Patent GB-1528452-A (GB)
Title: Apparatus for cutting and/or welding thermoplastics sheet
IPC: B29C65/24
Filed: 1975-09-26
Abstract: 1528452 Seaming non-metallic sheet material; making bags; cutting by laser UNITED KINGDOM ATOMIC ENERGY AUTHORITY 26 Sept 1975 [3 Oct 1974] 39644/75 Divided out of 1528451 Headings B5K B5D and B4B Apparatus for cutting and/or welding a workpiece 10 made of thermoplastic sheet material by a laser 13, comprises a backing member 6 made of a material capable of absorbing laser energy of the particular wavelength of a laser to be used, means for maintaining the workpiece in intimate thermal contact with the backing member, means for bring- ing radiation from the laser to a focus, and means for scanning the focused laser radiation over the workpiece.  Apparatus for making plastics bags comprises a driven drum 1, Fig. 1, having dividing plates 4 forming pockets 5 spaced around the periphery of a hub 2. Alternate plates 4 are radially longer or shorter, the longer plates 4 having indentations in the outer edges thereof and the shorter plates 4 being capped with a backing material 6. Flattened tubular thermoplastic material 10, e.g. high-density polythene, is drawn from a supply by driven rollers 8a and is fed into a guide duct 9 by driven rollers 8b, the rollers 8a, 8b being driven in synchronism with the rotation of the drum 1. The drum hub 2 is perforated and suction is applied thereto by a fan 11. A laser 13 is arranged to be tracked across the end of a plate 4 therebeneath and has a power cross-section to weld and cut simultaneously. A take-off device 15 for completed bags comprises curved steel spikes 16 and a rotating paddle 19. The duct 9 includes a device for positively directing the thermoplastic material 10 into successive pockets 5 and including a rotor 33 having twin lobes 34. Air is supplied from a plenum chamber 30 and the lobes 34 direct air first into one pocket 5 and then into a succeeding pocket 5. In operation, as each pocket 5 is presented to the duct 9, the thermoplastic material 10 is fed to form a loop 20 therein and is tensioned by the suction applied to the hub 2. The size of the loop 20 is determined by the relation of the thermoplastic material speed to the drum angular velocity. The laser 13 is operated each time a shorter plate 4 lies therebeneath and may be moved physically but preferably remains stationary whilst the radiation beam is moved by optical means. Due to the motion of the drum 1, the beam must have a velocity component along the length of the thermoplastic material 10 equal to the linear velocity of the relevant plate 4. To produce two bags simultaneously, Fig. 5 (not shown), cross-cuts (22) and weld seals (23) are produced on the shorter plates 4 leaving a sealed length (24) folded across each longer plate 4. The spikes 16 are forced into the folded region of each sealed length and into the indentations of the longer plate 4 and the sealed lengths are moved by the device 15 with the welded ends all together. When sufficient sealed lengths have been collected, they are removed, tack-welded and provided with perforations near the folds to enadle separate bags to be torn off. To produce single bags, Fig. 6, alternate pockets 5 are wide and narrow so that long loops 20 are formed in the wide pockets only. A perforated plate 61 is preferably provided near the outer end of each narrow pocket 5. If the thermoplastic material is polythene, a carbon dioxide laser having a wavelength of 10À6 microns and a power density of more than 40 wats per sq. mm. is used. The caps 6 are made of fused silica or a glass-ceramic material. A small diameter fused silica tube may be used and may be pre-heated by a resistance wire passing therethrough.  A servo-control circuit, Fig. 7 (not shown), is described. In another embodiment, Fig. 8, a flattened polyethylene tube 101 passes over a fused silica rod 103 capping a plate 102 having passages 104 therein to which suction is applied by a fan 106, the suction longitudinally tensioning the tube 101. A radiation beam 109 from a carbon dioxide laser 110 is focused by an optical system 111 to a line focus having an aspect ratio of approximately 10 : 1. The focused beam 109 is arranged to have a Gaussian energy profile so as to cut the tube 101 along the axis of the focused beam but, at the edges of the beam, to have only sufficient energy to weld the out edges.