Investment Materials

Question 1

main use of IMs

Answer 1

to produce metal / alloy inlays, onlays, crowns and bridges

Question 2

what technique is used in IMs

Answer 2

casting process which requires casting a molten alloy under pressure by centrifugal force

Question 3

describe the casting process of IMs i.e. lost wax technique

Answer 3

1. wax pattern made of required prosthesis e.g. crown / inlay - this is the positive replica
2. IM poured around wax pattern and allowed to set (mould) - this is the negative replica
3. wax is eliminated by either boiling water or burning in oven to leave a cavity of required shape
4. molten alloy is forced into mould cavity vacated by wax via channels (sprues) prepared in the IM

Question 4

things to consider during casting process

Answer 4

• pressure must be applied to ensure no gases or voids form within it so IM must be strong enough to withstand this
• however, when cast gases are produced and these must be allowed to escape & be captured by IM or else alloy will be porous and have voids
• on cooling, alloy contracts so it won’t be shape shape as determined by mould cavity

Question 5

4 types of IMs

Answer 5

1. dental stone / plaster - acrylic dentures
2. gypsum bonded materials - gold casting alloys
3. phosphate bonded materials - base metals / cast ceramics
4. silica bonded materials - base metal alloys

Question 6

requirements of IM (8)

Answer 6

1. expand - compensate for cooling shrinkage if alloy
2. porous - allow escape of trapped gases on casting (back pressure effect)
3. storage - at room temp: ease of handling (known as green strength) & casting temp: withstand casting forces
4. smooth surface - for easy finishing
5. chemically stable - porosity, surface detail
6. easy removal from cast - technician time
7. handling - not complicated
8. relatively inexpensive - as it is destroyed

Question 7

typcical contraction from alloy mpt to room temp

Answer 7

gold alloy - 1.4%
Ni / Cr alloys - 2.0%
Co / Cr alloys - 2.3%

Question 8

2 main components of IMs

Answer 8

binder
refractory

Question 9

what is the binder part of IMs

Answer 9

3 types of binders: gypsum / phosphate / silica - these determine what type of IM it is & purpose is to form coherent solid mass

Question 10

what is the refractory part of IMs

Answer 10

usually a type of silica (quartz / cristobalite) that withstands high temps & crucially undergoes expansion

Question 11

describe the inversion expansion of silica

Answer 11

below 570oC silica exists as alpha quartz with a squished crystalline lattice structure and above 570oC it is beta quartz with a maximum volume lattice structure

Question 12

gypsum IM composition

Answer 12

powder mixed with water
- silica (60-65%) refractory component
- calcium sulfate hemihydrate
- reducing agent for oxides
- boric acid & NaCl (to inhibit heating shrinkage & control setting time)

Question 13

gypsum IM setting reaction

Answer 13

as in gypsum products: calcium sulfate hemihydrate becomes calcium sulfate dihydrate

Question 14

gypsum IM dimensional changes

Answer 14

silica - thermal & inversion
gypsum - setting expansion ( hygroscopic expansion, contraction below 320oC)

Question 15

explain hygroscopic expansion that gypsum IM undergoes

Answer 15

water molecules are attracted between crystals (in this case, crystals of calcium sulfate hemihydrate) by capillary forces, forcing crystals apart & this can be up to 5x during expansion

Question 16

factors increasing hygroscopic expansion

Answer 16

• lower powder : water ratio
• increased silica content
• higher water temp
• longer immersion time

Question 17

gypsum contraction above 320oC - how is this avoided

Answer 17

causes water loss and is significantly reduced by NaCl and boric acid

Question 18

gypsum IM properties

Answer 18

1. expansion - total expansion sufficient for gold alloys; 1.4% by volume
2. smooth surface - fine particles
3. manipulation - easy & setting time controlled
4. porosity - good
5. strength - adequate if correct powder / liquid ratio and correct manipulation

Question 19

describe heat soaking in gypsum IM

Answer 19

an unwanted reaction above 700oC produces CO & SO2 which must escape - so IM undergoes heat soaking where it is held at a high temp for some time to enable gases to gradually escape

Question 20

gypsum IM chemical stability

Answer 20

below 1200oC - satisfies requirements
above 1200oC - CaSO4 reacts with SO2 to produce SO3 that causes porosity in casting and contributes to corrosion
limited to alloys with a mpt < 1200oC

Question 21

phosphate bonded IM composition

Answer 21

powder - silica / magnesium oxide / ammonium phosphate
liquid - water / colloidal silica

Question 22

purpose of colloidal silica solution

Answer 22

increases strength & gives hygroscopic expansion (2%) which will compensate for alloy shrinkage when its cooled to room temp

Question 23

phosphate IM setting

Answer 23

forms magnesium ammonium phosphate

Question 24

phosphate IM heating

Answer 24

1000-1100oC
at 330oC - water & ammonia liberated
at higher temps - complex reactions with silico-phosphates formed to give increased strength

Question 25

phosphate IM properties

Answer 25

1. high green strength - don’t need metal casting ring for support
2. easy to use
3. high strength
4. porous
5. chemically stable

Question 26

silica IM stages of production

Answer 26

stage 1 - prepare stock solution
stage 2 - add powder (quartz or cristobalite), gelation
stage 3 - drying, tightly packed silica particles

Question 27

properties of silica IM

Answer 27

strength - sufficient
not porous - therefore needs vents
complicated manipulation