Gypsum model and Dies Flashcards
Models and dies are needed to make … in a …
- indirect devices
- lab
Define ‘model’
- replica of soft tissues, associated bony tissue and remaining teeth
Define ‘die’
- replica of the prepared tooth/teeth
- for device manufacture
Requirements of model and die material
- accurate
- dimensionally stable
- compatible with impression materials
- colour contact
- ease of use
- cost
- strength, toughness and stiffness
- hardness
- fine detail reproduction
Why do model and die materials need to be accurate?
- minimal dimensional change on setting
- free of voids on setting
Why do model and die materials need to be dimensionally stable?
- accuracy after storage
Why do model and die materials need to be compatible with impression materials?
- no interaction - adhesion can cause damage
- no rejection - model must flow into impression
Why do model and die materials need to have colour contrast?
- easy to read
- aids in device manufacturer
Why do model and die materials need to be strong, tough and stiff?
- resist fracture
- and deformation
Why do model and die materials need to have hardness?
- resist scratching and abrasion
Why do model and die materials need to have fine detail reproduction?
- retain features recorded in impression
What is gypsum?
- common names for calcium sulphate dihydrate
- CaSO42H2O
How is gypsum used in dentistry?
- supplied in dry state
- based on calcium sulphate hemihydrate
- mixed with water to set
- 2CaSO42H2O -> (CaSO4)2 + 3H2O
Uses of gypsum in dentistry
- model and die material
- impression material
- investment material
Types of hemihydrate
- plaster/Plaster of Paris
- stone (Kaffir D, Calestone)
- special stone
Features of plaster gypsum
- heat to 120 degrees in open vessel (calcination)
- calcined hemihydrate
- large, irregular, porous
Explain stone hemihydrate
- heat to 120-130 degrees under pressure/steam autoclave
- autoclaved hemihydrate
- smaller, regular, dense, less porous
Explain special stone hemihydrate
- heat in boiling solution of calcium chloride or magnesium chloride
- densite
Plaster is normally what colour?
Stone is normally what colour?
- plaster is white
- stone contains coloured dye
Setting of hemihydrate
- reacts with water to form 2CaSO42H2O
- setting occurs through reacting powder with water
- need the most water for plaster, then stone
Explain the reaction of hemihydrate with water
- on mixing with water, some hemihydrate dissolves (low solubility)
- hydration occurs immediately
- dihydrate crystallises (very low solubility), comes out of solution and starts to solidify
- crystals are spherulitic
- excess water gives porosity
What does spherulitic mean?
crystals grow from impurities or gypsum
Physical changes during gypsum setting
- hardening
- further hardening by standing/drying out
- exothermic reaction
- dimensional change
How does gypsum harden?
- initial mix is fluid - can be poured
- initial set time - loses fluidity, excess can be carved away
- final set time can begin working on model
Why do we let additional gypsum hardening by standing out happen?
- allow to age for safety
What does the exothermic setting reaction of gypsum relate to?
- can be monitored
- correlates with increasing hardness
How does gypsum undergo dimensional change on setting?
- setting expansion occurs
- outward thrust of crystals - correlates with temp change
- can be used to monitor setting
Compare dimensional change in plaster and stone gypsum
- 0.3-0.5% for plaster
- 0.05-0.2% for stone
How to control setting time and expansion?
- added nuclei
- accelerators/retarders
- anti-expansion solutions
What does added nuclei do to gypsum?
- accelerate setting
- can be added by manufacture or from debris in mixing bowl
Explain accelerators/retarders
- potassium sulphate accelerates setting but can be too fast
- borax compensates for it
- reduces setting expansion
- can be added to the powder
Explain role of anti-expansion solutions
- some manufacturers add potassium sulphate and borax to water
- may be used in place of water
Effect of temp on setting of gypsum
- limited effect
- increased temp leads to increase of solution process
- leads to decrease in crystallisation
Increasing the W/P ratio causes what?
Why?
- slower setting
- dilution effect
- decreases concentration of crystallisation nuclei
What accelerates setting? Why?
- increasing mixing time
- mixing breaks up dihydrate crystals and disperses them
- more nuclei formed
Gypsum products are rigid and brittle/ soft and supple
rigid and brittle
…. of gypsum depends on porosity. Porosity depends on …
These properties improve when?
- strength, toughness and hardness
- W/P ratio
- more water means more space between crystals so sotne is always tougher/harder than plaster
- after drying
Applications of gypsum
- working models/dies (use stone)
- study models/mounting articulators (use plaster)
Advantages of gypsum
- inexpensive
- good colour contrasts
- can reproduce fine detail
- good dimensional stability
Disadvantages of gypsum
- need good technique (careful with mixing, vibration on pouring, low dimensional change)
- mechanical properties not ideal (common fractures)
- some compatibility problems (agar, plaster impressions both water based and adhere to gypsum models - separating sprays available too)
