Gypsum-bonded investment and dental precision casting (II) investment for the quick casting technique

Part I of this study presented a hypothesis that the thermal expansion of gypsum-bonded investment should become higher under heating conditions favorable for the formation of ahemihydrate. The present study aimed at proving this hypothesis by simulating the rapid heating condition employed in the quick casting technique. The expansion values obtained were 1.72-1.93% after 30 min in three investments, two rapid heating and one conventional types. Under a comparable measuring pressure a common laboratory measurement (5 degrees C/min) gave significantly lower expansions of 1.32-1.36% at 700 degrees C for the same investments (p<0.01). The shrinkage observed at 350-400 degrees C in this measurement was not observed in the rapid heating. Realistically large molds prepared from the conventional gypsum-bonded investment need to be investigated for their potentially high thermal expansion.