Covers the gypsum products used to make working models, starting with how beta and alpha calcium sulfate hemihydrate differ in crystal shape, density, water demand, strength and detail reproduction. Next it sets out a table of five model-material types, giving hemihydrate form, water/powder ratio, expansion and main use, with a bar chart of compressive strength. It then lists requirements for model materials and methods for strengthening plaster models, such as borax, alum, filling pores and surface water glass coating.
Topic 25 of 52 in the MU Sofia syllabus for Dental Materials (Dental Medicine, Year 1). The free AceMed note covers it in 5 sections, about 220 words (2 min of reading), with 14 flashcards.
In this note
Introduction
Alpha and beta hemihydrate
The five types
Requirements for model materials
Making plaster models stronger
Introduction
Model materials are auxiliary laboratory materials used to make working models: positive copies of the prosthetic field, poured into the impression taken from the patient's mouth. Gypsum products are by far the most common. Resin and epoxy models are stronger, but they shrink as they set and so add to the shrinkage of the impression.
Alpha and beta hemihydrate
All dental gypsum is calcium sulfate hemihydrate. How it is heated decides the shape of its crystals, and the crystals decide its properties.
The five types
Less water means fewer pores and a stronger model. This is why dense alpha crystals, which need little water, make the hardest products.
Requirements for model materials
Reproduce the finest details of the impression. This depends on wetting, the grain size of the powder and the viscosity of the mix.
Expand just enough to make up for the shrinkage of the impression.
Have enough strength for their purpose.
Differ in color from the impression material, and be color-coded by type.
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