Patent GB-947677-A (GB)
Title: Treatment of thermoplastic films
IPC: B29C48/92
Filed: 1962-04-11
Abstract: 947,677. Stretching films of thermoplastic polymeric material. E. I. DU PONT DE NEMOURS & CO. April 11, 1962 [April 20, 1961], No. 14080/62. Heading B5B. A continuous process for reducing the variations in thickness which are present from edge to edge of a flat film of thermoplastic polymeric material in a direction at right angles to the longitudinal axis of the film, which film has been formed by melt extrusion of the polymeric material through a slot die, and thereafter has been immediately quenched, but has not been stretched, comprises causing the film to travel first through a heating zone in which the longitudinal lanes of greater thickness are preferentially heated, and then through a stretching zone wherein the unsupported film is stretched in the longitudinal and/or transverse direction while the longitudinal lanes of greater thickness are still hotter than the remainder of the film. The heated film is preferably stretched first in the longitudinal direction and then in the transverse direction but may be stretched first in the transverse and then in the longitudinal direction. The preferential heating may be controlled by a thickness sensing device operating on the film after it has been stretched. The film must be in a substantially amorphous condition immediately before stretching. Any amorphous thermoplastic crystallizable polymer, particularly polyethylene terephthalate, may be used in the manufacture of films according to this invention, e.g. polyethylene-2,6-naphthalate; polytetramethylene - 1,2 - dioxybenzoate; polyethylene - 1,5 - naphthalate; polyhexamethylene adipamide or sebacamide; polycaprolactam and copolymers of ethylene terephthalate and ethylene isophthalate. Films of amorphous non-crystallizable polymers such as polystyrene can also be treated according to the invention. The preferential heating of the longitudinal lanes of greater thickness is preferably effected by means of a heater assembly comprising a number of tubular element electrical heaters disposed parallel to each other in the longitudinal direction. A typical individual heater (Fig. 3) is attached to a supporting framework (not shown) by a support 14. A radiant heating element 15 is encased in a tubular enclosure 16 and surrounded on the side away from the film web by a reflecting shield 17, the primary purpose of which is to reduce transverse radiation and limit the effect of the heater as much as possible to the specific lane involved. The heaters are preferably disposed for the full width of the film web and spaced with a centre-to-centre distance of one inch. The heaters, which are mounted so that their positions can be adjusted transversely, are about ¥ inch wide. The distance of the heater above the film surface can also be adjusted and is usually less than one inch. As shown in Fig. 4, the heater 18 may be controlled by a voltage regulating device such as an auto-transformer 19 which is capable of controlling the output of the heater from zero to its maximum by variation of the input voltage supplied from the line 20. The individual auto-transformers can be controlled manually or by an automatic device which senses thickness deviation in the web, converts the deviation signal to a functional signal which operates a controller which varies the voltage in appropriate manner according to the deviation. The automatic operation of preferential heating involves, as known in the art and associated fields, means to traverse and scan the web of film, a thickness sensing combination and amplification and control circuits.