Investment Materials Flashcards
What are the functions of investment materials in dentistry?
to produce metal/alloy inlays, inlays, crowns and bridges
Outline the process by which prosthesis are made with investment materials
- wax pattern made
- positive replica of prosthesis
- investment material set around wax
- negative replica of prosthesis
- mould
- wax eliminated
- boiling water or burning oven
- molten alloy forced into mould
- alloy forced in through sprues
- pressure used to prevent void formation
What happens to investment material on heating?
- experiences large forces
- expands
- must capture gases released
What are dental stone and plaster used to cast?
acrylic dentures
What are gypsum bonded materials used to cast?
gold casting alloys
What are phosphate bonded materials used to cast?
- base metals
- cast ceramics
What are silica bonded materials used to cast?
base metal alloys
What are the requirements of an investment material?
- expansion
- to compensate for cooling shrinkage
- porosity
- allow escape of cases
- back pressure effect
- strength
- green strength (at room temperature)
- casting forces
- smooth surface
- ease of finishing
- chemically stable
- porosity
- surface detail
- easy removal from cast
- technician time - handling
- ease of use - expense
- should be relatively inexpensive
- destroyed after use
What is green strength?
the strength of a material at room temperature
What happens is an investment material doesn’t allow gas to escape?
voids form in the material, weakening the prosthesis
What are the contraction percentages of gold, Ni/Cr and Co/Cr alloys?
Gold - 1.4%
Ni/Cr - 2%
Co/Cr - 2.3%
What are the two components in all investment materials and what are their functions?
- binder
- gypsum, phosphate, silica
- forms a coherent solid mass
- refractory
- silica (quartz or cristobalite)
- withstand high temperatures
- provides expansion
Does quartz or cristobalite have greater thermal expansion?
Cristobalite
At what temperature does inversion expansion occur for silica?
573 degrees
- quartz changes from alpha to beta
- beta quartz is at maximum volume
What is the composition of gypsum bonded investment material?
silica - 60-56%
calcium sulphate hemihydrate - 30-35%
reducing agent for oxide
boric acid, NaCl (inhibit heating shrinkage)
What is the setting reaction for gypsum bonded investment material?
calcium sulphate + water = calcium sulphate dihydrate
What dimensional changes do silica and gypsum experience?
- silica
- thermal expansion
- inversion expansion
- gypsum
- setting expansion
- hygroscopic expansion
- contraction above 320 degrees
- setting expansion
what is hygroscopic expansion?
- capillary forces pulling water molecules into the gaps between crystals of calcium sulphate hemihydrate, forcing them apart
- mechanism not fully understood
- up to 5x setting expansion
- variable increase in volume
What factors increase hygroscopic expansion?
- lower powder/water ratio
- increased silica content
- higher water temperature
- longer immersion time
Why does gypsum contract above 320 degrees?
- water loss
- significant reduction by sodium chloride and boric acid
What are the properties of gypsum bonded investment material?
- expansion
- sufficient for gold alloys
- smooth surface
- fine particles
- manipulation
- easy
- controlled setting time
- porosity
- good
- strength
- adequate
- correct powder/liquid ratio
- correct manipulation
What is heat soaking?
- an unwanted reaction in gypsum bonded investment above 700 degrees between calcium sulphate and carbon in the presence of wax or graphite residue in the investment material
- carbon monoxide/sulphur dioxide produced
- gases must be able to escape
- must be held at high temperature
Discuss the chemical stability of gypsum bonded investment
- chemically stable below 1200 degrees
- above 1200 degrees not chemically stable
- calcium sulphate reacts with silica
- sulphur trioxide produced
- porosity in prosthesis
What is the composition of phosphate bonded investment?
- powder
- silica
- magnesium oxide
- ammonium phosphate
- liquid
- water
OR - colloidal silica
- water
Why would colloidal silica solution be used in place of water for phosphate bonded investment?
- increases strength
- gives hydroscopic expansion
- 2%
- compensates for alloy shrinkage
What is the phosphate bonded investment setting reaction?
ammonium phosphate reacts with magnesium oxide and water to produce magnesium ammonium phosphate
What happens to phosphate bonded investment on heating?
330 degrees - water and ammonia liberated
> 330 degrees - complex reactions
- with silica-phosphates
- increased strength
What are the properties of phosphate bonded investment?
- high green strength
- do not need metal casting ring for support
- easy to use
- high strength
- porous
- chemically stable
What are the three stages of silica investment?
- prepare stock solution
- add powder (gelation)
- drying (tightly packed silica particles)
What are the dimensional changes experienced by silica?
- contraction during early heating
- water and alcohol loss from cell
- substantial thermal and inversion expansion
- lots of silica present
What are the properties of silica>
- strength
- strong enough
- porosity
- not porous
- requires specialist equipment to form vents
- risk of porous alloy
- usability
- complicated manipulation
