Explains what a master cast is and why every error in it carries into the restoration. Covers gypsum types such as impression plaster, model plaster, dental stone and improved stone, with a table of their uses, plus mixing ratios, pouring technique and the mixing, working and setting times. Lists the requirements for a good working cast and a good die, then compares three types of master cast in a table and walks through making a cast with removable dies, with a labelled diagram.
Topic 32 of 75 in the MU Sofia syllabus for Preclinics of Dental Prosthetics (Dental Medicine, Year 2). The free AceMed note covers it in 5 sections, about 363 words (4 min of reading), with 13 flashcards.
In this note
Introduction
Gypsum products
Requirements
Types of master cast
Making a cast with removable dies
Introduction
The master (working) cast is the positive copy of the prepared teeth, the edentulous ridges and the rest of the arch, poured from the final impression. The restoration is made on it, so every error in the cast becomes an error in the mouth.
Gypsum products
Casts are made of gypsum: calcium sulphate hemihydrate powder that sets when mixed with water.
Mixing rules
Measure powder and water; for die stone about 100 g powder to 20–25 ml water. The more accurate the ratio, the better the cast.
Put the water in the bowl first, sprinkle the powder in, then spatulate.
Pour in small amounts from one place, with vibration, to drive out air bubbles.
Requirements
A good working cast
Reproduces every detail captured in the impression.
Has no air bubbles, voids or distortion.
Is trimmed at the preparation margins so the wax pattern can be carved there.
Can be mounted precisely on an articulator.
A good die
A die is the positive copy of one prepared tooth, on which the wax pattern is made and finished. It must reproduce the preparation exactly, be free of bubbles, return to exactly the same place on the cast, and stay stable even when the cast is turned over.
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