11: Investments and casting


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


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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
Jan 1, 2015 | Posted by in Dental Materials | Comments Off on 11: Investments and casting
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