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Abstract

A vacuum furnace for heating dental reconstructionproducts using sintered powder metal that includes a sealed vacuum tube made of a material having a relatively low thermal shock resistance characteristic. The vacuum tube is desirably relatively short. The vacuum tube chamber has a heating chamber and end seals for the vacuum tube have a maximum use temperature of less than 200.degree. C. Insulation is positioned around and connected to the tube proximate the ends of the tube, and heating elements are placed around the heating chamber in an annular insulation chamber formed in the insulation between the insulation and the vacuum tube. Opposed annular clearances extending to positions proximately spaced from the ends of the tube and opening to the central annular insulation chamber are formed between the insulation means and the vacuum tube. The insulation prevents heat from passing to the ends of the tube and overheating the seals there. The annular clearances controls the rate of heat emanating from the heating elements during the heatng process so that the heat generated by the heating elements is absorbed by the tube at a gradual controllable temperature gradient along the length of the tube that is less that the rate of heat gain that would be beyond the tolerance of the thermal shock resistance characteristic of the tube, so that cracking of the tube during heating is avoided.

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First Claim