Abstract: An eccentrically positioned rotary piston (11) in an almost round casing (10) is driven by a pressure controlled hydraulic motor (27) with radial slots (12), guided rams (13).The radial position of the rams (13) is set with a control valve (15).A removable insert (16) in the housing (10) serves to alter the pre-compression of the material. This ensures that gas escapes from ground meat intended for molding patties and that the material is pressed at an even pressure into the mold plate opening (23). No changes are necessary when altering the mold plate size because the driving pressure is limited and this determines the filling amount.