Requirements of dental cast materials
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 sparingly soluble salt followed by crystallization of another. Increasing the temperature accelerates the solution process but retards the crystallization. Thus the two effects tend to cancel out. Increasing him W/P ratio retards setting by decreasing the concentration of crystallization nuclei. Increasing mixing time has the opposite effect. This accelerates setting by breaking up dihydrate crystals during the early stages of setting, thus producing more nuclei on which crystallization can be initiated.
Alterations in W/P ratio and mixing time have only a minimal effect on setting expansion.
Advantages and disadvantages
Gypsum model and die materials have the advantages of being inexpensive and easy to use. The accuracy and dimensional stability are good and they are able to reproduce fine detail from the impression, providing precautions are taken to prevent blow holes. The mechanical properties are not ideal and the brittle nature of gypsum occasionally leads to fracture - particularly through the teeth, which form the weakest part of any model. Problems occasionally arise when gypsum model and die materials are used in conjunction with alginate impression. The surface of the model may remain relatively soft due to an apparent retarding effect which hydrocolloids have on the setting of gypsum products. It is not certain whether the retarding effect is due to borax in the hydrocolloid or to the absorption of hydrocolloid onto the gypsum crystals which act as nuclei of crystallization. Despite these observations it cannot be said that gypsum products are incompatible with alginate impression materials since problems arise very infrequently.