Investment materials

Question 1

What is a refractory die?

Answer 1

a dental cast which is heat resistant, upon which we can form in wax complex structures so when we lose the wax in the lost-wax technique we have a shape of the object that was formed in wax, and also a heat resistant mould upon which the molten metal can be poured to form the structure which wears originally modelled in wax

Question 2

What is a refractory die used to support?

Answer 2

used to support materials during processing that would deform/disintegrate if not present e.g porcelain/ceramic veneers, cobalt chrome frameworks etc

Question 3

What must a refractory die withstand?

Answer 3

the rigours of processing (heat and pressure)

Question 4

What do refractory dies tend to be formed from?

Answer 4

phosphate bonded investments that are both fixed and glazed before use

Question 5

What are the 4 types of investment materials?

Answer 5

• dental plaster or stone
• gypsum bonded
• silica bonded
• phosphate bonded

Question 6

What type of investment material is used when making full or partial dentures?

Answer 6

dental plaster or stone

Question 7

What type of investment material is not heat resistant?

Answer 7

dental plaster or stone
- therefore not used in alloy casting

Question 8

What type of investment material is used when using gold casting alloys?

Answer 8

gypsum bonded

Question 9

What type of investment material is used when using base metal casting alloys?

Answer 9

silica bonded
- expensive, rarely used

Question 10

What type of investment material can be used when using either base metal or gold casting alloys?

Answer 10

phosphate bonded

Question 11

What type of investment material is used when using cast ceramics and glasses?

Answer 11

phosphate bonded

Question 12

What investment material is used for refractory dies in ceramic build ups?

Answer 12

phosphate bonded

Question 13

What are the applications of dental plaster or stone?

Answer 13

moulds for acrylic dentures

Question 14

What are the applications of gypsum bonded?

Answer 14

moulds for gold casting alloys

Question 15

What are the applications of silica bonded?

Answer 15

moulds for base metal casting alloys

(rarely used)

Question 16

What are the applications of phosphate bonded?

Answer 16

• base metal and gold casting alloys
• cast ceramics and glasses
• also used for refractory dies in ceramic build ups

Question 17

What are the ideal properties of an investment material?

Answer 17

• accurate - shape, size, surface detail
• temperature shape stable (>1000 degrees C)
• high compressive strength
• compensate for casting shrinkage - balance of setting and thermal expansions
• alloy compatible

Question 18

For alloy, porcelain and glass casting, what are the principal components of the investment materials?

Answer 18

• binder
• refractory material usually silica (heat resistant material)

Question 19

What happens to alloys/ceramics as they cool?

Answer 19

shrink on casting, as they cool

Question 20

Why must shrinkage be compensated for?

Answer 20

for accurate fit of restoration

Question 21

How is shrinkage compensated for?

Answer 21

usually investment expands, meaning the investment mould is actually larger than required to compensate for alloy shrinkage

Question 22

What are the different mechanisms of investment expansion?

Answer 22

• thermal
• setting expansion of the binder
• hygroscopic

Question 23

What is hygroscopic expansion?

Answer 23

in the presence of water the mould will expand
- place mould into water at initial set
- lining casting ring with damp asbestos equivalent

Question 24

What does silica exists in different forms of?

Answer 24

different allotropic forms (alpha and beta)

Question 25

What is the mechanism of expansion of silica?

Answer 25

crystalline inversion of allotropic forms of silica
- quartz alpha —> beta at 575 degrees C
- cristobalite Low —> High at 210 degrees C

Question 26

At what temperature do gypsum bonded investments decompose?

Answer 26

above 1200 degrees C

Question 27

What happens when gypsum bonded investments decompose?

Answer 27

liberates sulphur trioxide leading to porosity

Question 28

What are the benefits of phosphate bonded investments?

Answer 28

• higher strengths means more popular
• may be used for all current dental alloys

Question 29

What makes silica bonded unlike gypsum/phosphate bonded?

Answer 29

it lacks porosity
- inhibits escape of air
- back pressure generated
—> incomplete casting as mould does not fill
—> porosity

Question 30

How is wax burnt out of a metal casting ring?

Answer 30

put in furnace (temperature held) for 30 mins to 1 hour prior to casting

Question 31

What temperature are gold alloys held at for casting?

Answer 31

700-750 degrees C

Question 32

What temperature are palladium silver alloys held at for casting?

Answer 32

730-815 degrees C

Question 33

What temperature are base metal alloys held at for casting?

Answer 33

815-900 degrees C

(INCLUDES COBALT CHROMIUM)

Question 34

How are alloys melted?

Answer 34

• gas torch
• oxyacetylene torch
• electrical induction
• electrical resistance
• furnace

Question 35

How are alloys forced into moulds?

Answer 35

• gravity
• air pressure
• steam pressure
• centrifugal force
• vacuum
  (usually a combination of these 2)

Question 36

What are the stages of an alloy being cast?

Answer 36

• correct amount of alloy weighed out
• casting ring put in furnace and held into place
• casting ring removed from furnace and placed in a casting machine
• molted alloy forced into investment mould inside the casting ring (within the casting machine)
• alloy cooled and investment material removed
• metal sprue removed and cast alloy finished

Question 37

What are possible casting faults?

Answer 37

• finning and bubbling
• incomplete casting
• porosity in casting
• over/under sized casting

Question 38

What is finning/bubbling?

Answer 38

• extra appendages on casting
• due to investment mould cracking

Question 39

What is incomplete casting?

Answer 39

• poor space design or failure of the space (relates to sprue)
• alloy not molten
• lack of force
• back pressure
• cooling shrinkage (too rapid cooling)

Question 40

What is porosity?

Answer 40

• embedding of fractured investment in casting
• gaseous due to alloy entrapment of oxygen taken up by Cu, Au, Ag, Pt and Pd in alloys when molten

Question 41

What is undersized/oversized casting?

Answer 41

• impact of total process chain failure