Investment Procedure Flashcards
Function of liner for investing
Provide space for lateral expansion of investment
How should liner be placed in investment ring
2mm short of ring. Rough surface exposed to grip investment material to confine it and reduce expansion in longitudinal direction
Why should you remove excess surface acrive agent with dry brush from wax pattern before investing
Affect water powder ratio at that area
What is a con of using modified hand investment
Wax pattern may dislodge
Pros of vacuum investing
Minimise entrapment of air voids andair bubbles on wax pattern
Batter adaptation of invesetment to wax pattern, smoother casting
What is control water added technique
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
Why should minimal time be employed for investing procedure
Prevent thickening of investment
Prevent thermal contraction of pattern due to cooling effect brought about my evaporation of water from investment
How to control shrinkage compensation of investment
Thermal expansion:
1. Water powder ratio: more water/powder ratio, less thermal expansion
- Quartz cristobalite ratio. More quartz, less thermal expansion
- Hardness of wax
Hygroscopic:
- Time immersed in water
- Amount of water added during setting
- Water bath temperature
- Burnout temperature
How should wax pattern be orientated in investment
Ring centred in horizontal direction
Thin edges of casting orientate in direction of trailing edge
When shoud you remove the spure from the investment
After investment has hardened for at least one hour
Heat sprue and remove in direction parallel to sprue
What are the two methods to eliminate wax
Flush with boiling water or dry heat
What are the cons of using boiling water to eliminate wax
Fine details of mould lost by solution or disintegration of gypsum
Why should you start burnout while mould is still wet
Water in investment reduces absorption of wax and flushes out wax when water boils
Type 1 investment heat to
650-700ºC
Type 2 investment heat to
482ºC. Keep thermal expansion at a minimum
What happens if investment is heated to too high a temperature
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
Rate of ehating for type 1 investment
Slowly heat from room temperature to required temperature. Proper heating rate to produce smooth casting
What happens if investment is heated too rapidly
Flaking or fracture of investment due to steam pressure
Cracking due to uneven heating esp cristobalite
Alter size of casting esp quartz
Purposes of heating investment
Eliminate wax from mould
Obtained required amount of thermal shrinkage
Remove residual carbon from absorbed wax by converting to CO or CO2
How should sprue hole be positioned in the furnace
Facing down
Why should you avoid keeping investment overnight
Prevent drying out of investment material
