Investment strength as a function of time and temperature

Objectives: The purpose of this investigation was to evaluate and compare t he compressive strength characteristics of selected investments and to dete rmine if these change as a function of time and temperature after mixing. Methods: Two phosphate-bonded investments and one gypsum-bonded investment were selected and 288 cylindrical specimens were fabricated (n = 9). Baseli ne compressive strength values were determined for each material at room te mperature and compressive strength measurements were made for all materials at 700 degrees C, and at 872 degrees C only for phosphate-bonded investmen ts. Each material was tested at 2, 6, 12, and 24 h after mixing. Specimens were heated at 15 degrees C/min and heat soaked for 5 min in a burnout oven at the applicable testing temperature. They were transferred to a preheate d chamber on an Instron testing machine and subjected to a compressive load at a crosshead speed of 2 mm/min until failure. The resulting strength dat a are presented in MN/m(2) Results: The gypsum-bonded investment did not exhibit marked differences in strength as a function of either time or temperature. The phosphate-bonded investments, on the other hand, demonstrated considerable changes in compr essive strength and differed in compressive strength characteristics at 700 degrees C and 872 degrees C respectively. Conclusions: At elevated temperatures, all materials approximated peak stre ngth 2 h after initial mixing. At room temperature, the phosphate-bonded in vestments in this study were not significantly stronger than the gypsum-bon ded investment material. However, they exhibited increased compressive stre ngth as a function of time and temperature that was considerably higher tha n that exhibited by the gypsum-bonded investments. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.