Patent GB-555645-A (GB)
Title: Method and apparatus for compressing and electrically heating thermo-plastic materials
IPC: H05B6/78
Filed: 1942-02-25
Abstract: 555,645. Supply systems employing high-frequency currents. BRITISH INSULATED CABLES, Ltd., WALTON, G. H., QUAYLE, J. C., and JONES, P. Feb. 25, 1942, No. 2488. [Class 38 (iv)] [Also in Groups XI, V, XXIII and XXXVII] A hand tool for heating and joining thermoplastic materials comprises a pair of relatively movable electrodes 1, 2, Figs. 1 and 3, in insulating housings 3, 4 carried by the jaws 5, 6 of the tool. The electrodes are of square cross section and are connected to a switch 17 between the handles which controls the supply of high frequency energy to the electrodes. The strips of thermoplastic material 18, 19 to be joined are located by spring pressed stops 20 which are pressed into their recesses as the jaws approach one another when the material softens. Additional similar stops may be formed on the ends of the housings 3, 4, to control the width of the joint. A spherical or cylindrical mould in two parts may be fitted to the apparatus instead of the stripjointing mould, and small side channels may be provided to receive surplus material forced out of the mould, Fig. 6 (not shown). The insulating housings and stops may be of steatite, ebonite, or a borate glass containing mica such as " Mycalex " (Registered Trade Mark). A similar apparatus, Fig. 7 (not shown) is operated by a pedal instead of by hand pressure. The switch 17 is preferably inserted in the supply line to an oscillator generator or other frequency converter supplying high frequency energy to the electrodes and may comprise an oil filled tube 51, Fig. 8, the cap of which carries a spring loaded plunger 53 operated by the movement of the handles of the tool to close contacts 54, 55, the latter of which is carried by a spring loaded plunger 56 mounted in insulating guides 61. As the material becomes plastic under heat, plunger 56 descends and plate 58 is depressed against spring 63, contacts 59, 60 remaining closed until compression of the material is almost complete, when the spring contacts 60 have no further travel and further movement of the olunger 56 breaks the circuit and cuts off the high frequency field. On releasing the pressure, the plate 58 moves unward slowly, due to oil resistance, and contacts 54, 55 are opened before contacts 59, 60 are closed again. In a modification. Fig. 9 (not shown), the arrangement is such that the time at which the contacts 59, 60 open is independent of the rate of approach of the electrodes, since movement is transmitted between plunger 56 and plate 58 by a spring. In an alternative control, Fig. 10, a simple switch 17 actuated by movement of the mould parts is in series with a thermal delay switch 65 which closes the supply circuit 66, and then opens it after a period determined by the heater 70. The apparatus is suitable for materials such as polyvinyl chloride, the strength of field being adjusted to the nature and thickness of the material. The strength may be varied during heating by controlling the supply frequency voltage to the frequency changing apparatus by a rotary potentiometer, Fig. 11 (not shown) actuated by a bimetallic helix which is heated when the switch 17, Fig. 10, is closed. The field strength may be gradually increased or decreased, or first increased and then decreased as in Fig. 12. The output of the frequency converter is controlled bv a group of impedances 75a, 75d each controlled by a thermal delay switch 76a, 76d which successively short circuit the corresponding impedances 75a, 75b, and open the impedances 75c, 75d. Alternatively the helix 74. Fig. 11, could be replaced by a motor. Instead of controlling the supply of energy to the oscillator, the reactance of the tuning circuit may be varied.