impressions, waxes, casting Flashcards
what is gypsum and it’s use
Gypsum = calcium sulphate dihydrate (CaSO4)2H2O
Gypsum products used for models, dies, moulds.
Gypsum gets converted into the hemihydrate (CaSO4)H2O which (depending on technique) makes plaster, stone, or improved stone.
diff between plaster and stone
Plaster consists of large, irregular, porous particles. Plaster is used for models since it is cheap and soft.
Stone/improved stone consist of small, regular, non-porous particles. Stone is used for dies since they are harder. Improved stone is more expensive.
Properties of gypsum products:
No dimensional change once set
Compressive strength dependent on using the right amount of water
Great storage
Tensile strength (low for plaster, double that for stones)
products used for gypsum setting
Accelerators speed up gypsum setting by providing extra sites for crystal growth
K2SO4
CaSO4.H2O
NaCl (<20%)
Retarders slow down gypsum setting by interfering with crystal formation
NaCl (>20%)
Borax
Potassium citrate
how do you manipulate the properties of gypsume
Changing ratio of powder and liquid ->
More water = slower setting time + model will be weaker
More powder = hard to mix + creates lots of porosity
Increasing spatulation time -> faster setting time since it makes more sites for crystal growth to occur
Increasing temperature -> setting time is slower and eventually doesn’t occur if temperature is that high
define model, die, mould, refractory investment
Model = replica of fitting surfaces of oral cavity
Dies = replica of individual teeth
Mould = used for denture construction
Refractory investment = mould material for casting of wax/alloys or for creating indirect restorations
list impression material options
HYDROCOLLOIDS
- Alginate (irreversible)
- Agar (reversible)
ELASTOMERS
- Sillicons (condensation and addition) (addition comes in different bodies)
- Polysulphides
- Polyethers
NON ELASTIC
- ZOE zinc oxide eugonel
- Impression plaster
- Impression compound
compare condensaition and addition sillicons
Condensation silicones
Pros stronger than alginates, more elastic, good tear strength
Cons shrink in air, hydrophobic, expand in solution, limited shelf life
Addition silicones
Pros best dimensional stability, good elastic recovery
Cons gives a porous model, hydrophobic, setting impaired when using natural rubber gloves
strongest elastomer imp material
polysulphides
pros and cons of polyethers as imp materials
Pros -> good air dimensional stability, quick setting, not messy
Cons -> permanent deformation, poor dimensional stability with water so disinfection is difficult
pros and cons of polysulphides as imp materials
Pros strongest of all
Cons slow setting, bad smell, poor elastic recovery, poor dimensional stability so shrinkage
describe 3 types of non elastic imp materials
3 types of NON-ELASTIC imps materials:
Impression plaster – mucostatic, no tray needed, only for edentulous cases
Impression compound – mucocompressive, high viscosity, can be used for primary imps for edentulous cases
Zinc oxide eugenol – mucostatic, accurate in thin section, brittle when set, used in edentulous cases
diff between mucodisplasive/mucocompresive and mucostatic
Mucodisplasive/mucocompressive = viscous, displaces tissues, records impression of mucosa under load – this gives a denture with wider distribution of load so stable!
E.g. Impression compound, high viscosity alginates/elastomers e.g. polyether
Mucostatic = more fluid, does not displace tissues much, so records an imp of un-displaced mucosa – so gives better retention for denture
E.g. Impression plaster, ZOE, low viscosity alginates/elastomers, light body addition silicones
what imp materials do you use on dentate vs edentulous pts
Materials used for
dentate patients:
Alginate
Agar
Non-aqueous elastomers
Materials used for edentulous patients:
Impression compound
Impression plaster
Zinc oxide eugenol
Alginate
Non-aqueous elastomers
whats the diff between imp materials for fixed vs rem pros
Materials list is same for imps for removable pros – main difference is techniques.
ideal imp material
Ideal imp materials should be hydrophilic, flowable, nice taste/smell, snap setting, high tear strength, good recovery from deformation, dimensionally stable, can be disinfected, compatible with gypsum, good reproduction of details when being cast
impression techniques for fixes pros
Impression techniques using different viscosity materials (wash can be light-bodied silicone)
Dual viscosity – general term for using different viscosities
Putty and wash 1 stage
Wash material around tooth, putty into tray, insert tray
Not great since can’t control thickness of wash and putty might move wash so detail is compromised
Putty and wash 2 stage
Take putty imp before preparation of tooth, then once tooth is prepared, put wash on tooth, then insert the putty impression on top
Not great since needs 2 imps and difficult to reposition tray for the second go
What are dental waxes
Dental waxes = thermoplastic moulding materials that are solid at RT
room temp
list the types of waxes and their use
PATTERN WAX
Inlay wax – used for inlays and crowns (made first in wax then converted to metal)
Casting wax – used in patterns for denture framework
Modelling wax – easily mouldable, easy to carve, no residue after removing via boiling
PROCESSING WAX
Boxing wax – used to build up vertical walls around an impression and as beading wax (adapted around impression borders)
Sticky wax – used for the temporary joining of items e.g. to align fractured parts of a denture
IMPRESSION WAX
Impression wax – used for occlusal register (subtype is corrective wax which is for edentulous impressions)
how does casting and investing affect the outcome of an indirect resto
Casting and investing can produce errors in the steps of making indirect restorations.
Casting includes significant shrinkage during cooling, so the investing process must produce an oversized mould to cancel out the shrinkage.
describe the mechanism for expansion in the casting process
= setting expansion of binder, thermal expansion, hygroscopic expansion
what are investment materials
ceramic materials used to form moulds for dental casting
requirments for investment material
fast set, easily break away from casting, not react with alloy, can compensate for shrinkage, cheap since they are single use, can withstand high temps/pressures, porous to allow gases to escape
general composition of investment materials
Refractory – form of silica used to withstand high temps and compensate shrinkage
Binder – to hold refractory and other particles together
Other additions – to modify physical properties
types of investment material
Gypsum bonded – used for gold alloys up to 700 degrees C
Cheap, strong, and porous but need a metal ring for support
Phosphate bonded – up to 1000 degrees C
Stronger so no metal ring needed, finer particle size so smoother
Silica bonded – up to 1200 degrees C
Used for high melting point metal alloys, no setting expansion, expensive
requirments for die materials
accurate, compatible with imps material, dimensionally stable
Die materials are often improved stone, type IV, type V, surface hardened die stones.
Dies = positive reproductions of prepared teeth
