Patent GB-940367-A (GB)
Title: Method and apparatus for stretching thermoplastic sheeting
IPC: B29C55/04
Filed: 1961-03-06
Abstract: 940,367. Stretching thermoplastic film and thin sheeting. PITTSBURGH PLATE GLASS CO. March 6, 1961 [March 8, 1960], No. 8024/61. Heading B5B. An apparatus for continuously differentially stretching in the longitudinal direction a film or thin sheeting of organic thermoplastic material in web form comprises a power-driven cylindrical roll 12 (Figs. 1 and 2) mounted on a supporting structure 11, first and second powerdriven tapering rolls 13 and 14 mounted on the same supporting structure, angularly disposed relatively to one another with their smaller ends adjacent to each other, the smaller end of the first tapering roll being adjacent to the cylindrical roll and the larger end of the first tapering roll being spaced substantially farther away from the cylindrical roll, means to apply tension to the web when moving in a sinuous path of travel partially round the rolls 12 to 14, the rolls being so positioned with respect to one another and with respect to the tensioning means to define a path of travel for the moving web, the angle subtended by the web on each contacted rotating surface being at least about 180 degrees, means to heat the web during its contact with rolls 12 and 13 and means to cool it during its contact with roll 14. As shown in Figs. 1 and 2, the rolls 12 to 14 are driven by a motor 21 through a chain 22 engaging sprockets 15 to 17 keyed on shafts 18 to 20 connected to the rolls by universal joints 51 and 52 in the case of rolls 13 and 14. The apparatus is provided with a hood 23. The axes of rolls 12 to 14 are disposed relative to one another in such a manner that the web of sheeting or film W fed partially round the rolls does not slip on them and so that the stretching of web W occurs when it passes from roll 12 to roll 13. The web W is fed from a roll R (Fig. 2), rotatably mounted on a supporting stand 24 by means of shaft 25, discs 26 and pairs of guide rolls 27 through a slot 28 in hood 23 and after stretching and cooling passes out through a slot 28a in hood 23. The web W after passing through slot 28 passes over cylindrical roll 30 rotatably mounted on arms 30a pivotally mounted on brackets 31. Roll 30 is urged against roll 12 by springs 42 connected to arms 30a whereby roll 30 serves as a feed-in tension roll. The web W passes in a sinuous path partially around and in contact successively with rolls 12 to 14 and then over a tapered roll 29 and through slot 28a. The roll 29 is rotatably mounted on arms 29a pivotally mounted on brackets 43 and is urged by springs 45 connected to arms 29a against roll 14 whereby roll 29 serves as a tension take-off roll. The rolls 12 to 14 are hollow and have end plates 60, one of the end plates 60 of each of rolls 13 and 14 having the same diameter as end plates 60 of roll 12 and the diameter of the other end plates of rolls 13 and 14 being larger. The tapered rolls 13 and 14 preferably have the same included angle. The rolls 12 and 13 are so mounted that the angle between their axes is equal to the included angle of the cone of which tapered roll 13 is a frustum. Rolls 12 and 13 are also mounted so that the distance between them at the smaller end of roll 13 is not much greater than the thickness of web W. The roll 14 is mounted with respect to roll 13 so that at both ends of these rolls the distance between them is not much greater than the thickness of web W, but the distance between rolls 13 and 14 at their larger ends is slightly greater than the distance between these rolls at their smaller ends. The rolls 13 and 14 should be so disposed with respect to one another that the apices of their cones concide. The smaller end of roll 14 may be less in diameter than the smaller end of roll 13, and in this case the included angle of roll 14 is less than that of roll 13. The rolls 12 to 14 are rotated at such respective angular speeds that the surface speed of roll 13 at its smaller is about 7% greater than the surface speed of roll 12 and that the surface speed of roll 14 at its smaller end is about 10% greater than the surface speed of roll 12. The web W is thereby stretched as it travels from roll 12 to roll 13, the stretching of web W increasing towards the edge closer to the larger end of roll 13. The higher surface speed of roll 14 compared with that of rolls 12 and 13 serves to prevent any longitudinal contraction of web W before it is cooled by roll 14. The rolls 12 and 13 may be heated by passing a fluid heating medium through them and the roll 14 may be cooled by passing a fluid cooling medium through it, and an embodiment of fluid circulating systems for effecting this is described and illustrated. The rolls 12 and 13 may alternatively be heated by internal electric heaters. The apparatus may be used for heat-stretching thin sheeting or film of a thermoplastic material (such as a plasticized polyvinyl butyral) having a dyed band with a straight cut-off line separating the band from the colourless remainder of the sheeting, so as to produce a curved cut-off line, the finished sheeting being useful as an insert for a laminated curved glass windscreen.