The dynamic electric field causes the molecules in polar thermoplastics to oscillate. Depending on their geometry and dipole moment, these molecules may translate some of this oscillatory motion into thermal energy and cause heating of the material. A measure of this interaction is the loss factor, which is temperature and frequency dependent.
Polyvinylchloride (PVC) and polyurethanes are the most common thermoplastics to be welded by the RF process. It is possible to RF weld other polymers including nylon, PET, PET-G, A-PET, EVA and some ABS resins, but special conditions are required, for example nylon and PET are weldable if preheated welding bars are used in addition to the RF power.
High frequency welding is generally not suitable for PTFE, polycarbonate, polystyrene, polyethylene or polypropylene. However, due to the impending restrictions in the use of PVC, a special grade of polyolefin has been developed which does have the capability to be HF welded.
The primary function of high frequency welding is to form a joint in two or more thicknesses of sheet material. A number of optional features exist. The welding tool can be engraved or profiled to give the entire welded area a decorative appearance or it can incorporate an embossing technique to place lettering, logos or decorative effects on the welded items. By incorporating a cutting edge adjacent to the welding surface, the process can simultaneously weld and cut a material. The cutting edge compresses the hot plastic sufficiently to allow the excess scrap material to be torn off, hence this process is often referred to as tear-seal welding.
A typical plastic welder consists of a high frequency generator (which creates the radio frequency current), a pneumatic press, an electrode that transfers the radio frequency current to the material that is being welded and a welding bench that holds the material in place. The machine could also have a grounding bar that is often mounted behind the electrode, which leads the current back to the machine (grounding point). There are different types of plastic welders, the most common being tarpaulin machines, packaging machines and automated machines.
By regulating the machine’s tuning, the field strength can be adjusted to the material being welded. When welding, the machine is surrounded by a radio frequency field that, if too strong, can heat up the body somewhat. This is what the operator needs to be protected from. The strength of the radio frequency field also depends on the type of machine being used. Generally, machines with visible open electrodes (unshielded) have stronger fields than machines with enclosed electrodes.
When describing radio frequency electromagnetic fields, the field’s frequency is often mentioned. The permitted frequencies for plastic welders are 13.56, 27.12, or 40.68 megahertz (MHz). The most popular industrial frequency for HF welding is 27.12MHz.
The radio frequency fields from a plastic welder spread out around the machine, but most often it is only right next to the machine that the field is so strong that precautions need to be taken. The field’s strength decreases sharply with distance from the source.
The strength of the field is given in two different measurements: the electric field strength is measured in volts per metre (V/m), and the magnetic field strength is measured in amperes per metre (A/m). Both of these must be measured to get an idea of how strong the radio frequency field is. The current that goes through you if you touch the equipment (contact current) and the current that that goes through the body when welding (induced current) must also be measured.
Maximum permitted levels according to EU regulations:
Average over 6 minutes
Electric field strength: 61 V/m
Magnetic field strength: 0.16 A/m
Induced current: 100 mA
Contact current (not averaged): 40 mA
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