Effect of burnout temperatures on strength of phosphate-bonded investments--Part II: Effect of metal temperature.

OBJECTIVES It had previously been found that the strength of phosphate-bonded investments is temperature sensitive. However, while the effect of heat from the cast metal is expected to have some effect on continued reactions and melting, there is no published report dealing with this. It was the purpose of this study to consider further the effects of burnout temperature on strength and the contribution of metal casting temperature and investment composition. METHODS The disc-rupture test (Luk, H. W.-K. and Darvell, B. W., Strength of phosphate-bonded investments at high temperature. Dental Materials 1991, 7, 99-102) was employed to determine the effect of burnout temperature (ranging from 400 to 1000 degrees C) on the 'actual' strength of six phosphate-bonded investments; the cast metal temperature was controlled to be the same as that of the mould. Atomic absorption was employed to investigate aspects of the composition of the investments. RESULTS The 'actual' strength obtained in this test was always higher than the 'service' strength observed in similar testing with the metal at a fixed temperature (1460 degrees C). The 'actual' strength was also temperature sensitive, but with a different pattern of variation compared with the 'service' values. All investments tested were found to behave plastically at high temperatures. Calcium, sodium, zinc and iron were found in appreciable quantities in the investments. CONCLUSION Heat from the high-temperature casting metal has a material effect in decreasing the strength of phosphate-bonded investments. Such heating increases the plastic behaviour of the investment and this, together with the casting pressure, may result in a distorted mould and thus inaccurate castings, a hitherto unrecognized source of error. 'Actual' strength gives no guide to the 'service' strength, emphasizing the need for tests under service conditions. Composition affects investment high-temperature strength substantially.

[1]  T. Miyazaki,et al.  Casting of Titanium with Calcia Investment , 1987 .

[2]  K Asgar,et al.  Reaction of Cobalt-Chromium Casting Alloy with Investment , 1966, Journal of dentistry research.

[3]  F. A. Hummel,et al.  Phase Equilibria in the System ZnO ‐ P 2 O 5 , 1958 .

[4]  S. Okawa,et al.  Layered structure of cast titanium surface. , 1989, Dental materials journal.

[5]  S. Tsutsumi,et al.  Effect of magnesia investments in the dental casting of pure titanium or titanium alloys. , 1982, Dental materials journal.

[6]  H. Luk,et al.  Casting system effectiveness--measurement and theory. , 1992, Dental materials : official publication of the Academy of Dental Materials.

[7]  R. C. Weast.  CRC Handbook of Chemistry and Physics , 1973 .

[8]  R Neiman,et al.  Setting and Thermal Reactions of Phosphate Investments , 1980, Journal of dental research.

[9]  M. Markley.  COMPENSATION BY THERMAL EXPANSION , 1953 .

[10]  G. Mumford,et al.  The fit of certain dental casting alloys using different investing materials and techniques. , 1966, The Journal of prosthetic dentistry.

[11]  Hanns Wentrup.  Beitrag zum System Eisen-Phosphor-Sauerstoff , 1935 .

[12]  F. A. Hummel,et al.  Phase relations in the system SrO-P2O5 and the influence of water vapor on the formation of Sr4P2O9 , 1967 .

[13]  H. Luk,et al.  Effect of burnout temperature on strength of phosphate-bonded investments. , 1997, Journal of dentistry.

[14]  R. Earnshaw,et al.  The effect of potential investment expansion and hot strength on the fit of full-crown castings made with a gypsum-bonded investment. , 1995, Dental materials : official publication of the Academy of Dental Materials.

[15]  G. P. Kelly.  Study of Porosity and Voids in Dental Gold Castings , 1970, Journal of dental research.

[16]  K Ida,et al.  Studies on fundamental and practical factors in dental casting. 3. Procedures for determining practical casting conditions. , 1969, The Journal of Osaka University Dental School.

[17]  E. Lautenschlager,et al.  Casting Pure Titanium into Commercial Phosphate-bonded SiO2 Investment Molds , 1990, Journal of dentistry research.

[18]  H. Luk,et al.  Strength of phosphate-bonded investments at high temperature. , 1991, Dental materials : official publication of the Academy of Dental Materials.