STRESS-STRAIN AND THERMAL-EXPANSION CHARACTERISTICS OF A PHOSPHATE-BONDED INVESTMENT MOLD MATERIAL FOR DENTAL SUPER PLASTIC FORMING

Objectives. The stress-strain and thermal expansion characteristics of a phosphate-bonded investment material were measured to determine its suitability as a die material for superplastic forming of dental appl iances. Titanium alloy denture bases and implant superstructures have been fabricated successfully using this investment material, but a gre ater understanding of its properties was sought to optimize the formin g technique before exploitation could proceed. The effect of a boron n itride powder on the properties of phosphate-bonded investment was als o investigated. Methods. Three-point bend test specimens were cast in the phosphate-bonded investment material. Four groups of test samples were produced at specified water-to-powder ratios (w/p ratios). For on e group 5 wt.% boron nitride powder was added to the investment powder before mixing with water. For stress-strain measurements specimens we re heated to 920 degrees C and held for 30 min prior to the applicatio n of load. For thermal expansion measurements specimen length was meas ured for the entire heating and cooling cycle. Results: Curves of stre ss versus strain and thermal expansion were recorded for all four test groups. For increasing initial w/p ratio failure stress decreased wit h lowest failure stresses for specimens containing boron nitride. Stra in to failure also decreased with increasing initial w/p ratio, except for the group containing boron nitride which showed relatively large elongations to fracture. Specimen surfaces with higher initial w/p rat ios were observed to have larger surface porosity. Thermal expansion c urves for three groups were very similar, but differed considerably fr om the group containing boron nitride. Conclusions. The lowest initial w/p ratio should be used for highest hot strength. Hot deformation of phosphate-bonded investment dies for superplastic forming is an issue that requires further investigation because of potential die deformat ion and the resulting loss of fit between the superplastic superstruct ure and implant components. Boron nitride could be used in phosphate-b onded investment to decrease thermal expansion, but the consequences o f reduced hot strength and increased strain to failure may make the mo dified material unsuitable as a die for superplastic forming. (C) 1998 Elsevier Science Ltd. All rights reserved.