Patent GB-986160-A (GB)
Title: Improvements in or relating to the manufacture of flexible films
Inventor: BERRY WILLIAM
IPC: B29C48/12
Filed: 1962-09-12
Abstract: 986,160. Films. BRITISH CELLOPHANE Ltd. Aug. 29, 1963 [Sept. 12, 1962], No. 34817/62. Heading B5B. A film having substantially parallel lanes of varying thickness in the longitudinal direction but capable of being wound into a substantially distortion-free roll is made by extruding a film-forming material through a slot-shaped extrusion orifice and before the material is set to form a solid film having substantially parallel lanes of varying thickness in the longitudinal thickness at least one thick band is formed in the film and is caused substantially to follow a path at an angle to the direction of travel of the parallel lanes in the film. Each thick band may follow a wavy line down the length of the film, e.g. using the apparatus of Figs. 1 and 2 or of Figs. 5 and 6, or alternatively each thick band may consist of a series of discontinuous thick portions lying in echelon, e.g. using the apparatus of Fig. 3 or of Fig. 4. In Figs. 1 and 2, a casting hopper 1 consists of a base plate 2 to which two lips 3, 4 are bolted to form an extrusion orifice 5, the width of which can be varied along the length of the orifice by means of the adjusting bolts 6. A reciprocating rod 13 mounted within the enclosure 9 has a series of spaced deflecting fingers 15 mounted thereon which protrude almost to the exit of the orifice 5 and restrict the flow of film-forming material at each position occupied by a finger 15. The extruded film 11 has longitudinal lanes 12 of varying thickness, owing to the difficulty of making the orifice gap exactly uniform. As the rod 13 reciprocates, a series of bands 16, a pair corresponding to each finger 15, follows a substantially sinusoidal path along the length of the film 11. In Fig. 3, the hopper 1 is similar to that of Fig. 1, but the reciprocated rod 13 and adjusting bolts 6 are omitted, being replaced by a series of heat-expandable bolts 6<SP>1</SP>, each fitted with an internal electric heating coil and energized through an electric controller 18 which operates in accordance with a predetermined programme. When a bolt 6<SP>1</SP> is energized it becomes hot and expands, constricting the orifice 5 in the region of the bolt 6<SP>1</SP> owing to pressure on the lip 4. Thus, by energizing, at any one time, the majority of the bolts 6<SP>1</SP>, the thickness of the film 11 will be reduced in the regions adjacent to the energized bolts but will increase at the regions adjacent to the deenergized bolts, due to the input of film-forming material remaining constant, giving rise to thick portions 16<SP>1</SP> in the film. The controller 18 is programmed so that bolts 6<SP>1</SP> are de-energized and energized in succession across the hopper I<SP>2</SP> giving rise to a series of discontinuous thick portions 16<SP>1</SP> lying in echelon, so forming in effect a thick band lying along a path oblique to the direction of travel of the film 11. In Fig. 4, the heat-expandable bolts of Fig. 3 are replaced by adjusting bolts 6, but a plurality of inlet pipes 19 for the film-forming material are fitted, each operated by a valve 20 controlled by an electric controller 18<SP>1</SP> which operates in accordance with a predetermined programme, so that a series of discontinuous thick portions 1611 lying in echelon are formed across the film, in a manner similar to that of the apparatus of Fig. 3. In Figs. 5 and 6, a conventional viscose hopper 21 is used, and the film on passing through the coagulating bath 22 is subjected to the action of jets 25 of coagulating liquid forced through holes 26 in a pipe 27 reciprocated transversely across the film 23, which cause a thinning of the film at the points of impact giving rise to thin bands bounded by thick bands 28. The invention is particularly described in relation to film made by the wet-casting of viscose into a coagulating bath, but may be applied to the making of films by melt-extrusion. In this latter case, the apparatus of Figs. 5 and 6 is modified so that the film is subjected to the action of jets of air or other gas. Specification 827,381 is referred to.