Verfahren und Vorrichtung zum Herstellen von Hohlkoerpern mit abwechselnd aufeinander-folgenden Wandabschnitten unterschiedlicher Staerke

906,597. Moulding thermoplastic sheet material. HEDWIN CORPORATION. April 17, 1959 [Dec. 12, 1957], No. 13131/59. Class 87(2). An article such as a hollow liner for a barrel is produced from a thermoplastic material e.g. polythene, with selected strengthened areas by heating a polythene sheet to a forming temperature, gripping the sheet at spaced positions and cooling the gripped portions to below said forming temperature, cooling selected portions of the sheet which lie intermediate the gripped portions to below the forming temperature, stretching the sheet to effect a differential stretching of the cooled and uncooled portions of said sheet, and subsequently cooling the entire sheet while stretched to below said forming temperature. A male mould set comprises a head 11 supported on a plate 12, a base 13 supported by a piston rod 14 connected to the piston of a fluid pressure cylinder 15, and a series of movable discs 26, not shown in Fig. 5, which are arranged between the head 11 and the base 13. The mould set at room temperature is lowered centrally on to a heated sheet 24 of polythene in a frame 25 and the polythene is stretched to a position Y, Fig. 5. Vacuum is then applied to the head and base to draw the sheet from a position 24a to a position 24b and the thickness of the sheet is fixed at the areas contacting the head, the base, and the peripheries of the discs 26. A valve 19 is then manipulated and the base 13 is forced away from the head 11, Fig 4 (not shown) and the uncooled material stretched. The stretched material is then cooled and the base 13 retracted. The head 11 and base 13 may be water-cooled if necessary.