Patent GB-792547-A (GB)
Title: Improvements relating to the production of thermoplastic sheet material
IPC: B29C48/03
Filed: 1955-04-20
Abstract: Apparatus for the production of thermoplastic films by extrusion comprises a slot orifice die, a contact member having a stationary surface for engaging the film as it leaves the die, means for cooling the surface, and means for drawing the film over the surface.  As shown in Fig. 1, heat-softened thermoplastic material is extruded from the slot orifice 14 in the face 13 of a heated die 12 attached to the head of the conventional extruder 11 to form a film 15 which is drawn over the working faces 24, 26 of the copper tubes 18, 19 which are mounted in the chill bars 16, 17, extend more than the entire width of the die orifice, and have an area of contact with the film in the form of a strip extending across the film at right angles to its <PICT:0792547/IV (a)/1> <PICT:0792547/IV (a)/2> <PICT:0792547/IV (a)/3> length and of uniform width.  A cool heat-transfer fluid is circulated through the tubes 18, 19.  Each tube 18, 19 has fixed thereto a stiffening member 21, 22 respectively which extends substantially the entire length of each tube.  The working faces 24, 26 are machined along their entire length and then finished by blasting to produce a matt finish to reduce the tension required to pull the film over the working faces and to improve the surfaces of the film.  The positions of the tubes 18, 19 with respect to each other and to the film may be adjusted by means of the mechanism shown in Fig. 3.  This comprises a circular plate 31 adapted to support at its centre an extension of the lower chill bar tube 18.  The plate 31 is adjustably mounted in a circular opening of a block 32, being held by clamps 33 in its adjusted position in the block.  The block 32 is in turn adjustably mounted in a frame 34, itself adjustably mounted in a stationary support 36.  Adjusting screws 37, 38 are provided for setting the horizontal position of the block 32 and the vertical position of the frame 34 respectively. Mounted on the plate 31, by means of an adjusting screw 39, is a carriage 41 for supporting an extension of the upper chill bar tube 19, there being a radial slot 42 in the plate for receiving the extension.  Fig. 2 shows a form of chill bar particularly adapted to be brought very close to the front face 13 of the die 12.  The radius of curvature of the working face 48, 49 of each chill bar 51, 52 is considerably larger than the radius of the passage 53, 54 through which the cooling liquid flows, e.g. about 8 times the radius, and the centres of curvature of the working faces are at about the rear corners 56, 57 of the respective chill bars.  The lower and upper chill bars are each mounted to be adjustable vertically and horizontally and the upper chill bar may also be rotated.  In Fig. 2, 61 is a driven take-up roll for drawing the extruded film from the die over the chill bars and for winding the film into roll form, and 62 is a vertically adjustable idler roll.  At the start of the extrusion operation and when it is necessary to repair breaks in the film, the chill bars 16, 17 or 51, 52 are moved away from the face 13 of the die 12, manually or by a mechanism, to give free access to the film, and when satisfactory extrusion is resumed the chill bars are returned to the normal working position in which they engage the film.  Preferably at least one of the chill bars is brought close to the front face 13 of the die 12 so that the extruded film only travels a short distance, e.g. 1/8 to 3/16 inch before it engage the working face of the chill bar.  Thermoplastic materials which may be used include plasticized cellulose esters, cellulose ethers, polyethylene, polyvinyl acetals, polyvinyl chloride and copolymers thereof with vinyl acetate.  In an example, using the apparatus shown in Fig. 2, a cellulose acetate composition is extruded through a die having a slit orifice 5 inches wide and 10 mils. thick and the film is passed over chill bars cooled by oil circulating therethrough. The temperature of the die is 455 DEG  F., that of the oil entering the bars is 37.4 DEG  C., and that of the oil leaving the bars is 45.5 DEG  C.  The apparatus reduces necking down of the film.