Investment Materials

• Making an investment mould - for dentures:

  • The maximum temperature is below around 150°C
  • So, gypsum can be made to make the mould
• Making an investment mould - for metallic devices (crowns, partial dentures, etc):

  • Temperatures around 800°C - 1,500°C are used
  • Gypsum breaks down at temps above around 200°C 
• Making an investment mould - alloys - gold, Co/Cr, Ni/Cr, etc:

  • Temperature around 800°C - 1,500°C
  • Gypsum alone would be unstable
  • So refractory materials are needed
  • Refractory materials - stable at high temperatures
  • Silica (silicon dioxide) is the most common in dentistry
• Investments for alloy casting - silica is most commonly found as particles:

  • We need some form of binder to stick the particles together to form the mould
  • Form a composite structure by mixing binder with silica
• Investments for alloy casting - composite structure:

  • Continuous phase - binder
  • Allows initial setting and mould shape forming - wax pattern can be placed into it, material flows around it and will eventually set to form a stable mould
  • Used to characterise investment
  • Discrete phase - silica
  • Provides high temperature stability
  • Strengthen the mould - needed to survive stresses during casting
  • Contributes to dimensional accuracy of casting

  
• Investments for alloy casting:

  • Commonly used investment materials are:
  • Gypsum-bonded
  • Phosphate-bonded
  • Labs buy the investment material in powder form
  • Powder contains the binder and refractory
  • Powder is mixed with a liquid - either water or water-based
  • Forms paste that can be poured around pattern
• Investments for alloy casting - requirements of the materials:

  • Stability at the casting temperature
  • Reproduce the detail in the model accurately
  • Compensate for dimensional changes that occur during casting
• Gypsum bonded investments:

  • Gypsum - binding agent (continuous phase)
  • Setting reaction covered previously
  • Water + gypsum creates dental stone/plaster via a crystallisation reaction
  • (CaSO₄)₂H₂O + 3H₂O -> 2CaSO₄2H₂O
  • Silica - refractory agent (discrete phase)
  • Undergoes a structural change at high temps
  • Forms low-form (α form) at low temps
  • As temp increases transforms to high-form (β form)
  • Atoms move further apart to form more traditional hexagons
  • Temp this occurs at = inversion temp
  • An allotropic material - different forms of same material - eg carbon (graphite and diamond)

  
• Dimensional changes that occur during casting:

  • Casting involves pouring molten metal into mould
  • Heating liquids -> expansion
  • So casting (cooling down) -> shrinkage
  • Gold alloys ~ 1.5%
  • Base alloys ~ 2-2.5%
  • Need to compensate for this shrinkage during casting so the device will fit - therefore mould needs to expand as it's heated sufficiently so when alloy is poured into it, mould = big enough so when it cools it shrinks to the right size
  • Compensation needs to be correct for alloy type
  • Over- (expand too much) or under-compensation (doesn't expand enough) = device doesn't fit
• A variety of dimensional changes can occur that can compensate for the shrinkage:

  • Thermal expansion
  • All materials expand when heated
  • Therefore, the mould space will be bigger at the heating temp
  • Setting expansion
  • As for gypsum plaster & stone - crystal growth
  • Inversion
  • Change in silica structure due to change in form
  • Can be controlled by amount of quartz & cristobalite
  • Amount of expansion can be altered by different amounts of quartz & cristobalite
  • Hygroscopic expansion
  • Mould placed in water
  • Water enters spaces between crystals causing expansion
  • When high expansion required

  
• Thermal stability of gypsum bonded investments:

  • Gypsum bonded investments become unstable as temperature increases
  • Above 700°C the gypsum can react with carbon
  • CaSO₄ + 4C -> CaS + 4CO
  • 3CaSO₄ + CaS -> 4CaO + 4SO₂
  • These reactions lead to a weakening of the mould and the production of gases
  • Most common source of carbon is from the wax used to make the pattern
  • Can be prevented by heat soaking
  • Hold at the casting temperature to allow all carbon to burn off before pouring in the molten alloy
• Thermal stability of gypsum bonded investments:

  • Above 1200°C gypsum degrades
  • CaSO₄ + SiO₂ -> CaSiO₃ + SO₃
  • This results in the mould becoming significantly weaker and for porosity to form
  • Gypsum bonded investments
  • Used for low-melting alloys
  • Unsuitable for high-melting alloys (e.g. Ni/Cr)
• Phosphate-bonded investments:

  • Need to replace gypsum as binding agent - magnesium ammonium phosphate used
  • Silica still used as refractory agent (discrete phase)
  • Powder can be mixed w/ water/aqueous colloidal silica (special liquid) - ratio controls expansion
  • Setting reaction = NH₄.H₂.PO₄ + MgO + 5H₂O -> Mg.NH₄.PO₄.6H₂O
  • Setting expansion occurs
  • On heating - 2Mg.NH₄.PO₄.6H₂O -> Mg₂P₂O₇ + 2NH₃ + 13H₂O
  • Excess NH₃ & H₂O driven off
  • Thermal expansion + inversion occur
  • Complex silicophosphates form at high temps
  • Strengthen mould (enhanced w/ colloidal silica)
• Applications of investment materials:

  • Gypsum (stone and plaster)
  • For acrylic curing
  • Low temperature
  • Gypsum-bonded investment
  • For casting gold alloys
  • Phosphate-bonded investment
  • For casting base alloys