Refractory Models and investment materials

Question 1

What is a refractory model?

Answer 1

cast made form refractory investment material

supports delicate wax patterns prior to casting into metal alloy

Question 2

Does the refractory model need to withstand high temps?

Answer 2

yes

retains shape and strength at high temps

resistant to thermal shock

Question 3

What are some requirements of the refractory model?

Answer 3

Question 4

What determines the type of investment used?

Answer 4

melting point of alloy

Question 5

What are the 3 ingredients of the investment materials?

Answer 5

refractory material

binder

other chemicals:

sodium chloride
potassium sulphate
magnesium oxide
boric acid
graphite

Question 6

How do additives modify the physical properties of the investment material?

Answer 6

reduce the thermal expansion of the investment or to reduce oxides being formed on the metal

Question 7

What metal alloys demand a higher melting temp?

Answer 7

base metal alloys

precious metal alloys

Question 7

Are refractory chemical all the same for casting investment materials?

Answer 7

yes - with one of various forms of silicon dioxide (silica)

Question 8

What are the 3 types of silica in the refractory material?

Answer 8

quartz
crystobalite
tridymite

Question 9

What is the effect of quartz, crystobalite and tridymite in the refractory material?

Answer 9

influence on the changes in dimension during the casting process

Question 10

What is the role of the binder in the investment material?

Answer 10

holds the refractory material together

conveys strength

contributes to mould expansion as it expands on setting

Question 11

What are the 3 types of investment material based on the binder?

Answer 11

gypsum bonded

phosphate bonded

silica bonded

Question 12

What is the binder for gypsum bonded investment materials

Answer 12

a-calcium sulphate hemihydrate

Question 13

What is the binder for phosphate bonded investment materials?

Answer 13

magnesium oxide

Question 14

What is a key feature of the investment material at the end of wax melting?

Answer 14

the dimensions of the cast are the same from start to finish

Question 15

What are other features of the investment material that are desirable?

Answer 15

• 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

Question 16

Why must the investment material be porous?

Answer 16

to permit air and gases to escape through the investment while casting is carried out

Question 17

What are the effects of heating the investment?

Answer 17

Question 18

What binder behaves in a similar way to heating the investment?

Answer 18

calcium phosphate-based investments

Question 19

How to mix the investment material together?

Answer 19

in a vacuum to eliminate air inclusion

Question 20

What is the casting ring lined with?

Answer 20

ceramic material

Question 21

What dies the casting ring lined with ceramic material help with?

Answer 21

helps with expansion of the investment during setting and burnout

contributes to hydroscopic expansion

Question 22

When pouring investment into the ring, how do you do this?

Answer 22

on a vibrating plate

Question 23

What is the burnout temp?

Answer 23

500-600

Question 24

What are factors affecting burnout time?

Answer 24

size of ring

burnout temp

number of casting rings in the furnace

Question 25

Do you ait for the cast to cool after thermal burnout?

Answer 25

no, you should cast when the investment is hot to prevent cracking

Question 26

What is the primary cause of investment failure?

Answer 26

damage during the heating process

Question 27

What does rapid heating cause?

Answer 27

cracking, as the residual water is converted to steam