Radio Frequency (RF) Welding

Radio frequency (RF) welding or sealing is also known as high frequency (HF) or dielectric welding or sealing. By any name, it allows the joining of dielectric polymer films and sheets using high-frequency electromagnetic energy.

Heating dielectric materials

A dielectric material is an electrical insulator whose molecules can be polarized when an electro-magnetic field is applied. These materials consist of permanently dipolar molecules with positive and negative charges on opposite sides of them. They include water as well as several thermoplastic polymers. In the absence of an electro-magnetic field, those molecules are randomly oriented.

But when subjected to an electromagnetic field, the polar molecules tend to orient themselves with it—as shown in the illustration of water molecules. Even though polymers are made up of long chain molecules, those that are dielectric have their atoms arranged such that two sides of the chain have opposite charges. So, they also tend to oscillate similarly to the way water molecules do in the presence of an electromagnetic field.

If the field alternates at a high enough frequency, the friction caused by the rapid, internal molecular movement creates heat throughout the material. It works for water in the microwave oven and for dielectric polymers in the RF welding process.

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RF welding process

The RF welding process employs two major components:

An RF Power Generator with its associated controls. This converts incoming electric current to the high-voltage high-frequency current needed for the welding process.
A set of upper and lower dies that act as electrodes and are connected to the RF Power Generator. This forms an electrical circuit with the upper die positive and the lower die negative or ground. The thermoplastic material being welded can act as the insulator needed to prevent damaging arcing between the dies.

The dies are mounted in a press that applies pressure to the weld area while the RF energy creates the needed electro-magnetic field in the space between the dies. That field heats the material between the dies to the point of melting the polymer components which are then allowed to cool under pressure to form the weld.

The upper die is in the shape of the desired weld—which may be linear in the case of a lap weld to join two sheets or may follow the outline of the closed item being produced. The lower die is often a flat platen to support the material but may also be contoured to match the upper die.

A typical process sequence would be:

Load material into the die opening.
Press stroke to close the dies and apply pressure to the parts.
Apply RF energy to the dies for the short interval needed to melt the polymers.
Hold the pressure until the polymers cool to form the weld.
Open the dies to remove the joined item.

Material placement and removal from between the dies can range from manual operation to partially or fully automated higher speed operations.

RF weld bonds

Welding rules attached to the upper die plate establish not only the shape of the weld but also the type of weld bond that will be produced. The illustrations show three common types of rules and welds in cross section.

Plain seam welds use a flat-ended rule to join two or more thicknesses of material. The finished weld, while thinner than the combined parent materials, still serves as insulation to prevent arcing between the dies.

Tear/seal welds use a knife-edge rule to produce a very thin seam weld that can be torn after welding. This allows parts to be separated from the surrounding material without an extra cutting process. But it also involves the risk of arcing between the dies because of the thinned-out material under the weld rule and the close approach of the rule to the lower die. Therefore, a non-polar barrier material or buffer needs to be placed on the lower die under the work material to prevent arcing damage to the dies.

Tear/seal welds produce a narrow weld width that is less robust than a plain seam weld. To produce a higher-strength weld and still have the tear/seal advantage, a combined die rule can be used. The same risk of arcing that exists in tear/seal welds applies to these combined welds as well. So, a buffer also needs to be provided between the material being welded and the lower die as is done with tear/seal welds.

Polymers suited to RF welding

The following materials are all good candidates for RF welding and sealing:

Ethyl vinyl acetate – EVA
Polyethylene terephthalate – PET
Polyethylene terephthalate glycol –PETG
Polyurethane – PU
Polyvinyl chloride – PVC
Thermoplastic elastomer films – TPE
Thermoplastic polyurethane films – TPU
TPU coated nylon or polyester.

Advantages of RF welding

A clean non-additive process with no solvents or adhesives in the welded area.
The liquid and gas-tight bonds are unsurpassed with seam strength greater than the base material.
Fast cycle times. The electromagnetic field heats the material in a matter of seconds.
Customized shapes can be produced, and bonds made with dedicated tooling.
Ideally suited to flat and thin components.
Capable of welding multiple layers simultaneously.
Good weld appearance with minimal flash.
The process operates at ambient temperatures with no external heat source needed.
Can bond thicker materials with less distortion than Thermal Impulse welding or a Constant Heat process.
Although most fabric materials themselves are invisible to electro-magnetic fields they can be RF welded if they are coated with suitable polymers.
Non-dielectric polymers can sometimes be RF welded by including a layer of dielectric polymer between them in the bond.

Limitations of RF welding

While RF welding provides many advantages, it does have a few limitations.
RF welding is practically limited to joining thin sheets and films because a strong enough electro-magnetic field can only be generated when the gap between the electrodes is small—on the order of 1/8” or less.
If the electrodes are further apart, the field density is not adequate to effectively heat and melt the polymer.
Bond thickness is limited to the maximum effective gap between the electrodes.
Only materials with polar molecules respond to the RF process.
Specific welding configurations often require dedicated tooling.

RF welding is not ultrasonic welding

Ultrasonic welding is another established process for joining thermoplastics. However, it should not be confused with RF welding even though both generate heat using relatively high frequencies.

The RF process generates heat within the materials to be joined by using an alternating electromagnetic field to create molecular motion. Thereby, the internal molecular friction produces heat. Whereas the Ultrasonic process generates heat using high-frequency sound waves to induce mechanical vibration of the adjacent parts. This generates heat at the joint interface by means of friction between the adjacent surfaces rubbing against one another.

Both processes can be applied to thermoplastic materials and both act by locally melting the parts to allow their molecules to intermingle and create a bond upon cooling. However, the two processes are different, have distinct applications, and require specific material characteristics and weld designs.