Characteristics of the Thermal Expansion of Gypsum-bonded Investments

During casting procedure, expansion of investment should match the shrinkage of the casting alloy to achieve an accurate casting. Objectives: The objectives were to determine the thermal expansion behavior up to 1,000^oC, and to investigate the influence of different water/powder (W/P) ratio of gypsum-bonded casting investments on the thermal expansion. Methods: Four gypsum-bonded investments (PCT: Plum Cristobalite, PCV: Plum Crivest-CF, WMC: Whip-Mix Cristobalite, WMP: Whip-Mix Prestobalite) were studied, and one high temperature investment (HTP: Hi-Temp) was used as a control. Each material was mixed with water or special liquid with the W/P ratio recommended by the manufacturer, and 10 or 20 % more water or liquid was added. Thermal expansion was measured with a thermal dilatometer (DIL 402C, Netzche, Germany) with the temperature increase of 5^oC/min. Calibration material was alumina. Results: With normal W/P ratio, maximum expansion occurred around 284.5-341.4^oC, and the amount of expansion for each material was 0.748% in PCT, 0.599% in PCV, 0.723% in WMC, and 0.565% in WMP. HTP showed the maximum expansion of 0.786% at 611^oC. The amount of expansion at 700^oC was 0.503% in PCT, 0.397% in PCV, 0.515% in WMC, and 0.497% in WMP. At above 700^oC, gypsum-bonded materials demonstrated rapid shrinkage. With 10% more water, the ratio of decrease in expansion was in the range of -2.65 to 10.96%; and that was 10.79 to 26.69% with 20% more water. Hence, the amount of maximum thermal expansion was significantly influenced by the W/P ratio. Conclusions: Maximum thermal expansion of gypsum-bonded investments decreased by 16.28% with 20% more water. Therefore, it can be concluded that the proper W/P ratio recommended by the manufacturer should be followed.