refractory dies and investments Flashcards
what is the purpose of a refractory die?
to support materials during processing that would deform or disintegrate without its presence e.g. cobalt chrome frameworks
what is one requirement of a refractory die?
ability to withstand the rigours of processing
what are refractory dies composed of?
phosphate bonded investments
what are the 4 types of investment materials?
dental plaster/stone, gypsum bonded, silica bonded. phosphate bonded
what may investment materials be used for?
moulds for acrylic dentures (dental plaster), gold casting alloys (gypsum or phosphate), base metal casting alloys (silica or phosphate), cast ceramics and glasses (phosphate) and for refractory dies in ceramic build ups
what are the ideal properties of an investment material?
accurate, temperature shape stable, high compressive strength, compensate for casting shrinkage (expand), alloy compatible
what are the mechanisms of investment expansion?
thermal. setting and hygroscopic
why are gypsum bonded investments porous?
they liberate sulphur trioxide
why are phosphate bonded investments popular?
high strengths
what are silica bonded investments lacking in and how does this affect the resulting cast?
it lacks porosity so air can’t escape causing back pressure and casting won’t be complete because the mould won’t fill, resulting in porosity of the cast
how is an alloy forced into a mould for casting?
gravity, air pressure, steam pressure, centrifugal force, vacuum
name 4 casting faults
finning and bubbling, incomplete casting, porosity in casting, over/undersized casting
what is finning/bubbling due to?
cracking of the investment mould
why may casting be incomplete?
back pressure, alloy not molten, lack of force, cooling shrinkage
what is under/oversized casting a result of?
total process chain failure
