GB1137821A - Method and apparatus for deep-stretching initially flat-heat-stretchable plastics sheet material to the shape of a deep-draft open-topped receptacle - Google Patents

Abstract

1,137,821. Sheet forming. LILY CUPS OVERSEAS Ltd. 6 Dec., 1965 [4 Dec., 1964; 6 April, 1965], No. 51701/65. Heading B5A. Flat, heat-stretchable plastics sheet material is deep-stretched to the shape of a deep, opentopped receptacle (e.g. to line a paper drinking cup) by holding it along a periphery corresponding to that of the receptacle's open top, applying fluid pressure and/or vacuum normal to the plane of the sheet to prestretch it to dome shaped configuration whilst heating an annular peripheral zone progressively inwards, discontinuing the fluid pressure or vacuum and applying it in the opposite direction to stretch the sheet to the shape of the receptacle whilst heating or maintaining heat in the sheet. Preferably the peripheral zone is stretched more at its outer periphery during the pre-stretching step to provide a thickness gradient. Apparatus (Fig. 7) comprises a lower cavity die 45 and an upper cavity die 46 vertically movable relative to the lower die. The lower die socket 52 receives the receptacle to be lined, across the top of which a sheet of lining material has been heat-tacked. The upper die socket 68 is dome shaped and of aluminium and is heated by hot oil flowing in spiral channels 80 and circular channels 81. The cavity sidewall 68f is of truncated conical configuration; the slope 68c of the side wall being approximately 70‹ to give optimum results. An air pressure connection 73 and a vacuum connection 74 are provided in the mounting plate 65, the chamber 75 acting as an equalizing plenum for both positive pressure and vacuum, and communicating with the inlet chamber via the channels 76 and with the cavity via channels 77. The lower die is similarly heated and has a vacuum inlet 61 communicating with vacuum chamber 62. An ejector 56 is provided. Procedure. A preformed receptacle 21, e.g. of wound and bonded paper strip is supported on a holder 31 (Fig. 6) which may be mounted on a conveyer or turret and may be heated e.g. by element 32. A vertically movable electrically heated plunger 34 associated with a cutting die 35 and backing die 36 (details of construction being described, Fig. 6a, not shown) cuts out a portion of plastic film 23 substantially the size of the rim 26 of the receptacle, heats it and presses it onto thereceptaclerim. The receptacle with liner tacked on is then transferred to the lower die cavity 45 and the upper die is closed thereon thus clamping the edge, both dies having been pre-heated to about 250‹ F. along their interior surfaces excepting the top of the dome-shaped upper cavity which is of semiinsulating material and is thus only at about 200‹ F. After a period a vacuum is applied to the upper cavity to draw up the film which is further heated by contact with the side-walls of the cavity in successive increments as it is brought into contact therewith so that this prestretching operation produces a thickness gradient. Vacuum is now termiuated at the upper cavity and applied to the lower cavity thus drawing the film down into conforming engagement with the receptacle and subsequently positive air pressure is applied, e.g. at 35 p.s.i.g. to the upper cavity. With lower cavity vacuum still applied, positive air pressure is terminated and the die opened to allow the liner to cool and the bonding to the receptacle to set. Cooling water may be poured in. The liner material may be a two-ply film of polypropylene treated with corona discharge and polyethylene; the relative softening points being such that the polyethylene acts as an adhesive layer. Appropriate thicknesses and dimensions of receptacle and liner are given, the use of this type of two-ply liner being subject matter of the divisional application.