Device on pressing piston

A device for compressing a wax-type material which comprises a chamber containing a piston movably disposed therein, at least a portion of the lateral surface of said piston containing a plurality of projecting members which extend from said lateral surface into sliding engagement with the wall of said chamber, said projecting member defining a plurality of channels therebetween for venting of gases produced during the compressing operation, wherein said piston is provided with a head portion containing a skirt having a continuous surface and said continuous surface of said skirt being provided with said projecting members, and said projecting members are arranged in a plurality of substantially parallel rows, with the projecting members in adjacent rows being staggered with respect to each other.