CASTING INVESTMENTS (DENTAL MATERIALS)

CASTING INVESTMENTS
PROPERTIES REQUIRED OF AN INVESTMENT
1- Easily manipulated.
2- Sufficient strength at room temperature: To permit ease in handling and provide enough strength at higher temperatures to withstand the impact force of the molten metal.
3- Stability at higher temperatures: Investment must not decompose to give off gases that could damage the surface of the alloy.
4- Sufficient expansion: Enough to compensate for shrinkage of the wax pattern and metal that takes place during the casting procedure.
5- Beneficial casting temperatures: Preferably the thermal expansion versus temperature curve should have a plateau of the thermal expansion over a range of casting temperatures.
6- Porosity: Porous enough to permit the air or other gases.
7- Smooth surface.
8- Ease of divestment
9- Inexpensive.

COMPOSITION
In general, an investment is a mixture of three distinct types of materials:
1- Refractory Material: This material is usually a form of silicon dioxide, such as quartz, tridymite, or cristobalite, or a mixture of these.
2- Binder Material: Because the refractory materials alone do not form a coherent solid mass, some kind of binder is needed.
3- Other Chemicals: Usually a mixture of refractory materials and a binder alone is not enough to produce all the desirable properties required of an investment material.


CALCIUM SULFATE- BONDED INVESTMENT
The investments suitable for casting gold alloys contain (65-75 %) quartz or cristobalite, or a blend of the two, in varying proportions, (25-35 %) of ?-calcium sulfate hemihydrate, and about (2-3 %) chemical modifiers. The calcium sulfate-bonded investment is usually limited to gold castings, and is not heated above 700°C. The calcium sulfate portion of the investment decomposes into sulfur dioxide and sulfur trioxide at temperatures over 700°C, tending to embrittle the casting metal. Therefore, the calcium sulfate type of binder is usually not used in investments for making castings of palladium or base metal alloys.

PHOSPHATE-BONDED INVESTMENT
It is the most common type of investment for casting high-melting point alloys. This type of investment consists of three different components. One component contains a water-soluble phosphate ion. The second component reacts with phosphate ions at room temperature. The third component is a refractory, such as silica. Different materials can be used in each component to develop different physical properties.

SILICA-BONDED INVESTMENT
Another type of binding material for investments used with casting high-melting point alloys is a silica bonding ingredient. This type of investment may derive its silica bond from ethyl silicate, an aqueous dispersion of colloidal silica, or from sodium silicate. One such investment consists of a silica refractory, which is bonded by the hydrolysis of ethyl silicate in the presence of hydrochloric acid.