Radio frequency welding

Radio frequency welding, also known as dielectric welding and high frequency welding, is a plastics joining process that utilizes high-frequency radio waves to heat plastic parts to the point they form a melt layer. After the development of the melt layer, the parts are pressed together and then allowed to cool causing fusion. This process is capable of producing high quality joints in a range of plastics. Advantages of this process are fast cycle times, easily automated, repeatable, and good weld appearance. While this process has some great advantages, there are some limitations. 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.

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.^[1] Permanent molecular dipoles can form due to differences in electron densities between the atoms of a molecule. This can leave regions of negative charge where electron densities are high and positive charge where electron densities are low. A simple example of this is a water molecule where the high electronegativity of Oxygen creates a negative pole by the Oxygen atom and a positive pole by the Hydrogen atoms. Using this same principal when analyzing a (C₂H₃Cl)_n PVC molecule, a higher electron density would exist near the Cl atom in each unit of its polymer chain leading to a non-uniform distribution of charges and therefore a dipole would exist in PVC molecules. This makes PVC compatible for RF Welding. Other plastics commonly RF Welded include PET, nylons, thermoplastic polyurethanes, cellulose acetate, EVA, and PVDC.^[2] It is possible to weld non-polar plastics by using a conductive-composite implant.

Heating mechanism

The heating mechanism in Radio Frequency plastic welding is dielectric heating. When an electric field is applied to a dipole molecule, the polarity of the molecule will cause it to align itself with the electrical field. When an alternating electrical field is applied, the molecule will continuously try to align itself with the alternating electrical field 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 electrical field resulting in random motion as the molecule attempts to follow the electrical field. This motion causes intermolecular friction which leads to heat generation.^[2] 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.^[2] 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.^[3]

Procedure and process

The RF Welding procedure can be broken down into three main parts. Before the start of the procedure, proper part alignment is necessary. The first stage of the procedure is the pre-welding time which is used to reach the optimal pressure for welding. Arcing or material flashing and burning is possible if power is applied before the desired pressure is reached. This is similar to the first step in resistance spot welding of metals where a clamping time is used to ensure the desired force is reached before current is applied to avoid arcing. The second part of the procedure is when RF energy is applied to induce the dielectric heating mechanism in order to heat the joint to the melting temperature leading to fusion. At a certain point during heating, an equilibrium state can be reached when the heat generated from the applied power is equal to the heat loss through the dies used for clamping. Thinner parts will have more heat loss through the dies than thicker parts and different materials are more responsive to the dielectric heating mechanism than others. Therefore, the power used for welding must be tailored to the material that is being welded and the thickness of the joint in order to reach a heating equilibrium at the desired temperature. The final part of the procedure is the cooling stage which is the time allowed for the part to cool before removing the pressure applied from the dies. Some of the key process parameters are the clamping force, RF power, heating time, cooling time, and electrode or platen temperature (electrode and lower platen can heat up after performing many welds in a row which can impact the other parameters).

There are six major parts of RF Welding machines. They are the RF power generator, control unit, press, enclosure, handling mechanism, and electrode.^[2] The RF power generator is what supplies the power to be used for welding. The power needed for welding is based on the area of the weld, thickness, and the material. The control unit is the system used for operating the machine. The control unit is responsible for processing the information on the desired welding inputs such as force, power, and heating time, and instructing the other components of the machine to satisfy these process parameters. The press is what supplies the force to the joint that is to be welded. These are most commonly pneumatic but can also be hydraulic for larger scale applications. An RF enclosure or a cage that goes around the electrodes and open areas is used to protect the operator from injury and from the high voltages emitted from the electrodes. The handling mechanism is used to transport the plastic parts to and from the RF Welding machine. It also is responsible for properly placing and aligning the parts under the electrodes. The electrode is responsible for pressing the parts together using the force supplied by the press and transferring the RF energy into the joint. They are usually made of either Brass, Copper, and Bronze. An additional optional component of an RF Welding system is a barrier material that can be used and placed between the parts to be welded and the lower platen in order to reduce the risk of arcing and reduce the heat loss from the joint.^[2]

Applications

The most common application for RF Welding is sealing thin sheets of plastic such as PVC. Some products that typically use RF Welding include beach balls, airbeds, life jackets, book covers, and loose-leaf binders. RF Welding is also commonly used for medical items such as blood bags, disposable clothing, blood pressure cuffs, and packaging for certain items.^[2] It can be seen through the common applications of RF Welding that it is designed primarily for the welding or sealing of thin sheets so they are leak free.

References

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