High-frequency welding

Frequently asked questions about high-frequency (hf) welding

Joining plastic parts or components using welding technologies is only possible with thermoplastics. These are mainly flexible plastics that can be joined using a number of methods, including high-frequency or radio-frequency welding, and ultrasonic welding.

How does high-frequency welding work?

This process uses an electromagnetic field to assemble different layers or parts of thermoplastics. The principle is as follows: an HF voltage heats the various parts subjected to it. The heat emitted causes the different materials to melt under pressure and bond together. After the modified surface has been cooled and held under pressure, the resulting seam is at least as strong as the adjacent surfaces.

The application is carried out using high-frequency welding machines structured according to the following principle:

a high-frequency electric field generator, typically 27.12 MHz,
a pneumatic press for holding the parts to be assembled under pressure,
electrodes that create a high-frequency electric field, causing the molecules to vibrate and heating the materials,
a welding bench that holds the parts to be welded in place.

What materials are suitable for high-frequency welding?

While it is technically possible, under certain conditions, to weld metal at high frequencies, the HF welding process is generally used to join plastics. More precisely, it is only used for thermoplastics, the only category of plastics that are deformable when heated. However, even some thermoplastics cannot be welded using this method, such as polyethylene (PE) and polypropylene (PP).

The range of polyvinyl chloride (PVC) derivatives, polyurethane (PU) and polyamides (PA) are the plastics for which high-frequency welding is most widely used. These thermoplastics are used in the manufacture of many everyday objects:

magazine racks and notebook protectors,
blisters and plastic packaging,
tarpaulins and canvas of various sizes
etc…

What's the difference between ultrasonic and high-frequency welding?

Both technologies are used for joining thermoplastics and operate on the same principle: heating the material from the center outwards by vibrating the molecules. This results in rapid fusion of the contact areas of the elements to be welded, with a clean, crisp finish, even before the material has been sanded. The main difference lies in the way the material is heated. Ultrasonic welding is based on the use of an electric current (at a frequency of 20 to 70 kHz) which causes a metal part called a sonotrode to oscillate. These vibrations are transmitted by direct contact with the surface of the plastic. This technology can only be used for thermoplastics with relatively low melting temperatures. Its use for welding high-temperature plastics would require a great deal of rapid oscillation, which could damage the press. In contrast, the high-frequency welding machine is capable of creating a high-frequency electromagnetic field (of the order of 27.12 MHz). For this purpose, it is equipped with a generator in the form of an electrode emitting a powerful electric current that generates a dielectric effect on the materials to be joined. This allows vibrations to be transmitted without direct contact with the material, thus limiting the impact on the welding machine. This process can be used for a wide range of thermoplastics, thanks to the equipment’s capacity to generate and transmit energy.

What are the advantages of high-frequency welding?

HF welding is widely used for plastic assemblies because of its undeniable advantages:

Precision : high-frequency welding is clean and precise, thanks to the possibility of heating only the contact areas of the objects to be joined.
Made-to-measure: any shape can be used to join 2 parts or 2 components. Some machine models can handle large-format welds (for advertising posters, truck tarpaulins, sails, etc.).
Joining different types of plastic: fast, powerful heating enables different types of plastic to be combined.
Speed and rhythm: the HF welding press increases production cycles thanks to its high output rates.
Joint strength: thanks to the heating mode, the bond is stronger on the inside of the material than “on the outside, with no change after cooling”.

What are the potential dangers of high-frequency welding?

The basic principle of HF welding is the use of electromagnetic waves with a frequency of 27 MegaHertz. This produces radiation in the vicinity of the welding machine, which can have long-term harmful consequences for people working nearby. In the event of long-term exposure to this type of radiation, dizziness and nausea can occur, along with an increase in body temperature. To mitigate these health risks, there are a number of ways of protecting the area around the device from radiation. Aluminum can act as a wave absorber, in the form of boxes or generator shielding. The heat dissipated by the radiation can also be partially absorbed by the aluminum. As with all work involving industrial equipment, incidents can occur if an HF welder is misused. During a conventional welding cycle, the danger of burns is eliminated, as the raw material being heated by the machine is not accessible to staff or visitors until it has cooled down.