Investment Materials

Question 1

What can we use investment materials in Dentistry to produce?

Answer 1

Metal/alloy inlays, onlays, crowns and bridges.

Question 2

What are the stages involved in casting an alloy (lost wax technique)?

Answer 2

1. Wax pattern made (gives a positive replica)
2. Investment material poured around the wax pattern and allowed to set (forms negative replica)
3. Wax is then eliminated (burning or boiling water)
   This leaves a cavity/shape surrounded by investment material. The molten alloy is poured into mould cavity by wax through a sprue in the investment material.

Question 3

Why is pressure added to an alloy and why must the gases produced be eliminated?

Answer 3

To ensure there are no gaps or voids in it. The gases produced when casting an alloy MUST escape otherwise it would be porous.

Question 4

*Remember during cooling the alloy contracts and it wont be the same shape determined by the mould cavity.

Answer 4

.

Question 5

What are the 4 investment material types?

Answer 5

1. Dental stone plaster (acrylic dentures)
2. Gypsum (gold casting alloys)
   3, Phosphate bonded materials (base metals/cast ceramics)
3. Silica bonded materials (base metal alloys).

Question 6

What are the IDEAL properties of an investment materials?

Answer 6

Expand (compensate for shrinking of alloy)
Porous (allow escape of trapped gases on casting, back pressure effect)
Strong:
Room Temp- ease of handling (Green Strength)
Cast temp- withstand casting forces

Smooth surface (so there isn't much extra work needed to do by the clinician/technician)
Chemically stable 
Easy removal from cast
Handling
Relatively inexpensive.

Question 7

What combats the shrinking of the alloy during the casting process?

Answer 7

Investment materials expand to help combat the alloy shrinkage during casting.

Question 8

What are the typical contractions (by volume) from alloy melting point to room temperature?

Answer 8

Gold alloys 1.4%
Ni/Cr 2%
Co/Cr 2.3%.

Question 9

What are the two components of investment materials?

Answer 9

Binder- determines what kinds of investment material it is- forms coherent mass to provide substance (gypsum, phosphate and silica. They have different setting reactions to give different characteristics.
Refractory (silica)- withstands high temp and undergoes expansion.

Question 10

What is the expansion for Quartz and Cristobalite?

Answer 10

Quartz 0.8%

Cristobalite 1.3%.

Question 11

Why does quartz have a linear expansion until 560 degrees centigrade then shoots up rapidly?

Answer 11

Linear expansion until about 560 degrees centigrade then it climbed more rapidly. Below temp exists as alpha-quartz but beyond this temp the structure changes to beta-quartz where it has expanded to its maximum volume.

Question 12

What is the composition of gypsum bonded investment materials?

Answer 12

Powder (mixed with water)- -silica (refractory component)
-calcium sulphate hemihydrate
reducing agent for oxides
-chemicals to inhibit heating shrinkage and control setting time (NaCl and boric acid).

Question 13

What is the setting reaction for gypsum bonded investment?

Answer 13

Calcium sulphate hemihydrate added to water which produces calcium sulphate dihydrate.

Question 14

What are the dimensional changes in a gypsum-bonded investment material.

Answer 14

• Silica undergoes thermal and inversion expansion

- Gypsum (setting expansion) undergoes hygroscopic expansion and contraction above 320 degrees centigrade.

Question 15

What is hygroscopic expansion?

Answer 15

• Not fully understood -Expansion considered to be capillary forces (pulls water molecules into gaps in crystals of calcium sulphate hemihydrate forcing them apart)
• Can be up to 5x fold in volume within gypsum bonded investment material.

Question 16

What factors increase hygroscopic expansion?

Answer 16

• Lower powder/water ratio
• Increased silica content
• Higher water temperature
• Longer immersion time.

Question 17

What will happen to gypsum above 320 degrees celsius?

Answer 17

Above 320 degrees Celsius gypsum will contract because of water loss and presence of sodium chloride and boric acid.

Question 18

What are the properties of a gypsum bonded investment material?

Answer 18

• Expansion= expands by 1.4% which is sufficient for casting gold alloys as they shrink by this same percent
• Smooth surface= fine particles
• Manipulation- east and setting time is controlled
• Porous= good to uptake gases releases when casting
• Strength is adequate if correct powder/water ratio and correct manipulation.

Question 19

What is the unwanted reaction in gypsum bonded investment material (occurs about 700 degrees centigrade)?

Answer 19

Any wax residue in IM a reaction between calcium sulphate and carbon can release calcium monoxide. Calcium sulphide produced reacts with calcium sulphide to produce sulphur dioxide gas. IMPORTANT that these escape. Undergoes heat soaking (allows gases to escape).

Question 20

What does heat soaking allow to escape from the investment material?

Answer 20

Undergoes heat soaking (allows gases to escape).

Question 21

Can gypsum be used for an alloy melting above and below 1200 degrees?

Answer 21

Below- yes

Above- no as calcium sulphate reacts with silicon dioxide to generate sulphur trioxide (produces voids).

Question 22

What is the composition of phosphate bonded investment material?

Answer 22

Powder (silica, magnesium oxide and ammonium phosphate)

Liquid (water or colloidal silica).

Question 23

Why is colloidal silica mixed with phosphate bonded investment material?

Answer 23

Increase strength

To achieve 2% expansion (compensates for alloy shrinkage when cooled to room temperature).

Question 24

What is the setting reaction of phosphate bonded investment material?

Answer 24

Ammonium phosphate with magnesium oxide and water to produce magnesium ammonium phosphate.

Question 25

What happens to phosphate bonded investment material when you heat it to around 1000 degrees?

Answer 25

At 300 degrees water and ammonium are liberated | At higher temperatures there are complex reactions which increases its strength.

Question 26

What properties does phosphate bonded material have?

Answer 26

``` High green strength Easy to use High strength Porous Chemically stable. ```

Question 27

What is a materials "green strength"?

Answer 27

Strong when handled at room temperature.

Question 28

What are the properties of silica?

Answer 28

Strength Complicated to use Not porous (would make alloy porous and therefore weak).

Question 29

What investment materials are used for gold and CoCr alloys?

Answer 29

Gypsum- gold | Phosphate bonded- other alloys such as CoCr.

Question 30

Why must an investment material be strong?

Answer 30

To withstand pressures during casting.