models and dies Flashcards
properties which are good for dental models
1) cheap
2) easy to form (from an impression)
- i.e. not react with anything
3) safe
4) robust
5) rigid, no flex
6) will not chemically react with wax, imps ect
7) resistant to wear
8) resistant to heat (about 100 C)
modelling materials
1) plastics
2) organic materials ie wood, bone
3) clay based
-
4) Playdo
- i.e
5) Plaster set
- i.e. plaster of paris
good properties of plaster of paris or gypsum
1) Cheap
2) robust
3) easy to form
4) safe
5) rigid, no flex
6) will not react to wax or impressions
- does however react to some materials*
7) resistant to heat
Not as resistant to wear
plaster of paris uses
1) models and dies
2) impression materials (mucostatic)
3) moulds for denture construction
4) binder in refractory investments for casting
production of plaster
1) Open kettle at 110-120
- normal plaster
- large porous particles
- calcium sulphae B- hemihydrate
2) Pressure vessel at 125
- stone
- drive more water out of the plaster, higher temperature
- small dense particles
- calcium sulphate A hemihydrate
3) In the presence of deflocculants >
- improved stone
- small very dense particles
- the additions allow it to set in a more regulated way
- makes a very hard model
- calcium sulphate A hemihydrate
as you move to improved stone the properties alter..
- strength improves
- setting time gets longer
- more expensive
- hardness improves
model
whole mouth
die
a tooth (section of the model) removeable to be worked on
dental plaster info
- working time is 3-4mins, initial set in 10 mins
- slight expansion on setting of 0.2 to 0.3%
- exothermic setting reaction, cool once set
- softest but quickest setting
dental stone info
yellow form
- working time3-4mins, initial set in 10 mins
- slight expansion on setting of 0.2 -03.%
- exothermic setting reaction, cool once set
- stronger than PoP but takes longer to set
- often used for everyday dental models (mixed with 50/50DP to reduce cost and setting time)
improved stone info
coloured powder
- finer more regulated particles
- working time is 5 mins, initial set in 30 mins
- even less expansion on setting of 0.05-0.07%
- harder than PoP, takes longer to set
- usually vacuum mixed
- used where accuracy and wear resistance is critical eg crown and bridge models
chemistry of plaster
Calcium sulphate dihydrate (gypsum)
- with heat, water is removed
grt to calcium sulphate hemihydrate (powder)
- give water, returns to the original
setting process for plaster
1) start with the powder which is hemihydrate, add it to water
2) hemihydrate dissolves in water to form a solution (forms cystals which look like spherulites. which grow and interlock together)
3) super saturated solution forms
4) dihydrate precipitates out
5) more hemihydrate dissolves
6) crystal growth
what is expansion of plaster due to
crystals pressing agaisnt each other
what do accelerators and retarders do
reduce the setting expansion
easier to work with and more progressive
