An Introduction to High-Frequency Welding

This article will serve as an introduction to High-Frequency Welding. HF Welding, also known as radiofrequency (RF) or dielectric welding, is the process of fusing materials together by applying a rapidly alternating electric field, applied between two bars, in the joint area. HF welding only works with thermoplastics that have a high dielectric constant, a factor determined by the molecule’s geometry and dipole moment.

When exposed to alternating current, the polymer’s dipole molecules attempt to flip around to realign themselves to the new field orientation. This high-speed, relative movement causes intermolecular friction, which creates heat. When the polymer reaches its glass transition temperature (melts), the pressure is applied at the joint by the bars until the material cools. During this process, the molecules from both materials intermingle and weld forms. The resulting weld can be as strong as the original materials.

Molecules of materials with a low dielectric constant do not flip quickly or at all. Consequently, they will not generate the required heat to form a weld.

HF Welding Machines:

An HF welder consists of the following:

• a high-frequency generator,
• a pneumatic press,
• an electrode that transfers the radiofrequency current to the material that is being welded, and
• a welding bench that holds the material in place.

Operators adjust the field strength based on the material being welded and the type of machine being used. The permitted frequencies for thermoplastic welders are 13.56, 27.12, and 40.68 megahertz (MHz). The most popular industrial frequency is 27.12MHz.

Operator Safety

HF fields do radiate from the welding machine. Generally, machines with visible open electrodes (unshielded) have stronger fields than machines with enclosed electrodes. The field’s strength decreases sharply with distance from the source. However, if the field is too strong, it will heat up a human body.  Operators right next to or touching the machine, where the field is strongest, need to be protected. Governments usually regulate the maximum exposure for operators of HF equipment.

Advantages

• Decorative appearances or embossing on the welded items are possible by engraving or profiling the plates.
• By incorporating a cutting edge adjacent to the welding surface, the process can simultaneously weld and cut material. This is often called “tear-seal welding” because the cutting edge compresses the hot plastic sufficiently to allow the excess scrap material to be torn off.
• HF welding focuses the heat at the welding target so that the surrounding material does not have to be super-heated and risk burning in order to achieve target temperature at the joint.
• With HF, heat is generated only when the field is energized. This allows for greater control over the amount of energy that the material sees over the entire cycle. When the material can remain under pressure without introducing more heat, it is possible to both instantly heat, weld, and cool the material under compression thus increasing the weld strength.
• HF-generated heat does not radiate off the plates like other heated surfaces used in welding. This prevents heat-degradation of the material abutting the weld.

For more on the production advantages of welding read here.

Training

To operate high-powered HF welders safely, operators must receive thorough training from experienced in-house personnel or manufacturers’ representatives, who will often conduct free training at your site. An investment in more in-depth knowledge pays off quickly through increased production quality, reduced shutdown time, reduced repair costs, and increased employee safety.

Applications

Typical HF weld cycle times range from two to five seconds and can handle material from approximately 1/1000” to 50/1000” thick (25.4 μm to 1270 μm). With many other types of welding, if the materials are too thick, they will not bond in the middle because the core will not melt, while the outside layers can become overheated or burned. HF welding, on the other hand, heats from the inside out and is the best method for forming airtight seams.

In the technical textile industry, the technique is used on materials coated with PVC, EVA alloys, TPU, and polyurethanes (PU). 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 HF weldable if the welding bars are preheated.

HF welding is generally not suitable for PP, PE, 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.

HF welding is commonly used to manufacture medical supplies, many of which use TPU-coated nylon or less commonly polyester. Applications include protective clothing, gel- and foam-filled cushions, lumbar support, hydration reservoirs, and medical air and water mattresses, as well as covers for stretchers and beds.

Industrial applications include flexible storage tanks, inflatable life jackets (PFDs), flexible ducting, bellows, curtains, conveyor belts, water weights, lifting bags, and conveyor belts. HF welding is well suited to the TPU-, PVC-, EVA-, and PU-coated materials used in inflatable boats, life jackets, life rafts, and other buoyancy equipment. HF welding is also used in the construction of tents and temporary inflatable structures

For more information on welding techniques that are right for your application, download the Erez Ultimate Guide to Welding Coated Technical Textiles.