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.