Investment materials
Investment is defined as a ceramic material which is suitable for forming mold and in this mold of an alloy is appropriately casted.
Following the assembly of a wax pattern by either the direct or indirect method, subsequent stage in many dental procedures involve the investment of the pattern to make a mould. A sprue is attached to the pattern and therefore the assemblage is found during a casting ring. Investment material is poured round the wax pattern whilst still during a fluid state. When the investment sets hard, the wax and sprue former are removed by softening and burning out.
Requirements of investments for alloy casting procedures
1- The investment should be capable of reproducing the shape, size, and detail recorded in the wax pattern.
2- the investment should have a sufficiently high value of
compressive strength at the casting temperature so that it can withstand the stresses set up when the molten metal enters the mold.
3- One function of the investment mold is to compensate for this casting shrinkage. This is generally achieved by a combination of setting expansion during the hardening of the investment mold and thermal expansion during the heating of the mold to the casting temperature.
4- The main factors involved in the selection of investment material are the casting temperature to be used and the sort of alloy to be cast. Some gold alloys are cast at relatively low casting temperatures of around 900°C whilst some chromium alloys require casting temperatures of around 1450°c The investment is best ready to retain its integrity at the casting temperature and ready to provide the required compensation for casting shrinkage is chosen.
5-The particles of the investment material show are very fine to go smooth surface to the mold alsoThe investment material should be easy to manipulate and suitable sitting time
Composition of investment materials
In general, investment materials is a mixture of three distinguishing types of material:
1. Refractory material: This material is usually a form of silicon dioxide, such as quartiz, tridymite, or cristobalite, or a mixture of these. Silica is a refractory material that is capable of withstanding very high temperature that is used during casting it is also responsible for much of expansion necessary to compensate for casting shrinkage of the alloy.
2. Binder material: because the refractory material alone does not form a coherent solid mass, some kind of binder is needed. The common binder is used for the dental cast is a calcium sulphate hemihydrate, phosphate, and ethyl silicate. Investment can be classified according to the type of binder into:
Gypsum bonded investment. • Silica bonded investment.
Phosphate bonded investment.
3. Other chemical material: usually a mix of refractory material and therefore the binder alone doesn't enough to supply all desired properties required for investment.
Other chemicals, such as sodium chloride, boric acid, potassium sulfate, graphite, copper powder or magnesium oxide are often added in small quantities to modify various physical properties.
Available materials Investment materials consist of a mixture of refractory material, normally silica, which is capable of withstanding very high temperatures without degradation, and a binder which binds the refractory particles together. The nature of the binder characterizes the material. There are three groups of investment material in common use. They are referred to as gypsum-bonded, silica bonded or phosphate-bonded.