DMS - Investment Materials

Question 1

what restorations/appliances are investment materials used for in the construction process? (4)

Answer 1

• Metal/alloy inlays
• Onlays
• Crowns
• Bridges

Question 2

what investment material do we use for making gold alloys?

Answer 2

gypsum

Question 3

what investment material do we use for base metal alloys (except gold)? (2)

Answer 3

silica bonded materials

phosphate bonded materials

Question 4

how do we use investment materials? (1)

Answer 4

The Lost Wax Technique:

Question 5

describe the lost wax technique. (4)

Answer 5

1. Make a wax pattern of (e.g.) a crown = positive replica
2. Pour the investment material around the wax patter and allow to set = mould of a negative replica
3. Remove the wax with boiling water = leaves a cavity of the desired shaoe surrounded by the investment material
4. Pour the molten alloy into the cavity

Question 6

what types of materials are used as investment materials? (4)

Answer 6

• Dental stone/plaster – acrylic dentures
• Gypsum bonded materials – gold alloy
• Phosphate bonded materials – base metals/cast ceramics
• Silica bonded materials – base metal alloys

Question 7

what investment material do we use for acrylic dentures?

Answer 7

Dental stone/plaster

Question 8

what investment material do we use for cast ceramics/base metals?

Answer 8

Phosphate bonded materials

Question 9

What are the requirements of an investment material? (8)

Answer 9

• When an alloy is being cast pressure must be applied to ensure no voids – since the investment material is surrounding this alloy it must be able to withstand the pressure.
• It must be porous so that it’s able to capture the gases produced and let them escape
• Chemically stable so that it doesn’t degrade
• Must be able to expand via thermal, hygroscopic or inversion means to compensate for the cooling shrinkage of the alloy

(above 4 in bold)

• It must also be strong at room temp when being handled = green temp
• Smooth surface: so that the alloy wont need much finishing.
• Easily removed from the cast
• Inexpensive

Question 10

what is the shrinkage of a Co/Cr alloy after casting?

Answer 10

2.3%

Question 11

what is the shrinkage of a Ni/Cr alloy after casting?

Answer 11

2%

Question 12

what is the shrinkage of a gold alloy after casting?

Answer 12

1.4%

Question 13

what are the components that make up investment materials? (2)

Answer 13

1. Refractory – type of silica i.e. quartz or cristobalite
2. Binder – i.e. gypsum, phosphate, silica
   - determines what kind of investment material is made
   - Forms a coherent mass to provide substance

Question 14

what components of investment materials determines what kind of investment material is made?

Answer 14

the binder

Question 15

at what temperature does silica exist in a form called alpha quartz?

Answer 15

< 573 degrees

Question 16

at what temperature does silica exist in a form called beta quartz and is expanded to its maximum volume?

Answer 16

> 573 degrees

Question 17

what dimensional changes does silica undergo? (2)

Answer 17

inversion expansion

thermal expansion

Question 18

what dimensional changes does gypsum undergo?

Answer 18

hygroscopic expansion

contraction expansion

Question 19

what materials significantly reduce the hygroscopic expansion and contraction expansion of gypsum?

Answer 19

sodium chloride

boric acid

Question 20

What factors affect hygroscopic expansion in gypsum bonded investment materials? (4)

Answer 20

• Lower powder/water ratio
• Increased silica content
• Using water at a higher temp
• Immersing the powder in water for longer

Question 21

what are the properties of gypsum bonded investment materials? (5)

Answer 21

1. Expands by 1.4% - sufficient for casting gold alloys
2. Smooth surface from fine particles
3. Easily manipulated and the setting time is controlled
4. Sufficiently porous to allow gases to escape when casting the alloy
5. Adequate strength when the correct powder to liquid ratio is used.

Question 22

describe the unwanted reaction occurs at > 700 degrees in gypsum bonded investment materials? what products are produced? (2)

Answer 22

There is a reaction between the calcium sulphide and the carbon to release carbon monoxide

There is also a reaction calcium sulphide and calcium sulphate to produce sulfur dioxide gas.

Question 23

how do we overcome the gases produced by the unwanted reaction that occurs in gypsum bonded investment materials?

Answer 23

undergo heat soaking (held at high temperatures for some time) to allow the gases to escape and prevents voids forming in the cast alloy

Question 24

why can gypsum bonded investment materials not be used in alloys which require heating above 1200 degrees?

Answer 24

the calcium sulphate reacts with silica

= sulphur trioxide produced which leaves voids in the cast alloy.

Question 25

why is colloidal silica used in phosphate bonded investment materials? (2)

Answer 25

• To increase it’s strength

- Achieve 2% hygroscopic expansion to compensate for the alloy shrinkage when cooled to room temp.

Question 26

what temperature are phosphate bonded investment materials heated to in order to increase their strength?

Answer 26

1000-1100 degrees

Question 27

What are the properties of phosphate bonded investment materials?

Answer 27

1. High strength
2. Sufficiently porous
3. Chemically stable
4. Easy to use and strong whilst being prepared.