PubMed · 3267464
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G Derrien. 1988. [Polyurethane polymers for impression casting].. https://pubmed.ncbi.nlm.nih.gov/3267464/
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Recently, several new products of investments for "quick heating" have been put on the Japanese market. The total casting procedure time for this quick heating method involves only one hour; 30-minutes waiting after the start of mixing before placing the mold directly into the 700 degrees C furnace and 30-minutes heating in the furnace. The purpose of this study was to evaluate two variables effecting casting accuracy using these new investments. The effect of thickness of the casting liner inside the casting ring and the effect of waiting time before placing the mold into the 700 degrees C furnace were evaluated. A stainless-steel die with a convergence angle of 8 degrees was employed. Marginal discrepancies of the crown between the wax patterns and castings were measured. The size of the cast crown became larger when the thickness of the ring liner was thick and when the waiting time before placing the mold into the furnace was long. These results suggest that these new investments have the advantage of providing sound castings using short-time casting procedures. However, it is necessary to pay careful attention to the casting conditions for obtaining reproducible castings.
The authors examine the detail reproduction capacity and surface roughness of 11 commercially available materials [6 plasters (stones?), 2 epoxy resins and 3 polyurethane resins] indicated for the construction of working models with individually extractable dies. The study was performed in vitro according to the method outlined by the ADA in specification no. 19 relating to materials for elastomeric imprints. This specification describes the use of a steel test-block with which it is possible to prepare a master imprint in polyvinylsiloxane. This imprint allows the grooves of the test-block to be reproduced in the form of angular crest which represent the details to be reproduced. Both the master imprint and the samples to be tested were prepared according to the manufactures instructions. The study was performed in three stages: macroscopic analysis, microscopic analysis, profilometric analysis. The macroscopic analysis did not show any differences between the different materials tested. The microscopic analysis showed that when enlarged 40 times the resins revealed a smoother surface than the plasters. The surface quality of plasters was improved by using a hardening solution recommended by the manufacturer as an alternative to water, or by using spacer paint. The plaster materials gave excellent angular definition contrary to the epoxy and polyurethane resins. The use, where advised, of the hardening solution as an alternative to water did not alter this parameter. The electronic profilometer used for the profilometric analysis comprised a diamond sensor which, when run along the surface, recorded all roughness, translating it into chart form.(ABSTRACT TRUNCATED AT 250 WORDS)
The present study describes refractories which contribute to formation of skin holes of Ti casting. Plates consisting of quartz (Qu) and cristobalite (Cr) powders and Qu- and Cr-MgO mixtures were fired at high temperatures and used as phosphate-free molds. The cross-section of the castings was investigated using an optical microscope and an electron probe microanalyzer. Whereas the castings into the Qu molds were almost complete, those into the Cr molds were incomplete. The surface reaction zone of the Qu molds formed a typical layered structure with fewer skin holes. That of the latter extended deeply into the interior with a net-shaped Ti-Si phase, cracks, and holes. Addition of MgO to Cr effectively improved the castability and changed the reaction zone to the typical layered structure. However, slender holes occurred along the mold wall. The overall area around the holes was contaminated by Si and Mg. Probably, this resulted from a reaction of forsterite with Ti. The reactivity difference between Qu and Cr was not related to the fired strength of mold.