Casting

1. Casting Libyan International Medical University

3. Metal casting is made using a refractory mold made from a wax replica or pattern. • The procedure includes the following steps: • Prepare the tooth or teeth to receive a cast restoration. • Make an impression of the prepared tooth. • Pour gypsum to form cast or die. • Make wax pattern. • Sprue the wax pattern.

4. 6. Invest the wax pattern. 7. Eliminate the wax pattern by burning the wax out of the investment in a furnace, thus making the mold. 8. Force molten metal into the mold. 9. Clean the cast. 10. Remove the sprue from the casting. 11. Finish and polish the casting on the die. 12. Cement the finished cast restoration on the prepared tooth.

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

6. Wax pattern prepared directly in a patient's mouth will shrink about 0.4% when cooled from oral temperature. • In the indirect method of preparing the wax on die, the wax shrinkage is about 0.2%. • Wax pattern should be invested soon after its formation to avoid wax distortion. • Setting, thermal and hygroscopic expansion of investment to compensate wax and metal alloy shrinkage.

7. A liner is placed inside the ring to allow lateral expansion of the investment. • Ceramic paper liner acts as cushion against which expansion can take place. • Moreover, greater expansion caused by the uptake of water from liner referred as hygroscopic expansion. • 3 mm of clearance is allowed at each end of the ring so the mold is sealed.

8. Spruing • To form a mount for the wax pattern and fix the pattern in a space so a mold can be made. • To create a channel for elimination of wax during burnout. • To form a channel for molten alloy during casting. • Sprue should be attached to the bulkiest portion of the wax pattern to minimize wax pattern distortion

9. The sprue must be large and short enough to allow rapid filling of the mold cavity. • The sprue pin is mounted on a clean sprue-crucible-former base. • The wax pattern should be 6 mm from the end of the ring. • If less than 6 mm…..there is no enough thickness of investment to keep the molten alloy from breaking through. • If more than 6 mm, the alloy will solidify before the entrapped air can escape, resulting in incomplete casting or mold fracture.

10. Investment Materials • Investment are ceramic materials used as models for casting molten metals or holding metal appliances in correct relation during soldering procedures. • They are made from ceramics because they must be able to withstand the high temperature associated with the casting process. • In addition, they must not chemically react with the cast metal. • Investment consist of binder and refractory. • Binder…materials hold the investment together. • Refractory… material resists heat..e.g. silica

11. Requirements of ideal investment materials • It should gives accurate details • Expands to compensate wax and alloy shrinkage. • Easy to manipulate (working and setting time). • Resist high temperature. • Have pores to allow air to escape as the molten-alloy entering the mold. • It should have a strength to withstand the force of molten-alloy entering.

12. Types of investments Investments classified according to binder 1- Gypsum-bonded investment, composed of calcium hemihydrate as binder or matrix and silica in the form of quartz serves as refractory to withstand the high temperature. 2- Phosphate bonded investment, composed of silica refractory as in the gypsum bonded investment and the binder is made from an acidic phosphate and magnesium oxide. Its used for casting high melting point metals and alloys. 3-Silica bonded investment, used for metals that melt at very high temperature.