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Download the presentation for better view as few slides are not being shown completely.Investment Materials

IT IS RECOMMENDED TO FOLLOW McCABE APPLIED DENTAL MATERIALS ALONG WITH THIS LECTURE PERSENTATIONReferences FOR THIS LECTURE: Restorative Dental Materials, CraigNotes on dental materials, CombesDental Materials, PhillipsWhat is an investment MaterialA material suitable for forming a mold into which a metal or alloy is cast

Wax patterns (of appliances or restorations) are invested in investment material...assembly is heated to remove wax...the mold space formed is then occupied by the material of which the restoration or appliance is to be made (Acrylic or alloy)

Requirements for investment materialsEasily manipulatedSufficient strength at room temp.Stability at high temperatureSufficient expansion to compensate for wax pattern shrinkage and casting shrinkage of metalPorous enough to permit air and gasses to escape mold cavitySmooth surfaceEase of divestmentSuitable setting timeinexpensive

Composition of investment materialsA refractory material A material that can withstand high temperatures and will not disintegrate on heating. E.g. quartz, tridymite, crystobaliteA binder A material that will set and bind the refractory material together. E.g. Gypsum (alpha hemihydrate), phosphate, ethyl silicateOther constituents:Sodium chlorideBoric acidPotassium sulphateMagnesium oxidegraphiteTypes (based on binder)Gypsum bonded I.MSilica bonded i.mPhosphate bonded i.m

Types (based on silica/refractory material)Quartz investmentCristobalite investmentGYPSUM Bonded I.Msupplied as powders which are mixed with water COMPOSITION:

BinderCalcium sulphate alpha-hemihydrate25% - 45%Binds the components together and provide rigidity and strengthRefractory materialSilica (quartz and cristobalite)Regulate thermal and inversion expansionOther minor componentsPowdered graphite/carbon or powdered copper and various modifiers Control setting time, act as reducing agent

Types of gypsum bonded i.m(ada specification)

USES: Used as investment material to formcast gold alloy inlays, onlays, crown and bridgeGypsum (binder)Calcium sulphate Alpha hemi hydrate as binder used for investment used in casting gold alloys with melting ranges below 1000C or 1800FThermal expansions and contractionsBetween 200C and 400C all forms shrink because of dehydrationBetween 400C and 700C slight expansionAfter 700C large shrinkage because of decomposition and release of sulfur dioxide gas. (some texts say above 1200C)Silica (refractory material)Provide refractory component during heating of investment and regulate thermal expansionInversion expansion:When heated quartz changes from low alpha form to high beta form at 575C and Expansion occursSimilarly, cristobalite changes from low alpha form to high beta form at 200C 270C and expansion occurs

OVERALL EXPANSIONs IN Gypsum bonded i.mto compensate for casting shrinkage. Molten alloys shrink on solidification/cooling. Mold must be made correspondingly larger than original wax patternNormal setting expansion- occurs as investment hardens in air silica particles interfere with the interlocking of crystals, the outward thrust of the crystals increases the expansion of investment. Maximum expansion in air 0.6%Factors effecting: accelerators and retarders, manipulative conditions like low w/p ratio Hygroscopic expansion- occurs as investment hardens while immersed in watera continuation of normal setting expansion b/c immersion in water encourages continued growth of crystals. Expansion range 1.2-2.2% Factors effecting: composition, w/p ratio, spatulation, time of immersion, life of investment, confinement, amount of added waterThermal and inversion expansion- occurs when investment is heatedThermal expansion: 1% - 1.6% Factors effecting: w/p ratio, chemical modifiersHygroscopic expansioninvestment mould is placed into water at the initial set stage. known as the water immersion technique and can result in an expansion of five times the normal setting expansion. the water added technique in which a measured volume of water is placed on the upper surface of the investment material within the casting ring. This produces a more readily controlled expansion.lining the casting ring with a layer of damp asbestos which is able to feed water to a large surface area of the investment mould.Decomposition reactions on heatingGypsum-bonded investments decompose above 1200C by interaction of silica with calcium sulphate to liberate sulfur trioxide gas.CaSO4 + SiO2 CaSiO3 + SO3Another reaction which may take place on heating gypsum-bonded investments above 700C is that between calcium sulphate and carbon:CaSO4 + 4C CaS + 4COThe carbon may be derived from the residue left after burning out of the wax pattern or may be present as graphite in the investment. Further reaction can occur liberating sulphur dioxide:3CaSO4 + CaS 4CaO + 4SO2My lord, increase me in knowledgeAameen

Phosphate bonded i.mAvailable as powder that is mixed with water or colloidal silicaComposition of powder:

USES:Investment for base metal and gold casting alloys used to make copings and frameworks for metal-ceramic prosthesisfor cast ceramicsRefractory die for ceramic build-up

Refractory materialSilica (quartz or cristobalite or combination)80%High thermal shock resistance and thermal expansionBinderMagnesium oxide and Ammonium phosphate.otherscarbonToo produce clean casting and ease divestingSetting reactionOn mixing the powder with water, following reaction takes place:NH4H2PO4 + MgO + 5H2O + MgNH4PO46H2OThis binds the silica together to form the set investment mould.

At a temperature of about 300C ammonia and water are liberated by the reaction: 2(MgNH4PO46H2O) Mg2P2O7 + 2NH3 + 13H2O

Expansions in phosphate bonded i.mSetting expansion (more when colloidal silica is used instead of water)Inversion expansion and thermal expansionHygroscopic expansionTHERMAL CONTRACTIONS

Types of phosphate bonded i.m

SILICA bonded i.mCompositionBinder: Silica gel that reverts to silica (cristobalite) on heatingRefractory: Quartz or cristobalite powderThe binder solution is generally prepared by mixing ethyl silicate with a mixture of dilute HCL and industrial spirit. A slow hydrolysis of ethyl silicate occurs producing a sol of silicic acid with the liberation of ethyl alcohol as a byproduct.(C2H5O)4Si + 4H2O Si(OH)4 + 4C2H5OHThe silicic acid sol forms silica gel on mixing with quartz or cristobalite powder under alkaline conditions which shrinks on setting. Uses: casting high fusing metal partial denture alloys

Green shrinkageSilicic acid sol is mixed with quartz or cristobalite and a gel of polysilicic acid is formed which shrinks on setting.This gel is dried at temp below 168CDuring this drying, gel loses alcohol and water and forms conc. Hard gel and volumetric contraction occurs which reduces size of mold.This is green shrinkage that occurs in addition to setting shrinkageThermal expansion in silica bonded i.mContinued heating causes considerable expansion due to the close packed nature of the silica particles. A maximum linear expansion of approximately 1.6% is reached at a temperature of about 600C.No hygroscopic or setting expansion takes place.Thanks!Visit our facebook page for more detailshttps://www.facebook.com/e.dental.official

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