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.
Although small dimensional changes during setting can often be tolerated and even compensated for, changes occurring during storage are a more serious problem. Hence, the dimensional stability after setting should be as good as possible.
The material should, ideally, be fluid at the time it is poured into the impression so that fi ne detail can be recorded. A low contact angle between the model and impression materials would help to minimize the presence of surface voids on the set model by encouraging surface wetting.
The set material should be suffi ciently strong to resist accidental fracture and hard enough to resist abrasion during the carving of a wax pattern. 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.
