Models and dies Flashcards
What do we need in a dental modelling material?
Cheap Easy to form (from an imp) Safe Robust Rigid, no flex Will not chemically react with wax, imps etc. Resistant to wear Resistance to heat (around 100 degrees)
What is plaster of paris, or gypsum
Models and dies
Impression material
Moulds for denture construction
Binder in refractory investments for casting
Plaster won’t set in presence of
Agar
But alum soln can be used as barrier
What is plaster of paris?
Natural mineral that is mined
Dehydrated gypsum known as plaster of Paris
Upon addition of water, plaster of Paris becomes resgular gypsum again, causing material to harden or set (reversible reaction)
Production of plaster
Open kettle at 110-120C -calcium sulphate beta hemihydrate -large porous particles Pressure vessel at 125C -alpha hemihydrous -small dense particles In presence of deflocculants (CaCl2 and MgCl) -calcium sulphate alpha hemihydrate -more expensive -small very dense particles
Models and dies
Model - the whole mouth
Die - a tooth )a section of the model), removable to be worked on
Dental plaster - key info
White powder in lab
Working time 3-4 mins, initial set 10 mins
Slight expansion on setting of 0.2-0.3%
Exothermic setting reaction, cool once set
Softest but quickest setting
Rarely used on its own as a mould. Used as denture model
Dental stone key info
Yellow powder in lab
Working time 3-4 mins, intial set 10 mins
Slight expansion on setting 0.2-0.3%
Exothermic setting reaction, cool once set
Stronger than PoP, takes longer to set
Often used for everyday dental models
-mixed 50/50 with DP to reduce cost and setting time
Improved stone key info
Coloured powder in lab Finer, more regular particles Working time 5 mins, intial set 30 mins Expansion on setting 0.05-0.07% Harder than PoP, but takes longer to set Usually vaccum mixed Used where accuracy and wear resistance critical - e.g. crown and bridge models
Chemistry of plaster
Calcium sulphate dihydrate
CaSO42H2O –heat–> (CaSO4)2H2O + 3H2O (hemihydrate)
Setting process for plaster
Hemihydrate dissolves in water Super saturated solution forms Dihydrate precipitates out More hemihydrate dissolves Crystal growth -arms like pom poms until they all meet
Expansion of plaster
Interlocking crystals with water in the pores
-uncontrolled ~0.6%
-plaster 0.2-0.3%
-stone 0.08-0.1%
Accelerators and retarders also reduce setting expansion (anti-expansion agents)
Bonus properties of PoP
Easy to trim or grind
Can be coloured (appealing shades)
Can be polished to high shean
Sticks to itself
Separating plaster
Sodium alginate
When making moulds, you do not want plaster to bond to itself
Control of setting characteristics
Water/ powder ratio -plaster 50ml H2O / 100mg powder -stone 20-35ml H2O / 100mg powder Spatulation (over and under vacuum) Temp Retarders -borax -potassium citrate -sodium chloride (large amounts) Accelerators -potassium sulphate -gypsum -sodium chloride (small amounts, causes pink spots)
