Dental Casting Procedures and Investment Material Science

Casting Process for Oral Appliances

Casting is the process whereby the appliance is formed from an alloy in the space left by the wax pattern to manufacture different types of oral prosthetic devices.

Key Stages in the Dental Casting Process

1. Achievement of the wax pattern.
2. Investing the wax pattern.
3. Wax removal (burnout).
4. Melting the alloy and casting.
5. Finishing and polishing.

Refractory Foil Rings

These rings are used during the investment process.

Types of Refractory Foil Rings

• Asbestos (now largely replaced)
• Cellulose
• Ceramics

Functions of Refractory Foil Rings

• Insulation: Avoids abrupt warming of the investment material and facilitates the removal of the ring after casting.
• Expansionist: Contributes to expansion (setting, thermal, and hygroscopic).

Dental Investment Materials

Investment material is a refractory substance used to create the mold, into which a metal or alloy is cast for the manufacture of prosthetic devices.

Handling Techniques and Instrumentation

Instrumentation

• Spatula for plaster
• Rubber cup
• Cup with mechanical mixing device

Water/Powder Ratio (A/R): Always follow the manufacturer's instructions precisely.

Setting and Working Time Parameters

Mixing Time: Typically 9 to 18 seconds (must be no less than 5 seconds or more than 30 seconds).

• Initial Set: Occurs when the material loses its luster, allowing the removal of the ring base.
• Final Set: Allows the placement of the investment ring in the burnout oven.

Classification of Investment Materials

Classification by Temperature and Alloy Type

• Low Temperature (800°C – 1010°C): Used for casting noble alloys. These are typically calcium sulfate-bonded.
• High Temperature (up to 1400°C): Used for casting non-noble alloys. These are typically phosphate-bonded or silicate-bonded.

Low Temperature Investment (Calcium Sulfate-Bonded)

Classification by Expansion Type

• Type I: Inlays and crowns (relying primarily on thermal expansion).
• Type II: Inlays and crowns (relying primarily on hygroscopic expansion).
• Type III: Removable partial dentures (relying primarily on thermal expansion).

Commercial Presentation Formats

Investment material is typically presented as a powder:

• Bulk: Large pots and jars.
• Pre-measured: Packets and small bags.

Detailed Chemical Composition

• Refractory Element (55–75%): Silica (quartz or cristobalite). Provides heat resistance and expansion.
• Binder/Glue (25–45%): Plaster (calcium sulfate hemihydrate). Provides strength, controls setting time, and setting expansion.
• Reducing/Deoxidizing Substances (up to 5%): Copper, carbon, or graphite.
• Regulators (up to 5%): Boric acid or sodium chloride. Regulates working time, coating expansion, and minimizes casting contractions.
• Colorants: Added for identification.

Chemical Reaction During Setting

The setting reaction involves the hydration of the binder:

Calcium Sulfate (Hemihydrate) + H₂O → Crystals of Calcium Sulfate (Dihydrate)