Abstract Flexible fold lines in semi-rigid and rigid plastic sheet material (e.g., polyvinyl chloride) are provided with stressrelief indentations so that the tensile and compressive strains normally produced at the folds are substantially relieved, thereby permitting efficient production of a variety of sheet plastic articles such as folding containers and the like. The stress-relief indentations are simultaneously formed on opposite surfaces of the sheet material at the fold line by an apparatus having a die containing a heated scoring blade and a platen which can be preheated by contact with the heated scoring blade prior to insertion of the plastic sheet material between the scoring blade and platen. The temperature of the scoring blade and platen and the pressure with which they are brought to bear on the plastic sheet material disposed between them during the scoring operation are such that the thickness of the sheet material is substantially reduced at the fold line by compression and removal of material without any substantial lateral displacement flow thereof away from the fold line. In this way, parallel indentations are formed on both surfaces of the sheet material at the fold line which serve to relieve the tensile strains on the outer fold surface and compression strains on the inner fold surface normally produced when the material is bent about the fold line.
A container comprising walls of plastic sheet material joined to each other by fold lines comprising heat-scored linear indentations on opposite faces of the sheet material, the thickness of the sheet material between the indentations being substantially less than the thickness of the sheet material away from the fold lines, the density of the sheet material between the indentations being substantially greater than the density of the sheet material away from the fold lines, and the quantity of plastic material along each of said fold lines being less than the quantity of material through an equal cross-sectional area of sheet material located away from the fold lines, said fold lines being further characterized by the absence of ridges of laterally displaced plastic material along the heat-scored linear indentations.
Claim 2
Dependent Claim
128 words
A container comprising walls of plastic sheet material joined to each other by fold lines comprising heat-scored linear indentations on opposite faces of the sheet material, the thickness of the sheet material between the indentations being substantially less than the thickness of the sheet material away from the fold lines, the density of the sheet material between the indentations being substantially greater than the density of the sheet material away from the fold lines, and the quantity of plastic material along each of said fold lines being less than the quantity of material through an equal cross-sectional area of sheet material located away from the fold lines, said fold lines being further characterized by the absence of ridges of laterally displaced plastic material along the heat-scored linear indentations.
Claim 3
Dependent Claim
47 words
A container according to claim 1 wherein: the plastic sheet material is thermoplastic sheet material; and the thickness of the sheet material between the heat-scored linear indentations is between about 25 and about 75 percent of the thickness of the sheet material away from the fold lines.
Claim 4
Dependent Claim
53 words
A container according to claim 2 wherein: the thermoplastic sheet material is selected from the group consisting of poly(vinyl chloride), polycarbonate and polyester; and the thickness of the sheet material between the indentations is between about 40 and about 60 percent of the thickness of the sheet material away from the fold lines.