Patent GB-793185-A (GB)
Title: Improvements relating to the surface treatment of thermoplastic film
Inventor: STIMSON EDWARD ERIC
IPC: B29C59/04
Filed: 1954-06-15
Abstract: A surface of a thermoplastic film is modified, e.g. polished, finely embossed to give a matt effect or more coarsely embossed, by pressing the surface, by means of fluid-pressure acting directly on the other surface while the film is in a heat-softened condition, against a suitable impressing surface and cooling the film, while still in contact with the surface, to a temperature at which it retains the texture imparted by the surface after removal therefrom.  Apparatus for this purpose comprises an impressing member in the form of a band or drum, the outer surface of which constitutes the impressing surface, which member is arranged to be driven in such a way that the impressing surface moves along a closed path; the apparatus also comprises driving means for the impressing member, a casing embracing part of the path and having inlet and outlet apertures for the film to be treated, means for causing the film to pass continously through the inlet aperture into the casing and into contact with the impressing surface, means for excluding air from between the surface and the film, means for causing the film to travel within the casing in contact with the surface towards the outlet aperture and to emerge through the aperture, means for supplying a fluid under pressure to the casing so that it acts on the exposed surface of the film to hold the film against the impressing surface, sealing means adjacent to the apertures, the sealing means co-operating with the film to limit leakage of the fluid through the apertures, and means for controlling the temperature of the film so that, during at least part of its travel in contact with the surface, it is in a heat-softened condition and, before leaving the surface, is cooled to a temperature sufficiently low for the texture derived from the impressing surface to be retained.  As shown in Fig. 1, a film 24, e.g. of cellulose acetate plasticised with triacetin or diethyl phthalate, is drawn from a supply roll 49 at room temperature into contact with the upper surface of an endless flat flexible metal band 13 which passes over a drive roll 46 and an idle roll 47. The film is carried by the band through a steel casing 10 of rectangular crosssection, the temperature of the film being raised to 110-130 DEG  C. before reaching the nip 50 between the entry rolls 11, 12 by means of the radiant heater 40 supplying heat to the under <PICT:0793185/IV (a)/1> <PICT:0793185/IV (a)/2> surface of the band.  The temperature of the film is then raised to 150-180 DEG  C. after passing the nip 50 by means of the radiant heaters 41, 42 and then gradually reduced during its passage beyond these heaters to 80-100 DEG  C. at the nip 51 between the exit rolls 11, 12.  The air between the film and the band is squeezed out by the entry rolls 11, 12, and the film is held firmly against the surface of the band during its travel to the exit roll nip by a superatmospheric pressure of, for example, 1-5 atmospheres maintained in the casing 10.  The film is preferably subjected to this pressure for 1-5 minutes or more.  The film, after passing through the exit-roll-nip 51, is further cooled during its passage to the stripping roll 48, where it is stripped from the band 13 and runs to a take-up roll (not shown).  The rolls 11, 12 are positively driven at the same linear speed as the band 13.  The shafts 14, 15 of the rolls (Fig. 2) extend through apertures 16, 17 in the side-plates 18, 19 of the casing.  The lower rolls 12 of each pair are journalled in fixed bearings carried on the outside of these side-plates.  The bearings for the shafts of the upper rolls of each pair are carried by blocks sliding in ways carried by the outside of the casing, these bearing-blocks being movable by screws so that the upper roll of each pair can be held with considerable pressure against the corresponding lower roll.  The bearings &c. are not shown.  Each roll has a covering 20 of heat-resistant rubber which projects slightly beyond the length of the roll into contact with an 8-shaped sealing boss 21 on the inside of each side-plate having two apertures 22, 23 through which extend the shafts 14, 15 of the rolls. From each sealing boss there is a rearward extension 25, 26 to the ceiling 27 and floor 28 respectively of the casing.  These extensions carry vertical recesses 29, 30 which provide guides for the sealing member.  Each of these members comprises an end-plate 31, 32 sliding at its edges in the vertical recesses 29, 30 and having attached to its forward surface a sealing wedge 33, 34.  Each wedge extends across the whole width between the sealing boss extensions 25, 26. Each sealing member plate 31, 32 passes through the casing 10 into a downward-facing recess 35 in a block 37 and an upward facing recess 36 in a block 38 running across the top and bottom respectively of the casing.  At intervals along the length of these blocks setscrews 39 project into this recess and engage the edges of the sealing member back-plates 31, 32, thus providing a means of forcing the sealing members inwards so that each sealing wedge 33, 34 engages the rubber covering 20 of its respective roll 11, 12, making a substantially air-tight contact therewith.  The metal band 13 is slightly narrower than the length of the rolls, but slightly wider than the width of the film to be treated.  Under the downward pressure exerted on the upper roll, the gaps between the edges of the film and of the band and the sealing bosses are sealed by distortion of the rubber coverings of the rolls.  Compressed air automatically maintained at a predetermined pressure is supplied through an inlet pipe 43 to the space within the casing.  Owing to the band not extending across the whole width of the casing, the air pressure operates on both the upper and the lower surfaces of the band, thus facilitating the supporting of the band within the casing by a series of spaced idler rolls 44, 45 of which the first, 44, supports the band 13 only at the edges and the remainder, 45, extend across the whole width of the casing 10.  These remaining rolls 45 both support the band and conduct heat away from it. They may be cooled by circulating cold water through them.  A list of plasticisers which may be present in cellulose acetate sheet material suitable for the treatment is given.  The treatment may also be applied to sheet material of cellulose esters of other paraffinic monocarboxylic acids containing up to 4 carbon atoms, or of thermoplastic polymerized vinyl or vinylidene compounds such as polythene, polyvinyl chloride-acetate, and copolymers of vinyl chloride and/or vinylidene chloride with methacrylonitrile, or of polyhexamethylene adipamide, polyamino-caproic acid and polyethylene terephthalate.