Dental materials: Investment materials, Die and Casting Flashcards
What are investment materials used for?
Support materials like Porcelain/ CoCr (Made from phosphate bonded investments that are fixed + glazed before use) during processing that would disintegrate otherwise
What types of investment materials are there and what are they used in?
Dental plaster-> acrylic dentures
Gypsum bonded-> gold casting alloys
Silica-> base metals (rarely used)
Phosphate-> base metal + gold alloys, ceramics + glass, also used as refractory dies in ceramic build ups
What are the 2 components in investment needed for porcelain, alloy and glass casting and why are they important?
-Binder
-Refractory material like silica
porcelain, alloy and glass shrink on casting so need an accurate fit + must compensate for investment expanding at the same time
What are the properties of investment materials?
High compressive strength
Alloy compatible
Accurate
Must keep shape stable at high temps.
Compensates for casting shrinkage
What are the investment expansion mechanisms?
Thermal
Setting expansion of binder
Hygroscopic
Changing silica
What is the hygroscopic mechanism for investment expansion?
- place mould into water at initial set
- then line the casting ring with damp asbestos equivalent
What is the silica conversion mechanism for investment expansion?
It has allotropic forms which are crystalline inverted
1) Quartz alpha is converted into its beta form @ 575 degrees
2) Cristobalite goes from Low to High @ 210 degrees
What are the properties of gypsum as an investment material?
decomposes at high temps of 1200 degrees
which liberates sulphur trioxide which reduces porosity
What are the properties of phosphate as an investment material?
It has higher strength therefore more popular
used in all current alloys
What are the properties of silica as an investment material?
lacks porosity!
inhibits the escape of air-> creates back pressure which leads to incomplete casting as the mould does not fill
What 2 things must be done to an alloy prior to casting?
1) Melting alloy
2) Forcing alloy into mould
What are the casting temps. for gold, palladium silver and base metals?
Gold= 700-750
Palladium silver= 730-815
Base metal= 815- 900
this temp is held 30min-an hour prior to casting
What are the methods for melting alloys?
-Oxyacetate torch
-Gas air torch
-Electrical induction
-Electrical resistance
-Furnace (most used)
What are the methods for forcing an alloy into a mould?
-Gravity
-Air pressure
-Steam pressure
-Centrifugal force
-Vacumn (most used)
What types of casting faults can you have?
Firming + bubbling
Incomplete casting
Porosity in casting
Over/under sized casting
