Abstract: A sandwich of sheet material and a plastic film on at least one side thereof is laminated together by advancing the sandwich in one direction between an opposed pair of rigid heating and pressure plates having mutually facing planar surfaces for contacting the sides of the sandwich. The plates of the pair being biased together under a pressure which increases in the direction of the sandwich advance while the pressure level across the plates transverse to the direction of advance at any locus along the plates is substantially uniform, and passing the thus heated and compressed sandwich between the nip of a pair of pressure rollers to complete welding of the sandwich together.