Gypsum
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What is a study model
a POSITIVE REPLICA of dentition, produced from the impression (negative representation) of a patient’s dentition
what are the purposes of study models
• records the position, shape of teeth • aids visualisation/ assessment of dentition • enables manufacture of dental prostheses
what material is often used to create study models
gypsum
what are the uses of gypsum
• cast (plaster/stone) • die (stone/ improved stone) • mould material (stone) • investment binder (stone)
what determines the type of gypsum you end up with
manufacturing conditions
What is the manufacturing process of gypsum
Calcium sulphate dihydrate, heats it, produces calcium sulphate hemihydrate and water
What are the different types of gypsum
• plaster (beta-hemihydrate) • dental stone (alpha-hemihydrate) • densite (improved stone)
what determines the properties of the gypsum
crystalline structure
How do you manufacture gypsum to get plaster (beta-haemihydrate)
• heated in open vessel
How do you manufacture gypsum to get dental stone (alpha hemihydrate)
• heat in autoclave
How do you manufacture gypsum to get densite (improved stone)
• heat in the presence of Ca and Mg chloride
What is the crystalline structure like
large porous, irregular crystals
What is the crystalline structure of dental stone like
non-porous, regular crystals, requires less water
What is the crystalline structure of densite like
compact smoother particles
what is the basic reaction gypsum undergoes during the manufacturing process
dihydrate –> hemihydrate
what is the basic setting reaction gypsum undergoes
hemihydrate –> dihydrate Powder + water = gypsum (study cast) (reverse of manufacture)
What is the mixing ratio for plaster
water = 50-60ml powder = 100g
What is the mixing ratio for stone
water = 20-35ml powder = 100g
what is the theoretical ratio for the mixing ratio
water = 18.6ml powder = 100g
are the mixing ratios of powder and water the same for all types of gypsum
no
what is excess water needed for
a workable mass, affects properties
what happens during the the setting process
• hemihydrate dissolves • dihydrate forms • dihydrate solubility low-supersaturated solution • dihydrate crystals precipitate on impurities as crystals • more hemihydrate dissolved • continues until all hemihydrate dissolved Initial set • dihydrate crystals come into contact i.e. push apart • expansion starts • properties of weak solid and will not flow • can be carved Final set • strong and hard enough to be worked • strength continues to develop During setting: excess water is trapped in the powder mass On completion of setting: • excess water evaporates • voids produced (porosity)
How are initial and final setting times measured
using gilmore needles
How is the strength of gypsum
• compressive (~20-35MPa) - low hardness