a hinge for thermoplastic material particularly suitable for loose - leaf binder covers is constructed by embedding an embossed welding die into the thermoplastic material to a depth less than the full height of the embossment so that no vertical lines are produced in the themoplastic by the longitudinal edges of the die . the resulting surface of the thermoplastic is a hinge containing indentations corresponding to the pattern of the embossed die , which permit the thermoplastic to be folded at the hinge area .