Investment Material Properties for Precision Casting

Casting Operation Requirements

In all casting operations, the following elements are involved:

1. A wax pattern object to be reproduced.
2. A suitable material for manufacturing the mold, known as a coating, which is placed around the pattern and allowed to harden.
3. Suitable material for burning out the wax patterns and heating the mold.
4. The appropriate means to melt and drop the alloy.

One can estimate that the coating is a ceramic material suitable to form a mold within which to place a metal or alloy.

Coating Properties Needed

The ideal mold coating must possess several critical characteristics:

1. Ease of Use: Must be easy to handle and have a relatively short curing time.
2. Mechanical Strength: The mold coating must have sufficient strength at room temperature for easy handling and resistance to high temperatures during preheating.
3. Thermal Stability: When heated to high temperatures, it should not decompose, releasing gases that may damage the alloy surfaces.
4. Expansion Control: The investment should expand sufficiently upon heating.
5. Permeability: It must be porous enough to allow the escape of air trapped in the cavity during the casting process.
6. Surface Finish: It must result in a smooth surface and capture fine details in the casting.
7. Post-Casting Removal: When casting is complete, the coating must break away easily from the metal surface and not react chemically with it.
8. Cost-Effectiveness: Since the mold is always destroyed in the casting process, the coating material should be inexpensive.

These are the requirements of an ideal coating material.

Investment Composition

The investment material typically consists of three main components:

Refractory Material

This material must withstand the environmental conditions in which it is immersed without major alterations in its physical-chemical properties for a profitable economic period. It is usually silicon dioxide, such as quartz, tridymite, or cristobalite.

Binder

The binder is responsible for bringing the refractory material mass together, setting, and hardening in a short period. Since refractory materials alone do not form a solid, consistent mass, a binder is necessary. The most commonly used binder for casting dental gold alloys is alpha calcium sulfate hemihydrate. At higher temperatures, phosphate binders and ethyl silicate can also be used.

Modifiers

These serve to improve the qualities of the coating. For instance, sodium chloride and boric acid increase the thermal expansion of coatings bonded with calcium sulfate.

Bonded Coating with Calcium Sulfate: Composition

This specific composition is often used:

• Silica: 60–65% (Quartz, Cristobalite, or a mix of both in varying proportions).
• Calcium Sulfate Hemihydrate: 30–35%.
• Modifiers: 5%.

These coatings are easy to handle and are often used for casting gold alloys that are not heated above 700 ºC. Above this temperature, the material decomposes to form calcium sulfate dioxide and sulfur trioxide.