Flashcards in 2 - Investment Materials Deck (19):
4 types of investment materials?
- dental plaster/stone
- gypsum bonded
- silica bonded
- phosphate bonded
applications of different types of investment materials:
- dental plaster?
- gypsum bonded?
- silica bonded?
- phosphate bonded?
- acrylic dentures
- gold casting alloys
- base metal casting alloys (rarely used)
- base metal and gold casting alloys, also used for refractory dies in ceramic buildups
ideal properties of investment moulds? x5
- accurate: shape, size, surface detail
- temperature shape stable: maintain integrity
- high compressive strength
- compensate for casting shrinkage (balance of setting and thermal expansions)
- alloy compatible
investment materials for alloy, porcelain and glass casting - principal components and what does each one do?
- binder: binds refractory particles together
- refractory material (usually silica): withstands high temperatures without degradation
what happens to alloys/ceramics on casting? and how does this affect the investment mould?
- alloys/ceramics shrink on casting
- investment mould must compensate for this. done by expanding
mechanisms of investment expansion?
- thermal: setting expansion of binder
- hygroscopic expansion: place mould into water at initial set so more expansion occurs during setting
why is hygroscopic expansion useful?
increases the setting expansion of gypsum bonded investment materials
investment properties: gypsum bonded
- decompose above?
- liberates what to result in porosity?
- sulphur trioxide
investment properties: phosphate bonded
- higher str means?
- may be used for?
- means more popular
- used for all current dental alloys
investment properties: silica bonded
-what does it lack that gypsum/phosphate doesn't?
- what does this mean?
- what is generated?
- escape of air prevented
- back pressure generated. incomplete casting as mould does not fill
wax burn out: temperature held for how long before casting?
- gold alloy?
- palladium silver alloy?
- base metal alloy?
- 30mins to 1 hour
casting: melting alloy
- gas air torch
- electrical induction
- electrical resistance
- oxyacetylene torch
casting: forcing alloy into mould- done by?
- air pressure
- steam pressure
- centrifrugal force
list the casting faults? x4
- finning and bubbling
- incomplete casting
- porosity in casting
- over/under sized casting
finning and bubbling: describe + what causes it?
- extra appendages on casting
- due to investment mould cracking
incomplete casting - caused by?
- poor space design or failure
- alloy not molten
- lack of force
- back pressure
- cooling shrinkage
porosity: embedding of? occurs due to?
- 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
under/oversized casting: due to?
it is an impact of total process chain failure