11
Investments and casting
Figure 11.1 Thermal transformations of silica.
Figure 11.2 Polymerized silica.
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
Table 11.1 Expansion requirements for gypsum-bonded investments
Table 11.2 Thermal expansions of crystallographic forms of SiO2
| Crystallographic form | Expansion (%) | Temperature (°C) |
|---|---|---|
| Quartz | 1.4 | 600 |
| Cristobalite | 1.6 | 400 |
| Tridymite | <1 | 600 |
Table 11.3 Effect of manipulation variables on investment expansion
| 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 |
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