Investment materials

• To produce metal/alloy inlays, onlays, crowns and bridges • Technique involves casting molten alloy under pressure by centrifugal force • Need a mould cavity of required shaped • Alloy surrounded by investment material

• Wax pattern made e.g. crown/ inlay etc • Investment material poured around wax pattern and allowed to set (mould) • Wax is eliminated (by boiling water or burning in oven) • Molten alloy forced into mould cavity by wax via channels (sprues) prepared in the investment material

• As alloy is cast gases are produced • They must escape and be captured by investment material • Otherwise the alloy will have voids and it will be porous

• Upon cooling the alloy contracts and shrinks • Not same shape upon devestment

• Dental stone or plaster • Gypsum bonded materials • Phosphate bonded materials • Silica bonded materials • Acrylic dentures • Gold casting alloys • Base metals/cast ceramics • Base metal alloys

• Must expand to compensate for cooling shrinkage of alloy • Must be porous to allow escape of trapped gases on casting • Must be strong at room temp for ease of handling and referred to as Green strength • Must be strong on casting temp to withstand casting forces • Must have smooth surface for easy finishing • Chemically stable • Easy removal form cast to reduce technician time • Handling not complicated • Relatively inexpensive as it is destroyed

Gold alloys - 1.4% Ni/Cr alloys - 2% Co/Cr alloys - 2.3%

• Binder • Refractory

• Silica (quartz or critsobalite) • Withstands high temps • Undergoes expansion

• Gypsum or Phosphate or Silica • Form a coherent mass to provide substance • Each has different setting reaction which yields different material characteristics and pin points what type of investment material it is

• Below temp of 570C it exists as alpha-quartz and has squashed crystalline lattice structure • Above temp of 570C it exists as beta-quart where it explodes to max vol of crystalline lattice structure

• It is a powder mixed with water • Silica 60-65% • Calcium sulphate hemihydrate 30-35% • Reducing agent for oxides • Chemicals to inhibit heating shrinkage and control setting time (boric acid, NaCL)

Calcium hemihydrate and water produces calcium sulphate di-hydrate

• Silica undergoes thermal expansion and inversion expansion • Gypsum undergoes setting expansion that has two aspects • Hygroscopic expansion and contraction above 320oC

• The mechanism is not fully understood • Water molecules attracted between calcium sulphate hemi-hydrate crystals by capillary forces • Forces crystals apart • Can be up to 5X setting expansion

• Lower powder/ water ratio • Increased silica content • Higher water temp • Longer immersion time

• Above 320oC • Due to water loss and • Presence of sodium chloride and boric acid

Expansion - total expansion sufficient for casting gold alloys - 1.4% by volume Smooth surface - Fine particles give good surface Manipulation - Easy and setting time is controlled Porous which is good as it takes in gases released when casting alloys Strength is adequate if correct powder/liquid ratio and has correct manipulation

• If any wax residue or graphite in the investment material then reaction between CaCO4 and carbon occurs • Release carbon monoxide and CaS • CaS react with CaCO4 to produce SO2 (sulphur dioxide gas) • Crucial that CO2 and SO2 escape • Heat soaking used so investment material is held at high temp for some time so gases can gradually escape

• Below 1200oC satisfies requirements • Above 1200oC causes problems • CaSO4 reacts with silica to generate sulphur trioxide • This produces voids in cast alloy and not suitable • If alloy melts above 1200oC then can’t use Gypsum bonded investment

Powder - Silica - Magnesium oxide - Ammonium phosphate Liquid - Water or colloidal silica

• Increases strength • Gives hygroscopic expansion of 2%

Ammonium phosphate reacts with magnesium oxide and water - Produces magnesium ammonium phosphate

• Heated to 1000-1100oC • Water and ammonia liberated at 330oC • At higher temps other reactions occur with silico-phosphates which generate increased strength