Dental Investment and Casting Process

Casting Process

• Steps include spruing, investing, and casting.
• Attach sprue former to the wax pattern before removal from the die.
• Place the wax pattern with the former in a casting ring lined with ceramic paper.
• Invest the wax pattern, allowing the investment to set.
• Remove the crucible former and place the ring in a burnout furnace to eliminate wax.
• Melt the alloy and cast it into the mold created by the investment.

Definition of Investment Material

• Investment materials are ceramics used to form molds for casting metals or alloys.
• Components include: crucible, sprue, reservoir, wax pattern, casting ring, ceramic paper liner, and investment material.

Composition of Dental Investment

• Composed of refractory materials, binders, and additives.
• Refractory typically involves silica (SiO2), existing in three forms: quartz, cristobalite, tridymite.
• Refractories control thermal expansion to compensate for the shrinkage of the alloy and gypsum.

Types of Dental Investments

• Classified based on the type of binder:
  • Gypsum bonded investments: Uses gypsum as binder and silica for thermal expansion. Suitable for gold alloys.
  • Phosphate bonded investments: Utilizes phosphate as a binder, silica as the refractory material, suitable for high melting alloys.
  • Ethyl silicate bonded investments: Primarily used for base metal alloys but now less common.

Expansions of Dental Investments

• Essential to compensate for casting shrinkage:
  • Setting Expansion: Adjust water-to-powder ratio to influence setting expansion; decreased water increases expansion.
  • Hygroscopic Expansion: Achieved by submerging the setting investment in water to enhance expansion.
  • Thermal Expansion: Occurs during burnout; preferable to use investments with minimal setting expansion and higher thermal expansion.

Casting Ring and Its Techniques

• The casting ring secures the investment during setting.
• Ring-less Technique: Allows unlimited external expansion, requires careful control of expansion timing.
• Ring Technique: Includes a ring liner to manage external expansion and provide support.

Ring Liner

• Liner should match the casting ring's inner diameter; thickness should be no less than 1mm.
• Liner placement aids in uniform investment expansion and minimizes distortion of the wax pattern.
• Traditional options included asbestos; now, other materials like aluminum silicate ceramic and cellulose paper are preferred.

Investing Procedures

• The process embeds the sprued wax pattern in an investment to form a ceramic mold.
• Clean the wax pattern using a cleanser and secure it in a sprue base.
• Mix liquid and powder carefully to avoid air entrapment.

Investing Techniques

• Hand Technique: Utilizes a brush for detailed application, filling the investment under vibration.
• Vacuum Technique: Investment performed under vacuum conditions for minimized air entrapment, leading to better casting quality.

Wetting Agent

• A debubblizer spray should be applied on the wax pattern prior to investing, ensuring complete wetting.
• Bubbles between the wax and investment compromise casting quality, causing nodules on the surface.

This quiz covers the steps involved in the casting process in dentistry, including spruing, investing, and casting. It explores the procedures immediately after completing the wax pattern.