Investment Material Flashcards
What is investment material?
Required for casting process of indirect restorations made of alloy.
Of required shape to contain molten alloy and withstand high temperatures (dimensionally stable)
Describe the process of casting an alloy
Wax patterns of required prosthesis is created (positive replica)
Investment material placed around wax pattern and is set (negative replica)
Wax is burned off to create a cavity of the required shape
Molten alloy poured into investment material via sprue
What are ideal properties of investment material?
Porous (allow gases that are released to be captures and stored within, while maintaining shape)
Strong
Smooth
Chemically stable
Inexpensive as destroyed after use
What is the alloy shrinkage of:
Gold
NiCr
CoCr
Gold- 1.4%
NiCr- 2%
CoCr- 2.3%
What are the 2 components of investment material?
Refractory- silica. This withstands high temperatures and undergoes expansion.
Binder- forms coherent mass to provide substance. Can be gypsum, phosphate, silica-bonded.
What is the composition of gypsum investment material?
30% calcium sulphate, 60% silica
Reducing agent to inhibit shrinkage/ control setting time
What is the composition of phosphate investment material?
Powder- silica, magnesium oxide and ammonium phosphate
What are the advantages of gypsum bonded and phosphate bonded investment materials?
Porous- traps gases released during casting
Expands via thermal expansion to counter alloy shrinkage on cooling
Sufficient strength to withstand pressure while casting
Stable at high temperatures.
Above which temperature does gypsum undergo expansion?
320 degrees
What is the purpose of heat soaking?
Investment material is held at high temperature for a period of time to allow gases to gradually escape.
