Spruing and Investing

Dr. Rewaa Abou El Hassen

Lecturer of Fixed Prosthodontics
• The lost-wax casting technique  convert wax patterns to cast metal.
• The process consists of surrounding the wax pattern with a mold made
of heat-resistant investment material, eliminating the wax by heating,
and then introducing molten metal into the mold through a channel
formed by the sprue.
Steps
• When the wax pattern has been completed the
margin has been reflowed, it is carefully evaluated
for smoothness, finish, and contour.
• A sprue is attached to the pattern, then removed
from the die and fitted to a crucible former.
The wax pattern should be invested
immediately because any delay leads
to distortion of the pattern due to
stress relief of the wax
 SPRUING
• Sprue design varies according to: type of restoration being cast, the
alloy used, and the casting machine.
Requirments:

1. The sprue must allow the molten wax to escape from the mold.

2. The sprue must enable the molten metal to flow into the mold.

3. The metal within it must remain molten slightly longer than the alloy that has
filled the mold provides a reservoir to compensate for the
shrinkage that occurs during solidification of the casting.
Sprue can be wax, plastic,or metal

Wax sprues are preferred as they melt at the same rate as the pattern and thus
allow easy escape of the molten wax.
Solid plastic sprues soften at a higher temperature that may block the escape of
wax, resulting in increased casting roughness.
• Plastic sprues can be useful when casting fixed partial dentures (FPDs) in one piece
because their added rigidity minimizes distortion.
• Hollow plastic sprues are available that permit the escape of wax
If a metal sprue is used, it should be made of nonrusting metal to avoid possible
contamination of the wax.
• Hollow to increase contact surface area and strengthen the attachment between
the sprue and pattern.
• Removed from the investment at the same time as the crucible former.
Spue design:
a. Direct spruing: sprue former attached directly to the crucible former.
It is used with single crown (disadvantages suck -back porosity)

b. Indirect spruing: sprue former is attached to runner bar before attached to the
sprue former.
It is used with multiple units bridges (enhance control of casting porosity)
Diameter
• large-diameter sprue is recommended because this improves the flow of molten metal
into the mold and ensures a reservoir during solidification.

• A 2.5-mm (10-gauge) sprue is recommended for molar and metal-ceramic patterns.

• A smaller 2.0-mm (12-gauge) is adequate for premolars and partial coverage

restorations.

N.B: Narrower sprue prevents the molten metal from flowing into the mold prematurely.
Sprue former length:
• With direct spruing: the sprue formers should be long enough to position the
wax patterns outside the heat center of the investment ring.
• With indirect spruing: the pattern should be placed off the runner bar in a
location just outside the heat center of the investment.

: the pattern should be placed off the runner bar in a location just outside the heat center of the
investment.
Location
• The sprue should be attached to the bulkiest part of the pattern, away from margins and
occlusal contacts Normally the largest noncentric cusp is used.
• The point of attachment should permit a stream of metal to be directed to all parts of
the mold.
• The sprue must also allow for proper positioning of the pattern in the ring. This can be
critical because expansion within the mold is not uniform.' For example, spruing on
the cusp tip can give good results, but spruing on the proximal contact may produce a
casting that is too wide mesiodistally and too short occlusocervically.
• Reservoir should be located at the heat center.
• Attachment.
• The sprue's point of attachment to the pattern should be carefully smoothed to
minimize turbulence.
• 90⁰ not recommended increases casting porosity and reduces mold filling."
Venting:
• Small auxiliary sprues or vents have been recommended to improve
casting of thin patterns. Their action may help gases escape during
casting' or ensure that solidification begins in critical areas by acting as a
heat sink“
CRUCIBLE FORMER ( sprue former)

 The sprue is attached to a crucible former,* usually made of rubber, which constitutes the base of
the casting ring during investing.
 The crucible former is tall to allow use of a short sprue and allow the pattern to be positioned
near the end of the casting ring.
CASTING RING AND LINER
• The casting ring holds the investment in place during setting and restricts the expansion of the
mold.
• Normally a liner is placed inside the ring to allow for more expansion.
• Asbestos was used as the liner, but this has been replaced by other materials to avoid the health
risks.
• Wetting the liner increases the hygroscopic expansion of the mold.
(Increased expansion can be obtained by placing the mold in a water bath)
RINGLESS INVESTMENT TECHNIQUE
With the use of higher-strength, phosphate-bonded investments.
The method uses a paper or plastic casting ring and is designed to allow unrestricted
expansion of the investment thus the plastic ring was removed before casting
 Investing

• Investment consist of a refractory material (usually silica) and a binder

material, which provides strength. Additives are used by the manufacturer
to improve handling characteristics
• Classification of investments according to binder:

Gypsum-bonded phosphate-bonded silica-bonded investments.

Applications:
• The gypsum-bonded investments are used for castings made from ADA
Type II, Type III, and Type IV gold alloys.
• The phosphate-bonded materials are recommended for metal-ceramic
frameworks.(high concentration of silica)
• The silica-bonded investments are for high-melting base metal alloys used
in removable partial denture castings.
• Types of expansions occurs in investments:
Because most metal-ceramic alloys fuse at around 1200° C (2300° F), additional shrinkage occurs when
the casting cools to room temperatureTo compensate for this, a larger mold is necessary.

A. Setting Expansion:
As the gypsum investment sets after mixing, it expands and slightly enlarges the
mold and the metal casting.
• The water-powder ratio can be altered to reduce or increase the amount of setting
expansion.
• The use of less water increases the setting expansion and results in a slightly larger
casting.
• Use of an additional ring liner increases the setting expansion.
• Slight increase in mixing time.
• If a smaller casting is desired, ↑ water can be used or the liner can be eliminated.
B. Hygroscopic Expansion.
• It occurs when water is added to the setting gypsum investment immediately after the ring
has been filled.
• Usually this is accomplished by submerging the ring in a water bath at 37° C (100° F) for
up to 1hour immediately after investing.
• A wet ring liner also contributes hygroscopic expansion to that portion of the mold with
which it is in contact.
C. Thermal Expansion
When the mold is heated to eliminate the wax, thermal expansion occurs. The silica
refractory material is principally responsible for this because of solid-state phase
transformations.
• An ideal investment should incorporate the following features:

1-Expansion to compensate precisely for shrinkage of the cast alloy during cooling.
2-The ability to produce smooth castings with accurate surface reproduction without
nodules.
3-Chemical stability at high casting temperatures.
4-Adequate strength to resist casting forces.
5-Sufficient porosity to allow for gas escape.
INVESTING:
• Vacuum mixing of investment materials is highly recommended for minimal
surface defects, especially when phosphate-bonded investments are used.
• Good results are possible with brush application of vacuum-mixed
investment or when the investment is poured into the ring under vacuum.