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.
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.
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
α-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.
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.
Another type of binding material for investments used with casting highmelting 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.