11
Investments and casting
Box 11.1 Requirements of investment materials
Easy manipulation
Room-temperature (RT) and elevated temperature strength
Rapid hardening
High-temperature (HT) stability
Thermal expansion over range of temperatures
Sufficient porosity for gas escape
Smooth surface finish
Easy separation from the casting
Nonreacting with the cast metal
Moderate-to-low cost
Crystallographic form | Expansion (%) | Temperature (°C) |
---|---|---|
Quartz | 1.4 | 600 |
Cristobalite | 1.6 | 400 |
Tridymite | <1 | 600 |
Factor | Setting and hygroscopic expansion | Thermal expansion |
---|---|---|
Increase in W/P ratio | Decreased | Decreased |
Increased spatulation time | Increased | No effect |
Increased spatulation rate | Increased | No effect |
Increased age of investment | Decreased | No effect |
Delayed immersion | Decreased | — |
Higher water bath temperature | Increased | — |
Wax strength | More distortion | Less effect |