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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?

- 1200degrees
- 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?

- porosity
- escape of air prevented
- back pressure generated. incomplete casting as mould does not fill


wax burn out: temperature held for how long before casting?
temperature for:
- gold alloy?
- palladium silver alloy?
- base metal alloy?

- 30mins to 1 hour
- 700-750deg
- 730-815deg
- 815-900deg


casting: melting alloy

- gas air torch
- electrical induction
- furnace
- electrical resistance
- oxyacetylene torch


casting: forcing alloy into mould- done by?

- gravity
- air pressure
- steam pressure
- centrifrugal force
- vacuumn


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


how to reduce chance of casting faults? x4

- understand process
- understand investments
- adherence to instructions
- attention to detail