Dental Materials · MU Plovdiv

73. Plasters and stones. Setting reaction.

Inside: 12 sections · 3 labelled pictures · 2 tables · 28 flashcards · 5 min

Topic 73 of 85 in the MU Plovdiv syllabus for Dental Materials (Dental Medicine, Year 1). The free AceMed note covers it in 11 sections, about 584 words (5 min of reading), with 28 flashcards.

In this note

  1. Introduction
  2. Alpha and beta hemihydrate
  3. The five types
  4. Requirements for model materials
  5. Making plaster models stronger
  6. Manufacture
  7. Setting reaction
  8. Expansion
  9. Strength
  10. Controlling the setting time
  11. Types

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

  1. Reproduce the finest details of the impression. This depends on wetting, the grain size of the powder and the viscosity of the mix.
  2. Expand just enough to make up for the shrinkage of the impression.
  3. Have enough strength for their purpose.
  4. Differ in color from the impression material, and be color-coded by type.

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