apparatus for simultaneously severing and heat sealing of overlapping portions of thermoplastic sheets between a fixedly mounted anvil and a reciprocable heated blade has an uncooled hold down device with two metallic plate - like members which flank the blade and are confined in layers of silicone so that they are thermally insulated from the blade and need not come in direct contact with the thermoplastic material . those portions of the silicone layers which contact the exposed side of the adjacent sheet opposite the anvil at the respective sides of the blade tend to stretch the sheets in a direction at right angles to the cutting plane to thus ensure immediate separation of the welded seams which are formed by the blade during severing of the sheets .