Patent GB-664787-A (GB)
Title: Process and apparatus for producing polished thermoplastic sheets
IPC: B29C69/02
Filed: 1949-05-12
Abstract: 664,787. Endless-belt conveyers. BRITISH CELANESE, Ltd. May 12, 1949. [May 12, 1948] No. 12702/49. Class 78 (i). [Also in Groups V and XXXV] Polished thermoplastic sheet material is made bv feeding a continuous sheet of hot thermoplastic material to the nip between two endless metal belts having highly polished outer surfaces. The belts travel at the same speed and carry the material along a straight path over the whole of which the sheet is subjected to pressure between the polished surfaces of the two belts. Whilst the sheet travels along the straight path it is subjected to progressive controlled cooling by the maintenance of a belt-temperature gradient along the path, the belt-temperature at the beginning of the path being higher than the Olsen flow-temperature for the material and at the end being lower than the said flow-temperature. The belttemperature gradient is maintained by heating the two belts as they approach the straight path and cooling them by projecting air on their inner surfaces near the mid-point of the path. The Olsen flow-temperature (O.f.t.) is defined in A.S.T.M. specification D596-43. The belttemperature may be 10 to 35‹C. above the O.f.t. at the beginning of the straight path, and at least 50‹C. below the O.f.t. at the end of the path. The belt-temperature at the midpoint may be from 10‹C. above to 10‹C. below the O.f.t. In an example, using a cellulose acetate composition having an O.f.t. of 120‹C., the composition is continuously extruded in sheet form from an extrusion head kept at 200‹C., to belts which are heated to 132‹C., at the receiving point. The belts are at 116‹C. at the mid-point and 60‹C. at the end. In another example, using ethyl cellulose having an O.f.t. of 161‹C., the corresponding temperatures are 222‹C., 177‹C., 154‹C. and 104‹C. A sheet 20, Fig. 1, of thermoplastic material extruded at 21 passes between the polished outer surfaces of a pair of endless metal belts 22, 23, which run around heated pulleys 24, 25, 26, 27. The belts are cooled by air fed through a manifold tube 150, Fig. 8, to spaced laterally disposed pipes 154 having apertures through which air passes and impinges on the belts. Valves 152 are disposed between the manifold tube and the pipes. The pipes are rotatable to vary the direction of flow of the air against the belts. Pulley 26 is driven from electric motor 28 via gearing that includes a vertical shaft 38, Fig. 2, carrying a cone pinion gear 39 which meshes with gear 40 keyed to shaft 31 of pulley 26. Pulley 24 is driven from shaft 38 by a sliding connection including a sleeve 41 keyed to shaft 38. A bushing 42 is splined to sleeve 41 and keyed to a shaft 43, which, via gears 44, 45, drives shaft 46 of pulley 24. This arrangement allows the distance between pulleys 24, 26 to be varied without disturbing the drive. The distance between pulleys 24, 26 may be varied by raising or lowering pulley 24 by turning a hand wheel 47, Fig. 3, this turning being translated, via shaft 48, worms 49, &c., into a vertical movement of shafts 54 which carry bearing blocks 55 in which shaft 46 is journalled. A corresponding variation in distance between (idler) pulleys 25, 27, is effected by raising or lowering pulley 25. This is done by turning hand wheel 69, Fig. 1, thus turning shaft 68 and traversing screw-threaded.. sleeves 67 to close or open toggle linkages 66 and thus raise or lower the toggle hinges 66<SP>1</SP>. On the toggle hinges rests a bed 61, and on this bed is supported, on rollers 62, a carriage 60 which supports bearing blocks 57 in which the shaft 58 of pulley 25 is journalled. The pulley 27 also is supported by a carriage 79 having rollers 84 which run on a bed 83, but this bed is secured to the machine framework 176. The belts 22, 23 are tensioned by a leftward pull applied to the carriages 60, 79, the idler pulleys being thereby urged away from the driving pulleys. The leftward pull is applied by weights 70 attached to cables 71 running over pulleys 72, each cable being attached to a link 73 pivoted to the carriage. The pull may be relieved by suspending the weights from a cross-beam 86. When the weights are to be suspended the tension may be maintained in the belts by locking the carriages to the machine frame by engaging locking feet 190 with channels 191 in the carriages. The machine normally rests on supporting blocks 182, but may be lowered by turning a screw-threaded shaft 180, the weight of the machine being thus transferred to castors 175 running in ways 177, whereupon the machine may be rolled away laterally to afford access to the extrusion head 21. If the belts wander laterally they are re-aligned by means including photo-electric scanners 107, 108, 144, 115, Fig. 5. Thus, if belt 22 shifts to the left a narrow strip of the highly polished upper surface of belt 23 will be exposed at 105 and another similar strip of belt 22 will be exposed at 106. These exposures are detected by scanners 107, 108, and the impulses thus generated are caused to operate solenoid valves 110, 111 to admit hydraulic fluid to cylinders 102, whereby piston rods 101<SP>1</SP> move shafts 98 about pivots 100. These shafts carry rolls 92, 94, and the pivoting'movements bring the rolls into contact with belts 22, 23 to realign them. If belt 22 shifts to the right, scanners 114, 115 operate valves 116, 117 and rolls 91, 93 to obtain the same result. If both belts shift to the left or the right, microswitches 118, 120 operate the valves 110, 116 or 111, 117 respectively to force the belts back. The belts are guided also by spring-pressed grooved rollers which engage their edges, Fig. 6 (not shown). The inner unpolished surfaces of the belts are cleaned by scrapers 164, Fig. 10, and brushes 166 co-operating with suction nozzles 167. Apparatus is described for removing the belts from the machine, Figs. 11 and 12 (not shown).