High-frequency welding (High Frequency / Radio Frequency), or dielectric welding, refers to the process of joining materials by using electric field energy focused on the area that is being joined.
Due to the high-frequency welding technology, it is possible for the resulting weld to be as strong as the original material. The high-frequency welding process relies on the specific properties of the material to produce heat in an alternating electromagnetic field. This means only certain materials may be welded with this technique.
High-frequency welding consists of subjecting the parts to be joined by applying a high-frequency electromagnetic field (usually 27,12 MHz), which is typically deployed between two metallic electrical conductors. These conductors – electrodes – apply pressure as they heat and cool. The dynamic electric field induces oscillations of the molecules of polar thermoplastics. And depending on their geometry and dipole moment, these molecules may convert some of this oscillatory motion into thermal energy and result in the heating of the material. A measure of this interaction can be found in the dielectric loss factor, which depends on temperature and frequency.
Polyvinyl chloride (PVC) and polyurethanes represent the most common thermoplastics which are welded using a high-frequency process. It is possible to weld other polymers, as well, including nylon, PET, PET-G, A-PET, EVA and some ABS resins, however, this would require special conditions. For example, nylon and PET can be welded if additionally preheated electrodes are used.
High frequency welding may not be suitable for PTFE, polycarbonate, polystyrene, polyethylene or polypropylene. However, given the impending restrictions on the use of PVC, a special grade of polyolefin has been developed that is capable of being welded using this method.
The primary function of HF welding involves creating a permanent bond between two or more layers of material. There are a number of additional options during welding.
The electrode can also be engraved or profiled in order to give the welded surface a decorative appearance. The weld can be produced using an embossing technique in order to place lettering, logos or decorative effects on the components. With a cutting edge adjacent to the weld surface, this process can combine and cut material at the same time. The cutting edge compresses the hot plastic sufficiently to enable the excess waste material to be torn off, so the process is often referred to as break welding.
A typical plastic welder comprises a high-frequency generator (generating radiofrequency current), a pneumatic press, a conductive electrode and a work table that holds the material in place. The machine may also incorporate a grounding foot, often mounted behind the electrode. The foot guides the current back to the machine (grounding point).
There are various types of plastic welding machines, with the most common being tarpaulin machines, packaging machines and automatic. Adjusting the machine parameters, it is possible to adapt the field strength to the material to be welded. During welding, the machine in question is surrounded by a radio-frequency field, against which the machine operator must be protected. Radio-frequency field emissions will also depend on the type of machine. Machines that have visible open electrodes (unshielded) emit stronger fields compared to machines with closed electrodes.
Permitted frequencies available for plastic welders are 13.56, 27.12 or 40, 68 MHz (MHz). The most common industrial frequency for HF is 27,12 MHz.
The electromagnetic field produced by plastic welding machines propagates around the machine, however, it is usually only right next to the machine that the field is so strong that safety measures must be taken. The strength of the field decreases drastically depending on the distance from the source. Field strength is reported in two different measurements: electric field strength is measured in volts per metre (V / m) and magnetic field strength in amperes per metre (A / m). It is necessary to measure both to find out how strong the electromagnetic field is. The current passing through a person when touching the machine (contact current) and the current flowing through the body during welding (induced current) should also be measured.
Maximum permissible levels as per EU legislation:
On average during 6 minutes
Electric field strength: 61 V / m
Magnetic field strength: 0,16 A / m
Induced current: 100 mA
Contact current (non-averaged): 40 mA
Plastic welding represents a process that requires precise control. It consists of fusing two or more layers of plastic together using the energy of an electric field generated by a high-frequency alternating current of 27.12 Mhz. After the completion of the welding process, the welded plastics undergo a quality check to ensure that the joint is strong and durable.
Plastic welding has a wide range of applications in different industries. It is often used in the production of packaging, in the automotive industry, in the creation of items such as car mats, sun visors or side tarpaulins, and in the conveyor belt production process. Plastic welding is often also used in the medical industry, for instance to manufacture rehabilitation accessories or pouches for general medical use. Depending on requirements, the welding process may be adapted to various types of plastic and product shapes.
Plastic sealing, although effective, is not a process without its challenges. One of the biggest problems is to control and stabilise the process parameters. If the parameters are not set correctly, overheating and damage to the plastic and electrode can occur, or the resulting seal would not be strong enough. Another challenge is to ensure consistent seal quality, and this requires precise process control and regular quality checks.
Furthermore, not all plastics are suitable for sealing. Some, such as polyethylene, polypropylene and polycarbonate, cannot be sealed with high frequency. Despite these challenges, high-frequency plastic sealing is still one of the most effective processes to join materials in many industries.
HF sealing occurs from the inside out by using the material itself as a heat source. The heat is focused at the weld target so that the surrounding material does not have to be super-heated to arrive at a target temperature at the joint.
With HF heat is generated only when the field is energized. Once the generator cycles, the heat is turned off. This allows for greater control over the amount of energy that the material sees over the entire cycle. In addition, HF-generated heat does not radiate off the die like on a heated die. This prevents heat-degredation of the material abutting the weld.
HF tooling is usually run "cold". This means that once the HF is turned off, the material stops being heated, but remains under pressure. In this fashion it is possible to both instantly heat, weld, and cool the material under compression. More control over the weld results in more control over the resulting extrusion, thus increasing the weld strength.
RF welds are "clean" because the only material needed to produce an HF weld is the material itself. There are no adhesives or by-products involved in HF welding.
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