L13 Investment

In ves tm en t mate ri als Dr Layla Abu-Naba’a PhD, MFD Irl Assistant Professor of Prosthodontics Dent 301

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Investments Indirect fabrication: wax pattern, sprue -pour mould - cast - finish prosthesis Used for: Casting alloys Acrylic dentures Ceramic crowns

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Investments Requirements: Accuracy and detail production during pouring Detail production during casting at high temp High compressive force - force of molten metal

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Material

Refractory material + binder Silica gypsum Phosphate Silica Silica expansion Quartz α & β thermal, inversion Cristobalite thermal, more inversion Tridymite not commonly used Inversion Low form High form

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Gypsum bonded -Calcium sulphate hemi-hydrate di-hydrate

-Functions - binder - Produces expansion

-+ Powder graphite or powder

copper -+ Sodium chloride + boric acid -

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Gypsum bonded Expansion: -Setting expansion -Hygroscopic expansion - Immersion in water -Add water to the surface of ring -Add a liner that is wetted 12

Silica bonded -Powder silica or powder crystobalite

-Functions - binder - Produces expansion

-+ Magnisium oxide - increase ph -+ Ethyl silicate + industrial spirit increase mixing ability -+Hydrochloric acid - to produce silica gel

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Phosphate bonded -Powder silica -+ Magnesium oxide -+ Ammonium phosphate -= Magnesium ammonium phosphate -Can be mixed with colloidal silica solution or water

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Properties of investment material Thermal stability: retain integrity at high temp and strength Gypsum-bonded used for casting of gold alloys and low melting temp base alloys. Gypsum reaction at 700c produces sulphur dioxide gas- to prevent porosities in prosthesis then leave in the heat for a time (heat soaking) before the cast metal is poured High temp base alloys use phosphate and silica bonded investment material No need for a casting ring in

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Properties of investment material Porosity: To allow the escape of air during casting Gypsum and phosphate bonded are porous Silica bonded is tightly packed - so make vents to prevent ‘back pressure’.

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Properties of investment material Compensating expansion: Shrinkage ? gold 1.4, Ni/Cr 2.0 Cr/Co 2.3% Thermal , hygroscopic 0.3 to 1.3% with gypsum bonded 1.6% silica bonded 2.0% ph. Bonded

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Thank you

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