Refractory Models and investment materials Flashcards
What is a refractory model?
cast made form refractory investment material
supports delicate wax patterns prior to casting into metal alloy
Does the refractory model need to withstand high temps?
yes
retains shape and strength at high temps
resistant to thermal shock
What are some requirements of the refractory model?
What determines the type of investment used?
melting point of alloy
What are the 3 ingredients of the investment materials?
refractory material
binder
other chemicals:
sodium chloride
potassium sulphate
magnesium oxide
boric acid
graphite
How do additives modify the physical properties of the investment material?
reduce the thermal expansion of the investment or to reduce oxides being formed on the metal
What metal alloys demand a higher melting temp?
base metal alloys
precious metal alloys
Are refractory chemical all the same for casting investment materials?
yes - with one of various forms of silicon dioxide (silica)
What are the 3 types of silica in the refractory material?
quartz
crystobalite
tridymite
What is the effect of quartz, crystobalite and tridymite in the refractory material?
influence on the changes in dimension during the casting process
What is the role of the binder in the investment material?
holds the refractory material together
conveys strength
contributes to mould expansion as it expands on setting
What are the 3 types of investment material based on the binder?
gypsum bonded
phosphate bonded
silica bonded
What is the binder for gypsum bonded investment materials
a-calcium sulphate hemihydrate
What is the binder for phosphate bonded investment materials?
magnesium oxide
What is a key feature of the investment material at the end of wax melting?
the dimensions of the cast are the same from start to finish
What are other features of the investment material that are desirable?
- stable over a full rage of casting temperatures
- no radical change o the expansion behaviour of the investment should occur around the casing temp of the alloy
must be porous to permit air and gases to escape through the investment while casting is carried out
Why must the investment material be porous?
to permit air and gases to escape through the investment while casting is carried out
What are the effects of heating the investment?
What binder behaves in a similar way to heating the investment?
calcium phosphate-based investments
How to mix the investment material together?
in a vacuum to eliminate air inclusion
What is the casting ring lined with?
ceramic material
What dies the casting ring lined with ceramic material help with?
helps with expansion of the investment during setting and burnout
contributes to hydroscopic expansion
When pouring investment into the ring, how do you do this?
on a vibrating plate
What is the burnout temp?
500-600
What are factors affecting burnout time?
size of ring
burnout temp
number of casting rings in the furnace
Do you ait for the cast to cool after thermal burnout?
no, you should cast when the investment is hot to prevent cracking
What is the primary cause of investment failure?
damage during the heating process
What does rapid heating cause?
cracking, as the residual water is converted to steam
