[Studies on the accuracy of the castings. (Part 10) On the effects of starting time of the heat to phosphate bonded investment mold (author's transl)].

Properties of phosphate bonded investments are superior than that of plaster bonded investments in mechanical properties, expansional values for compensating to the casting shrinkage of alloys, and refractory resistance. Therefore, phosphate bonded investments have been available to casting for high fusing alloys. But, their properties are varied easily by manipulating. In this report, two commercial products of Cerami Gold (Whip--Mix Corp.) and Ceravest (G--C Dental Industrial Corp.) were used. They were studied on setting expansion, thermal expansion, compressive strength, change of pH value caused by setting reaction, decrement of weight, casting accuracy and roughness of castings in process of time. Results were follows; 1. The weight decrement of investment mold were about 1.5% on Cerami Gold and 3.5% on Ceravest at 24 hours after the mold had been prepared. 2. pH value of Cerami Gold investment mold changed to 8.5 from 7.0 at 5 hours after mixing, but that of Ceravest investment mold changed to 9.0 from 7.0 at 3 hours. 3. The setting expansion of Cerami Gold investment was caused about 0.9% at 3 hours and that of Ceravest investment was about 0.3% at 1 hour ater mixing. Since then, they were to be constant. 4. The thermal expansion of Cerami Gold investment was decreased by elapsed time until 3 hours after mixing, and then since passing at 5 hours, they were increased. But, that of Ceravest investment was decreased in process of time. 5. The compressive strength of both investments were increased as long as elapsed time after setting. Heating at 800 degrees C in the oven, it was 100--110 Kg/cm2 regardless of time on Ceravest investment, and other was 170 kg/cm2 which was increased until passing 5 hours. 6. The roughness of castings surface were similar to the mold surface and they were H3 12--16 microns which were decreased as elapsed time after investment was mixed. 7. Accurate castings were produced by the mold heated after 24 hours, using with Cerami Gold investment. On the Ceravest investment, it was 30 minutes of 5 hours after mixing.