Machine for making bags

A machine for making bags from thermoplastic sheet material of indefinite length, having an entry end for said sheet material and an outlet end for bags, comprising, in a direction from said entry end toward said outlet end, a folder for folding said sheet material lengthwise about a fold line into two layers, and apparatus for thermowelding said layers together along lines extending transversely of the sheet material and for severing or perforating said sheet material along said lines, wherein said apparatus comprises a heated die on one side of the material, means moving the heated die toward and away from the material, an unheated die on the other side of the material and movable only by the heated die and resiliently urged toward the heated die, the unheated die being in two portions with a knife between them, the knife extending transversely of the material and being respectively shielded and exposed by the unheated die in two end positions of movement of the unheated die, the heated die being movable toward and away from the material first to press the material against the unheated die when the unheated die shields the knife and then to move the material and the unheated die together until the unheated die exposes the knife and the knife severs or perforates the material, wherein the machine applies heat to the material only from the side of the material opposite the knife.