Investment Procedure

Question 1

Function of liner for investing

Answer 1

Provide space for lateral expansion of investment

Question 2

How should liner be placed in investment ring

Answer 2

2mm short of ring. Rough surface exposed to grip investment material to confine it and reduce expansion in longitudinal direction

Question 3

Why should you remove excess surface acrive agent with dry brush from wax pattern before investing

Answer 3

Affect water powder ratio at that area

Question 4

What is a con of using modified hand investment

Answer 4

Wax pattern may dislodge

Question 5

Pros of vacuum investing

Answer 5

Minimise entrapment of air voids andair bubbles on wax pattern

Batter adaptation of invesetment to wax pattern, smoother casting

Question 6

What is control water added technique

Answer 6

Use a RUBBER RING

Add specific amount of water on top of investment and allow investment to set at room temperature

May reduce confining effect of rubber ring by placing ring in 38ºC water bath but keep water level below top of ring

Question 7

Why should minimal time be employed for investing procedure

Answer 7

Prevent thickening of investment

Prevent thermal contraction of pattern due to cooling effect brought about my evaporation of water from investment

Question 8

How to control shrinkage compensation of investment

Answer 8

Thermal expansion:
1. Water powder ratio: more water/powder ratio, less thermal expansion

2. Quartz cristobalite ratio. More quartz, less thermal expansion
3. Hardness of wax

Hygroscopic:

1. Time immersed in water
2. Amount of water added during setting
3. Water bath temperature
4. Burnout temperature

Question 9

How should wax pattern be orientated in investment

Answer 9

Ring centred in horizontal direction

Thin edges of casting orientate in direction of trailing edge

Question 10

When shoud you remove the spure from the investment

Answer 10

After investment has hardened for at least one hour

Heat sprue and remove in direction parallel to sprue

Question 11

What are the two methods to eliminate wax

Answer 11

Flush with boiling water or dry heat

Question 12

What are the cons of using boiling water to eliminate wax

Answer 12

Fine details of mould lost by solution or disintegration of gypsum

Question 13

Why should you start burnout while mould is still wet

Answer 13

Water in investment reduces absorption of wax and flushes out wax when water boils

Question 14

Type 1 investment heat to

Answer 14

650-700ºC

Question 15

Type 2 investment heat to

Answer 15

482ºC. Keep thermal expansion at a minimum

Question 16

What happens if investment is heated to too high a temperature

Answer 16

Decomposition of investment: gypsum give off sulfur gases when heated above 700ºC. Sulfur gas will contaminate gold colour. Dark colour penetrate casting and cannot remove by polishing

Early wear of rings and heating element

Rough casting due to disintegration

Question 17

Rate of ehating for type 1 investment

Answer 17

Slowly heat from room temperature to required temperature. Proper heating rate to produce smooth casting

Question 18

What happens if investment is heated too rapidly

Answer 18

Flaking or fracture of investment due to steam pressure

Cracking due to uneven heating esp cristobalite

Alter size of casting esp quartz

Question 19

Purposes of heating investment

Answer 19

Eliminate wax from mould

Obtained required amount of thermal shrinkage

Remove residual carbon from absorbed wax by converting to CO or CO2

Question 20

How should sprue hole be positioned in the furnace

Answer 20

Facing down

Question 21

Why should you avoid keeping investment overnight

Answer 21

Prevent drying out of investment material