Investment Materials

to produce metal / alloy inlays, onlays, crowns and bridges

casting process which requires casting a molten alloy under pressure by centrifugal force

• 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

• 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

• dental stone / plaster - acrylic dentures • gypsum bonded materials - gold casting alloys • phosphate bonded materials - base metals / cast ceramics • silica bonded materials - base metal alloys

• 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

gold alloy - 1.4% Ni / Cr alloys - 2.0% Co / Cr alloys - 2.3%

binder refractory

3 types of binders: gypsum / phosphate / silica - these determine what type of IM it is & purpose is to form coherent solid mass

usually a type of silica (quartz / cristobalite) that withstands high temps & crucially undergoes expansion

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

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)

as in gypsum products: calcium sulfate hemihydrate becomes calcium sulfate dihydrate

silica - thermal & inversion gypsum - setting expansion ( hygroscopic expansion, contraction below 320oC)

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

• lower powder : water ratio • increased silica content • higher water temp • longer immersion time

causes water loss and is significantly reduced by NaCl and boric acid

• 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

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

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

powder - silica / magnesium oxide / ammonium phosphate liquid - water / colloidal silica

increases strength & gives hygroscopic expansion (2%) which will compensate for alloy shrinkage when its cooled to room temp

forms magnesium ammonium phosphate

1000-1100oC at 330oC - water & ammonia liberated at higher temps - complex reactions with silico-phosphates formed to give increased strength