The influence of handling technique on the strength of phosphate-bonded investments

Objectives: This study examined the influence of six handling techniques on the strength of four phosphate-bonded investment (PBI) materials. The size and distribution of pores produced by the handling techniques were investigated and their effect on room temperature strength determined. Methods: Specimens were tested for strength using four point bend at a cross-head speed of 1 mm min^-1. The fracture surface of randomly selected specimens was viewed through a light microscope to determine the pore size. A three-parameter Weibull distribution was used to describe strength giving values of Weibull modulus and characteristic strength. Results: The handling technique that produced the smallest pore size resulted in the highest strength. Significant differences in strength (90% confidence level) were found between most handling techniques. The reduction of pore size by mechanical spatulation and setting under pressure was found to be the most effective method of increasing the strength of PBI. Significance: Previous studies have related handling technique to the inclusion of air bubbles in dental investments but have not reported any consequent effect on strength. PBI materials are used in the production of a range of dental appliances with a variety of different material forming processes and are required to withstand the loads applied without fracture or distortion to ensure accuracy of fit of these prostheses. The use of a handling technique that reduces pore size can significantly increase the strength of PBIs, thereby decreasing the possibility of die fracture or degradation.