MICROWAVE DEWAXING AND FIRING OF REFRACTORY LININGS FOR CAST JEWELRY USING A CENTRIFUGAL MACHINE

Abstract

This research develops a specific methodology for de-waxing and firing refractory lining molds to obtain cast jewelry pieces using microwave technology. These processes, in the traditional technique, require slow and progressive thermal curves with a duration of 7 to 12 h using electric muffles. The hypothesis of this proposal is based on the fact that the 40 % of water necessary for the elaboration of refractory pastes is a perfect ally for an effective dielectric heating by microwaves and, therefore, this technology would be viable for the production of metal jewelry. In order to carry out the study, it was necessary to design a method based on appropriate time/power ramps so that these negatives gradually lose the contained water, without the thermal expansion generating cracks or crumbling of the mold due to non-uniform humidity profiles.  In the research it was determined that microwave heating is effective for these compounds, however, with advanced dehydration, their microwave absorption capacity is exponentially devaluated, making it essential to use a susceptor that makes it possible to increase their temperature. For this purpose, a muffle doped with SiC was fabricated to subject the samples to hybrid microwave heating after dehydration, making it possible to design gradual thermal curves until the firing was completed. The quality of the thermal processing was evaluated by subjecting the samples to mechanical stress using a centrifugal machine for the injection of the molten metal. At the end of the study, 99 % wax collection and effective recording quality for artistic models were obtained with a reduction of 66 % with respect to the time used in the traditional technique.  The results of this research demonstrate that the viability of microwaves as an energy source contributes to simplify and economize the production of metal jewelry