2 - Investment Materials

Question 1

4 types of investment materials?

Answer 1

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

Question 2

applications of different types of investment materials:

• dental plaster?
• gypsum bonded?
• silica bonded?
• phosphate bonded?

Answer 2

• acrylic dentures
• gold casting alloys
• base metal casting alloys (rarely used)
• base metal and gold casting alloys, also used for refractory dies in ceramic buildups

Question 3

ideal properties of investment moulds? x5

Answer 3

• accurate: shape, size, surface detail
• temperature shape stable: maintain integrity
• high compressive strength
• compensate for casting shrinkage (balance of setting and thermal expansions)
• alloy compatible

Question 4

investment materials for alloy, porcelain and glass casting - principal components and what does each one do?

Answer 4

• binder: binds refractory particles together

- refractory material (usually silica): withstands high temperatures without degradation

Question 5

what happens to alloys/ceramics on casting? and how does this affect the investment mould?

Answer 5

• alloys/ceramics shrink on casting

- investment mould must compensate for this. done by expanding

Question 6

mechanisms of investment expansion?

Answer 6

• thermal: setting expansion of binder

- hygroscopic expansion: place mould into water at initial set so more expansion occurs during setting

Question 7

why is hygroscopic expansion useful?

Answer 7

increases the setting expansion of gypsum bonded investment materials

Question 8

investment properties: gypsum bonded

• decompose above?
• liberates what to result in porosity?

Answer 8

• 1200degrees

- sulphur trioxide

Question 9

investment properties: phosphate bonded

• higher str means?
• may be used for?

Answer 9

• means more popular

- used for all current dental alloys

Question 10

investment properties: silica bonded

• what does it lack that gypsum/phosphate doesn’t?
• what does this mean?
• what is generated?

Answer 10

• porosity
• escape of air prevented
• back pressure generated. incomplete casting as mould does not fill

Question 11

wax burn out: temperature held for how long before casting?
temperature for:
- gold alloy?
- palladium silver alloy?
- base metal alloy?

Answer 11

• 30mins to 1 hour
• 700-750deg
• 730-815deg
• 815-900deg

Question 12

casting: melting alloy

methods?

Answer 12

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

Question 13

casting: forcing alloy into mould- done by?

Answer 13

• gravity
• air pressure
• steam pressure
• centrifrugal force
• vacuumn

Question 14

list the casting faults? x4

Answer 14

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

Question 15

finning and bubbling: describe + what causes it?

Answer 15

• extra appendages on casting

- due to investment mould cracking

Question 16

incomplete casting - caused by?

Answer 16

• poor space design or failure
• alloy not molten
• lack of force
• back pressure
• cooling shrinkage

Question 17

porosity: embedding of? occurs due to?

Answer 17

• 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 18

under/oversized casting: due to?

Answer 18

it is an impact of total process chain failure

Question 19

how to reduce chance of casting faults? x4

Answer 19

• understand process
• understand investments
• adherence to instructions
• attention to detail