Casting

• Factors that influence casting:

  • Selection of the correct type of investment
  • The type of casting machine/equipment
  • The design of mould
• Investment composed of:

  • Refractory material - silica
  • Binder - gypsum or phosphate
  • Mix investment with water-based liquid - pour around wax-pattern and allow to set
  • For accurate casting: total shrinkage by alloy = total expansion by mould
  • Compensation for casting shrinkage:
  • Gold alloys ~1.5%
  • Base alloys ~ 2-2.5%
• Expansion for mould in investment comes from:

  • Setting of investment (crystal growth)
  • Hygroscopic expansion caused by adding water to investment during setting
  • Expansion when heating the mould to casting temperature
  • Inversion of silica (quartz or cristobalite)
  • For phosphate-bonded investments: ratio of water to "special liquid"

  Important: different manufacturers require different procedures to get expansion
• Selection of the correct type of investment:

  • Needs to compensate shrinkage: possible error - casting too big or too small
  • Needs thermal stability: possible error - sulphide deposits (gypsum investment breakdown)
  • Must heat investment slowly:
  • Investments are insulators, cracks can form during heating
  • Possible casting errors - fins
  • Investment must not have large pores at the mould surface
  • Need to use a mixture of investment particle size
  • Possible casting error - bubbling on casting surface

  
• Casting metals and alloys:

  • Metal/alloy must be fully molten when poured into mould
  • Must heat to above the liquidus line
  • The higher the temperature the greater the chance of air incorporation
  • This could lead to porosity in the casting
  • NOTE: base metal alloys have higher melting temperatures than gold alloys
• Casting metals and alloys:

  • The metal/alloy must flow into all of the mould
  • BUT liquid metals/alloys have high surface tensions
  • Gravity will not be enough to force the metal/alloy into the whole mould - this would lead to "short" castings
  • The metal/alloy must displace the air in the mould
  • Need air to escape
  • The investment material will have some pores
  • Mould base thickness affects amount of air that can escape
  • If air doesn't fully escape
  • Some gas will bubble back - causing porosity in the casting

  
• Casting machines:

  • Machines needed to force the molten alloy into the mould
  • Centrifugal force commonly used
  • Traditionally casting was performed by hand
  • Suitable for small castings made from gold alloys
  • High density provided high force - thrust
  • Base alloys have low density - insufficient force
  • Modern casting machines
  • Electric motor allows higher speeds

  
• Method of melting the alloys:

  • Gas/air flame
  • Provides enough heat for gold alloys
  • Too low for base metal alloys
  • Oxy/acetylene flame
  • Higher temp possible - suitable for base metal alloys
  • Ratio of oxygen:acetylene must be carefully controlled
  • Too much O₂ leads to oxidation of the casting
  • Too much acetylene leads to carbide embrittlement of the casting
  • Induction heating
  • Better control of the heating rate and temperature possible
  • Most commonly used currently
• Design of the mould - investment base thickness (x)

  • Must be thick enough to provide stable mould
  • Too thick can lead to poor air flow - air cannot escape the mould properly
  • Potential problems: rounded edges and loss of detail
  • Solution: thin base and for phosphate bonded investments add a vent

  
• Design of the mould - sprue required to allow alloy to enter the mould:

  • Allows the flow of the material to be controlled
  • Must be removed after casting - trimmed by technician

  
• Design of the mould - for successful casting:

  • The material at the extremity of the casting should solidify first
  • The material in the sprue should solidify last
  • If material in sprue solidifies too early insufficient alloy will enter the mould
  • Potential problem: casting porosity

  
• Complicated castings:

  • For larger castings (multi-unit devices, bridges, etc.)
  • Moulds are more complex
  • Harder for alloy to penetrate mould extremities whilst molten
  • The chance of solidifying in the sprue increases
  • Multiple sprues
  • Molten alloy enters mould in multiple locations
  • Easier for alloy to penetrate extremities
  • Requires more post-processing (polishing)
  • Sprue reservoir
  • Thinner sections cool faster than thicker sections
  • Reservoir thicker than sprue and casting
  • Solidifies last

  
• Casting errors:

  • Problem = too small
  • Likely cause = too little mould expansion
  • How to avoid fault = use correct temperature and investment materials
• Casting errors:

  • Problem = too large
  • Likely cause = too much mould expansion
  • How to avoid fault = use correct temperature and investment materials
• Casting errors:

  • Problem = distorted
  • Likely cause = stress relief of wax pattern
  • How to avoid fault = warm wax thoroughly before creating pattern
• Casting errors:

  • Problem = rough surface
  • Likely cause = breakdown of investment, air bubbles on wax pattern, weak investment
  • How to avoid fault =
  • Do not overheat mould or alloy. Use correct investment.
  • Use wetting agent and/or vacuum-investing technique.
  • Avoid using too much water when mixing investment.
• Casting errors:

  • Problem = fins
  • Likely cause = cracking of investment
  • How to avoid fault = avoid heating investment too rapidly
• Casting errors:

  • Problem = irregular voids
  • Likely cause = casting shrinkage of alloy, turbulent flow of molten alloy, inclusion of particles of investment
  • How to avoid fault =
  • Place sprues of correct diameter and reservoir at (or near) the bulkiest section of the pattern
  • Heat mould upside down so that any loose particles fall out
• Casting errors:

  • Problem = spherical voids
  • Likely cause = gases dissolve in molten alloy and form bubbles when it cools
  • How to avoid fault = do not overheat alloy for too long
• Casting errors:

  • Problem = oxidation of surface
  • Likely cause = overheating in air
  • How to avoid fault =
  • Do not use an oxidising flame or heat for too long
  • Use a flux to protect molten alloy
• Casting errors:

  • Problem = sulphide deposits
  • Likely cause = breakdown of investment (particularly gypsum-bonded)
  • How to avoid fault =
  • Do not overheat the investment
  • Use correct investment
• Casting errors:

  • Problem = rounded margins
  • Likely cause = back-pressure of air due to low porosity of mould
  • How to avoid fault =
  • Place pattern near end of the casting ring. Use porous investments (or vents)
  • Ensure there is no wax left and cast with sufficient force
• Casting errors:

  • Problem = short castings
  • Likely cause = alloy deficiency; mould too thin, too cold, or had blocked sprues; insufficient casting force
  • How to avoid fault =
  • Use enough alloy; make sure completely molten
  • Use correct diameter sprues and heat to correct temperature
  • Ensure casting machine applies sufficient force