CastingCasting

Libyan International Medical UniversityLibyan International Medical University

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• Metal casting is made using a refractory Metal casting is made using a refractory mold made from a wax replica or pattern.mold made from a wax replica or pattern.

• The procedure includes the following The procedure includes the following steps:steps:

1.1. Prepare the tooth or teeth to receive a cast Prepare the tooth or teeth to receive a cast restoration.restoration.

2.2. Make an impression of the prepared tooth.Make an impression of the prepared tooth.

3.3. Pour gypsum to form cast or die.Pour gypsum to form cast or die.

4.4. Make wax pattern.Make wax pattern.

5.5. Sprue the wax pattern.Sprue the wax pattern.

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6. Invest the wax pattern.6. Invest the wax pattern.

7. Eliminate the wax pattern by burning the wax out of 7. Eliminate the wax pattern by burning the wax out of the investment in a furnace, thus making the mold.the investment in a furnace, thus making the mold.

8. Force molten metal into the mold.8. Force molten metal into the mold.

9. Clean the cast.9. Clean the cast.

10. Remove the sprue from the casting.10. Remove the sprue from the casting.

11. Finish and polish the casting on the die.11. Finish and polish the casting on the die.

12. Cement the finished cast restoration on the prepared 12. Cement the finished cast restoration on the prepared tooth. tooth.

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Setting, thermal and hygroscopic expansions of Investment and shrinkage of wax and alloy

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Wax pattern prepared directly in a patient's Wax pattern prepared directly in a patient's mouth will shrink about 0.4% when cooled from mouth will shrink about 0.4% when cooled from oral temperature.oral temperature.

In the indirect method of preparing the wax on In the indirect method of preparing the wax on die, the wax shrinkage is about 0.2%.die, the wax shrinkage is about 0.2%.

Wax pattern should be invested soon after its Wax pattern should be invested soon after its formation to avoid wax distortion.formation to avoid wax distortion.

Setting, thermal and hygroscopic expansion of Setting, thermal and hygroscopic expansion of investment to compensate wax and metal alloy investment to compensate wax and metal alloy shrinkage.shrinkage.

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• A liner is placed inside the ring to allow lateral A liner is placed inside the ring to allow lateral expansion of the investment.expansion of the investment.

• Ceramic paper liner acts as cushion against which Ceramic paper liner acts as cushion against which expansion can take place.expansion can take place.

• Moreover, greater expansion caused by the Moreover, greater expansion caused by the uptake of water from liner referred as uptake of water from liner referred as hygroscopic expansion.hygroscopic expansion.

• 3 mm of clearance is allowed at each end of the 3 mm of clearance is allowed at each end of the ring so the mold is sealed.ring so the mold is sealed.

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SpruingSpruing

1.1. To form a mount for the wax pattern and fix the To form a mount for the wax pattern and fix the pattern in a space so a mold can be made.pattern in a space so a mold can be made.

2.2. To create a channel for elimination of wax To create a channel for elimination of wax during burnout.during burnout.

3.3. To form a channel for molten alloy during To form a channel for molten alloy during casting.casting.

Sprue should be attached to the bulkiest portion Sprue should be attached to the bulkiest portion of the wax pattern to minimize wax pattern of the wax pattern to minimize wax pattern distortiondistortion

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• The sprue must be large and short enough to allow rapid The sprue must be large and short enough to allow rapid filling of the mold cavity.filling of the mold cavity.

• The sprue pin is mounted on a clean sprue-crucible-former The sprue pin is mounted on a clean sprue-crucible-former base.base.

• The wax pattern should be 6 mm from the end of the ring.The wax pattern should be 6 mm from the end of the ring.• If less than 6 mm…..there is no enough thickness of If less than 6 mm…..there is no enough thickness of

investment to keep the molten alloy from breaking through.investment to keep the molten alloy from breaking through.• If more than 6 mm, the alloy will solidify before the If more than 6 mm, the alloy will solidify before the

entrapped air can escape, resulting in incomplete casting or entrapped air can escape, resulting in incomplete casting or mold fracture.mold fracture.

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Investment MaterialsInvestment Materials Investment are ceramic materials used as models for casting Investment are ceramic materials used as models for casting

molten metals or holding metal appliances in correct relation molten metals or holding metal appliances in correct relation during soldering procedures.during soldering procedures.

They are made from ceramics because they must be able to They are made from ceramics because they must be able to withstand the high temperature associated with the casting process.withstand the high temperature associated with the casting process.

In addition, they must not chemically react with the cast metal.In addition, they must not chemically react with the cast metal. Investment consist of binder and refractory.Investment consist of binder and refractory. Binder…materials hold the investment together.Binder…materials hold the investment together. Refractory… material resists heat..e.g. silicaRefractory… material resists heat..e.g. silica

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Requirements of ideal investment materialsRequirements of ideal investment materials

1.1. It should gives accurate detailsIt should gives accurate details

2.2. Expands to compensate wax and alloy Expands to compensate wax and alloy shrinkage.shrinkage.

3.3. Easy to manipulate (working and setting time).Easy to manipulate (working and setting time).

4.4. Resist high temperature.Resist high temperature.

5.5. Have pores to allow air to escape as the Have pores to allow air to escape as the molten-alloy entering the mold.molten-alloy entering the mold.

6.6. It should have a strength to withstand the force It should have a strength to withstand the force of molten-alloy entering.of molten-alloy entering.

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Types of investmentsTypes of investments Investments classified according to binderInvestments classified according to binder

1- 1- Gypsum-bonded investment, Gypsum-bonded investment, composed of calcium composed of calcium hemihydrate as binder or matrix and silica in the form of hemihydrate as binder or matrix and silica in the form of quartz serves as refractory to withstand the high quartz serves as refractory to withstand the high temperature.temperature.

2- Phosphate bonded investment, 2- Phosphate bonded investment, composed of silica refractory as composed of silica refractory as in the gypsum bonded investment and the binder is made in the gypsum bonded investment and the binder is made from an acidic phosphate and magnesium oxide. Its used from an acidic phosphate and magnesium oxide. Its used for casting high melting point metals and alloys.for casting high melting point metals and alloys.

3-3- Silica bonded investment, Silica bonded investment, used for metals that melt at very used for metals that melt at very high temperature.high temperature.

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