![]() ![]() This produces irregular, porous particles which are sometimes referred to as β-hemihydrate particles ( Fig. Gypsum is heated to a temperature of about 120☌ in order to drive off part of the water of crystallization. Plaster is produced by a process known as calcination. The various types of gypsum product used in dentistry are chemically identical, in that they consist of calcium sulphate hemihydrate, but they may differ in physical form depending upon the method used for their manufacture.ĭental plaster (plaster of Paris): Dental plaster is indistinguishable from the white plaster used in orthopaedics for stabilizing fractured limbs during bone healing. The hemihydrate is mixed with water and reacts to form the dihydrate. Gypsum products used in dentistry are formed by driving off part of the water of crystallization from gypsum to form calcium sulphate hemihydrate.Īpplications of gypsum products in dentistry involve the reverse of the above reaction. It should give a good colour contrast with the various waxes which are often used to produce wax patterns. For example, the set model should easily be removed from the impression without damage to its surface and fracture of teeth. The material should be compatible with all the other materials with which it comes into contact. ![]() ![]() The set material should be sufficiently strong to resist accidental fracture and hard enough to resist abrasion during the carving of a wax pattern. 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 material should, ideally, be fluid at the time it is poured into the impression so that fine detail can be recorded. Hence, the dimensional stability after setting should be as good as possible. 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.Īlthough small dimensional changes during setting can often be tolerated and even compensated for, changes occurring during storage are a more serious problem. 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 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. The main requirements of model and die materials are dimensional accuracy and adequate mechanical properties. 3.2 Requirements of dental cast materials The Type 1 material will be discussed in Chapter 17 (Non-elastic Impression Materials). The current ISO Standard for Dental Gypsum Products identifies five types of material as follows: Type 1ĭental stone, die, high strength, low expansionĭental stone, die, high strength, high expansion Many materials have been used for producing models and dies but the most popular are the materials based on gypsum products. The morphology of the hard and soft tissues is recorded in an impression and models and dies are prepared using materials which are initially fluid and can be poured into the impression, then harden to form a rigid replica. The term die is normally used when referring to a replica of a single tooth. The term model is normally used when referring to a replica of several teeth and their associated soft tissues or, alternatively, to an edentulous arch. ![]() Many dental restorations and appliances are constructed outside the patient’s mouth using models and dies which should be accurate replicas of the patient’s hard and soft tissues. Their main uses are for casts or models, dies and investments, the latter being considered in Chapter 5. Gypsum products used in dentistry are based on calcium sulphate hemihydrate (CaSO4 2) 2 Gypsum is a naturally occurring, white powdery mineral with the chemical name calcium sulphate dihydrate (CaSQ 4♲H 2O). Chapter 3 Gypsum Products for Dental Casts 3.1 Introduction ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |