inductors for the induction welding of complicated welding patterns have been produced up to now mostly from copper plates which were provided with slits in the desired patterns by spark - cutting and embedded into plastic material . this method of manufacture is expensive and does not allow production of inductors with a very detailed pattern because , among other things , the slits must have a minimum width determined by the spark - cutting . by making the inductors in accordance with the present invention having a thin copper foil which is applied onto the surface of an insulating material , these difficulties are overcome and the desired inductor pattern can be produced in a simple manner by etching with the help of a suitable acid . this makes possible great accuracy of detail so that detailed inductors , e . g . for the packaging industry , can be manufactured at low costs .