Arriving at a completed casting after fabrication of
wax pattern involves three steps:
(1) investing-surrounding the wax pattern with a
material that can accurately duplicate its shape and
(2) burnout-removal of the wax pattern so that a
mold is created into which the molten alloy can be
(3) casting-introducing the molten alloy into the
previously prepared mold.
Sprue Former Attachment:
The sprue former is a small-diameter pin or tube made of
wax, plastic, or metal. A 10-gauge (2.6 mm diameter) sprue
form can be used on most patterns, while the 12 gauge (2.0
mm diameter) is used on small premolar patterns. One end
of the sprue former is attached to the wax pattern and the
other end to the crucible former (a conical rubber base).
After the investment has hardened, the crucible former is
removed from the ring, leaving a funnel-shaped entrance to
the mold. The channel lift by the sprue former following
burnout is the sprue, an inlet for the gold that will be forced
into the mold
A sprue former as large as possible should be used on each
pattern. If the sprue is too thin or too long, the gold may
solidify in the sprue before it goes in the larger cavity
formed by the wax pattern.
The sprue former should be attached to the wax pattern as
its point of greatest bulk, avoiding centric occlusal contacts
if possible. It is attached at an angle to allow the incoming
gold to flow freely to all portions of the mold. If the sprue is
directed at a right angle to a flat wall of the mold, a “hot
spot” may be created at that point.
Select a hollow plastic sprue former and place it inside the
crucible former and casting ring for measurement. The
sprue former should be just long enough so that the highest
point on the wax pattern will be 6.0 mm from the end of the
Remove the sprue former from the crucible former and
shorten it with a sharp knife if necessary.
If the hole in the crucible former is too large to firmly grasp
the sprue former, fill the hole with soft wax. With a PKT
No.1 instrument. Place the sprue former into the molten
bead of sticky wax.
Melt in the sticky wax around the sprue former-wax pattern
junction with the PKT No.1 instrument to provide a smooth
conduit for the molten alloy. Do not expose the wax pattern
to prolonged heat during this procedure. Avoid over
bulking the sprue former attachment because this will
increase the risk of shrink-spot porosity and also make
removal of the sprue from the casting more difficult. The
sprue former also should not be constricted at its
attachment to the wax pattern. The best castability and
least porosity is produced by a sprue former-wax pattern
junction that is either straight or slightly flared.
During the time period between removal of the wax pattern
from the die and hardening of the investment, deformation
will occur as stresses in the wax are released. To minimize
this time, all the armamentarium must be at hand, the ring
liner should be in place, and the water should be measured
out before the pattern is removed.
To produce uniform expansion, surround the pattern on all
sides with investment that is as uniform in thickness as
possible. The closer to the centre of the ring that the
pattern is placed, the greater will be the expansion. With
pliers, push the sprue former down into the soft wax in the
top of the crucible former until the top of the pattern is 6.0
mm below the end of the ring. To provide adequate bulk of
gold during solidification, the sprue itself should be no
longer than 6.0 mm (it can be shorter). To correct any
discrepancy in length, add soft wax onto the sprue former,
thus lengthening the cruicible former and shortening the
exposed sprue former, Smooth the wax around the base of
the sprue former.
An investment must fulfill three important
It must reproduce precisely the detailed form of
the wax pattern.
It must provide sufficient strength to withstand the
heat of burnout and the actual casting of the
It must expand sufficiently to compensate for the
solidification shrinkage of the alloy
The molten alloy used for dental restorations shrink upon
solidification: gold alloys by approximately 1.5% and
nickel-chromium alloys by as much as 2.4%. If the mold is
not made correspondingly larger than the original wax
pattern, the resultant casting will be that much smaller.
For crowns, it is necessary to compensate for the
solidification shrinkage of the specific alloy used by
expanding the mold enough to at least equal the shrinkage.
There are four mechanisms that can play a role in
producing an expanded mold:
(1) setting expansion of the investment,
(2) hygroscopic expansion,
(3) wax pattern expansion, and
(4) thermal expansion.
Setting Expansion: It occurs as a result of normal crystal
growth. The expansion probably is enhanced by silica
particles in the investment interfering with the forming
crystalline structure of the gypsum, causing it to expand
outward. This type of expansion, in air, normally is about
0.4%, but expansion is partially restricted by the metal
Hygroscopic Expansion: It may be employed to
augment normal expansion. The investment is allowed to
set in the presence of water, producing additional
expansion. The water in which the investment is immersed
replaces the water used by the hydration process.
Hygroscopic expansion ranges from 1.2% to 2.2%. More
controlled amounts of hygroscopic expansion may be
achieved by adding a measured amount of water to the
Hygroscopic expansion does occur in an unrestricted
trough or an expandable investment ring. However, in a
lined, rigid, metal ring, the expansion attributed to
hygroscopic expansion is more likely due to expansion of
the wax pattern caused by the elevated temperature of the
water in which the pattern has been immersed.
Wax Pattern Expansion: Expansion of the wax pattern
while the investment is still fluid occurs when the wax is
warmed above the temperature at which it was formed. The
heat may come from the chemical reaction of the
investment or from a warm water bath in which the ring is
The low-temperature burnout technique employs a
combination of wax pattern expansion and thermal
expansion of the mold. After the investment-filled ring is
removed from a 100°F (38°C) water bath, the ring is heated
to only 900°F (48°C) before casting to produce the
additional expansion needed.
Thermal Expansion: Thermal expansion of the
investment occurs when the investment is heated in the
burnout oven. The high-temperature burnout technique
relies primarily on thermal expansion of the mold. The
investment around the wax pattern is allowed to harden in
air at room temperature, and then is heated to
approximately 1,200°F (650°C).
Regardless of which technique is employed, a precise
routine for investing, burning out, and casting must be
adhered in order to achieve consistent results.
Two kinds of investments are in common usage for the
fabrication of cast restorations: those bonded with gypsum
are used for alloys that fuse below 1,975°F (1,080°C), and
those bonded with phosphate are used for higher-fusing
alloys. The manufacturer’s instructions for a particular
brand should be followed.
Investing Material Types
A. Gypsum-Bonded Investments
B. Phosphate-Bonded Investments
The gypsum-bonded investments are used with types I, II
and III gold alloys. These investments are themselves
classified as type I for use with the high-temperature
technique, or type II for the low-temperature technique.
After setting, both types of investment are composites
containing a matrix of gypsum with silica as a refractory
filler, and certain chemical modifiers. The gypsum matrix,
α-calcium sulfate hemihydrate, comprises 30% to 35% of
the investment and acts as binder. The refractory material,
either quartz or cristobalite, makes up 60% to 65% of the
investment and provides the thermal expansion for the
The rigid metal ring in which setting takes places must be
lined with a compressible material to allow setting
expansion to occur in a radial direction. Expansion can be
controlled to some extent by varying the thickness of the
liner. Thermal expansion is achieved in the burnout oven
through the normal expansion that occurs upon heating the
silica as well as through phase changes that occur in the
200cc Vac-U-Spat bowl and lid.
Vac-U-Vestor (Whip Mix Corp. Louisville, KY).
Rubber crucible former.
Casting ring (32 mm diameter).
Plastic water measure.
PKT (Thomas) waxing instruments (no. 1 and no.4).
Sprue formers (hollow plastic).
One package (50 g) of investment.
Four-inch (10 cm) strip of cellulose ring liner.
For a single crown or onlay, use metal casting ring with an
outside diameter of 32 mm. Place a resilient liner on the
inside of the ring to provide a buffer of pliable material
against which the investment can expand to enlarger the
An alternative method uses a split plastic casting ring that
offers no resistance to the setting expansion. The
technique allows easier escape of gas from the mold during
casting, but the mold is more vulnerable to cracking.
Cellulose, ceramic papers are now used as substitutes for
Ceramic material does not readily absorb water except
Place a dry strip of cellulose liner approximately
9.5 cm long into a 32-diameter casting ring,
carefully adapting the strip to the walls of the ring
with no overlap. The liner should be 3.0 mm short
of both ends of the ring. Dip the ring into water to
wet the liner, then gently shake off the excess. Do
not compress the wet liner against the ring because
its cushioning effect will be reduced. Rotate the
ring firmly onto the crucible former, being careful
to avoid snapping movements or contact of the
wax pattern with the ring.
The procedure for investing a pattern for a single-tooth
restoration to be cast in type II or III gold with the vacuum
mix, vacuum pour technique is as follows:
Place the assembled ring and crucible former into the hole
at the top of the Vac-U Spat investor. Hold the lid by the
spindle with the paddle toward you and the inlay ring to the
bottom. Look into the aperture through which the
investment will flow into the ring, and make sure that the
internal portion of the wax pattern is visible.
Connect once end of the clear plastic vacuum tubing to the
vacuum outlet on the Vac-UI vestor. Insert the metal
connector on the other end of the tubing into the hole in
the lid of the Vac-U-Spat. Turn on the Vac-U-Vestor briefly.
Pour the recommended amount of room-temperature water
into the bowl. This must be carefully measured, since the
water-powder ratio has a critical effect on expansion. Add a
package of investment to the water and mix it with a handheld spatula until all of the investment has become wet.
Place the lid on the bowl and make sure it is firmly sealed.
Turn on the Vac-U-Vestor and insert the spindle of the lid
of the Vac-U-Spat into the smaller of the two drive chucks
on the bottom of the unit. The gauge should register a
Power-spatulate for 15 seconds. Since the length of
spatulation can affect expansion of the investment measure
the time of spatulation precisely. Overspatulation will
increase thermal expansion.
Remove the spindle from the drive chuck. Do not turn off
the Vac-U-Vestor, and do not disconnect the vacuum at
this point. Place the drive nut of the Vac-U-Spat spindle on
the vibrator knob. More sure that the shaft is horizontal and
the casting ring is in the lowest position on the
circumference of the lid. Hold the Vac-U-Spat in this
position for a few seconds until the investment has run to
the lower side of the bowl.
Slowly invert the Vac-U-Spat until the shaft points straight
down, keeping the drive nut in contact with the vibrator. It
should take slightly less than 30 seconds to traverse the 90
degree arc from the horizontal to the vertical position.
Remove the drive nut from the vibrator knob, keeping the
Vac-U-Spat inverted. While it is still in this position, turn
off the vacuum pump and disconnect the vacuum hose.
Then remove the casting ring and crucible former from the
Vac-U-Spat lid. Place the crucible former on the vibrator
knob for a few seconds to settle any investment that might
have spilled during separation of the ring from the lid. Do
not overvibrate; this may cause air to slip around the seal
between the ring and the crucible former, rising up and
lodging on the underside of the pattern.
If a high-temperature (1,200°F, 650°C) burnout technique
will be used, place the casting ring and crucible former into
a humidor (a covered plastic container or sealed plastic bag
with wet paper towels in the bottom) and let set at room
temperature. If a low-temperatur (900°F, 480°C) burnout
technique is to be used, immediately immers the ring in a
100°F (38°C) water bath to produce expansion of the wax
pattern. Allow the investment to set for a minimum of 30
minutes. Leave the ring in the humidor until you are ready
for burnout and casting.
To prevent clogging the drains with accumulated
investment, empty the unused portion of investment
remaining in the Vac-U-Spat bowl into the investment
envelope. Fold over the top of the package so the waste can
be disposed off neatly. Use a brush and running water to
clean the bowl, lid, and paddle before the investment
hardens on them.
Investment of Inlay and Dowel-Core Patterns:
Less mold expansion is required for dowel cores and inlays
than for crown. Omitting the ring or increasing the
investment water-powder ratio by 1.0 mL will result in a
slightly undersized casting that will fit more easily into the
cavity prepared in the tooth.
The following technique is recommended for investing and
casting a dowel-core pattern. Invest the pattern in
Beautycast using the standard water-powder ratio, without
a ring liner. Burn out at 1,200°F (650°C). A gas-oxygen
torch or electric induction casting machine must be used to
melt the alloy.
It is possible for an experienced operator to cast and
cement gold inlays and dowel cores on the same day that
the teeth are prepared by using the following accelerated
technique for investment and burnout.