1. Field of the Invention
This invention is related to apparatus utilized in performing high frequency welding operations on workpieces.
2. Description of the Prior Art
The term "high frequency operation" where used herein is to be understood as referring to an operation in which a workpiece is heated by the application of a high frequency electric field. Examples of such operations are:
1. A "welding" operation: where a workpiece comprises an assembly formed by two or more layers, the assembly is welded using a suitably shaped tool under conditions of heat and pressure. In carrying out such a welding operation, if the layers of the assembly are of thermoplastics material, they are inherently weldable but, if the layers are not of thermoplastics material, the assembly may also comprise a layer of heat-activatable adhesive sandwiched between the other layers, which adhesive acts to "weld" the other layers together. If, for example, an overall weld is required, the tool may be a flat plate. If, on the other hand, welding is only required in specific areas, then the tool may be in the form of a die having blade portions defining said area.
2. An "embossing" operation: the surface of a workpiece, whether the workpiece comprises one or more layers, is embossed, that, is, provided with a decorative pattern, using a pattern-carrying tool under conditions of heat and pressure. In such an operation, the surface of the workpiece is rendered plastic by the effect of the heat thereon so that the pattern of the tool can be impressed thereon.
3. An operation in which a workpiece comprising an assembly formed of two or more layers is both welded and embossed by the use of a suitable tool under conditions of heat and pressure.
An apparatus which comprises means for generating a high frequency field and means for pressing a workpiece while a high frequency operation is being performed thereon is generally known as a high frequency welding machine. High frequency welding machines which are presently in use are usually comprised of a high frequency generator comprising a triode tube and a resonant cavity coupled to a press having platens which apply pressure to a workpiece in carrying out a high frequency operation and act as electrodes for applying a high frequency field formed from high frequency energy supplied to the press by the generator to the workpiece. In this kind of machine, the platens act like aerials and emit stray radiation which may cause interference with radio communication. To avoid any such interference, it is necessary that such a machine should operate at a frequency of 27.12+0.6% MHz., which frequency range is reserved for industrial machinery. This limitation on frequency makes it necessary to use a tuning capacitor to adjust the operating frequency of the machine for each different workpiece.
A machine of the above type suffers from two separate disadvantages. First, the need to tune the machine for different workpieces is time-consuming. Second, the triode tube of such a machine is rarely used under optimum working conditions, as recommended by the manufacturers. This is because the working conditions of the tube are altered from workpiece to workpiece. This shortens the life of the expensive tube.
U.S. Pat. No. 3,742,180, assigned to the assignee of the present invention, overcomes some of the disadvantages cited above wherein press platens are incorporated in the resonant cavity of the generator to operate upon the workpiece. The likelihood of interference is reduced since the radiation is contained within the resonant cavity and there is no need to operate at a specific frequency. The machine can operate at a natural frequency, which natural frequency is dependent on the nature of the workpiece. However, this machine has the disadvantage that, when power is applied to its triode tube, the initial current through the tube may exceed that which the tube can sustain without damage. This may occur before a high frequency field is built up in the resonant cavity so that virtually the entire current flow is through the tube.
It is one of the various objects of the present invention to provide an apparatus for performing high frequency operations on workpieces comprising means for insuring that, despite differing workpieces, a triode tube of the apparatus operates substantially under optimum conditions during operation of the apparatus.
It is another object of the present invention to provide means for protecting a triode tube of the apparatus from suffering an excessive initial current flow therethrough.