Investment materials Flashcards
What is a refractory die?
a dental cast which is heat resistant, upon which we can form in wax complex structures so when we lose the wax in the lost-wax technique we have a shape of the object that was formed in wax, and also a heat resistant mould upon which the molten metal can be poured to form the structure which wears originally modelled in wax
What is a refractory die used to support?
used to support materials during processing that would deform/disintegrate if not present e.g porcelain/ceramic veneers, cobalt chrome frameworks etc
What must a refractory die withstand?
the rigours of processing (heat and pressure)
What do refractory dies tend to be formed from?
phosphate bonded investments that are both fixed and glazed before use
What are the 4 types of investment materials?
- dental plaster or stone
- gypsum bonded
- silica bonded
- phosphate bonded
What type of investment material is used when making full or partial dentures?
dental plaster or stone
What type of investment material is not heat resistant?
dental plaster or stone
- therefore not used in alloy casting
What type of investment material is used when using gold casting alloys?
gypsum bonded
What type of investment material is used when using base metal casting alloys?
silica bonded
- expensive, rarely used
What type of investment material can be used when using either base metal or gold casting alloys?
phosphate bonded
What type of investment material is used when using cast ceramics and glasses?
phosphate bonded
What investment material is used for refractory dies in ceramic build ups?
phosphate bonded
What are the applications of dental plaster or stone?
moulds for acrylic dentures
What are the applications of gypsum bonded?
moulds for gold casting alloys
What are the applications of silica bonded?
moulds for base metal casting alloys
(rarely used)
What are the applications of phosphate bonded?
- base metal and gold casting alloys
- cast ceramics and glasses
- also used for refractory dies in ceramic build ups
What are the ideal properties of an investment material?
- accurate - shape, size, surface detail
- temperature shape stable (>1000 degrees C)
- high compressive strength
- compensate for casting shrinkage - balance of setting and thermal expansions
- alloy compatible
For alloy, porcelain and glass casting, what are the principal components of the investment materials?
- binder
- refractory material usually silica (heat resistant material)
What happens to alloys/ceramics as they cool?
shrink on casting, as they cool
Why must shrinkage be compensated for?
for accurate fit of restoration
How is shrinkage compensated for?
usually investment expands, meaning the investment mould is actually larger than required to compensate for alloy shrinkage
What are the different mechanisms of investment expansion?
- thermal
- setting expansion of the binder
- hygroscopic
What is hygroscopic expansion?
in the presence of water the mould will expand
- place mould into water at initial set
- lining casting ring with damp asbestos equivalent
What does silica exists in different forms of?
different allotropic forms (alpha and beta)
What is the mechanism of expansion of silica?
crystalline inversion of allotropic forms of silica
- quartz alpha —> beta at 575 degrees C
- cristobalite Low —> High at 210 degrees C
At what temperature do gypsum bonded investments decompose?
above 1200 degrees C
What happens when gypsum bonded investments decompose?
liberates sulphur trioxide leading to porosity
What are the benefits of phosphate bonded investments?
- higher strengths means more popular
- may be used for all current dental alloys
What makes silica bonded unlike gypsum/phosphate bonded?
it lacks porosity
- inhibits escape of air
- back pressure generated
—> incomplete casting as mould does not fill
—> porosity
How is wax burnt out of a metal casting ring?
put in furnace (temperature held) for 30 mins to 1 hour prior to casting
What temperature are gold alloys held at for casting?
700-750 degrees C
What temperature are palladium silver alloys held at for casting?
730-815 degrees C
What temperature are base metal alloys held at for casting?
815-900 degrees C
(INCLUDES COBALT CHROMIUM)
How are alloys melted?
- gas torch
- oxyacetylene torch
- electrical induction
- electrical resistance
- furnace
How are alloys forced into moulds?
- gravity
- air pressure
- steam pressure
- centrifugal force
-
vacuum
(usually a combination of these 2)
What are the stages of an alloy being cast?
- correct amount of alloy weighed out
- casting ring put in furnace and held into place
- casting ring removed from furnace and placed in a casting machine
- molted alloy forced into investment mould inside the casting ring (within the casting machine)
- alloy cooled and investment material removed
- metal sprue removed and cast alloy finished
What are possible casting faults?
- finning and bubbling
- incomplete casting
- porosity in casting
- over/under sized casting
What is finning/bubbling?
- extra appendages on casting
- due to investment mould cracking
What is incomplete casting?
- poor space design or failure of the space (relates to sprue)
- alloy not molten
- lack of force
- back pressure
- cooling shrinkage (too rapid cooling)
What is porosity?
- embedding of fractured investment in casting
- gaseous due to alloy entrapment of oxygen taken up by Cu, Au, Ag, Pt and Pd in alloys when molten
What is undersized/oversized casting?
- impact of total process chain failure
