3. Advantages
Preserve the original cast
Eliminating the danger of fracture or
abrading the surface of the original master
cast
Forming the wax or plastic pattern
The metal framework is cast against the
surface of refractory cast
25. “Ideal” Investment:
Expaned to compensate metal shrinkage
not decompose at high temperature
Strengthed at various temperature
Porous to allow the escape of gases
Consistent casting temperatures
26. “Ideal” Investment
Easy to manipulate.
Smooth castings.
Easy to break out.
Inexpensive.
27. TYPES OF INVESTMENTS
Based on the type of binder:
Gypsum-bonded or Calcium-sulfate bonded
Phosphate-bonded
28. Gypsum-bonded or Calcium-sulfate bonded
Composition
Binder: a-hemihydrate form of gypsum (25-45%)
Ca SO4 ½+3/2 h2o= Ca So4.2H2o
Not to be heated above 700oC decomposition
29. Phosphate-bonded investment
Composition:
Filler (refractory) = quartz or cristobalite or both (80%)
Binder = magnesium oxide and acid phosphate
Liquid = colloidal silica suspension
30. Factor
Setting hygroscopic
expansion
Thermal expansion
Water/powder ratio
increased
Decreased Decreased
Time of spatulation
increased
Increased No effect
Rate spatulation
increased
Increased No effect
Age of investment
increased
Decreased No effect
Delay before
immersion increased
Decreased
Water bath temperature
patterns
Increased
41. Casting wax : Composition
The ingredients are similar to inlay waxes
42. Casting wax : Properties
Flow
Breaking point : no fracture
43. Working properties
Pilable and readily adaptable
Adapt accurately to surface against which it pressed
not be brittle on cooling
Vapporize, leaving no film rather than carbon