Le 6 décembre 2016, 02:23 dans Humeurs • 0
High Frequency Welding, also known as radio frequency (RF) or dielectric welding, is the process of fusing materials together by applying radio frequency energy to the area to be joined. The resulting weld can be as strong as the original materials.
HF welding relies on certain properties of the material being welded to cause the generation of heat in a rapidly alternating electric field. This means that only certain materials can be welded using this technique. The process involves subjecting the parts to be joined to a high frequency (most often 27.12MHz) electromagnetic field, which is normally applied between two metal bars. These bars also act as pressure applicators during heating and cooling. 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. Also, it is possible to RF weld other polymers, such as 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. There are different types of plastic welders, while the most common being tarpaulin machines, packaging machines, and automated machines.
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