Method and device for sealing an insert in a molded part to avoid the escape of flash, the device includes an upper mold and a lower mold halves that mate together to define a cavity configured to receive the insert. A compressible material is coupled to the lower mold half to facilitate engagement of the inserted workpiece. The lower mold half has a slide, operated by a cam pin in the upper mold half when the upper and lower mold halves mate together. The upper mold half includes an upper plunger having first and second surfaces and a spring is coupled to the mating surface of the upper mold half. The spring compresses the compressible material and engages the inserted workpiece to form a flash barrier along the molding chamber.