Gypsum Products for Dental Casts

The main requirements of model and die materials are dimensional accuracy and adequate mechanical properties. The accuracy of fit of any restoration or appliance constructed outside the mouth depends inter alia on the dimensional accuracy of the replica on which it is constructed. Thus, the dimensional changes which occur during and after the setting of these model materials should, ideally, be minimal in order to produce an accurate model or die. The final fit of the appliance may depend upon a balancing of small expansions or contractions which occur at different stages in its construction and it would be unwise to consider, in isolation, dimensional changes occurring with the model and die materials.

The material should be compatible with all the other materials with which it comes into contact. For example, the set model should easily be removed from the impression without damage to its surface and fracture of teeth. It should give a good colour contrast with the various waxes which are often used to produce wax patterns.

Manipulation and setting characteristics

Plaster and stone powders are mixed with water to produce a workable mix. Hydration of the hemihydrate then occurs producing the gypsum model or die. In the case of the more dense material, dental stone, a ratio of about 0.3 is required to produce a workable mix, whereas for the more porous plaster a higher W/P ratio of 0.55 is required. Factors under the control of the operator are temperature, W/P ratio and mixing time. Surprisingly, temperature variation has little effect on the setting times of gypsum products. This is due to the fact that the setting involves dissolution of one sparing