Patent GB-1042843-A (GB)
Title: Temperature control means for electrically heated impulse sealing elements
IPC: B29C65/38
Filed: 1965-05-27
Abstract: 1,042,843. Seaming non-metallic sheet material. WELDOTRON CORPORATION. May 27, 1965 [June 10, 1964], No. 22632/65. Heading B5K. A device 32 to prevent the temperature of an electrically heated, heat-expansible element 30 for heat-sealing and cutting superimposed layers of thermoplastic film from rising excessively, comprises contact means 75, 92 operable under the control of the element 30 in response to a predetermined expansion thereof, when heated, to interrupt an electric circuit in which the element 30 is included. The element 30 is positioned on a sealing bar 22 having secured thereto a support 40 containing a cylinder 50 with a piston 52 reciprocally mounted therein and biased by a spring 53. Oil from a reservoir 54 can flow through a control valve assembly 56 to the cylinder 50. A one-way ball valve 69 permits unrestricted flow of the oil from the reservoir 54 through a main passage 64 but not return flow thereof. Return flow, if any, takes place through a small by-pass 66 adjustably restricted by a needle valve 68. A bracket 70 is adjustably attached to the connecting rod 72 of the piston 52 and an electrical contact 74 having a contact face 75 is screw-threaded on to an arm 71 of the bracket 70. The movement of the arm 71 towards the bar 22 is limited by a shaft 80 embedded in an insulator 82 at the extremity of the bar 22. The element 30 is supported by an insulator 84 at the opposite extremity of the bar 22 and passes over the insulator 82 to be secured to an electrically conductive expansion block 88 having a contact face 92 and slidably positioned on the shaft 80. A spring 94 urges the block 88 away from the bar 22 to tension the element 30. In operation, the element 30 is moved into contact with the thermoplastic layers to press them against a backing member and upon operation of a switch 118, Fig. 1 (not shown), a current impulse is passed through the element to cut and seal the layers. As the element 30 is heated it expands rapidly and the spring 94 moves the block 88 along the shaft 80 so that the faces 72, 92 come into contact and cutoff the current supply to the element 30. The interconnected piston 52, the rod 72 and the bracket 70 under the force of the spring 53 do not immediately follow the movement of the block 88 because of the valve 69 and the restriction of the valve 68 preventing rapid return of the oil from the cylinder 50 to the reservoir 54. As soon as the element 30 begins to cool, the block 88 moves away from the contact 74 and the element cannot be supplied with a further current impulse until after the switch 118 has been momentarily opened. Since the rate of contraction of the element 30 is slower than the rate at which this element was heated by the current, the spring 53 still moves the piston 52 and the bracket 70 very slowly away from the bar 22 but as the element 30 contracts the block 88 abuts an arm 98 of the bracket 70 to reverse the movement of the piston 52. As the element 30 develops a permanent elongation in use the block 88 will stop further away from the insulator 82, with the arm 98 in abutment therewith, in a new position. An electrical circuit for the operation is described, Fig. 3 (not shown). In a modification (not shown), the cylinder 50 and its associated parts are omitted and the bracket 70 is mounted on the bar 22. Sealing apparatus, Fig. 1 (not shown), incorporating the means 32, comprises sealing and cutting elements 28, 30 mounted on sealing bars 20, 22, respectively, actuated by a handle 26 to contact backing members 33, 31, respectively. Articles wrapped in thermoplastic film to be sealed are placed on a platform 18 and the sealing assembly moved downwardly about arms 16 to actuate a switch 118 to supply current to the elements 28, 30.