Investment Material

Question 1

what are investment materials used in the production of

Answer 1

inlays, onlays, crowns and bridges that are made of an alloy

Question 2

a casting method is required for inlays, onlays and bridges - what forces are required for this process

Answer 2

centrifugal forces

Question 3

what is an investment material

Answer 3

it is the required shape to contain the molten alloy and withstand the high temperatures involved and make sure the alloy’s dimensions are maintained

Question 4

what is the first stage in investing material

Answer 4

a wax pattern of the required prosthesis (crown/ inlay) is created - positive replica

Question 5

what is the second stage in investment material

Answer 5

investment material is placed around the wax pattern and allowed to set to form a mould - the negative replica

Question 6

what is the third stage in investment material

Answer 6

wax is removed - by burning or boiling water so there is a cavity of the required shape around the investment material

Question 7

what is the fourth stage in investment material

Answer 7

molten alloy is poured into the mould cavity via the sprue

Question 8

what must be applied when the alloy is cast and why

Answer 8

pressure so that no gaps or void form in it

Question 9

what will inevitably be produced when the alloy is cast

Answer 9

gases

Question 10

why is it important what gases are allowed to escape and captured by the investment material

Answer 10

so the alloy will not have voids and be porous

Question 11

what happens upon cooling of an alloy

Answer 11

it contracts, and will not be the same shape as determined by the mould cavity

Question 12

what does investment material need to be with regards to porosity and why

Answer 12

needs to be porous and allow the gases released to be captured/ stored within it which maintaining its own shape

Question 13

why do investment materials need to have a smooth surface

Answer 13

so the alloys surface wont need much finishing work to be done by the technician

Question 14

what alloy has the highest shrinkage upon setting and which has the lowest

Answer 14

gold has the lowest
cobalt chrome has the highest

Question 15

what are the two components in an investment material

Answer 15

refractory component and a binder

Question 16

what is the refractory component in an investment material

Answer 16

a silica - quartz or cristobalite which withstands high temperatures and undergoes expansion

Question 17

what is the binder in an investment material

Answer 17

it forms a coherent mass to provide substance

Question 18

what are the three types of binder available

Answer 18

gypsum, phosphate and silica bonded

Question 19

what occurs to quartz and cristobalite as the temperature is increased

Answer 19

they expand

Question 20

below which temperature does alpha quartz exist

Answer 20

below 570

Question 21

beyond which temperature does beta quartz exist

Answer 21

570

Question 22

what is alpha quartz

Answer 22

squashed crystalline lattice structure

Question 23

what is a component in gypsum bonded investment material

Answer 23

calcium sulphate hemi-hydrate in powder form

Question 24

what percentage of silica is in gypsum bonded investment material

Answer 24

60%

Question 25

gypsum bonded investment materials also have reducing agents, what do these agents do

Answer 25

inhibit heat shrinkage and controls the setting time

Question 26

what is the setting reaction for gypsum

Answer 26

calcium sulphate hemi-hydrate added to water which produces calcium sulphate di-hydrate

Question 27

what expansion does gypsum undergo

Answer 27

hygroscopic expansion

Question 28

what factors affect hygroscopic expansion

Answer 28

lower powder/ water ratio increased silica content using water at a higher temperature immersing the powder in water for longer

Question 29

why will gypsum contract above 320 degrees

Answer 29

water loss presence of sodium chloride and boric acid

Question 30

gypsum bonded investment material expands by 1.4%, what alloy shrinks by the same amount

Answer 30

gold alloys

Question 31

does gypsum bonded investment material have sufficient strength and is sufficiently porous

Answer 31

yes

Question 32

what is the unwanted reaction at about 700 degrees in casting gypsum bonded investment material

Answer 32

calcium sulphate and and carbon react to release carbon monoxide and calcium sulphide which can then produce sulphur dioxide

Question 33

it is crucial that carbon monoxide and sulphur dioxide gases escape, how is this ensured

Answer 33

investment material goes through heat soaking - which means its held at a high temperature for some time to allow gases to escape

Question 34

from what temperatures should gypsum bonded investment material not be used

Answer 34

over 1200 due to the calcium sulphate reacting with silica which produces sulphur trioxide which makes voids in the cast alloy

Question 35

what does phosphate bonded investment material powder contain

Answer 35

silica magnesium oxide ammonium phosphate

Question 36

what is phosphate bonded investment material powder mixed with

Answer 36

water or colloidal silica

Question 37

why is phosphate bonded investment material mixed with colloidal silica

Answer 37

to increase the strength and to achieve 2% expansion 5

Question 38

what is the setting reaction for phosphate bonded investment material

Answer 38

ammonium phosphate reacts with magnesium oxide and water to produce magnesium ammonium phosphate

Question 39

what are the properties of phosphate bonded investment material

Answer 39

high strength sufficiently porous chemically stable

Question 40

what investment material is not a porous material

Answer 40

silica bonded investment material