Investment Materials Flashcards
main use of IMs
to produce metal / alloy inlays, onlays, crowns and bridges
what technique is used in IMs
casting process which requires casting a molten alloy under pressure by centrifugal force
describe the casting process of IMs i.e. lost wax technique
- wax pattern made of required prosthesis e.g. crown / inlay - this is the positive replica
- IM poured around wax pattern and allowed to set (mould) - this is the negative replica
- wax is eliminated by either boiling water or burning in oven to leave a cavity of required shape
- molten alloy is forced into mould cavity vacated by wax via channels (sprues) prepared in the IM
things to consider during casting process
- pressure must be applied to ensure no gases or voids form within it so IM must be strong enough to withstand this
- however, when cast gases are produced and these must be allowed to escape & be captured by IM or else alloy will be porous and have voids
- on cooling, alloy contracts so it won’t be shape shape as determined by mould cavity
4 types of IMs
- dental stone / plaster - acrylic dentures
- gypsum bonded materials - gold casting alloys
- phosphate bonded materials - base metals / cast ceramics
- silica bonded materials - base metal alloys
requirements of IM (8)
- expand - compensate for cooling shrinkage if alloy
- porous - allow escape of trapped gases on casting (back pressure effect)
- storage - at room temp: ease of handling (known as green strength) & casting temp: withstand casting forces
- smooth surface - for easy finishing
- chemically stable - porosity, surface detail
- easy removal from cast - technician time
- handling - not complicated
- relatively inexpensive - as it is destroyed
typcical contraction from alloy mpt to room temp
gold alloy - 1.4%
Ni / Cr alloys - 2.0%
Co / Cr alloys - 2.3%
2 main components of IMs
binder
refractory
what is the binder part of IMs
3 types of binders: gypsum / phosphate / silica - these determine what type of IM it is & purpose is to form coherent solid mass
what is the refractory part of IMs
usually a type of silica (quartz / cristobalite) that withstands high temps & crucially undergoes expansion
describe the inversion expansion of silica
below 570oC silica exists as alpha quartz with a squished crystalline lattice structure and above 570oC it is beta quartz with a maximum volume lattice structure
gypsum IM composition
powder mixed with water
- silica (60-65%) refractory component
- calcium sulfate hemihydrate
- reducing agent for oxides
- boric acid & NaCl (to inhibit heating shrinkage & control setting time)
gypsum IM setting reaction
as in gypsum products: calcium sulfate hemihydrate becomes calcium sulfate dihydrate
gypsum IM dimensional changes
silica - thermal & inversion
gypsum - setting expansion ( hygroscopic expansion, contraction below 320oC)
explain hygroscopic expansion that gypsum IM undergoes
water molecules are attracted between crystals (in this case, crystals of calcium sulfate hemihydrate) by capillary forces, forcing crystals apart & this can be up to 5x during expansion
factors increasing hygroscopic expansion
- lower powder : water ratio
- increased silica content
- higher water temp
- longer immersion time
gypsum contraction above 320oC - how is this avoided
causes water loss and is significantly reduced by NaCl and boric acid
gypsum IM properties
- expansion - total expansion sufficient for gold alloys; 1.4% by volume
- smooth surface - fine particles
- manipulation - easy & setting time controlled
- porosity - good
- strength - adequate if correct powder / liquid ratio and correct manipulation
describe heat soaking in gypsum IM
an unwanted reaction above 700oC produces CO & SO2 which must escape - so IM undergoes heat soaking where it is held at a high temp for some time to enable gases to gradually escape
gypsum IM chemical stability
below 1200oC - satisfies requirements
above 1200oC - CaSO4 reacts with SO2 to produce SO3 that causes porosity in casting and contributes to corrosion
limited to alloys with a mpt < 1200oC
phosphate bonded IM composition
powder - silica / magnesium oxide / ammonium phosphate
liquid - water / colloidal silica
purpose of colloidal silica solution
increases strength & gives hygroscopic expansion (2%) which will compensate for alloy shrinkage when its cooled to room temp
phosphate IM setting
forms magnesium ammonium phosphate
phosphate IM heating
1000-1100oC
at 330oC - water & ammonia liberated
at higher temps - complex reactions with silico-phosphates formed to give increased strength
phosphate IM properties
- high green strength - don’t need metal casting ring for support
- easy to use
- high strength
- porous
- chemically stable
silica IM stages of production
stage 1 - prepare stock solution
stage 2 - add powder (quartz or cristobalite), gelation
stage 3 - drying, tightly packed silica particles
properties of silica IM
strength - sufficient
not porous - therefore needs vents
complicated manipulation
