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. [1] 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.
Four types of polarization can occur in materials subjected to high-frequency alternating electric fields: [2]
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. [1] 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. [3] The amount of heat generated by friction in the material is dependent on field strength, frequency, dipole strength, and free volume in the material. [1] 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. [3] 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. [4]
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. [5] 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. [1] 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. [1] [2]
Some plastics commonly welded with dielectric heating include: [1] [3] [6]
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. [2] It is possible to weld non-polar plastics by using a conductive-composite implant to improve dielectric loss. [1]
The RF welding procedure consists of five steps: [1]
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. [1]
The parameters used to control the welding process consist of: [1] [2]
The parameters listed are often interdependent, and a process window must be developed to tune the process for acceptable weld quality. [2]
Radio-frequency welding equipment, generally consists of: RF power generator, control unit, press, enclosure, electrodes, and sometimes a handling mechanism. [1] [2] [3] The RF power generator converts line power to high-frequency, high-voltage power for welding. Typical voltages are 1kVAC – 1.5kVAC at a frequency of 27.12 MHz. [1] The power needed for welding is based on the area of the weld, thickness, and the material. [2] 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. Some controllers are capable of monitoring outputs and adjusting parameters to ensure satisfactory welding. [1] The press (or actuator) supplies the clamping force pneumatically or hydraulically. [2] The electrodes are a pair of conductive structures that transmit the electric field through the members being joined. The electrodes contact the parts and apply the hold pressure prior to and during welding, and through solidification. Generally, the upper electrode projects from the upper fixture surface, while the lower electrode is a flat conductive surface. In some cases, the bottom electrode can project above the bottom fixture, to conform to geometry or to better localize melting through the reduction of stray electric field. Both electrodes can be fabricated with features to alter the finish of the welded surface. They are usually made of either brass, copper, or bronze. [1] An RF enclosure or a cage that goes around the electrodes and open areas is used to protect the operator from injury including radio frequency radiation. [1] Automated machines can be semi-automatic (requiring the operator to manipulate the parts) or fully automatic (where the machine is responsible for loading, transporting, and manipulating parts). [2]
The most common application for RF welding is sealing thin sheets of polar thermoplastics 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. [3] RF welding is most typically used in the construction of products that require a watertight or airtight seal. A welded seam or insertion tube sealing process creates seals that can withstand various types of requirements for certain liquid types or air pressures. An example of this would be for the medical industry, where ensuring the seal is airtight and fluid-tight is crucial. [7]
In electromagnetism, a dielectric is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor, because they have no loosely bound, or free, electrons that may drift through the material, but instead they shift, only slightly, from their average equilibrium positions, causing dielectric polarisation. Because of dielectric polarisation, positive charges are displaced in the direction of the field and negative charges shift in the direction opposite to the field. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarised, but also reorient so that their symmetry axes align to the field.
In physics, the term dielectric strength has the following meanings:
Welding is a fabrication process that joins materials, usually metals or thermoplastics, primarily by using high temperature to melt the parts together and allow them to cool, causing fusion. Common alternative methods include solvent welding using chemicals to melt materials being bonded without heat, and solid-state welding processes which bond without melting, such as pressure, cold welding, and diffusion bonding.
A thermoplastic, or thermosoftening plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.
Induction welding is a form of welding that uses electromagnetic induction to heat the workpiece. The welding apparatus contains an induction coil that is energised with a radio-frequency electric current. This generates a high-frequency electromagnetic field that acts on either an electrically conductive or a ferromagnetic workpiece. In an electrically conductive workpiece, the main heating effect is resistive heating, which is due to induced currents called eddy currents. In a ferromagnetic workpiece, the heating is caused mainly by hysteresis, as the electromagnetic field repeatedly distorts the magnetic domains of the ferromagnetic material. In practice, most materials undergo a combination of these two effects.
An electret is a dielectric material that has a quasi-permanent electrical polarisation. An electret has internal and external electric fields, and is the electrostatic equivalent of a permanent magnet.
Ultrasonic welding is an industrial process whereby high-frequency ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, and especially for joining dissimilar materials. In ultrasonic welding, there are no connective bolts, nails, soldering materials, or adhesives necessary to bind the materials together. When used to join metals, the temperature stays well below the melting point of the involved materials, preventing any unwanted properties which may arise from high temperature exposure of the metal.
Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat. Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.
In electronics, electrical breakdown or dielectric breakdown is a process that occurs when an electrically insulating material, subjected to a high enough voltage, suddenly becomes a conductor and current flows through it. All insulating materials undergo breakdown when the electric field caused by an applied voltage exceeds the material's dielectric strength. The voltage at which a given insulating object becomes conductive is called its breakdown voltage and, in addition to its dielectric strength, depends on its size and shape, and the location on the object at which the voltage is applied. Under sufficient voltage, electrical breakdown can occur within solids, liquids, or gases. However, the specific breakdown mechanisms are different for each kind of dielectric medium.
An electric arc is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An arc discharge is initiated either by thermionic emission or by field emission. After initiation, the arc relies on thermionic emission of electrons from the electrodes supporting the arc. An arc discharge is characterized by a lower voltage than a glow discharge. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".
Electric resistance welding (ERW) is a welding process in which metal parts in contact are permanently joined by heating them with an electric current, melting the metal at the joint. Electric resistance welding is widely used, for example, in manufacture of steel pipe and in assembly of bodies for automobiles. The electric current can be supplied to electrodes that also apply clamping pressure, or may be induced by an external magnetic field. The electric resistance welding process can be further classified by the geometry of the weld and the method of applying pressure to the joint: spot welding, seam welding, flash welding, projection welding, for example. Some factors influencing heat or welding temperatures are the proportions of the workpieces, the metal coating or the lack of coating, the electrode materials, electrode geometry, electrode pressing force, electric current and length of welding time. Small pools of molten metal are formed at the point of most electrical resistance as an electric current is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials.
Dielectric spectroscopy measures the dielectric properties of a medium as a function of frequency. It is based on the interaction of an external field with the electric dipole moment of the sample, often expressed by permittivity.
Dielectric heating, also known as electronic heating, radio frequency heating, and high-frequency heating, is the process in which a radio frequency (RF) alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric.
Corona treatment is a surface modification technique that uses a low temperature corona discharge plasma to impart changes in the properties of a surface. The corona plasma is generated by the application of high voltage to an electrode that has a sharp tip. The plasma forms at the tip. A linear array of electrodes is often used to create a curtain of corona plasma. Materials such as plastics, cloth, or paper may be passed through the corona plasma curtain in order to change the surface energy of the material. All materials have an inherent surface energy. Surface treatment systems are available for virtually any surface format including dimensional objects, sheets and roll goods that are handled in a web format. Corona treatment is a widely used surface treatment method in the plastic film, extrusion, and converting industries.
Electrosurgery is the application of a high-frequency alternating polarity, electrical current to biological tissue as a means to cut, coagulate, desiccate, or fulgurate tissue. Its benefits include the ability to make precise cuts with limited blood loss. Electrosurgical devices are frequently used during surgical operations helping to prevent blood loss in hospital operating rooms or in outpatient procedures.
Plasma activation is a method of surface modification employing plasma processing, which improves surface adhesion properties of many materials including metals, glass, ceramics, a broad range of polymers and textiles and even natural materials such as wood and seeds. Plasma functionalization also refers to the introduction of functional groups on the surface of exposed materials. It is widely used in industrial processes to prepare surfaces for bonding, gluing, coating and painting. Plasma processing achieves this effect through a combination of reduction of metal oxides, ultra-fine surface cleaning from organic contaminants, modification of the surface topography and deposition of functional chemical groups. Importantly, the plasma activation can be performed at atmospheric pressure using air or typical industrial gases including hydrogen, nitrogen and oxygen. Thus, the surface functionalization is achieved without expensive vacuum equipment or wet chemistry, which positively affects its costs, safety and environmental impact. Fast processing speeds further facilitate numerous industrial applications.
Hot plate welding, also called heated tool welding, is a thermal welding technique for joining thermoplastics. A heated tool is placed against or near the two surfaces to be joined in order to melt them. Then, the heat source is removed, and the surfaces are brought together under pressure. Hot plate welding has relatively long cycle times, ranging from 10 seconds to minutes, compared to vibration or ultrasonic welding. However, its simplicity and ability to produce strong joints in almost all thermoplastics make it widely used in mass production and for large structures, like large-diameter plastic pipes. Different inspection techniques are implemented in order to identify various discontinuities or cracks.
Laser welding of polymers is a set of methods used to join polymeric components through the use of a laser. It can be performed using CO2 lasers, Nd:YAG lasers, Diode lasers and Fiber lasers.
Advanced thermoplastic composites (ACM) have a high strength fibres held together by a thermoplastic matrix. Advanced thermoplastic composites are becoming more widely used in the aerospace, marine, automotive and energy industry. This is due to the decreasing cost and superior strength to weight ratios, over metallic parts. Advance thermoplastic composite have excellent damage tolerance, corrosion resistant, high fracture toughness, high impact resistance, good fatigue resistance, low storage cost, and infinite shelf life. Thermoplastic composites also have the ability to be formed and reformed, repaired and fusion welded.
Implant induction welding is a joining method used in plastic manufacturing. The welding process uses an induction coil to excite and heat electromagnetically susceptible material at the joint interface and melt the thermoplastic. The susceptible material can be contained in a gasket placed between the welding surface, or within the actual components of a composite material. Its usage is common for large, unusually shaped, or delicate parts that would be difficult to weld through other methods.
{{cite book}}
: CS1 maint: others (link){{cite book}}
: CS1 maint: others (link)