Dental Casting Process Overview

Expansion of Investment

• Expansion of investment is enhanced by silica particles interfering with the crystalline structure of gypsum, causing it to expand outward.
• Normal expansion in air is approximately 0.4%, but it is partially restricted by the metal investment ring.

Hygroscopic Expansion

• Hygroscopic expansion can be employed to augment normal expansion, producing additional expansion.
• Maximum expansion can be achieved by immersing an investment-filled ring in a 100°F (38°C) water bath.
• Hygroscopic expansion ranges from 1.2% to 2.2%.

Thermal Expansion

• Thermal expansion of the investment occurs when the investment is heated in the burnout oven.
• Heating of the mold also serves to eliminate the wax pattern and to prevent the alloy from solidifying before it completely fills the mold.
• The silica refractory material is principally responsible for thermal expansion due to solid-state phase transformations.

Ideal Properties of an Investment

• An ideal investment should have controllable expansion to compensate for shrinkage of the cast alloy during solidification.
• It should produce smooth castings with accurate surface reproduction without nodules.
• It must provide sufficient strength to withstand burnout and casting of the molten metal.
• It should have chemical stability at high casting temperatures.
• It should have sufficient porosity to allow for gas escape.
• It should allow for easy recovery of the casting.

Gypsum-Bonded Investments

• Gypsum-bonded investments satisfy most of the requirements for an ideal material.
• However, they are not suitable for casting metal-ceramic alloys because gypsum is unstable at high temperatures required.
• Factors that increase expansion of gypsum-bonded investments include:
  • Use of full-width ring liner
  • Use of two-ring liner
  • Use of dry liner
  • Prolonged spatulation
  • Lower water/powder ratio
  • Storage in 100% humidity
  • Hygroscopic technique

Phosphate-Bonded Investments

• Phosphate-bonded investments are more stable at high temperatures and are the material of choice for casting metal-ceramic alloys.
• They are mixed with a specially prepared suspension of colloidal silica water.
• Some phosphate-bonded investments have carbon and are gray in color.
• Carbon-containing materials should not be used for casting base metal alloys because carbon residue affects the final alloy composition.

Shrinkage and Condensation

• The molten alloys used for dental restorations shrink upon solidification: gold alloys by approximately 1.5% and nickel-chromium alloys by as much as 2.4%.
• If the mold is not made correspondingly larger than the original wax pattern, the resultant casting will be smaller.
• For crowns, it is necessary to compensate for solidification shrinkage of the specific alloy used by expanding the mold enough to at least equal the shrinkage.

Spruing and Venting

• The sprue former is attached to the wax pattern and the other end to the crucible former.
• The attachment area should not be restricted, as necking increases casting porosity and reduces mold filling.
• Small auxiliary sprues or vents have been recommended to improve casting of thin patterns.
• Their action may be to help gases escape during casting or to ensure that solidification begins in critical areas.

Learn about the critical aspects of spruing, investing, and casting in the dental casting process. Understand the importance of correct sprue placement to ensure uniform expansion within the mold and proper filling of the molten alloy.