Patent GB-597372-A (GB)
Title: Improvements in or relating to process and apparatus for the treatment of thermoplastic sheet material
IPC: B29C59/04
Filed: 1945-08-17
Abstract: A method for relieving stresses in a sheet or film of thermoplastic material formed by heat treatment, comprises pressing the thermoplastic material into contact with a smooth solid surface heated to a temperature at which the thermoplastic material adheres thereto, and while in contact with the said surface, heating the thermoplastic material in the absence of pressure to a temperature above that at which the sheet or film was originally formed, but below the temperature at which it decomposes for a time sufficient to remove the stress, cooling while in contact with the said surface and, when it is in a non-plastic condition, stripping it therefrom. In order to avoid cutting the material into pieces the treatment may be carried out continuously on an endless metal band 1 as shown in Fig. 1.  The band is driven by a roller 3 and kept in tension by an adjustable idler roller 2.  Burners 8 and 9 heat the metal band to a temperature high enough to cause the thermoplastic material 40 which is fed from a roller 15 to adhere to it.  These burners may be replaced by other heating devices such as infra-red lamps.  The material and band pass between a rubber coated idler roll 11 which applies pressure to prevent air-entrapment between the band and the film and to ensure good adhesion, and another idler roll 13 on a fixed shaft beneath the band.  Both rolls 2 and 13 may be cored to allow circulation of a heating fluid to maintain the temperature of the belt.  The roll 11 may also be coned, but in this case it is to allow the circulation of a cooling fluid to prevent undue heating of the rubber surface.  The band then passes through an opening 51 into a heating chamber 16 and out through a second opening 52.  Two electrical resistance heaters 17 and 18 are mounted within the chamber 16, the one above the belt having a higher heat capacity than the lower one.  This causes any volatile material such as a volatile plasticiser to be released preferentially from the upper surface of the film and prevents blistering.  Other heating means may be used, such as conductive and convective means or combinations such as electrical inductive heating of the belt and electrostatic heating of the plastic layer by high-frequency currents.  In order to prevent any accumulation of inflammable volatile material in the chamber 16, an inlet pipe 50 with a funnel-shaped end 53 is provided through which a constant stream of an inert gas such as nitrogen flows, the gas passing out through the openings 51 and 52.  ordinarily, the amount of plasticiser volatilized is insufficient to affect the physical properties of the film.  An adjustable pressure ironing mechanism 20 consisting of an idler roll 21 mounted beneath the band on a fixed shaft and an upper pressure-applying roll 22, which is internally cooled to prevent the hot plastic sticking to its surface, and is preferentially positively driven to have a peripheral speed synchronous with the belt speed.  Adjacent to the ironing mechanism is a cooling chamber 38 in which nozzles 19 spray <PICT:0597372/IV/1> water or other non-solvent cooling liquid on to both sides of the metal belt 1.  The material and belt then pass under another idler roll 24 mounted on a fixed shaft to serve as a fixed fulcrum point to enable the plastic film to be stripped from the belt.  It is then rewound on a roller 28.  The overflow from the cooling chamber passes down a pipe 25 into a collecting chamber 26 which also collects liquid dripping off the belt.  A squeegee 27 is mounted within the chamber to wipe off any adhering liquid prior to its return to pick up the thermoplastic film.  In an example, a resin formed from about 9 mols. of vinyl chloride and 1 mol. of vinyl acetate with ethyl hexyl phthalate as a plasticiser was calendered at 150 DEG  C. to form a film about 30 inches wide and 0.010 inch thick.  The metal belt was heated to 210 DEG  C. and the film placed on it and stress relieved in the heating chamber at a speed which subjected the film for 20 seconds to a temperature of 260 DEG  C.  Both surfaces were highly glazed by this treatment and in addition the transparency was increased.  The temperature of treatment depends upon the thickness of the film or sheet and the temperature at which the material was calendered or otherwise heat formed.  The film may be embossed while still in a plastic condition.