selves with the laws and the adverse outcomes if those
laws are violated, in full or part.
For the purposes of illustration, the required steps to cast
a three-unit fixed partial denture pattern are presented in
the following sections to illustrate several key points related
to the spruing, investing, and casting procedures.
Spruing
1st law of casting
Attach the pattern sprue former to the thickest part of
the wax pattern. As molten alloy enters the heated mold, it
moves from the reservoir to fill the pattern margins. To facil
itate this movement, alloy should flow from areas of greater
volume to areas of lesser volume (ie, the margins).
Before beginning the spruing process, it is best to take
a few moments to evaluate the size, geometry, and configu
ration of the wax pattern(s) to be cast to identify any special
requirements that may need to be considered (Fig 5-22a).
Select a prefabricated indirect sprue former of an appropri
ate gauge and length (Figs 5-22b and 5-22c). Lute a wax
pattern sprue former to the most practical portion of the
occlusal/incisal surface of each wax pattern (Fig 5-22d).
Warm the prefabricated indirect sprue former and gently
alter the shape to align it, as needed, with the curvature
of the arch (Figs 5-22e and 5-22f). If 18-gauge wax han
dles are to be applied (and they are highly recommended,
if a lingual metal collar is present), place them on the lin
gual surfaces of the metal-ceramic wax patterns while on
the master cast and before luting the patterns to the run
ner bar (Fig 5-23).
Except with thin anterior copings, do not place the pat
tern sprue former in a cutback area if an adjacent fully con
toured cusp is available. Molten metal flowing from a thin
region to a thicker region of the wax pattern may solidify
before the mold is completely filled, resulting in what is
referred to as a cold shut.
Penalties. The penalties for not obeying this law are cold
shuts, short margins, or incomplete castings.
2nd law of casting
Orient wax patterns so all of the restoration margins
face the trailing edge when the ring is positioned in the
casting machine. To identify the orientation of the wax pat
terns, add a wax dot to the crucible former so you know
how to position the ring in the casting cradle once those
patterns have been invested (see Fig 5-7).
Penalties. The penalties for not obeying this law are cold
shuts, short margins, and incomplete castings.
3rd law of casting
Position the wax patterns in a “cold zone” of the
investment mold and the reservoir in the heat center
of the casting ring. The coolest parts of the mold (ie,
cold zones) are located at the end of the ring and along
the ring periphery. The hottest part (ie, heat center) of
the casting ring is located near the center of the ring, as
determined by the location of the largest mass of metal in
the investment (see Figs 5-8d and 5-8e). Limit the amount
of investment covering the patterns to approximately 6.0
mm (0.25 inch), and position the reservoir in the heat cen
ter.
10,33
Adherence to this law increases the likelihood that
casting porosity will occur in the reservoir rather than in
the restoration.
Penalty. The penalty for not obeying this law is porosity in
the actual restorations.
4th law of casting
A reservoir must have sufficient molten alloy to accom
modate the shrinkage that occurs within the resto
rations. Ideally, alloy that fills the pattern (ie, restoration)
areas will solidify first. As that molten metal cools, it shrinks
Naylor_Chap_05.indd 100 9/6/17 12:07 PM
101
The Laws of Casting
Fig 5-22 (a) Three-unit fixed partial denture substructure (see chapter 4) after completion of the wax cutback. (b) This Ready Sprue (KerrLab) has a runner bar with a
diameter (6 gauge) and length sized to support this fixed partial denture wax pattern. (c) This 4-gauge indirect Ready Sprue former is too long, and the gauge size is too
large, which will require casting more alloy than needed for this fixed denture substructure. Rather than modify the sprue former, it would be advised to switch to a more
appropriately sized prefabricated pattern. (d) Wax pattern sprue formers of an appropriate diameter and length were selected and luted to each wax pattern. (e) The runner
bar portion of the indirect sprue former is luted to the three individual pattern sprue formers. Before being attached, the Ready Sprue can be warmed slightly, adjusted to
mimic the actual curvature of the arch (rather than to remain straight), and allowed to cool to room temperature before being attached to the wax-up.(f) The prefabricated
indirect sprue former (left) can also be modified by removing the central web of wax (right) to allow investment to surround the runner bar.
a b
c d
e f
Fig 5-23 Do not forget to attach 18-gauge wax to the lingual surfaces to create handles
on each casting. (This step is optional but highly recommended, as shown with these indi
vidual wax patterns.) These handles make it easier to hold the castings through all stages of
porcelain application and avoid potential damage to delicate metal margins from hemostats
and other similar metal instruments.
Naylor_Chap_05.indd 101 9/6/17 12:07 PM
102
The Fundamentals of Spruing, Investing, and Casting
5
through solidification shrinkage and creates a vacuum. For
complete casting to occur, the vacuum must be able to
draw additional molten metal from an adjacent source—
namely, the reservoir. A runner bar can be an effective res
ervoir provided the diameter of the bar is equal to or larger
than the thickest cross-sectional area of the wax pattern
to be cast (see Figs 5-6, 5-8e, 5-15c, and 5-22b). Units
sprued directly should include a round ball that potentially
can serve as a reservoir in a properly designed spruing
system (see Fig 5-6).
Penalties. The penalties for not obeying this law are
incomplete castings, porosity in the restorations, and/
or suck-back porosity.
5th law of casting
Do not cast a button if a runner bar or other internal res
ervoir is used (see Fig 5-9). With indirect spruing, the larg
est mass of metal should be the reservoir (see Fig 5-8e).
The presence of a button is counterproductive because it
can draw available molten alloy from the bar, shift the loca
tion of the heat center, and reduce the feed of metal to the
restorations (see Fig 5-11). Therefore, the volume of the
wax patterns should not be larger than that of the runner
bar, if the bar is to act as a true reservoir (see Figs 5-8e
and 5-12). Weigh the sprued patterns, and use the wax
pattern–alloy conversion chart (appendix F) to calculate
the amount of alloy needed based on the density of the
particular alloy to be cast (Fig 5-24).
Penalties. The penalties for not obeying this law are poros
ity in the restorations, suck-back porosity, and potential dis
tortion during porcelain firing.
6th law of casting
Turbulence must be minimized, if not totally eliminated.
Pathways for the flow of metal should be smooth, gradual,
and without impediments. Eliminate sharp turns, restric
tions, points, or impingements that might create turbulence
and occlude air in the casting. Restrictions or constrictions
can accelerate the metal’s rate of flow and abrade the mold
surface, possibly resulting in mold wash.
17
Penalties. The penalties for not obeying this law are mold
wash, voids in the casting, and surface pitting. Voids can
be created by the occlusion of air from turbulent metal flow.
Mold wash can remove investment particles from the mold’s
inner surface and carry them ahead of the alloy. These
entrapped particles can produce surface pits and incom
plete margins.
7th law of casting
Select a casting ring of sufficient length and diame
ter to accommodate the patterns to be invested. If a
casting ring is to be used, it should be of sufficient length
and diameter to permit the patterns to be located 6.0 mm
(0.25 inch) apart and 6.0 mm (0.25 inch) from the top of
the investment. A minimum of 9.0 mm (0.35 inch) of invest
ment should separate the patterns from the ring liner, if
used (see Fig 5-3). If too little investment covers the wax
patterns, the molten alloy is more likely to break through
the mold on casting. Conversely, filling the ring with too
much investment may position the wax patterns too close
to the heat center of the mold and impair the escape of
the gases produced by the investment material when heat
ing the mold to the desired high temperature for casting.
In addition, an insufficient amount of investment between
wax patterns and the casting ring (or exterior of the mold
with the ringless casting technique) or casting ring liner
can result in uneven investment expansion (setting and
thermal).
Penalties. The penalties for not obeying this law are
mold fracture, casting fins (Fig 5-25), and porosity in the
restorations.
Fig 5-24a Apply a thin layer of debubblizer (ie, wax pattern cleaner) to the wax
patterns and indirect spruing system and allow it to dry thoroughly before investing.
Fig 5-24b Weigh the assembly following the method described in Fig 5-10, and
calculate the amount of metal needed based on the density of the alloy (see appendix
F). An electronic scale provides more accurate weight information than a balance scale.
Naylor_Chap_05.indd 102 9/6/17 12:07 PM
103
Investing and wax elimination
8th law of casting
Reduce the surface tension and increase the wettabil
ity of the wax patterns. Proper surface tension reduction
is important because it enables the casting investment to
cover the wax patterns completely and thus reduce the
potential for air-bubble entrapment.
34
A wetting agent, such
as a wax pattern cleaner, should be brushed or sprayed
on the wax patterns and dried thoroughly before invest
ing
10
(see Fig 5-24a). Apply the liquid sparingly. Too much
wetting agent may create a surface film that can dilute and
weaken the investment in that area and produce bubbles or
fins on the casting (see Fig 5-25). On the other hand, too lit
tle wetting agent can result in inadequate reduction of sur
face tension and lead to increased air entrapment. When a
wetting agent is applied correctly, the result is a clean wax
surface, which improves the ability of the casting invest
ment to wet the patterns more completely (Fig 5-26).
Penalty. The penalty for not obeying this law is the formation
of bubbles on the surface of the patterns that become nodules
on the casting surface as a result of air entrapment (ie, too lit
tle wetting agent) or excess liquid (ie, too much wetting agent).
9th law of casting
Weigh any bulk investment and measure the investment
liquid for a precise powder-to-liquid ratio. The correct
proportions of investment powder to liquid and any required
dilution of the special liquid (ie, colloidal silica) with distilled
water should be established for each type of alloy. Two key
variables to consider relative to the powder-to-liquid ratio
are (1) the total volume of liquid, and (2) the concentration
of the special liquid.
In terms of liquid volume, the total powder-to-liquid
ratio to use initially is that recommended by the investment
manufacturer.
10
Changes can be made to any dilution ratio
based on actual outcomes in your dental laboratory (eg,
amount of expansion, quality of fit). Bear in mind that if
too little liquid is dispensed, the investment will be viscous
and may lack proper flow and workability. A thick mixture,
if workable, will produce increased setting expansion of
the investment, which ultimately may result in loose-fitting
castings. On the other hand, if too much investment liq
uid is used—whether it is 100% colloidal silica or a com
bination of special liquid and distilled water, in proportion
to the specified weight of powder—a thinner mixture will
result. That diluted investment mixture will reduce setting
and thermal expansion, and the castings produced may be
tighter fitting than desired or not fit at all.
Furthermore, care should be taken when dispensing the
special liquid because the concentration of this colloidal
silica also directly affects thermal expansion. For example,
if 100% of this liquid is used, more expansion occurs during
heating the investment as compared to a dilution using just
50% special liquid and 50% distilled water. In other words,
with the total liquid volume unchanged, the greater the vol
ume of colloidal silica, the greater the overall expansion
(setting and thermal).
Fig 5-25 This casting has fins caused either by excess wax pattern cleaner or by
improper heating of the mold that led to mold cracks filled by alloy on casting.
Penalties. The penalty for not obeying this law is ill-fitting
castings. Restorations can be too tight (ie, not enough spe
cial liquid) or too loose (ie, too much special liquid).
10th law of casting
Avoid the incorporation of air in the casting invest
ment and remove the ammonia gas by-product of
phosphate-bonded investments by mixing under vac
uum. Vacuum mixing the casting investment extracts air
and gaseous by-products, something hand spatulation
cannot do. Areas of the mold filled with dense bubble-free
investment, when set and heated, will expand more uni
formly than sections of investment containing large voids
created by air entrapped in the mix. The required mixing
time will depend on the type of investment used and the
mixing speed (slow versus high). In general, mechanical
mixing hastens the setting reaction compared to hand mix
ing, so expect the amount of working time to be reduced.
Penalties. The penalties for not obeying this law are small
nodules on the casting, a weak mold, or distortion of the
casting.
11th law of casting
Allow the casting investment to set completely before
initiating the wax elimination procedure. If setting of the
investment is not complete at the time a ring or mold (for
the ringless technique) is placed in the burnout oven, the
mold may be in the weak green state. If so, the investment
is not quite strong enough to withstand the pressure and
resist the expansion produced by the formation of steam in
the early stage of heating. Steam is a normal by-product of
heating the free water in the set investment during wax elim
ination. Also, the amount of setting expansion may be less
than what would be achieved if the invested ring were left
undisturbed for the entire recommended setting time. With
the investment still in the green state, it lacks early strength
and, therefore, could develop cracks or possibly fracture.
The Laws of Casting
Naylor_Chap_05.indd 103 9/6/17 12:07 PM
104
The Fundamentals of Spruing, Investing, and Casting
5
For best results, wax elimination should be initiated only
after the investment has been allowed to remain undis
turbed for the recommended setting time, typically 1 hour.
If wax patterns are invested at the end of the day or are not
to be cast immediately after setting is complete, keep the
invested ring hydrated by storing it in a sealed container or
plastic bag. Should the casting investment be allowed to
dry out for any reason, it is difficult to replace the free water
in the interior of the investment and achieve total rehydra
tion, even if the mold were soaked in water.
There are special investments that have a short bench
setting time, can be placed directly in a heated oven (rather
than a room-temperature furnace), and can be heated rap
idly to high temperature. One example is FastFire 15 (Whip
Mix), which has a minimum 15-minute setting time (30 min
utes for a mix of 60 g or less), as compared to the stan
dard 60-minute setting time for other phosphate-bonded
investments.
Penalties. The penalties for not obeying this law are mold
cracking/blowout or casting fins (see Fig 5-25).
12th law of casting
Use a wax elimination technique that is appropriate for
the type of patterns involved (wax versus plastic) and
recommended for the type of casting alloy selected.
Plastic sprue formers should be heated slowly to allow the
plastic to soften, melt, and gradually flow out of the mold
without exerting pressure on the investment walls. The
safest way to achieve this is by using the two-stage wax
elimination technique described earlier.
22
Set the burn
out oven for a slow-to-moderate rate of rise to permit the
heat to move through the investment slowly and achieve
uniform thermal expansion. If this process is rushed and
wax elimination is incomplete, the channels of the spru
ing system may be partially blocked by wax or plastic res
idue (carbon). In addition, air may not be able to escape
completely as the fluid casting alloy enters the mold.
22,23
With a two-stage technique, it is recommended to plan at
least a 30-minute heat soaking at 427°C (801°F) for the
first wax-elimination stage. After completion of this first
stage, raise the final temperature of the oven and heat
Fig 5-26 (a) Vacuum mix the investment to remove gaseous byproducts and produce a smooth mix. (Carbon-free phosphate-bonded investment shown.) (b) After vacuum
mixing, carefully add a small portion of the mixed investment to the inside of the individual wax retainers to prevent the entrapment of air and ensure complete wetting of the
patterns. The bristles of brushes can trap air and potentially transfer bubbles to the wax pattern, so some technicians prefer to apply the investment to the patterns with the tip of a
glass rod. (c) After all the wax surfaces have been covered by investment, place the casting ring in the crucible former. (d) Once a single wax pattern is secure in a round casting
ring, carefully add enough investment to cover the wax patterns without trapping air. This technique is the same for multiple patterns using large and small round and oval rings.
a b
c d
Naylor_Chap_05.indd 104 9/6/17 12:07 PM
105
the ring to the recommended high temperature setting.
Then heat soak the ring for the recommended hold time of
the metal-ceramic alloy manufacturer.
Penalties. The penalties for not obeying this law are cold
shuts, short margins, mold cracks, and/or casting fins (see
Fig 5-25).
Melting and casting
13th law of casting
Adequate heat must be available to melt and cast the
alloy properly.
1
The selected heat source should be capa
ble of melting a high-heat metal-ceramic alloy to the point
of sufficient fluidity for casting (see Fig 5-19). Exercise cau
tion, because prolonged heating, caused by an improperly
adjusted torch, can prevent the alloy from attaining the flu
idity needed for complete mold filling and compensation
for heat loss.
1
In fact, too much heat or too high a melting
temperature can burn off minor alloying elements through
vaporization and/or oxidation (ie, burned metal). Be mind
ful that ingots of high noble and noble alloys will slump and
flow together to form one molten mass in the casting cruci
ble. In sharp contrast, most base metal alloy ingots retain
their basic shape on heating and do not flow together to
form a single mass of molten metal (Fig 5-27). Therefore,
identifying the correct heating point to initiate the cast of
nickel- and cobalt-based alloys is a skill a dental laboratory
technician must expect to acquire over time.
Penalties. The penalties for not obeying this law are over
heated metal, loss of minor alloying elements, cold shuts,
short margins, rough castings, and/or investment break
down (too much heat exposure).
14th law of casting
When torch casting, use the reducing zone of the flame,
not the oxidizing zone, to melt the alloy.
2,5
An improperly
adjusted torch can permit carbon or oxygen to be absorbed
by an alloy while it is being heated. But the exclusive use
of the reducing zone of the casting torch flame minimizes
the likelihood of unwanted metal oxidation and gas absorp
tion and helps to ensure a proper melt is achieved
2,5
(see
Fig 5-19).
The Laws of Casting
Fig 5-27 (a) Most base metal alloys do not pool when heated, so you should use the fewest ingots possible and have them in contact with each other in the crucible. The alloy
instructions should describe the features to look for to identify when the alloy is ready to be cast. (b) Through mutual contact, the ingots can be heated uniformly. (c) Note how the
edges of the ingots have rounded and remain in contact, but the ingots have not pooled. (d) These ingots are now molten under an oxide surface skin and are ready to be cast. Learn
how to recognize the appearance of the molten state for each type of base metal alloy, because this point in the melting process is different from that of high noble and noble alloys.
a b
c d
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106
The Fundamentals of Spruing, Investing, and Casting
5
Penalties. The penalties for not obeying this law are porosity
(from gas absorption) and/or the possibility of an unplanned
change in the alloy’s coefficient of thermal expansion due to
alloy contamination.
15th law of casting
Provide enough force to cause the liquid alloy to flow
into the heated mold. Adjust the casting machine to the
requirements of each alloy. Low-density metals are lighter
and more fluid, so they typically require additional windings
of a centrifugal casting arm as compared to more slug
gish high-density gold-based alloys. However, do not over
wind the casting machine.
10,35
Penalties. The penalties for not obeying this law are cold
shuts, short margins, incomplete castings, mold fractures,
and/or fins (ie, too much force).
16th law of casting
Cast toward the margins of the wax patterns. Place the
heated ring in the casting cradle using the orientation dot
(see Figs 5-7 and 5-9c) or other landmark as a guide, so
the wax pattern margins face the trailing edge (the second
law of casting). In a centrifugal casting machine, the metal
will flow downward and to the right, taking advantage of the
centrifugal, rotational, and gravitational forces exerted on
the molten alloy.
17,36
Penalties. The penalties for not obeying this law are cold
shut, short margins, and otherwise incomplete castings.
17th law of casting
Do not quench the ring immediately after casting.
Once casting is complete, remove the ring from the cast
ing machine and set it aside undisturbed. You want to
allow the alloy and the investment to cool to room tem
perature. Uneven cooling and shrinkage between alloy
and investment can exert tensile forces on the casting.
37
After casting, the alloy may not possess sufficient strength
to resist these forces, and the restoration could tear if
quenched. Unlike copper-containing type III and type IV
gold alloys, metal-ceramic alloys are not formulated to be
heat softened (ie, quenched) and then later heat hardened
(ie, strengthened). Casting rings should be bench cooled.
Once they can be handled safely, divest the casting and
proceed with processing.
Penalty. The penalty for not obeying this law is hot tears in
the restoration.
Judging Success in Casting
Casting success typically is associated with an
intact, well-fitting restoration, coping, or substructure. How
ever, the true challenge in the casting process is in the
management of the thermodynamics—that is, to direct the
location of the casting porosity to a noncritical area.
With indirect spruing, the most ideal location for porosity
to reside is the undersurface of the runner bar opposite the
restorations (Fig 5-28). When direct spruing includes a res
ervoir ball, the side of the ball opposite the restoration is an
ideal location for casting porosity to occur with this spruing
technique (Fig 5-29).
Castability tests
Subjective castability testing using an abstract mesh pat
tern has not been shown to correlate well with actual cast
ing parameters (ie, high temperature for the mold and
melting range for alloys
23
) (Figs 5-30 and 5-31). Replicas
of metal-ceramic substructures should be used in lieu of
abstract patterns such as the Whitlock test
23
to evaluate
Fig 5-28a When no button is cast and the reservoir is the last portion of the casting
to solidify, the porosity should appear on the underside of the bar, away from the resto
rations (cast in a nickel-chromium-beryllium alloy).
Fig 5-28b If the runner bar is too small or a button is cast, the bar may be the best
part of the casting. However, there is an increased likelihood that the restorations will
be incomplete, as seen in this casting. Note the dense surface and absence of porosity
on the underside of the bar, an indication that the button supported the bar rather than
the patterns.
Naylor_Chap_05.indd 106 9/6/17 12:07 PM
107
Analyzing Casting Failures
the castability of a new alloy. It is advisable to evaluate that
new alloy under the conditions of your dental laboratory,
using the procedures and equipment you have on hand
to ensure the outcomes represent potential results in your
actual work environment.
Analyzing Casting Failures
Despite concerted efforts to follow the recommended pro
cedures outlined in this chapter, casting failures and mis
haps are bound to occur in the dental laboratory (see
Fig 5-28b). Errors in technique and, on occasion, material
failures can unexpectedly result in unsatisfactory castings.
By standardizing your technique and paying strict attention
to each step involved in spruing, investing, and casting, it
is possible to minimize the number of actual miscasts and
to control the location of the casting porosity (see Figs 5-28
and 5-29). Proper storage and rotation of casting invest
ment inventories also help to ensure consistent and accu
rate performance of your expendable materials.
When casting failures do occur, troubleshoot each mis
cast to diagnose the cause or causes of the problem so
corrective measures can be taken before making any
additional castings. Mackert
10
developed one of the most
comprehensive methods to assess casting failures from
preparation of a custom impression tray to waxing, spru
ing, investing, wax elimination, and ultimately, melting of
the alloy. Failure analysis is an important process to follow
when results occur outside an expected outcome.
Fig 5-29 (a) When the wax pattern is weighed and cast with the proper volume of metal for direct spruing with the buttonless casting technique, the pattern area can solidify
first and cast a complete restoration. The ball area is the largest mass of metal and functions as a reservoir. (b) This drawing illustrates how the presence of a button—the largest
mass of metal—shifts the location of the heat center from the ball to the button and can result in an incomplete casting with direct spruing. (c) In this example of direct spruing,
alloy solidification shrinkage occurs in the area of the reservoir ball, so the casting porosity should appear on the side opposite the restoration. In this case the porosity resides
on the underside of the reservoir ball. The large round base was thin with little mass and illustrates the principle depicted in Fig 5-29a.
a b c
Fig 5-30a A nickel-chromium-beryllium alloy repro
duced 100% of this abstract mesh pattern.
Fig 5-30b The alloy in Fig 5-30a not only reproduced
the entire abstract mesh pattern (left), it also produced a
complete metal-ceramic substructure pattern (right) when
tested using the same casting parameters.
Fig 5-31 The best castability performance of a nickel-
chromium beryllium-free alloy with the abstract test was
to cast 30.9% of the mesh pattern (left). However, this
alloy reproduced a complete metal-ceramic substruc
ture pattern (right) using the same casting parameters,
suggesting the mesh test is not a good predictor of alloy
castability.
23
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108
The Fundamentals of Spruing, Investing, and Casting
5
Summary
The very nature of the differences between metal-ceramic
alloys and conventional gold-based crown-and-bridge
metals necessitates certain adjustments to your waxing,
spruing, and investing techniques. Whether it is in under
standing the demands of low-density alloys, selecting an
appropriate spruing technique (indirect versus direct), or
determining how to properly cast a high-fusing alloy, knowl
edge of the theoretical aspects of waxing, spruing, and
investing is required. This chapter brings together the fun
damental principles of these technical procedures in the
laws of casting with emphasis on the buttonless casting
technique.
What’s Next?
Before proceeding to the discussion of metal finishing
(chapter 7), it is important to understand how dental por
celain attaches to a metal-ceramic alloy and what factors
ensure the ceramic veneer remains attached to a prop
erly designed metal-ceramic structure (chapter 6). In this
way, you will gain a better appreciation of the materials,
procedures, and techniques so essential to these aspects
of metal-ceramic technology.
References
To access annotations to this reference list, please go to:
www.quintpub.com/Metal-CeramicTechnology.
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The Fundamentals of Spruing, Investing, and CastingChapter584Success in the fabrication of metal-ceramic crowns and fixed partial dentures depends to a large extent on the abil-ity to obtain high-quality castings that are not only properly designed but also fit well. This chapter provides an over-view of important topics related to the fundamental princi-ples of spruing, investing, and casting. In addition, empha-sis is placed on the laboratory steps for the use of the buttonless casting technique.TerminologyWhile investing and casting wax patterns are procedures that have been part of dentistry for quite some time, it is not uncommon for several key terms involved in these pro-cesses to be used interchangeably but not always cor-rectly. Therefore, the following descriptions and explana-tions are presented to clarify the intended meanings and use of the terminology found in this chapter. Instead of an alphabetic list, the terms appear in a sequence that is intended to promote a better understanding of their mean-ing and recommended use. • Sprue former In its simplest application, a sprue former is a piece of dental wax or plastic, generally round in shape, that is attached to the thickest portion of a wax pattern at one end and a crucible former at the other.1 Sprue formers are used for direct spruing (ie, extended from the top of a cruci-ble former straight to the wax pattern) and indirect spruing (ie, oriented circuitously from the crucible for-mer to the pattern area). Today, prefabricated wax and plastic sprue former patterns are available for pur-chase in different shapes and sizes (ie, gauges) for both the direct and indirect spruing methods. • Pattern sprue former The sprue former that is attached directly to a wax pattern can be described as a pattern sprue former to dif-ferentiate it from other compo-nents of the sprue former net-work, such as the runner bar (ie, connector bar) and the ingate sprue formers that are part of indirect spruing. The length and gauge of a pattern sprue former are deter-mined by the size (ie, volume) and thickness of the wax pattern(s) to be cast.• Sprue way The channel (ie, void) in the set investment (ie, mold) created after the elimination of the wax or plastic sprue former network (ie, wax elimination or “burnout” procedure), is called the sprue way. A sprue way is formed with both direct or indi-rect spruing, but the sprue way network tends to be more complex with indirect spruing than with direct spruing, as the name alone would suggest. • Ingate sprue former This is a term preferred by the author to describe the large-diameter sprue formers that are attached to the crucible former at one end and the runner bar at the other end in the indirect spruing pro-cess. Ingate (ie, feeder) sprue Naylor_Chap_05.indd 84 9/6/17 12:07 PM 85Terminologyformers must be of sufficient diameter to permit needed volume of molten alloy to fill spaces created by the runner bar and the attached wax patterns. So ingate sprue formers would be a smaller gauge (ie, larger diameter) than the gauge of the pattern sprue formers. This makes more sense when you consider that ingate sprue formers are attached to the under-side of a runner bar and pattern sprue formers are attached to the superior side of the runner bar. And because molten alloy should flow from areas of large volume to areas of small volume, the sprue way net-work must be designed properly for the casting pro-cess to be successful.• Sprue Technically, a sprue is cast metal. More specifically, it is the portion of a metal casting that reproduced the sprue former. (Note: Some incorrectly use the word sprue when referring to the wax sprue former). In other words, a sprue is a part of a cast-ing while the sprue former is the wax or plastic form that creates the channel in the set investment after the wax (or in some cases, plastic) has been eliminated in a burnout furnace.• Direct spruing This is the name given to the technique in which a sprue former system creates a channel that extends from the opening in a mold (created by the crucible former) directly to a wax pattern area. • Indirect spruing The term indi-rect spruing describes a sprue former system that forms multiple channels in the mold that run indirectly from the open end of the mold to the patterns, with a runner bar strategically posi-tioned between the mold open-ing and the pattern area.• Runner bar The part of the wax indirect spruing system that is between the mold opening and the wax pattern area (see arrow). The role of a runner bar is to support multiple single wax patterns, a fixed partial denture, or a combination of both. The function of the runner bar is to create a large sprue way in the mold that can retain a reservoir of molten alloy during the casting process. When the thermodynam-ics of casting are controlled, as with buttonless casting (described later), molten metal in this large area serves as a reservoir of alloy to ensure the pattern areas are able to solidify completely and solidify before the run-ner bar. A runner bar also is referred to as a reservoir bar or a connector bar, and the term may be used to describe the wax component as well as the metal com-ponent once it has been reproduced in metal.• Reservoir That portion of a sprue former (in wax) or sprue system (in cast metal) that retains a large vol-ume of wax or alloy, respectively. A straight sprue for-mer with a round ball is an example of direct spruing with the ball portion serving as a reservoir. With indi-rect spruing, the runner bar creates a large space in the mold, after wax elimination, to hold molten metal. An effective reservoir is expected to be the largest vol-ume of metal cast and the area of the casting that solidifies last. A reservoir ball or runner bar can be a component in a spruing system but not actually func-tion effectively as a reservoir if the thermodynamics allow these areas to solidify early and a button (or other mass of alloy) to solidify last. The goal is to have molten alloy reproduce the patterns areas first, fol-lowed by the ball reservoir in direct spruing and the runner bar with indirect spruing, and not to produce a button with either method. • Crucible former The round or oval base to which a sprue for-mer or prefabricated sprue for-mer pattern (wax or plastic and direct or indirect) is attached is referred to as a crucible former. • Wax gauge This term is used to identify the diameter (ie, gauge) of the different wax sprue formers avail-able for use in the dental laboratory. The gauge num-ber is inversely related to the diameter of the wax, so the larger the gauge number, the smaller the diameter of the wax sprue former. The gauge sizes used in den-tistry generally range from 18 gauge (1.02 mm/0.04 inch) to 4 gauge (5.20 mm/0.21 inch).2 Popular sizes include 4 gauge, 6 gauge (4.12 mm/0.16 inch), 8 gauge (3.26 mm/0.13 inch), 10 gauge (2.59 mm/0.10 inch), and 12 gauge (2.05 mm/0.08 inch).• Porosity The term refers to the presence of voids (ie, spaces) in an otherwise solid structure. In a casting, porosity forms in the area of metal that solidifies last.• Solidification shrinkage At the end of the casting process, a molten alloy transitions from the liquid state to the solid state, and alloy shrink-age is inherent in that transition. In other words, solid-dification shrinkage (or casting shrinkage) is a normal phenomenon, one that technicians counterbalance through planned investment expansion. • Casting porosity Porosity occurs with every casting in the area that solidifies last, and that porosity (see arrow) most often is referred to as casting poros-ity or localized shrinkage porosity. When casting, the challenge is to understand and control the thermody-namics of the process, so the porosity that is going to occur resides in a noncritical portion of the sprue network rather than somewhere in the pattern area. Controlling porosity is best achieved by choosing an appropriate spruing method, creating smooth tran-sitions from the opening in the mold to the pattern abcminus equalsNaylor_Chap_05.indd 85 9/6/17 12:07 PM 86The Fundamentals of Spruing, Investing, and Casting5area(s), establishing an appropriate sprue design, avoiding sharp internal angles in the mold, and cast-ing with the correct volume of alloy. Too little or too much alloy can spell disaster for a casting regardless of the spruing design.• Buttonless casting technique This term refers to a casting technique for both direct and indirect spruing in which the wax pattern to be reproduced is weighed and the amount of alloy needed is calculated based on that weight and the density of the alloy. No button is cast, and the largest mass of metal is the runner bar (indirect) or ball reservoir (direct).Spruing Techniques A wax spruing system creates a channel or series of chan-nels in the set investment through which molten alloy can flow to reach the wax pattern areas. Consequently, one of the first decisions to make before preparing a wax pattern for investing is which type of spruing system to employ. For best results, this decision should be made on a case-by-case basis to ensure that sufficient molten alloy will be made available to reproduce all of the invested units. No single spruing method is universally accepted as the technique of choice. On the contrary, opinions among den-tal technicians and recommendations by alloy manufac-turers differ so widely that at times they may conflict with one another. For example, one manufacturer may suggest direct spruing, whereas others may insist that only indirect spruing should be used with their alloys. To complicate matters, reports in the dental literature offer differing views on the subject. Consequently, it is extremely important to understand the general principles of spruing, including spruing meth-ods (direct versus indirect), sprue placement, sprue gauge selection, sprue length, reservoir location, constricted spru-ing, sprue composition (wax versus plastic), and the value of prefabricated wax sprue formers. Such knowledge, coupled with hands-on practical experience, will greatly improve your chances for consistent, successful casting results. As explained in the terminology section, wax patterns can be sprued in one of two ways—directly or indirectly. Each method has its advantages and disadvantages, so it is important to understand the philosophy behind both techniques to ensure each is employed correctly when used.Direct spruing The basic concept with direct spruing is to permit mol-ten metal to flow directly to the pattern area in the heated investment mold. This method is less complex than the indi-rect technique and usually requires less time and effort to complete. With the direct spruing technique, one end of a straight sprue former is luted (ie, attached) to the thick-est cross-sectional part of the wax pattern (ie, the wax pat-tern sprue former), while the other end is luted to the top of the crucible former. Having a ball reservoir between the pattern and the cru-cible former is one way to modify the sprue former, and pre-fabricated direct wax sprue formers with a reservoir ball are available commericially.3–5 The purpose of any reservoir is to supply the molten metal needed to fill the pattern areas completely. Even with the added presence of a reservoir ball, the spruing method is still considered to be direct. Direct spruing is used most frequently to cast single units (Fig 5-1) and small multi-unit patterns. However, a basic weakness of this technique is the potential for suck-back porosity, which is evidenced by the presence of a void at the junction of the restoration and the sprue3 (Fig 5-2). This type of porosity is more likely to occur when the casting includes a button and no reservoir (see Fig 5 -1).Fig 5-1 Casting produced using direct spruing. The straight sprue former can be modified with a ball reservoir, but the spruing method is still direct.Fig 5-2 (a) One problem associated with direct spruing is the increased likelihood of suck-back porosity (arrow) at the pattern-sprue junction. Suck-back porosity cannot always be detected externally but may be obvious on examination of the intaglio surface of the casting. (b) Suck-back porosity is apparent (arrow) in this cross-sectional view of a maxillary central incisor coping that was cast directly.a bNaylor_Chap_05.indd 86 9/6/17 12:07 PM 87Spruing TechniquesIndirect spruing Once the casting process has been initiated, molten alloy does not flow straight from the casting crucible into the pat-tern area in the heated mold.4–6 Instead, the alloy takes a circuitous (ie, indirect) route to reach the pattern areas—hence the name indirect spruing (Fig 5-3). Pattern sprue formers are used to attach wax patterns to the superior surface of a round wax runner bar (typically 4 or 6 gauge). The void in the mold formed by the bar eventu-ally will be filled with molten alloy via the channels created by two large ingate sprue formers. The bar’s large volume houses molten metal for a sufficient length of time to per-mit the pattern areas to fill with metal first and draw on this additional molten alloy as needed to complete the solidifi-cation process.7 (This is why the runner bar is referred to as a reservoir bar.) If no reservoir is provided and a button is cast, alloy can be drawn from the pattern sprue, and metal in the pattern areas acts as a source (ie, reservoir) of molten alloy for the runner bar or even the button. This behav-ior is directly opposite of the intended casting dynamics. Again, because the molten metal cannot flow directly to the wax pattern areas, this method is referred to as indi-rect spruing. There is evidence that alloy composition influences the manner in which molten metal fills a mold during casting. For example, it has been shown that a palladium-silver alloy flows unidirectionally, whereas type III gold fills in a random or scattered fashion.7Opinions differ as to the value of indirect spruing for a single crown or multiple single units when direct spruing can produce acceptable results.8,9 Users of the indirect technique might cite the advantages of enhanced predict-ability and reliability in casting, better management of the volume of metal used to cast, and greater control of cast-ing porosity. Prefabricated sprue formersPrefabricated sprue formers of different lengths and diam-eters are available for both direct and indirect spruing (Fig 5-4). These products not only make it possible to stan-dardize a technique for greater consistency, they also help eliminate errors in sprue design and permit the standard-ization of reservoir location as compared to direct spruing. Users of high noble and noble alloys will find that the cast indirect sprue network can be sectioned into smaller com-ponents of a size that can be reused far more readily than can accumulated buttons (Fig 5-5). Fig 5-3 Indirect spruing. These general guidelines should be mod-ified to meet the requirements of each case.Fig 5-4 Prefabricated direct sprue formers with a reservoir ball of different gauges (bottom) and indirect sprue formers (top) with runner bars of different gauges (Whip Mix).¼ in / 6 mm (maximum)Wax patterns with attached sprue formersCasting ringInvestmentRing liner (if used)Runner barIngate sprue formerCrucible formerNaylor_Chap_05.indd 87 9/6/17 12:07 PM 88The Fundamentals of Spruing, Investing, and Casting5Alloys shrinkage versus alloy porosityProper spruing and casting methods are key to managing two facts of the casting process: (1) investment expansion is needed to compensate for alloy shrinkage, and (2) all castings contain porosity. As mentioned in the terminology section, there is an inherent shrinkage of a dental casting alloy as it transitions from a molten liquid to a solid state, and this change of state is referred as solidification shrink-age, and the region that solidifies last contains the casting porosity. The ever-present challenge in the dental labora-tory is to control the location of that porosity, but that can be achieved using proper spruing and casting methods, such as the buttonless casting techniques.Understanding the Spruing ProcessA multitude of factors must be considered before deciding if wax patterns are to be sprued directly or indirectly. The following section presents several key variables that lay the foundation for understanding the rationale for the button-less casting technique and the laws of casting that follow.Sprue former placementIdeally, the sprue former attached to the wax pattern (ie, the pattern sprue former as opposed to an ingate sprue former) should be luted to the thickest cross-sectional area of the pat-tern to allow the molten alloy to flow from regions of large vol-ume (ie, thick areas) to regions of lesser volume (ie, thin sec-tions).10 Placing the sprue former elsewhere on a wax pattern might result in an incomplete casting if a thin section under-goes solidification before the mold has filled completely. Of course, there are exceptions to every rule. With many anterior wax copings, this option is not available or advisable because the patterns may be small and thin. In such instances, the most practical sprue former location is the midincisal region. The same logic should be applied to locate an appropriate site on a molar coping that is to receive complete porcelain coverage. In both these exam-ples, make certain the attachment of the sprue former to wax pattern is smooth and uniform with no sharp edges,10 and flare this transition from sprue former to wax pattern11 (Fig 5-6a). Avoid forming ridges that later may become irregularities in the investment, which potentially could break off during casting and contaminate the molten alloy.Sprue former gaugeA pattern sprue former of sufficient diameter (ie, gauge) should be selected to supply the volume of alloy required by the patterns to be cast. It is important to remember that the larger the gauge number, the smaller the diameter of the sprue former. Manufacturers invariably include sprue gauge recommendations for their alloys, but these sugges-tions are made without any direct knowledge of the size, geometry, thickness, and number of patterns to be cast. So it is the laboratory technician’s responsibility to carefully assess the wax patterns, evaluate the particular require-ments of each case, and determine the appropriate gauge that is needed. With direct spruing, it is advisable to use a sprue former with a reservoir ball. With indirect spru-ing, select a prefabricated sprue former with a runner bar that is greater in diameter than the thickest cross-sectional area of the largest wax pattern to be cast. This requirement is especially critical when metal pontics and large molar retainers are to be cast (Fig 5-6b). Pattern sprue former lengthWith the direct method, the pattern sprue formers should be long enough to position the wax patterns outside the heat center of the investment and into a cold zone.5,12,13 The length of this sprue former will vary depending on the size of the wax pattern(s), the type and size of the crucible for-mer, and the length of the casting ring. With the indirect method, it is also recommended that the wax patterns be placed off the runner bar in a loca-tion just outside the heat center of the investment. Experi-ence has shown that a 5.0-mm-long pattern sprue former is often sufficient to provide the needed separation between the wax patterns and the runner bar (Fig 5-6c).11 Fig 5-5a Once cast, the indirect spruing network can be sectioned easily with a carborundum disk. The sections of metal can then be combined with 50% (by weight) of new alloy ingots for another casting.Fig 5-5b Buttons of cast high noble and noble alloys often remain unused and have to be sent to a refiner rather than recast with new alloy because of their large size. Base metal buttons (shown here) have no commercial value, are unusable, and can be discarded.Naylor_Chap_05.indd 88 9/6/17 12:07 PM 89Understanding the Spruing ProcessFig 5-6 (a) A smooth transition from sprue former to wax pattern prevents the forma-tion of irregularities in the set investment that could break off during casting. (b) In this type III gold casting, the large runner bar and the complete metal crowns (sectioned) were cast using a prefabricated Tri-Wax indirect sprue former (Ivoclar Vivadent). (c) This casting demonstrates that a sprue former with a runner bar of this size (4 gauge) would easily be able to support large metal-ceramic pontics and retainers. Note that the wax pattern was located off the runner bar by approximately 5 mm, as evidenced by the three pattern sprues attached to the runner bar. The pattern sprues are also offset from the two large ingate sprues as they should be with indirect spruing. Pattern position in relation to a runner bar is quite import-ant. Incomplete castings may result if the wax patterns are so far away from a runner bar that molten alloy solidifies in the sprue channel before the vacant pattern areas have filled completely. Conversely, wax patterns placed on or very close to the runner bar may be denied the opportu-nity to undergo orderly solidification. Either way, the pro-cess may be dominated by the premature solidification of the runner bar itself. In other words, you could wind up with a dense, completely cast runner bar with incomplete restorations attached to it. Recognizing the need for bal-ance in pattern placement between being too far from and too close to the runner bar comes with experience and thoughtful analysis of sprue former design, measured alloy use, and analysis of casting outcomes. But begin with a 5.0-mm-long pattern sprue former, see how that dimension works, and make adjustments from that starting point for future castings.Chill set versus handle The term chill set11 has been used to describe what essen-tially is an auxiliary sprue former or venting sprue former attached to the thin portion of a wax pattern. Some believe these extra attachments are beneficial and enable the mold to fill completely with molten alloy, thereby allowing the cast restorations to cool quickly. According to the theory behind chill set use, molten alloy flows to fill the chill set area (which is closer to the external portion of the mold), and solidifica-tion takes place here first. Then the wax pattern void fills and solidifies before the sprue or any reservoir area.4Certainly, placement of a handle14 or knob, as described by Yamamoto,15 on a wax pattern will make it easier for the ceramist to hold the metal castings during porcelain appli-cation. Recommended length for such a handle ranges from 3.0 mm14 to 4.0 mm, depending on the preference of the technician. The handles may have to be shortened and rounded for a subsequent intraoral try-in of either the metal substructure or the bisque-baked porcelain to avoid poten-tial patient injury.15 If a technician chooses not to place an external han-dle or knob on the wax pattern, some other way of grasp-ing the casting must be used. The two most common alter-nate methods are: (1) to clamp the casting with a hemostat modified to contact the metal above the marginal area and (2) to grasp the work with a special instrument with two tips that fit inside the casting against the walls of the res-toration. With the first method, care must be taken to avoid damage to the metal margin if clamping is not performed correctly. For the second method, the challenge is finding a way to position the work just to gain access to the inta-glio surfaces.As illustrated in chapter 4, the recommendation to add 18-gauge round wax to single wax patterns (see Figs 4-13c and 4-16d) and retainers of fixed partial den-tures (see Fig 4-11) probably does more to facilitate the handling of a cast coping or fixed partial denture frame-work during porcelain application than to serve as a chill a bca bcNaylor_Chap_05.indd 89 9/6/17 12:07 PM 90The Fundamentals of Spruing, Investing, and Casting5set in the casting process, provided the proper spruing technique is followed. Historically, textbooks included drawings and photo-graphs of castings produced using direct spruing with pattern sprue formers of different lengths (both long and short), large cast metal buttons, and chill sets of various geometries. However, one explanation for why chill sets are not essential to achieve a complete casting is the suc-cessful spruing and casting accomplished with the button-less casting technique. Dental laboratories would be better served by developing and employing standardized tech-niques that ensure consistent high-quality castings with spent alloy in a reusable form.Orientation of a wax pattern The casting of an otherwise properly sprued wax pattern can be jeopardized if that invested pattern is not oriented correctly in the casting ring. Recall the earlier recommen-dation to attach a pattern sprue former to the thickest por-tion of the wax pattern whenever possible. To avoid creating sharp 90-degree angles between the sprue former and the wax pattern in the sprue former sys-tem, flare the attachment of the pattern sprue former to the wax pattern. Any right angles in wax will be reproduced in set casting investment as sharp ridges. These delicate investment ridges can break off (entirely or in part) as the heavy molten alloy forcefully enters the sprue way, pushing contaminants forward to the critical thin areas formed by the wax pattern margins. Use of an orientation reference It is essential to avoid orienting wax patterns in such a way that molten alloy has to flow backwards, toward the mold entrance. You actually want to take advantage of centrifugal, rotational, and gravitational forces when using a broken-arm centrifugal casting machine, so the molten alloy is directed by the sprue way toward thinner sections of the pattern areas, such as the margins. This outcome can be achieved provided the invested waxed restorations have been placed in the casting machine so the pattern margins face the trailing edge of the casting arm (which is down and to the right). See if the crucible former you are using has any distinguishing features to help you orient the direction of the wax patterns. If not, simply place a wax dot on its base to create a reference mark and then invest your patterns (Fig 5-7). The resulting indentation in the invest-ment will be visible after wax elimination when the heated ring is placed in the casting cradle. Location of the reservoir The reservoir portion of a spruing system—whether it is in the form of a 4- or 6-gauge large runner bar (ie, indi-rect sprue former) or an 8-, 10-, or 12-gauge round ball (ie, direct sprue former)—should be oriented in what is to become the heat center of the ring5,12,16,17 (Fig 5-8). Such positioning permits the reservoir to remain molten long enough to furnish metal to the patterns until each solidi-fies completely. Aside from being in the heat center of the Fig 5-7 (a) If your crucible former has no reference features, a wax orientation dot can be placed directly on the base. (b) The wax orientation dot created a depression (arrow), and the technician knows to orient the heated ring in the cradle of the casting machine with that dot visible in the lower right quadrant. At this orientation, the pattern margins face down and to the right. (c) The wax orientation dot will transfer to the investment. After wax elimination, a dimple will be visible in the investment. This can be used to identify the location of the trailing edge of the invested wax patterns for casting.abcNaylor_Chap_05.indd 90 9/6/17 12:07 PM 91Buttonless Casting Techniqueheated mold, the reservoir should have the largest mass of any part in the spruing system (see Fig 5-8e). In other words, you do not want to cast a button if it can be avoided because the button would then compete with the reservoir to be the true heat center (Fig 5-9). This is an essential con-cept in the buttonless casting technique. Buttonless Casting TechniqueA button is solidified excess alloy that fills the mold in the area previously occupied by the upper portion of the cruci-ble former (see Figs 5-9a and 5-9b). The goal of the button-less casting technique is to cast everything except a button (see Figs 5-8e and 5-9c). One of the bestways to ensure that no button is cast is to weigh the entire assembly of wax patterns and supporting sprue system (Fig 5 -10). If a button is present or the invested patterns are larger than the run-ner bar, these components compete with any reservoir as the largest mass of metal18 and influence the location of the heat center in the casting ring5 (see Fig 5 -11).In contrast with direct spruing where no reservoir ball is used, a button serves as the source of molten metal for the pattern during solidification. In such a situation, the length of the pattern sprue former connecting the wax pattern to the button area is critical if the button is to serve as an effective reservoir. Should the pattern sprue former be too long, too short, or too narrow, there is greater likelihood of a miscast.Fig 5-9 As one is able to better estimate the amount of alloy needed in the buttonless casting technique, the size of the cast button (a) will get smaller and smaller (b) until it is completely eliminated (c). Note the presence of the small concavity (arrow) formed in the investment by the wax orientation dot. Geometric center = heat centerFig 5-8 (a) It may be incorrect to conclude that the geometric center (GC) and the heat center of the investment are one and the same. (b) The investment surrounding the crucible former (arrows) is less likely to contribute significantly to the heat center of the ring. (c) An area other than the GC of the ring may be the location of the ring’s true heat center in an area referred to as the center of mass of the investment. (d and e) The center of mass of the investment can be the heat center (H) of the ring (d) provided it contains the largest mass (ie, runner bar) of molten alloy (e). The location of H is influenced more by where the largest mass of alloys is than the position of the investment center of mass. aabbccd eG CCenter of massCenter of mass = Heat centerCenter of mass = Heat centerHHNaylor_Chap_05.indd 91 9/6/17 12:07 PM 92The Fundamentals of Spruing, Investing, and Casting5The buttonless casting technique can be accomplished simply by adhering to the following four simple steps: • Step 1: Place a prefabricated “button” insert in the cru-cible former opening, or dome the crucible former by manually filling in the opening with wax, then weigh the crucible former on a digital scale (preferred) (see Fig 5-10). Record the weight on the base of the cruci-ble former using an indelible pen so the crucible former does not have to be weighed again (see Fig 5-10c).• Step 2: Attach the wax patterns to a prefabricated sprue former (preferred) or hand-fashioned spruing network. Insert the spruing system in the crucible for-mer and secure it in place. • Step 3: Weigh the entire assembly on the scale (see Fig 5-10d).• Step 4: Calculate the difference between the two weight measurements. That difference is the weight of wax above the top of the crucible former (ie, the sprue system) (see Fig 5-10e), which is the only part that should be cast in the buttonless casting technique. Too much alloy may produce a button that is larger than the runner bar. Should this occur, the heat center shifts from the runner bar to the button itself (Fig 5 -11). Avoid casting buttonsAs mentioned earlier, it is ill-advised to cast in such a way that you end up with a collection of spent alloy in the form of buttons of different sizes and weights (see Fig 5-5b). In practical terms, cast buttons, even small ones, are diffi-cult to recycle because they invariably weigh more than the amount of alloy needed for your next casting. This is espe-cially true when trying to comply with the requirement to include at least 50% new alloy (ie, ingots) with spent metal to replenish lost minor alloying elements and stay within the calculated total volume of alloy needed for the next casting. Runner bar to wax pattern relationshipIf the wax patterns collectively have a larger volume than that of the wax runner bar you selected, or if the volume of the runner bar is too small relative to the weight of the waxed restorations, the heat center in the actual casting may be shifted upward toward the wax pattern area (Fig 5 -12). Controlling the heat center locationYou now know to expect to find porosity in some portion of the casting. So it is important to remember that correct placement of the runner bar in the mold, use of a calcu-lated volume of alloy, and not casting a button are critical to the successful use of the buttonless casting technique and controlling porosity location. Adherence to these rec-ommendations will help to ensure that the largest volume of alloy (ie, the reservoir) will remain molten longer than the other components. In this way you control the heat center location, and the reservoir is able to furnish needed molten Fig 5-10 (a) Wax buttons can be purchased to fill the opening in the top of the crucible former. (b) Fill the opening in the crucible former with wax (or add a wax button) and place a large wax dot in the base to aid in identifying the orientation of the wax patterns once they have been invested. Then weigh the crucible former. (c) After the crucible former (with wax in the opening and the wax orientation dot) has been weighed, record that weight on the base with an indelible marker. (d) A prefabricated wax sprue former (yellow wax), with attached wax patterns, is placed in the crucible former and weighed. (e) Technique used to arrive at the weight of the wax spruing system to be cast without a button according to the buttonless casting technique. The difference in weight between the entire spruing assembly (A) and the weight of the domed crucible former (B) is the weight of the wax pattern to be cast (C). a b cd eminusA B CequalsNaylor_Chap_05.indd 92 9/6/17 12:07 PM 93Spruing Considerationsmetal to the patterns to complete the pattern solidification process as designed. When the buttonless casting process is executed prop-erly that heat center location will be the area with the larg-est mass of metal to solidify last. If too much alloy is cast, as in Fig 5-11, the heat cen-ter will shift downward, and porosity is likely to occur in the button, leaving a densely cast runner bar, and the wax pat-terns could be left incomplete (Figs 5-13 and 5-14). Another scenario that might occur is where there is a large pon-tic and a smaller-diameter runner bar. In this example, the pontic probably will be the heat center and solidify last, making it the location of the casting porosity. When assess-ing the castings depicted in Figs 5-11 and 5-12, the runner bars will be dense, complete, and free of porosity, and the cast patterns will be incomplete or contain porosity—nei-ther of which are favorable outcomes (see Fig 5-13).Spruing ConsiderationsConstricted spruingTapering, rather than flaring, the sprue former at its point of attachment to the wax pattern is a practice referred to as constricted spruing.13 This taper was proposed to permit the sprue former to serve as a reservoir, thereby decreas-ing the likelihood of suck-back porosity.4,6,12 In practical terms, the constriction may be helpful in the mold-filling process, but this technique was originally advocated for low-density base metal alloys. It has been reported that the use of a narrow-diameter (1.0 mm) sprue former for patterns cast in a nickel-chromium alloy had more defects than sprue formers of 2.0- and 3.0-mm diameters, even when vents were added.19 Other research has shown that, as the density of a casting alloy increases, constricted Fig 5-11 The large button competed with the runner bar in functioning as the reservoir and caused the heat center (H) to shift downward toward the largest mass of metal (in this case, the button rather than the bar). This change in thermodynamics put the pattern area at risk of not filling completely.Fig 5-12 Likewise, the heat center (H) can shift upward to the area of the large patterns when no button is cast and the cast restorations weigh more than the runner bar. In this example, the runner bar is of the proper length but not the proper gauge to support restorations this large in size and volume.Fig 5-13 The buttonless casting technique was not used to produce this casting. In fact, too much metal was cast, as evidenced by the presence of a large button, and the heat center shifted from the reservoir bar to the button. Note the large button, a dense runner bar, and how the metal failed to fill all three crown patterns using the indirect technique. Fig 5-14 This casting exemplifies what can happen with direct spruing when a button is cast, even though ball reservoir sprue formers were used. Too much metal was cast, and a button formed that shifted the thermodynamics much like is illustrated in Fig 5-29b.Center of massH may shiftHeat center Center of massH may shiftHHNaylor_Chap_05.indd 93 9/6/17 12:07 PM 94The Fundamentals of Spruing, Investing, and Casting5spruing is more apt to interfere with mold filling and lead to increased porosity3,20,21 as noted in the laws of casting (described later in this chapter).5,17 This behavior might be explained by the fact that alloys with a high density tend to be sluggish when filling a heat mold compared to lower-density metals. Therefore, it is recommended that you follow this gen-eral rule: The greater the alloy density, the larger the sprue- pattern access. Keep in mind that some individuals will report success with constricted spruing for many alloy sys-tems, regardless of density levels. This is not to say that both techniques will not work. This recommendation to limit the use of constricted spruing is intended to promote a spruing methodology (ie, indirect spruing and the button-less casting technique) that is more likely to offer consistent and highly reproducible results.Sprue former composition: Wax versus plasticIt is not often emphasized, but the wax elimination process for the spruing network should be different for wax versus plastic sprue formers. Typically, casting wax melts readily in the normal course of heating invested patterns in a burn-out furnace, leaving little concern for the presence of car-bon residues after heat soaking at the recommended max-imum furnace temperature. In fact, casting waxes certified Fig 5-15 (a) Prefabricated wax sprue formers in blue (Ringless Casting, Whip Mix) and red (Tri-Wax, Ivoclar Vivadent). Note the variations in size for both the direct and indirect sprue formers. Refer to Table 5-1 for comparative product information on size, color, gauge, and diameter. (b) Prefabricated indirect Ringless Casting (left) and Tri-Wax (right) wax patterns may differ in gauge size, but they both position the runner bar in approximately the same vertical position in the crucible former and ring. (c) The red Tri-Wax sprue former has a 4-gauge runner bar and is a better choice to support large wax patterns than the smaller yellow Ready Sprue (KerrLab) with its 6-gauge runner bar. (d and e) Schematics of the Tri-Wax runner bar (d) and the Ready Sprue runner bar (e) show both positioned in the same general area in the mold. This illustrates how the spruing process can be standardized. a bc d eDirect and indirect prefabricated wax sprue formersDirect IndirectProduct (mfr) Size Color GaugeDiameter (mm) Size Color GaugeDiameter (mm)Length (cm)Ready Sprues – – – – – Yellow ~6 4.0 4.60 (KerrLab) – – – – – Green ~6 4.0 4.60– – – – – Orange ~4 5.0 4.60 – – – – – Red ~4 5.0 5.50Ringless Casting Small Blue 10 2.6 Small Blue 6 4.1 4.30 System (Whip Mix) Large Red 8 3.3 Large Red 4 5.2 4.30Tri-Wax Mini Red 12 2.1 – – – – – (Ivoclar Vivadent) Small Red 10 2.6 Small Red 6 4.1 3.75 Large Red 8 3.3 Large Red 4 5.2 4.30–, not applicable.Table 5-1Naylor_Chap_05.indd 94 9/6/17 12:07 PM 95High-Heat Casting Investmentsby the American Dental Association will not leave a residue of more than 0.1% of the specimen’s original weight. How-ever, there is no assurance that all wax sprue formers per-form equally well. On the other hand, plastic sprue formers and wax pat-terns with any plastic or acrylic resin component have to be handled differently. Plastic materials may not burn out com-pletely through the lower temperature range of the heating process. In fact, plastic actually has a greater potential to leave carbon residue in the mold. Moreover, plastic tends to undergo greater expansion before softening than does wax. Should this expansion occur when the investment is in the green state, it can result in mold cracking. More importantly, if the pathway for the escape of molten wax is blocked in any way by unmelted plastic or its residue, that wax may overheat (ie, boil) and erode the inner surface of the mold. The resulting castings may have a higher degree of surface roughness due to this damage to the mold’s inte-rior surface; this is sometimes referred to as mold wash.To remedy this problem, it has been suggested to apply a thin layer of waxover the entire surface of a plastic sprue former to create potential space and an escape mecha-nism for the melting wax patterns as the mold temperature increases. But adding wax in this manner is not practical because it not only requires more time, it can also increase the number of potential irregularities in the investment if the wax is not applied evenly and smoothly over the plastic components. Two-stage wax elimination With or without the application of a wax coating, a two-stage wax elimination process is recommended when plastic sprue formers16 are used or any time resin is a part of an invested wax pattern. As recommended by Tombasco and Reilly,22 begin the first stage of this two-stage wax elim-ination process with a 30-minute heat soaking at 427°C (801°F). At the end of this 30-minute heat soaking, reset the oven to the desired high temperature for the alloy being cast, and continue with wax elimination as if it were a single-stage process. Prefabricated sprue formers Experience has shown that when performed properly, spru-ing and investing can be made even easier and yield more consistent results using prefabricated wax sprue formers5 (Fig 5 -15). In particular, these ready-made wax formers are available with runner bars or a reservoir ball for both direct and indirect spruing to meet the needs of a wide variety of individual cases. Ready-made wax patterns offer a predict-able and timesaving method of spruing and standardize that methodology at the same time. The sprue formers with runner bars are available in different gauges and lengths and provide an element of stability to fixed partial denture wax patterns, thus helping to avoid pattern distortion while investing (see Figs 5-15a and 5-15c). Perhaps of greater importance, these prefabricated patterns enable technicians to avoid having to construct a personal interpretation of an appropriate indirect sprue design. Table 5-1 includes information about several dif-ferent brands of prefabricated wax sprue formers for both direct and indirect spruing. High-Heat Casting InvestmentsThe elevated melting ranges of metal-ceramic alloys (see Table 3-4) exceed the recommended upper limits for heating gypsum-bonded investments (704°C/1,299°F) and require high-heat, phosphate-bonded investments or the silica-bonded investments that can be used with alloys heated up to 1,315°C (2,399°F). In addition, alloys with a high melting range usually undergo more con-traction during solidification (ie, solidification shrinkage) than alloys with a lower melting range.2 The two variet-ies of phosphate-bonded investment—carbon-containing and carbon-free—are more widely used than the silica- bonded materials. In lieu of distilled water, most phosphate-bonded invest-ments require a mixture of a special liquid (ie, colloidal sil-ica) and distilled water. Concentrations of the special liquid may range from as little as 10% (for high palladium alloys) to as much as 100% (for cobalt-chromium alloys). Used full strength, this liquid provides added silica that thickens the mix, resulting in greater thermal expansion on heating. Diluting the special liquid with distilled water has the oppo-site effect; that is, less colloidal silica reduces the amount of potential investment expansion. Typically, manufacturer’s instructions include a sug-gested concentration of special liquid and distilled water as a starting point. It should be understood that the special liquid–to-water ratio will have to be adjusted for the condi-tions of each laboratory until the desired level of investment expansion required of each alloy type has been achieved. Carbon-containing phosphate-bonded investmentsA variety of carbon-containing phosphate-bonded cast-ing investments are available. These high-heat refractory materials are readily identifiable by their gray-black color, which is due to the presence of carbon even after wax elim-ination (Fig 5 -16). Divestment is easier and castings typi-cally are “cleaner,” meaning they have little or no surface oxidation, when retrieved from a carbon-containing invest-ment.2 Once mixed, these investments may appear coarse in texture compared to their gypsum-bonded investment counterparts. Generally, carbon-containing phosphate-bonded invest-ments are recommended for gold-based metal-ceramic alloys. They should not be used with palladium-, nickel-, or cobalt-based alloys; these alloy systems have the poten-tial to absorb available carbon, leading to the formation of carbides and/or porosity (due to physical carbon inclu-sions). Some manufacturers believe that with the use of an appropriate wax elimination technique, no carbon should remain after heat soaking at the high temperature setting. They contend that there is more risk of carbon contamina-Naylor_Chap_05.indd 95 9/6/17 12:07 PM 96The Fundamentals of Spruing, Investing, and Casting5tion when a torch is improperly adjusted than there is in a carbon-containing investment. However, a simple labora-tory test will show that even after a 90-minute heat soaking at 816°C (1,501°F), a substantial amount of carbon appears to remain in the critical interior area of a carbon-containing investment21 (see Fig 5-16).Carbon-free phosphate-bonded investmentsThe carbon-free phosphate-bonded investments are readily identifiable by their white color, both before and after they are mixed (see Fig 5-9). These products were developed to address concerns for the potential interaction of car-bon with the nickel- and cobalt-containing casting alloys, as well as the various palladium-based metals. Proponents of these investments prefer the added security of using a carbonless investment system and the avoidance of pro-longed high-temperature wax elimination (871°C/1,600°F) required to remove all residual carbon.23 Some carbon-free investments have a grainy tex-ture like their carbon-containing counterparts, but there are also a number of investments with a fine texture. For example, Cera-Fina (Whip Mix) is a fine-grained carbon- free phosphate-bonded investment that produces a smooth, creamy consistency and provides users with ample working time (8 to 10 minutes) to permit investing of multiple units and rings from the same mix (see Fig 5-9). Other similar products are available from leading invest-ment manufacturers. Investment–casting alloy interaction Variations can be seen in the performance of alloys with different investments.11,24,25 It is possible that the problems attributed to a given metal are, in fact, linked to the invest-ment. Excessive nodule formation on the casting surface (Fig 5 -17) and castings with fins may occur more frequently with one brand of investment than another. A number of manufacturers indicate that their alloys may be used with virtually any commercially available phosphate-bonded investment. That may not always be true, and there are studies to support the view that alloy-investment pairing can influence results.5,23–26 Therefore, it may be prudent to con-duct laboratory tests for potential adverse alloy-investment interactions before making a large purchase of any new investment product.23 Casting ring linersAsbestos had been the material traditionally used for lin-ing casting rings until its potential risk to the health of dental laboratory technicians was discovered.27,28 Evi-Fig 5-16a Carbon is still present in this carbon-containing investment even after wax elimination at 816°C (1,501°F) for 90 minutes. The reducing atmosphere it cre-ates is helpful to gold-based alloys (ie, reduces oxidation), but the residual carbon is a potential contaminant for palladium-, nickel-, and cobalt-based metal-ceramic alloys. Fig 5-16b Even after holding this carbon-containing phosphate-bonded invest-ment for 90 minutes at 871°C (1,600°F), carbon was clearly evident in the critical pattern areas. However, the white appearance of the top and bottom of the heated investment would suggest the carbon had been eliminated in those areas.Fig 5-17 A rough casting with multiple nodules may be caused by several factors, including a less-than-ideal alloy-investment pairing or a failure to evacuate the invest-ment’s gaseous by-products during mixing. To diagnose the problem, try another investment to determine if what occurred is due to a lapse in technique, an issue with the investment, or an adverse alloy-investment interaction.Naylor_Chap_05.indd 96 9/6/17 12:07 PM 97Melting and Casting Techniquesdently, the asbestos fiber bundles were found to pro-duce hazardous-sized respirable particles capable of causing lung disease (Fig 5 -18a). Alternative nonasbestos ring liner materials fall into three categories: ceramic (aluminum silicate), cellulose (paper), and a ceramic-cellulose combination.29 The microstruc-ture of these alternatives to asbestos varies (Figs 5 -18b to 5-18d). Furthermore, the relative safety of ceramic ring liners remains uncertain because aluminum silicate also appears capable of producing hazardous-sized respirable particles.29 These health and safety concerns can be elimi-nated by switching to a ringless casting system.Investing TechniqueIn metal-ceramic technology, the dental laboratory techni-cian relies on the same basic technique used to produce castings with crown-and-bridge alloys with the exception that high-heat phosphate-bonded investments are required in place of gypsum-bonded investments. When casting a new alloy, ask the manufacturer for the name of recom-mended investments, suggested liquid-to-powder ratios, and an initial special liquid concentration.Wax elimination techniqueThe wax elimination process, commonly known as the “burnout technique,” refers to the protocol followed to heat invested wax patterns in preparation for casting. The low- and high-temperature settings and rate of rise vary for alloys of different compositions. For example, high-temperature settings range up to 871°C (1,600°F) for base metal alloys and between 760°C (1,400°F) and 871°C (1,600°F) for noble and high noble alloys. It is always advisable to refer to the instructions on an alloy package for the recom-mended temperature settings (low and high) and wax elim-ination time for each specific metal. Also, check the invest-ment manufacturer’s suggested temperature rate of rise for the burnout furnace, and adhere to the rate established for each type of investment. Melting and Casting TechniquesAssuming that a case has been properly sprued and invested and that the wax elimination process has been completed, the next crucial steps involve melting and cast-ing the metal-ceramic alloy. A poorly adjusted torch or flawed casting technique can ruin all of your efforts to this point. Given the importance of these procedures, the fol-lowing additional topics are included to help you better understand the casting process.Casting torch selection There are two types of torch tips to choose from when select-ing casting equipment: a multiorifice tip and a single-orifice tip5 (Fig 5 -19). Multiorice tipOf the two torch tips, the multiorifice design is generally preferred for melting metal-ceramic alloys. Its principal advantage is that heat can be distributed over a wide area, which allows for more uniform heating of an alloy. This fea-ture is especially helpful for melting multiple ingots when casting a high-fusing base metal alloy.Single-orice tipThe single-orifice tip, on the other hand, allows the oper-ator to concentrate more heat in one area, but that area of heat is smaller than what is produced by the multiori-Fig 5-18 (a) Asbestos liner (original magnification 500). (b) Ceramic liner (original magnification 150). (c) Cellulose liner (original magnification500). (d) Ceramic- cellulose combination liner (original magnification 500).a b c dNaylor_Chap_05.indd 97 9/6/17 12:07 PM 98The Fundamentals of Spruing, Investing, and Casting5fice tip (see Fig-19). Controlled, limited heating provided by a single-orifice torch tip is desirable for torch soldering, especially presoldering procedures (see Fig 7-29a). Choice of fuels There are three fuel sources that potentially could be used to melt metal-ceramic alloys30: 1. Acetylene. This colorless gas has a distinc-tive garlic-like odor. It will burn in air and can generate a flame approaching 1,649°C (3,000°F). Unfortunately, acetylene is usually contaminated with carbon and other elements, so it should not be used to melt ceramic alloys.2. Natural gas. This fuel is the byproduct of the natural decomposition of organic matter in the ground. When mixed with air, the natural gas flame approaches a temperature of 1,204°C (2,199°F). But if oxygen is used to replace air, a natural gas flame can reach the tem-peratures required to melt the high-fusing noble and base metal alloys. So technically, natural gas is an acceptable fuel source when combined with oxygen, although it is not ideal. It is not regarded as an ideal heat source because natural gas has its own set of challenges. Inadequate gas line pressure, fluctuations in pressure levels, water contamination, and variations in composition among gas supply companies are just some of the potential problems encountered by natu-ral gas users. Despite these issues, natural gas is a widely used fuel.3. Propane. The problems associated with natural gas are avoided entirely by using bottled propane gas with oxygen. The constant, regulated mixture of pure uncontaminated propane—when combined with oxy-gen—provides a clean, consistent burn that allows the operator to obtain a more ideal melt, free of concerns for pressure fluctuations and alloy contamination.Casting equipmentThe high noble, noble, and base metal alloys can be cast either with a torch in a centrifugal casting machine or torch-less in an induction casting machine. Irrespective of the type of casting equipment selected, the low-density base metal alloys typically require an additional winding of the casting arm in a centrifugal casting machine to ensure ade-quate casting pressure. Casting cruciblesThree types of casting crucibles are available for dental casting alloys of different composition: (1) high-heat, (2) clay, and (3) carbon crucibles. Fig 5-19 (a) The proper flame pattern for a multi-orifice (left) and a single-orifice (right) torch tip differ in appearance. (b) Base metal alloys require a different flame configuration from that of noble alloys. Some alloy manufacturers may recommend increasing the inner blue cone length by up to 0.75 in.abMultiorifice tip Single-orifice tip Oxidizing zone Reducing zone (about 2.5 in)Sharp blue tips (about 0.25 in)Melt the alloy with the reducing flame only10.0 in0.5–0.75 inOxidizing zone Reducing zone (1.5–2 in)Sharp blue cone (1–1.25 in)Naylor_Chap_05.indd 98 9/6/17 12:07 PM 99Melting and Casting TechniquesHigh-heat crucibles. Crucibles made of zircon are cap able of withstanding the temperatures required to melt metal- ceramic as well as crown-and-bridge alloys. Both zircon and quartz crucibles are recommended for casting noble and base metal-ceramic alloys, but the zircon type gener-ally is the more durable of the two. Additionally, zircon cru-cibles are available in a variety of colors (yellow, pink, blue, and white), so a particular color can be dedicated to a spe-cific alloy to avoid potential contamination during the melt.After selecting the appropriate type of casting crucible (ie, zircon or quartz), it is advisable to heat the crucible in the burnout oven along with the casting ring or invested patterns, when using a ringless casting technique. Preheat-ing the crucible prevents spalling (ie, cracking) and helps prolong a crucible’s working life. Never cast different alloys in the same crucible because this also will increase the like-lihood of alloy contamination and may alter the properties of the resulting casting. Instead, use color-coded crucibles (Fig 5-20) or, in the case of white crucible formers, simply carve the alloy name in a visible portion of the crucible side or base to permit ready identification. The following two types of casting crucibles should not be used when melting metal-ceramic alloys.Clay crucibles. Clay crucibles are not recommended when melting any type of metal-ceramic alloy. They are accept-able for melting gold-based crown-and-bridge alloys, but they have a greater potential to deteriorate when subjected to the high temperatures required to cast ceramic alloys, which can possibly contaminate the melt.31 Carbon crucibles. Carbon crucibles should also not be used with metal-ceramic alloys. Although carbon crucibles are quite suitable when melting gold-based alloys, expo-sure to such a ready source of carbon can lead to significant contamination of palladium-, nickel-, and cobalt-based met-als. As mentioned previously, that contamination can result in carbide formation that can embrittle these alloys and/or cause porosity from carbon inclusions.Electric casting machinesElectric casting machines with carbon crucibles are appro-priate for gold-based alloys, but they are not recommended for palladium-, nickel-, and cobalt-based alloys because of the potential for carbon contamination. Compounds for airborne-particle abrasion Commercially available abrasive compounds such as alumi-num oxide (Al2O3), a general-purpose blasting compound, and glass beads are recommended to remove investment and surface oxides from a metal casting. A 50-μm grit, non-recycled (white) aluminum oxide abrasive is the material of choice for air-abrading porcelain-bearing surfaces and dental porcelain32 (Fig 5-21). Aluminum oxides in colors other than white may contain impurities such as iron. To check, you can run a magnet through the material if you suspect it is contaminated.The term sandblasting is very much engrained in the vernacular of dental laboratory technology, but the com-pounds used are not actually some form of sand (ie, sili-con dioxide). More than likely, technicians and clinicians are using an aluminum oxide (recycled or nonrecycled) or glass beads. So the phrase airborne-particle abrasion is Fig 5-20a Zircon Alumina Crucibles (KerrLab) are more durable than the quartz type. Color-coded crucibles permit assignment of different alloys to specific colors. The name of a particular alloy or alloy type can be scribed on the back of a crucible as well. Fig 5-20b A white quartz casting crucible in a centrifugal casting machine. Fig 5-21 Pure aluminum oxide (left) is white and less likely to contain contami-nants than is a comparable gray all-purpose abrasive (right).Naylor_Chap_05.indd 99 9/6/17 12:07 PM 100The Fundamentals of Spruing, Investing, and Casting5preferred and considered more accurate when describing this procedure.The Laws of CastingCasting dental alloys is both an art and a science as well as a process governed by numerous rules, or “laws.” Per-sonal interpretation of these laws is demonstrated in a tech-nician’s approach to the procedure—that is, the art of cast-ing. Integrating the theoretical principles supporting this art enables the user to blend science with art in dental technol-ogy. At times, casting problems can be attributed to poor technique, omitting or altering a step, and/or a failure to adhere to one or more of the basic concepts inherent to the casting process. Three key concepts of castingThe foundation of these laws is based on three key con-cepts: (1) alloy shrinkage is inherent in the casting process as the molten alloy transitions from a liquid state to the solid state (ie, solidification shrinkage), (2) every casting con-tains porosity attributed to the solidification shrinkage, and (3) porosity invariably occurs in the area of the casting that is last to solidify. The challenge faced by the technician is how best to use that knowledge of the science of casting to plan the location of that porosity in an area of the spruing system away from the actual restoration(s). Building on the historical work of Ingersoll and Wan-dling17 and their 14 laws governing the principles from spru-ing to casting, the author created an expanded set of 17 recommendations dealing with each phase of the casting process (spruing, investing, wax elimination, melting, and casting procedures) in 1992. Collectively, these guidelines are referred to as the laws of casting. Included in these laws are the principles behind the recommended button-less casting technique described in this chapter. As with any set of regulations, there are penalties when these laws are not followed, so technicians should acquaint them-selves with the laws and the adverse outcomes if those laws are violated, in full or part. For the purposes of illustration, the required steps to cast a three-unit fixed partial denture pattern are presented in the following sections to illustrate several key points related to the spruing, investing, and casting procedures. Spruing1st law of castingAttach the pattern sprue former to the thickest part of the wax pattern. As molten alloy enters the heated mold, it moves from the reservoir to fill the pattern margins. To facil-itate this movement, alloy should flow from areas of greater volume to areas of lesser volume (ie, the margins). Before beginning the spruing process, it is best to take a few moments to evaluate the size, geometry, and configu-ration of the wax pattern(s) to be cast to identify any special requirements that may need to be considered (Fig 5-22a). Select a prefabricated indirect sprue former of an appropri-ate gauge and length (Figs 5-22b and 5-22c). Lute a wax pattern sprue former to the most practical portion of the occlusal/incisal surface of each wax pattern (Fig 5-22d). Warm the prefabricated indirect sprue former and gently alter the shape to align it, as needed, with the curvature of the arch (Figs 5-22e and 5-22f). If 18-gauge wax han-dles are to be applied (and they are highly recommended, if a lingual metal collar is present), place them on the lin-gual surfaces of the metal-ceramic wax patterns while on the master cast and before luting the patterns to the run-ner bar (Fig 5-23). Except with thin anterior copings, do not place the pat-tern sprue former in a cutback area if an adjacent fully con-toured cusp is available. Molten metal flowing from a thin region to a thicker region of the wax pattern may solidify before the mold is completely filled, resulting in what is referred to as a cold shut. Penalties. The penalties for not obeying this law are cold shuts, short margins, or incomplete castings. 2nd law of castingOrient wax patterns so all of the restoration margins face the trailing edge when the ring is positioned in the casting machine. To identify the orientation of the wax pat-terns, add a wax dot to the crucible former so you know how to position the ring in the casting cradle once those patterns have been invested (see Fig 5-7).Penalties. The penalties for not obeying this law are cold shuts, short margins, and incomplete castings. 3rd law of castingPosition the wax patterns in a “cold zone” of the investment mold and the reservoir in the heat center of the casting ring. The coolest parts of the mold (ie, cold zones) are located at the end of the ring and along the ring periphery. The hottest part (ie, heat center) of the casting ring is located near the center of the ring, as determined by the location of the largest mass of metal in the investment (see Figs 5-8d and 5-8e). Limit the amount of investment covering the patterns to approximately 6.0 mm (0.25 inch), and position the reservoir in the heat cen-ter.10,33 Adherence to this law increases the likelihood that casting porosity will occur in the reservoir rather than in the restoration.Penalty. The penalty for not obeying this law is porosity in the actual restorations.4th law of castingA reservoir must have sufficient molten alloy to accom-modate the shrinkage that occurs within the resto-rations. Ideally, alloy that fills the pattern (ie, restoration) areas will solidify first. As that molten metal cools, it shrinks Naylor_Chap_05.indd 100 9/6/17 12:07 PM 101The Laws of CastingFig 5-22 (a) Three-unit fixed partial denture substructure (see chapter 4) after completion of the wax cutback. (b) This Ready Sprue (KerrLab) has a runner bar with a diameter (6 gauge) and length sized to support this fixed partial denture wax pattern. (c) This 4-gauge indirect Ready Sprue former is too long, and the gauge size is too large, which will require casting more alloy than needed for this fixed denture substructure. Rather than modify the sprue former, it would be advised to switch to a more appropriately sized prefabricated pattern. (d) Wax pattern sprue formers of an appropriate diameter and length were selected and luted to each wax pattern. (e) The runner bar portion of the indirect sprue former is luted to the three individual pattern sprue formers. Before being attached, the Ready Sprue can be warmed slightly, adjusted to mimic the actual curvature of the arch (rather than to remain straight), and allowed to cool to room temperature before being attached to the wax-up.(f) The prefabricated indirect sprue former (left) can also be modified by removing the central web of wax (right) to allow investment to surround the runner bar. a bc de fFig 5-23 Do not forget to attach 18-gauge wax to the lingual surfaces to create handles on each casting. (This step is optional but highly recommended, as shown with these indi-vidual wax patterns.) These handles make it easier to hold the castings through all stages of porcelain application and avoid potential damage to delicate metal margins from hemostats and other similar metal instruments.Naylor_Chap_05.indd 101 9/6/17 12:07 PM 102The Fundamentals of Spruing, Investing, and Casting5through solidification shrinkage and creates a vacuum. For complete casting to occur, the vacuum must be able to draw additional molten metal from an adjacent source—namely, the reservoir. A runner bar can be an effective res-ervoir provided the diameter of the bar is equal to or larger than the thickest cross-sectional area of the wax pattern to be cast (see Figs 5-6, 5-8e, 5-15c, and 5-22b). Units sprued directly should include a round ball that potentially can serve as a reservoir in a properly designed spruing system (see Fig 5-6).Penalties. The penalties for not obeying this law are incomplete castings, porosity in the restorations, and/or suck-back porosity. 5th law of castingDo not cast a button if a runner bar or other internal res-ervoir is used (see Fig 5-9). With indirect spruing, the larg-est mass of metal should be the reservoir (see Fig 5-8e). The presence of a button is counterproductive because it can draw available molten alloy from the bar, shift the loca-tion of the heat center, and reduce the feed of metal to the restorations (see Fig 5-11). Therefore, the volume of the wax patterns should not be larger than that of the runner bar, if the bar is to act as a true reservoir (see Figs 5-8e and 5-12). Weigh the sprued patterns, and use the wax pattern–alloy conversion chart (appendix F) to calculate the amount of alloy needed based on the density of the particular alloy to be cast (Fig 5-24).Penalties. The penalties for not obeying this law are poros-ity in the restorations, suck-back porosity, and potential dis-tortion during porcelain firing. 6th law of castingTurbulence must be minimized, if not totally eliminated. Pathways for the flow of metal should be smooth, gradual, and without impediments. Eliminate sharp turns, restric-tions, points, or impingements that might create turbulence and occlude air in the casting. Restrictions or constrictions can accelerate the metal’s rate of flow and abrade the mold surface, possibly resulting in mold wash.17 Penalties. The penalties for not obeying this law are mold wash, voids in the casting, and surface pitting. Voids can be created by the occlusion of air from turbulent metal flow. Mold wash can remove investment particles from the mold’s inner surface and carry them ahead of the alloy. These entrapped particles can produce surface pits and incom-plete margins.7th law of castingSelect a casting ring of sufficient length and diame-ter to accommodate the patterns to be invested. If a casting ring is to be used, it should be of sufficient length and diameter to permit the patterns to be located 6.0 mm (0.25 inch) apart and 6.0 mm (0.25 inch) from the top of the investment. A minimum of 9.0 mm (0.35 inch) of invest-ment should separate the patterns from the ring liner, if used (see Fig 5-3). If too little investment covers the wax patterns, the molten alloy is more likely to break through the mold on casting. Conversely, filling the ring with too much investment may position the wax patterns too close to the heat center of the mold and impair the escape of the gases produced by the investment material when heat-ing the mold to the desired high temperature for casting. In addition, an insufficient amount of investment between wax patterns and the casting ring (or exterior of the mold with the ringless casting technique) or casting ring liner can result in uneven investment expansion (setting and thermal).Penalties. The penalties for not obeying this law are mold fracture, casting fins (Fig 5-25), and porosity in the restorations. Fig 5-24a Apply a thin layer of debubblizer (ie, wax pattern cleaner) to the wax patterns and indirect spruing system and allow it to dry thoroughly before investing. Fig 5-24b Weigh the assembly following the method described in Fig 5-10, and calculate the amount of metal needed based on the density of the alloy (see appendix F). An electronic scale provides more accurate weight information than a balance scale. Naylor_Chap_05.indd 102 9/6/17 12:07 PM 103Investing and wax elimination8th law of castingReduce the surface tension and increase the wettabil-ity of the wax patterns. Proper surface tension reduction is important because it enables the casting investment to cover the wax patterns completely and thus reduce the potential for air-bubble entrapment.34 A wetting agent, such as a wax pattern cleaner, should be brushed or sprayed on the wax patterns and dried thoroughly before invest-ing10 (see Fig 5-24a). Apply the liquid sparingly. Too much wetting agent may create a surface film that can dilute and weaken the investment in that area and produce bubbles or fins on the casting (see Fig 5-25). On the other hand, too lit-tle wetting agent can result in inadequate reduction of sur-face tension and lead to increased air entrapment. When a wetting agent is applied correctly, the result is a clean wax surface, which improves the ability of the casting invest-ment to wet the patterns more completely (Fig 5-26). Penalty. The penalty for not obeying this law is the formation of bubbles on the surface of the patterns that become nodules on the casting surface as a result of air entrapment (ie, too lit-tle wetting agent) or excess liquid (ie, too much wetting agent). 9th law of castingWeigh any bulk investment and measure the investment liquid for a precise powder-to-liquid ratio. The correct proportions of investment powder to liquid and any required dilution of the special liquid (ie, colloidal silica) with distilled water should be established for each type of alloy. Two key variables to consider relative to the powder-to-liquid ratio are (1) the total volume of liquid, and (2) the concentration of the special liquid. In terms of liquid volume, the total powder-to-liquid ratio to use initially is that recommended by the investment manufacturer.10 Changes can be made to any dilution ratio based on actual outcomes in your dental laboratory (eg, amount of expansion, quality of fit). Bear in mind that if too little liquid is dispensed, the investment will be viscous and may lack proper flow and workability. A thick mixture, if workable, will produce increased setting expansion of the investment, which ultimately may result in loose-fitting castings. On the other hand, if too much investment liq-uid is used—whether it is 100% colloidal silica or a com-bination of special liquid and distilled water, in proportion to the specified weight of powder—a thinner mixture will result. That diluted investment mixture will reduce setting and thermal expansion, and the castings produced may be tighter fitting than desired or not fit at all. Furthermore, care should be taken when dispensing the special liquid because the concentration of this colloidal silica also directly affects thermal expansion. For example, if 100% of this liquid is used, more expansion occurs during heating the investment as compared to a dilution using just 50% special liquid and 50% distilled water. In other words, with the total liquid volume unchanged, the greater the vol-ume of colloidal silica, the greater the overall expansion (setting and thermal).Fig 5-25 This casting has fins caused either by excess wax pattern cleaner or by improper heating of the mold that led to mold cracks filled by alloy on casting. Penalties. The penalty for not obeying this law is ill-fitting castings. Restorations can be too tight (ie, not enough spe-cial liquid) or too loose (ie, too much special liquid). 10th law of castingAvoid the incorporation of air in the casting invest-ment and remove the ammonia gas by-product of phosphate-bonded investments by mixing under vac-uum. Vacuum mixing the casting investment extracts air and gaseous by-products, something hand spatulation cannot do. Areas of the mold filled with dense bubble-free investment, when set and heated, will expand more uni-formly than sections of investment containing large voids created by air entrapped in the mix. The required mixing time will depend on the type of investment used and the mixing speed (slow versus high). In general, mechanical mixing hastens the setting reaction compared to hand mix-ing, so expect the amount of working time to be reduced.Penalties. The penalties for not obeying this law are small nodules on the casting, a weak mold, or distortion of the casting. 11th law of castingAllow the casting investment to set completely before initiating the wax elimination procedure. If setting of the investment is not complete at the time a ring or mold (for the ringless technique) is placed in the burnout oven, the mold may be in the weak green state. If so, the investment is not quite strong enough to withstand the pressure and resist the expansion produced by the formation of steam in the early stage of heating. Steam is a normal by-product of heating the free water in the set investment during wax elim-ination. Also, the amount of setting expansion may be less than what would be achieved if the invested ring were left undisturbed for the entire recommended setting time. With the investment still in the green state, it lacks early strength and, therefore, could develop cracks or possibly fracture. The Laws of CastingNaylor_Chap_05.indd 103 9/6/17 12:07 PM 104The Fundamentals of Spruing, Investing, and Casting5For best results, wax elimination should be initiated only after the investment has been allowed to remain undis-turbed for the recommended setting time, typically 1 hour. If wax patterns are invested at the end of the day or are not to be cast immediately after setting is complete, keep the invested ring hydrated by storing it in a sealed container or plastic bag. Should the casting investment be allowed to dry out for any reason, it is difficult to replace the free water in the interior of the investment and achieve total rehydra-tion, even if the mold were soaked in water. There are special investments that have a short bench setting time, can be placed directly in a heated oven (rather than a room-temperature furnace), and can be heated rap-idly to high temperature. One example is FastFire 15 (Whip Mix), which has a minimum 15-minute setting time (30 min-utes for a mix of 60 g or less), as compared to the stan-dard 60-minute setting time for other phosphate-bonded investments. Penalties. The penalties for not obeying this law are mold cracking/blowout or casting fins (see Fig 5-25).12th law of castingUse a wax elimination technique that is appropriate for the type of patterns involved (wax versus plastic) and recommended for the type of casting alloy selected. Plastic sprue formers should be heated slowly to allow the plastic to soften, melt, and gradually flow out of the mold without exerting pressure on the investment walls. The safest way to achieve this is by using the two-stage wax elimination technique described earlier.22 Set the burn-out oven for a slow-to-moderate rate of rise to permit the heat to move through the investment slowly and achieve uniform thermal expansion. If this process is rushed and wax elimination is incomplete, the channels of the spru-ing system may be partially blocked by wax or plastic res-idue (carbon). In addition, air may not be able to escape completely as the fluid casting alloy enters the mold.22,23 With a two-stage technique, it is recommended to plan at least a 30-minute heat soaking at 427°C (801°F) for the first wax-elimination stage. After completion of this first stage, raise the final temperature of the oven and heat Fig 5-26 (a) Vacuum mix the investment to remove gaseous byproducts and produce a smooth mix. (Carbon-free phosphate-bonded investment shown.) (b) After vacuum mixing, carefully add a small portion of the mixed investment to the inside of the individual wax retainers to prevent the entrapment of air and ensure complete wetting of the patterns. The bristles of brushes can trap air and potentially transfer bubbles to the wax pattern, so some technicians prefer to apply the investment to the patterns with the tip of a glass rod. (c) After all the wax surfaces have been covered by investment, place the casting ring in the crucible former. (d) Once a single wax pattern is secure in a round casting ring, carefully add enough investment to cover the wax patterns without trapping air. This technique is the same for multiple patterns using large and small round and oval rings. a bc dNaylor_Chap_05.indd 104 9/6/17 12:07 PM 105the ring to the recommended high temperature setting. Then heat soak the ring for the recommended hold time of the metal-ceramic alloy manufacturer.Penalties. The penalties for not obeying this law are cold shuts, short margins, mold cracks, and/or casting fins (see Fig 5-25). Melting and casting 13th law of castingAdequate heat must be available to melt and cast the alloy properly.1 The selected heat source should be capa-ble of melting a high-heat metal-ceramic alloy to the point of sufficient fluidity for casting (see Fig 5-19). Exercise cau-tion, because prolonged heating, caused by an improperly adjusted torch, can prevent the alloy from attaining the flu-idity needed for complete mold filling and compensation for heat loss.1 In fact, too much heat or too high a melting temperature can burn off minor alloying elements through vaporization and/or oxidation (ie, burned metal). Be mind-ful that ingots of high noble and noble alloys will slump and flow together to form one molten mass in the casting cruci-ble. In sharp contrast, most base metal alloy ingots retain their basic shape on heating and do not flow together to form a single mass of molten metal (Fig 5-27). Therefore, identifying the correct heating point to initiate the cast of nickel- and cobalt-based alloys is a skill a dental laboratory technician must expect to acquire over time. Penalties. The penalties for not obeying this law are over-heated metal, loss of minor alloying elements, cold shuts, short margins, rough castings, and/or investment break-down (too much heat exposure). 14th law of casting When torch casting, use the reducing zone of the flame, not the oxidizing zone, to melt the alloy.2,5 An improperly adjusted torch can permit carbon or oxygen to be absorbed by an alloy while it is being heated. But the exclusive use of the reducing zone of the casting torch flame minimizes the likelihood of unwanted metal oxidation and gas absorp-tion and helps to ensure a proper melt is achieved2,5 (see Fig 5-19).The Laws of CastingFig 5-27 (a) Most base metal alloys do not pool when heated, so you should use the fewest ingots possible and have them in contact with each other in the crucible. The alloy instructions should describe the features to look for to identify when the alloy is ready to be cast. (b) Through mutual contact, the ingots can be heated uniformly. (c) Note how the edges of the ingots have rounded and remain in contact, but the ingots have not pooled. (d) These ingots are now molten under an oxide surface skin and are ready to be cast. Learn how to recognize the appearance of the molten state for each type of base metal alloy, because this point in the melting process is different from that of high noble and noble alloys. a bc dNaylor_Chap_05.indd 105 9/6/17 12:07 PM 106The Fundamentals of Spruing, Investing, and Casting5Penalties. The penalties for not obeying this law are porosity (from gas absorption) and/or the possibility of an unplanned change in the alloy’s coefficient of thermal expansion due to alloy contamination.15th law of castingProvide enough force to cause the liquid alloy to flow into the heated mold. Adjust the casting machine to the requirements of each alloy. Low-density metals are lighter and more fluid, so they typically require additional windings of a centrifugal casting arm as compared to more slug-gish high-density gold-based alloys. However, do not over-wind the casting machine.10,35 Penalties. The penalties for not obeying this law are cold shuts, short margins, incomplete castings, mold fractures, and/or fins (ie, too much force).16th law of castingCast toward the margins of the wax patterns. Place the heated ring in the casting cradle using the orientation dot (see Figs 5-7 and 5-9c) or other landmark as a guide, so the wax pattern margins face the trailing edge (the second law of casting). In a centrifugal casting machine, the metal will flow downward and to the right, taking advantage of the centrifugal, rotational, and gravitational forces exerted on the molten alloy.17,36 Penalties. The penalties for not obeying this law are cold shut, short margins, and otherwise incomplete castings.17th law of castingDo not quench the ring immediately after casting. Once casting is complete, remove the ring from the cast-ing machine and set it aside undisturbed. You want to allow the alloy and the investment to cool to room tem-perature. Uneven cooling and shrinkage between alloy and investment can exert tensile forces on the casting.37 After casting, the alloy may not possess sufficient strength to resist these forces, and the restoration could tear if quenched. Unlike copper-containing type III and type IV gold alloys, metal-ceramic alloys are not formulated to be heat softened (ie, quenched) and then later heat hardened (ie, strengthened). Casting rings should be bench cooled. Once they can be handled safely, divest the casting and proceed with processing.Penalty. The penalty for not obeying this law is hot tears in the restoration. Judging Success in CastingCasting success typically is associated with an intact, well-fitting restoration, coping, or substructure. How-ever, the true challenge in the casting process is in the management of the thermodynamics—that is, to direct the location of the casting porosity to a noncritical area. With indirect spruing, the most ideal location for porosity to reside is the undersurface of the runner bar opposite the restorations (Fig 5-28). When direct spruing includes a res-ervoir ball, the side of the ball opposite the restoration is an ideal location for casting porosity to occur with this spruing technique (Fig 5-29). Castability testsSubjective castability testing using an abstract mesh pat-tern has not been shown to correlate well with actual cast-ing parameters (ie, high temperature for the mold and melting range for alloys23) (Figs 5-30 and 5-31). Replicas of metal-ceramic substructures should be used in lieu of abstract patterns such as the Whitlock test23 to evaluate Fig 5-28a When no button is cast and the reservoir is the last portion of the casting to solidify, the porosity should appear on the underside of the bar, away from the resto-rations (cast in a nickel-chromium-beryllium alloy). Fig 5-28b If the runner bar is too small or a button is cast, the bar may be the best part of the casting. However, there is an increased likelihood that the restorations will be incomplete, as seen in this casting. Note the dense surface and absence of porosity on the underside of the bar, an indication that the button supported the bar rather than the patterns. Naylor_Chap_05.indd 106 9/6/17 12:07 PM 107Analyzing Casting Failuresthe castability of a new alloy. It is advisable to evaluate that new alloy under the conditions of your dental laboratory, using the procedures and equipment you have on hand to ensure the outcomes represent potential results in your actual work environment.Analyzing Casting FailuresDespite concerted efforts to follow the recommended pro-cedures outlined in this chapter, casting failures and mis-haps are bound to occur in the dental laboratory (see Fig 5-28b). Errors in technique and, on occasion, material failures can unexpectedly result in unsatisfactory castings. By standardizing your technique and paying strict attention to each step involved in spruing, investing, and casting, it is possible to minimize the number of actual miscasts and to control the location of the casting porosity (see Figs 5-28 and 5-29). Proper storage and rotation of casting invest-ment inventories also help to ensure consistent and accu-rate performance of your expendable materials. When casting failures do occur, troubleshoot each mis-cast to diagnose the cause or causes of the problem so corrective measures can be taken before making any additional castings. Mackert10 developed one of the most comprehensive methods to assess casting failures from preparation of a custom impression tray to waxing, spru-ing, investing, wax elimination, and ultimately, melting of the alloy. Failure analysis is an important process to follow when results occur outside an expected outcome.Fig 5-29 (a) When the wax pattern is weighed and cast with the proper volume of metal for direct spruing with the buttonless casting technique, the pattern area can solidify first and cast a complete restoration. The ball area is the largest mass of metal and functions as a reservoir. (b) This drawing illustrates how the presence of a button—the largest mass of metal—shifts the location of the heat center from the ball to the button and can result in an incomplete casting with direct spruing. (c) In this example of direct spruing, alloy solidification shrinkage occurs in the area of the reservoir ball, so the casting porosity should appear on the side opposite the restoration. In this case the porosity resides on the underside of the reservoir ball. The large round base was thin with little mass and illustrates the principle depicted in Fig 5-29a. a b cFig 5-30a A nickel-chromium-beryllium alloy repro-duced 100% of this abstract mesh pattern. Fig 5-30b The alloy in Fig 5-30a not only reproduced the entire abstract mesh pattern (left), it also produced a complete metal-ceramic substructure pattern (right) when tested using the same casting parameters.Fig 5-31 The best castability performance of a nickel- chromium beryllium-free alloy with the abstract test was to cast 30.9% of the mesh pattern (left). However, this alloy reproduced a complete metal-ceramic substruc-ture pattern (right) using the same casting parameters, suggesting the mesh test is not a good predictor of alloy castability.23 Naylor_Chap_05.indd 107 9/6/17 12:07 PM 108The Fundamentals of Spruing, Investing, and Casting5SummaryThe very nature of the differences between metal-ceramic alloys and conventional gold-based crown-and-bridge metals necessitates certain adjustments to your waxing, spruing, and investing techniques. Whether it is in under-standing the demands of low-density alloys, selecting an appropriate spruing technique (indirect versus direct), or determining how to properly cast a high-fusing alloy, knowl-edge of the theoretical aspects of waxing, spruing, and investing is required. This chapter brings together the fun-damental principles of these technical procedures in the laws of casting with emphasis on the buttonless casting technique. What’s Next?Before proceeding to the discussion of metal finishing (chapter 7), it is important to understand how dental por-celain attaches to a metal-ceramic alloy and what factors ensure the ceramic veneer remains attached to a prop-erly designed metal-ceramic structure (chapter 6). In this way, you will gain a better appreciation of the materials, procedures, and techniques so essential to these aspects of metal-ceramic technology.ReferencesTo access annotations to this reference list, please go to: www.quintpub.com/Metal-CeramicTechnology.1. Myers RA. Study of the causes of discontinuity of metal in sprues during pressure casting. J Am Dent Assoc 1936;23:554–568. 2. Anusavice KJ. Phillips’ Science of Dental Materials, ed 11. Phila-delphia: Saunders, 2003.3. Nielsen JP, Ollermann R. Suck-back porosity. Quintessence Dent Technol 1976;1:61–65.4. Naylor WP, Young JM. Non-Gold Base Dental Casting Alloys. Vol I: Alternatives to Type III Gold. Brooks AFB, TX: School of Aero-space Medicine, 1985:19–25.5. Naylor WP. Non-Gold Base Dental Casting Alloys. Vol II. Porcelain- Fused-to-Metal Alloys. Brooks AFB, TX: School of Aerospace Medicine, 1986:75–99.6. Rousseau CH. The Rousseau casting system: A foundation for esthetic restorations. Trends Tech Contemp Dent Lab 1984;1(3):26–29.7. Dootz ER, Asgar K. Solidification patterns of single crowns and three-unit bridge castings. Quintessence Dent Technol 1986;10:299–305.8. Young HM, Coffey JP, Caswell CW. Sprue design and its effect on the castability of ceramometal alloys. J Prosthet Dent 1987;57:160–164. 9. Young HM, Marguelles-Bonnet R, Mohammed H. The relationship of metal volume and sprue design to porosity in nonprecious castings. Quintessence Dent Technol 1987;11:399–404. 10. Mackert JR Jr. An expert system for analysis of casting failures. Int J Prosthodont 1988;1:268–280.11. Dykema RW, Goodacre CJ, Phillips RW. Johnston’s Modern Practice in Fixed Prosthodontics, ed 4. Philadelphia: Saunders, 1986:161–163.12. McLean JW. The Science and Art of Dental Ceramics, Vol II: Bridge Design and Laboratory Procedures in Dental Ceramics. Chicago: Quintessence, 1980:223–238.13. Weber K. Casting mould design and spread in precious metal castings [in German]. Dent Labor (Munch) 1977;25:363–367.14. Tamura K. Essentials of Dental Technology. Chicago: Quintessence, 1987:356,378.15. Yamamoto M. Metal-Ceramics. Principles and Methods of Makoto Yamamoto. Chicago: Quintessence, 1985:88–89.16. Alleluia VV. The parameters of better casting, Part 1. Dent Lab Rev 1980;55(10):18–23. 17. Ingersoll CE, Wandling RA. Laws of Casting. Harmony Notes, Jan 2–3. Amherst, NY: Williams Dental, 1986. 18. Civjan S, Huget EF, Godfrey GD, Lichtenberger H, Frank WA. Effects of heat treatment on mechanical properties of two nickel- chromium-based casting alloys. J Dent Res 1972;51:1537–1545.19. Wight TA, Grisius RJ, Gaugler RW. Evaluation of three variables affecting the casting of base metal alloys. J Prosthet Dent 1980;43:415–418.20. Compagni R, Faucher RR, Youdelis RA. Effects of sprue design, casting machine, and heat source on casting porosity. J Prosthet Dent 1984;52:41–45.21. Verrett RG, Duke ES. The effect of sprue attachment design on castability and porosity. J Prosthet Dent 1989;61:418–424.22. Tombasco T, Reilly RP. A comparison of burnout temperatures and their effects on elimination of plastic sprues. Trends Tech Contemp Dent Lab 1987;Dec:36–39. 23. Naylor WP, Moore BK, Phillips RW, Goodacre CJ, Munoz CA. Comparison of two tests to determine the castability of dental alloys. Int J Prosthodont 1990;3:413–424.24. Barreto MT, Goldberg AJ, Nitkin DA, Mumford G. Effect of invest-ment on casting high-fusing alloys. J Prosthet Dent 1980;44:504–507.25. Hinman RW, Tesk JA, Whitlock RP, Parry EE, Durkowski JS. A technique for characterizing casting behavior of dental alloys. J Dent Res 1985;64:134–138.26. Teteruck WR, Mumford G. The fit of certain dental casting alloys using different investing materials and techniques. J Prosthet Dent 1966;16:910–927.27. Davis DR. Potential health hazards of ceramic ring lining material. J Prosthet Dent 1987;57:362–369.28. Priest G, Horner JA. Fibrous ceramic aluminum silicate as an alternative to asbestos liners. J Prosthet Dent 1980;44:51–56.29. Naylor WP, Moore BK, Phillips RW. A topographical assessment of casting ring liners using scanning electron microscopy (SEM). Quintessence Dent Technol 1987;11:413–420. 30. Dental Laboratory Technology, U.S. Air Force Manual 162-6, Department of the Air Force, November, 1982:53–54. 31. Cascone P. Quartz or clay? . . . A never ending saga. Thermotrol Technician 1977;31:1, 4. 32. Winings JR. Using aluminous oxide abrasives in porcelain-bonded- to-metal fabrication. J Prosthet Dent 1981;46:345–347.33. Phillips RW. Studies on the density of castings as related to their position in the ring. J Am Dent Assoc 1947;35:329–342.34. Johnson A, Winstanley RB. Air-bubble entrapment as affected by investment technique, pattern angle, and use of a surface tension-reduction agent. Int J Prosthodont 1994;7:35–42.35. Goodsir L. Casting low density alloys. Thermotrol Technician 1973;27(5):2, 4. 36. Ogura H, Raptis CN, Asgar K. Inner surface roughness of complete cast crowns made by centrifugal casting machines. J Prosthet Dent 1981;45:529–535.37. Cascone P. Fractures in ceramic alloy. Thermotrol Technician 1976;30(4):1, 3. Naylor_Chap_05.indd 108 9/6/17 12:07 PM

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