Radio-frequency welding, also known as dielectric welding and high-frequency welding, is a plastic welding process that utilizes high-frequency electric fields to induce heating and melting of thermoplastic base materials. The electric field is applied by a pair of electrodes after the parts being joined are clamped together. The clamping force is maintained until the joint solidifies. Advantages of this process are fast cycle times (on the order of a few seconds), automation, repeatability, and good weld appearance. Only plastics which have dipoles can be heated using radio waves and therefore not all plastics are able to be welded using this process. Also, this process is not well suited for thick or overly complex joints. The most common use of this process is lap joints or seals on thin plastic sheets or parts.
Heating mechanism
Four types of polarization can occur in materials subjected to high-frequency alternating electric fields:
Electronic or electric polarization is the redistribution of electrons Ionic polarization is the redistribution of charged particles—cations and anions Maxwell–Wagner polarization is a charge buildup at the interfaces of non-homogeneous materials Dipole polarization is the realignment of permanent dipoles
Dipole polarization is the phenomenon that is responsible for the heating mechanism in Radio Frequency plastic welding, dielectric heating. When an electric field is applied to a molecule with an asymmetric distribution of charge, or dipole, the electric forces cause the molecule to align itself with the electrical field. When an alternating electrical field is applied, the molecule will continuously reverse its alignment, leading to molecular rotation. This process is not instantaneous, therefore if the frequency is high enough, the dipole will be unable to rotate quickly enough to stay aligned with the electric field resulting in random motion as the molecule attempts to follow the electrical field. This motion causes intermolecular friction which leads to heat generation. The amount of heat generated by friction in the material is dependent on field strength, frequency, dipole strength, and free volume in the material. Since the main driving force for dielectric heating is the interaction of the dipole of a molecule with the applied electrical field, RF welding can only be conducted on dipole molecules. The typical frequency range for dielectric heating is 10–100 MHz but normally RF Welding is conducted around 27 MHz. At too low of frequency, the dipoles are able to align themselves with the electrical field and stay in phase with the electrical current minimizing the intermolecular friction that is produced. This can also be described as having minimal power loss from the electrical field since the molecules will stay in phase and absorb minimal energy. As frequencies become high enough, power loss starts to increase as the dipoles are unable to align themselves at the rate of the reversing electrical field. The dipoles become out of phase absorbing energy and this is when heating occurs. At a certain frequency, a power loss maximum is reached to where higher frequencies will have decreased power loss and produce less heating. The maximum dielectric power loss is material dependent.
Compatible materials
The radio frequency heating mechanism relies on a dipole in the molecule in order to generate heat and therefore the plastics used in RF welding are limited to those whose molecules contain an electrical dipole. Permanent molecular dipoles can form due to differences in electronegativities between the atoms of a molecule. Negative charge is shifted toward atoms with higher electronegativity, resulting in more negatively charged regions surrounding more electronegative atoms, and positively charged regions surrounding less electronegative atoms. Because polyethylene consists of symmetric mer groups, no dipole forms, and polyethylene cannot be joined using radio-frequency welding. Like water, polyvinyl chloride (PVC) consists of asymmetrically distributed atoms of differing electronegativities, with a resulting dipole moment. Because of its strong dipolar moment (and other properties) PVC is considered an excellent material for radio-frequency welding. In addition to polarity, properties that contribute to good radio-frequency weldability are high dielectric constant, which reduces resistance to current flow; high dielectric strength, which prevents arcing through the joint members during welding; and high dielectric loss, which is a factor that describes the amount of heat generated by an electric field. Some plastics commonly welded with dielectric heating include:
Polyvinyl chloride (PVC) Chlorinated polyvinyl chloride (CPVC) Polyurethane Nylons Cellulose acetate Ethylene-vinyl acetate (EVA) Polyvinylidene chloride (PVDC) Polyethylene terephthalate (PET) Additional members can be added to a joint for a variety of reasons – improving thermal insulation, preventing sticking of parts to the welding equipment, preventing arcing, and buffering non-uniform clamping pressure or electric field. It is possible to weld non-polar plastics by using a conductive-composite implant to improve dielectric loss.
Procedure and process The RF welding procedure consists of five steps:
Loading parts Applying pressure Applying electric field Holding pressure Unloading parts Loading consists of placing the joint member into the welding machine. The welding operation begins with application of pressure on the members from the electrodes. Generally, the bottom electrode is fixed, and the actuator drives the upper electrode down with a prescribed force. The electric field is applied to the parts for a specified time while pressure from the electrodes is maintained. Dielectric heating causes the parts that are in intimate contact to melt, and the liquid polymers diffuse into each other at the interface. Diffusion and solidification of the joint occur while pressure is maintained for a specified time. Once the joint is cooled and the upper electrode is retracted, the part can be unloaded. The parameters used to control the welding process consist of:
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