Hot Plate Welding
Hot plate welding is known as a tolerant process capable of providing weld strengths of the parent polymer. Polyolefins and soft polymers can be easily welded with this process to produce high quality parts. In other industries, such as automotive, it has been used to join plastic battery cases, fuel tanks and fuel filler pipes. Infrastructure applications, such as gas and water distribution, sewage and effluent disposal pipes have used hot plate welding extensively.Hot plate welding operates by contacting the weld surfaces with a heated platen or tool to create a molten or plasticized region. These regions are then pressed together to make the weld. In practice, the process is operated on a hot plate welding machine in which the parts to be welded are clamped in holders. A platen, heated to a suitable temperature, advances between the parts and they are pressed against it. The part surface is melted away until contact is complete (matching). At this point, further movement is often stopped, and heating continued, to create a deeper molten or plasticized zone. After heating is completed, the parts are retracted from the platen, the platen moves out of the way, and the samples are forged together and held until the polymer cools. Mechanical or microprocessor-controlled stops are usually used to control the amount of displacement of the polymer from the weld zone, particularly during the heating phase.
If polymers are compatible, the hot plate process is capable of welding dissimilar thermoplastics with the use of two platens to compensate for different polymer melting or softening points. Hot plate welding can provide good welds with filled polymers providing the filler material and level allow the polymer to interdiffuse on forging. Weld times will vary with the volume of polymer to be fused and the thermal conductivity of the substrate. Welding times normally fall in the range from 5 to 60 seconds, although the mass of polymer that must be melted, and the corresponding cooling rates, will govern cycle times. With large masses of polymer (1.5 meter diameter pipes with wall thicknesses of 10 cm may take an hour, depending on equipment). The size of objects that can be hot plate welded is limited by the practical size of the hot plate, the mechanics of its removal from the weld zone and of moving the parts to forge them together.
Hot plate welding is limited because melted polymers tend to stick to the platen. Sticking can either create strings of polymer that end up in the product or tear out/deform sections of the melted polymer that can produce gaps in the bond line. Non-stick surfaces are used, but they tend to break down at temperatures in excess of 270 deg. C. This temperature limitation is not a problem for lower melting polymers, but polycarbonates, polymethacrylates, polyamides, and others, require higher welding temperatures. An alternate approach is to use hot plate temperatures that are high enough to decompose and evaporate sticking material from the platen surface. While this approach is used, it will produce weld fume from the decomposing polymers and, if the polymers do not decompose cleanly, particles of char remain on the platen and contaminate the following weld.
Ultrasonic Welding
When a rubber ball is dropped, it never bounces back to its drop height because some of the potential energy is converted to heat and sound. This type of process is involved in ultrasonic welding. Instead of a ball, a small projection(s) in the weld zone is flexed by an oscillating force at rates of 10,000 to 70,000 times per second (kHz). This causes the projection to melt and flow across the joint to create a weld. A metal tool (horn) that is oscillating vertically contacts the plastic part at a distance from the projection and is the means for delivering the energy. The part on the other side of the joint area rests on an anvil, ensuring the energy is spent in the weld zone. Frictional heating can also occur to some extent because transmission of the energy through the plastic parts is very complex.
The oscillating force is generated when alternating electrical power (at frequency) is applied to a train of tuned components that are sized to form a resonant system. The first component converts the electrical power to oscillations. This occurs when the power is applied to a sandwich of piezoelectric or magnetostrictive materials and metal blocks. These oscillations are amplified (or de-amplified) by a booster and the booster is connected to the horn. The horn can either amplify or de-amplify the oscillations, depending on the needs of the application. While the frequency of oscillations vary between 10 and 70kHz, the most common frequencies are in the range from 20 to 40kHz. Oscillation amplitudes range from 20 to 80 microns. Joints are designed to contain a molded projection of thermoplastic (usually triangular in shape and 0.2 to 0.3 mm high) that flexes in preference to the bulk of the plastic or composite. Modulus losses occurring in this projection, the energy director, cause the polymer to melt and flow through the bond line.
Ultrasonic welding can be accomplished at various distances from the horn ranging from only a fraction of a millimeter up to several centimeters. For distant welding the polymer must transmit the energy efficiently, i.e. not be too flexible or have too high a loss modulus. ABS and high impact polystyrene are among the easiest polymers to weld ultrasonically. Ultrasonic welding will usually join amorphous thermoplastics more readily than semicrystalline ones. However, the advent of more powerful machines has blurred this distinction, and semicrystalline polymers are welded routinely.
Joint design and location is very important in ultrasonic welding. Interference or shear types of joint involve welding a slightly larger part into a smaller one. Shear joint designs are commonly used for semicrystalline polymers, or when watertight seals or high joint strengths are needed.
Ultrasonic welding is probably the most commonly used thermoplastic welding process because it is very fast (fractions to a few seconds) and usually produces welds that are relatively free of flash. Often dust from fracture of joint components is produced. In addition, ultrasonic welding is relatively easy to automate and is particularly suitable for high volume production.
Rapid development of the ultrasonic welding machine has occurred in the last ten years. Basic functions, such as weld energy, collapse, trigger force, and pressure are now microprocessor-controlled. In addition, real time feedback and control of welding conditions is being offered, along with the ability to vary weld force and amplitude during the weld cycles.
The benefits of a amplitude control were reported recently. With a constant amplitude the rate of heat generation in the bond line is extremely rapid (approximately 3000C per second), and temperatures can reach those where polymer degrade. Degradation weakens the joint. If a high amplitude is used to initially generate the desired temperature, a lower amplitude can then be used to maintain it during the approximately 1 second weld cycle. Higher bond strengths have been observed and, in at least one case, polycarbonate weld samples yielded in the sample rather than the weld.
Infrared Heating
Infrared radiation is being developed as a non-contact alternative for hot plate welding. The infrared radiation is often supplied by high-intensity, quartz, heat lamps, producing radiation with wavelengths around 1 micron. When this radiation is applied to a polymer, melting occurs. In one mode of operation, the lamps are removed after melting has occurred, and the parts are forged together, as with hot plate welding.
Infrared is particularly promising for higher melting polymers since the parts do not contact the heat source. The causes of stringing and/or joint damage are not present. A recent report indicates that infrared welding of a glass-reinforced polymer (polyethersulfone) results in exceptionally high weld strengths (Weld Factor = 80+%) that are not achieved with other welding processes.
Another potential advantage of infrared welding is speed. Infrared radiation can penetrate into a polymer and create a zone of melt quickly. By contrast, hot plate welding involves heating the polymer surface and relying on conduction to create the required melt zone. As might be expected, however, the depth of penetration depends on many factors, and it varies strongly with only minor changes in polymer formulation. Consistent infrared welding is likely to require very close attention to batch-to-batch polymer uniformity.
Through-Infrared transmission Infrared Welding (TTIR)EWI has been developing a process called through-transmission infrared welding (TTIR) in which the radiation is passed through a transparent polymer to an absorbing interface that is in contact with the transparent polymer. Heat generation at the interface melts the transparent polymer. The heat source is outside the weld zone.
EWI has conducted Through-Transmission Infrared Welding (TTIR) both by joining a transparent structure to an opaque one or by using a thin interlayer film between two layers to be joined. Usually, only low forces are applied to the layers to keep them in contact, to allow heat transfer. Air, blowing over the upper layer, can both cool and apply the weld force.
Nearly all polymers are "transparent" to infrared energy in the near infrared part of the spectrum. Even polymers that are very opaque to visible light, such as TeflonÒ TFE, are highly transparent and can be welded using appropriate absorbing layers.
EWI has demonstrated that the infrared energy can be transmitted through optical materials such as quartz rods (light pipes) or through glass fiber bundles. For example, a bundle of quartz fibers with approximately the same area as the weld zone can receive the infrared energy and the exit of the fiber bundle can be shaped to conform to the weld zone. Very complex weld zones can be accommodated with this technique. Additional information is available to members of EWI.
Vibration Welding
Vibration or linear friction welding involves the rubbing of two thermoplastic parts together, under pressure and at a suitable frequency and amplitude, until enough heat is generated to melt and mix the polymer. After vibration is stopped, the parts are aligned and the molten polymer allowed to solidify, creating the weld. The process is similar to spin welding except the motion is linear rather than rotational.
In a welding cycle, the lower platen, holding a stationary part, rises to contact the part in the upper platen containing the other part. The parts are pressed together at a preset pressure (1 to 4 MPa). The upper platen is mounted on a resonant spring assembly and is vibrated by hydraulic or electromagnetic drivers. The resonant spring has several functions; it reduces the amount of energy needed, allows pressure to be applied and positions the parts accurately at the end of the weld before the polymer resolidifies.
This process is generally suitable for most thermoplastics, both amorphous and semicrystalline. Welds are produced in a matter of milliseconds and the process is more tolerant of moisture in the polymer, e.g., nylons. As might be expected, best results are obtained when the joints are planar although parts with angles 10 degrees off planar are considered weldable. Amplitudes generally range from 0.5 to a few millimeters and frequencies range from 100 to 500 Hz although frequencies of 100-300 Hz are most common. Research vibration welders capable of exploring higher frequencies have been built.
The parameters chosen for a particular application are usually determined by a test program. A certain amount of flash from the weld zone is desirable because contaminants on the surface of the substrates are usually removed from the weld zone along with the flash. Excessive amounts, however, may detract from the esthetics of the weld. If so, a flash trap or cavity located next to the joint but not visible, can be incorporated. As a general rule, higher amplitudes produce greater amounts of flash.
The process is applicable to most polymers, both amorphous and semicrystalline, whether extruded, blow molded, thermoformed or stamped. The technique is said to be particularly useful for welding of crystalline thermoplastics such as acetal, polyethylene, nylons and polypropylene which are not easily welded by ultrasonic or solvent welding.
Several new developments in equipment have been made during the last few years. One is the ability to vary the pressure during the welding cycle. This allows more of the melted polymer to remain in the bond line and produces, therefore, a wider weld zone. This tends to both reduce flash and increase weld strength, A second development is the development of equipment for orbital welding. In this friction process, one part is rotated against the other in an orbital motion. At present, this process is unable to handle parts of the size used in vibration welding, but it should be able to weld thin-walled parts more effectively.
Laser Welding of Plastics
Laser welding of polymers is under development as a high-speed, non-contact process for welding of thermoplastics. Laser radiation, in the normal mode of operation, is so intense and focused that it will very quickly degrade thermoplastics (see figure). It has been used to weld polyethylene by pressing the unwelded parts together and tracking a laser beam along the joint area. Decomposed polymer was squeezed from the weld zone and a strong joint was obtained through a thin layer of undecomposed polymer remaining in the bond line.
EWI has shown that Nd-YAG and similar wavelength energy is absorbed at levels useful for welding even when very low levels of absorbing pigment are present in the absorbing layer. EWI has demonstrated methods that form essentially invisible joints. For example, particles of silicon carbide embedded in a surface of a transparent polymer will provide enough absorption to form a weld but they are invisible to the eye. Similarly, compatible inks that dry to form nearly transparent coatings are also useful. Further information on this and other interactions with pigmented polymers is available to members of EWI.
Recently, the plastics welding group at The Welding Institute (TWI) in the United Kingdom reported on the high speed laser welding of polyethylene films using carbon dioxide and Nd-YAG lasers. Weld speeds of 500 meters/minute were demonstrated, but higher speeds were considered possible. Weld strengths were near parent material strength.
As with through-transmission infrared welding, laser radiation of suitable wavelengths can be passed through some polymers (transparent) and be absorbed by underlying, absorbing polymers.
Spin Welding Spin welding of thermoplastics was first reported in the 1930s. In this process, one of the substrates is fixed while the other is rotated with a controlled angular velocity. On contact, frictional heat causes the polymer to melt, and a weld is created on cooling.
Major welding parameters include rotational speed, friction pressure, forge pressure, weld time and burnoff length. As the heating effect depends on relative surface velocity, maximum heat is generated at the outer edge in solid components. The differential generation of heat can result in weld zone stresses. Hollow sections with thin walls are more satisfactory.
Spin welding has been performed with a drill press but better quality welds can be obtained by units that impart a controlled amount of energy to the spinning part. Usually, this is done by spinning a flywheel to a preset energy level. On contact with the fixed part, this energy is dissipated in the bond line. Other designs use driven fixtures to impart fixed amounts of energy.
Advantages of spin welding are said to be high weld quality, simplicity, speed and reproducibility. In most cases, little surface preparation is necessary. The main disadvantage is that, in its simplest form, it is only suitable for applications where at least one of the components is circular and where angular alignment is not required. However, units that are electrically-driven and which stop at a preset orientation are common.
One of the first applications of the spin welding process was to seal water-filled compasses. This was done by spinning the cover onto the base while the base was immersed in a fluid. Other applications include the manufacture of floats and aerosol bottles. Spin welding is useful for attaching studs to plastic parts.
Hot Gas and Extrusion Welding
In hot gas/air and extrusion welding, the weld joint is filled with a partially- or fully-molten polymer, respectively. The weld joint can be either a groove cut between two sheets, as in a butt weld, or in the corner made by two pieces of polymer positioned at right angles. Lap and other types of joints can be welded by these techniques.
In hot gas/air welding, heated gas or air is blown onto the surfaces of the joint area and, simultaneously onto a filler rod, to melt both surfaces. These surfaces are then pressed together using either manual pressure or a metal tab (pressure foot) that is built into the tip of the welding tool.
In extrusion welding, the heating method is sometimes similar (heated tools or infrared heaters can also be used). The difference is that a fully molten polymer is injected into the weld joint. The molten polymer is generated inside the welding tool and pumped into the weld joint as the tool is moved along the weld.
These processes are often used to weld long weld joints. Typical applications include welding of seams in environmental pond liners, assembly of large thermoplastic air ducts, manufacture of polymeric tanks, and attachment or repair of polymeric pipelines.RF or Dielectric WeldingRF welding is useful for joining polymers that have strong dipoles, such as polyvinyl chloride (PVC), polyurethanes, and polyamides. Application of a high intensity alternating electromagnetic field to these dipoles will tend to cause orientation with the field. The dipoles will try to alternate with the changing field polarity and, in the process, will convert some of the field energy into heat, creating a weld (see figure). In the USA, the most common RF welding frequency is set at 27.12 MHz, but frequencies can vary depending on country.
The high intensity field is normally applied to polymer by electrodes that are pressed against both sides of the film layers. Since field intensity decreases with distance, this process is normally most useful when the electrodes are close together, as with polymer films. Welding appears to occur at the interface between the films because the relatively cold electrodes draw heat from the film surface, but not as effectively from the more distant interface.
Concerns about the incineration of PVC disposable products has led to greatly increased interest in welding of polyester or multilayer films. For the latter multilayer materials, one of the layers is sensitive to the field while the other layers provide the properties required for the particular application. These newer materials generally heat less efficiently, but conditions can sometimes be found where acceptable welding occurs. Heating efficiency is material dependent. Polyolefins, such as polyethylene and polypropylene, have very weak dipoles are essentially insensitive to the field.
In the medical area, bags for fluids are probably the main application of RF welding. The bags, and the ports for entry into the bag, can be made in one step. Weld times range from fractions of a second to seconds, depending on the material, film thickness, and area being welded. Use of microprocessors and improved control during the weld cycle have led to both increased quality and speed.
Microwave Welding
Microwave welding is still a technology in a fairly early stage of development. Microwaves have higher frequencies than either induction or RF (dielectric) welding and, in the United States, the two common frequencies are 915 MHz and 2.45 GHz (kitchen microwaves). As with RF welding, heating is caused by interaction with dipoles, such as those found in water or similar compounds, or with ionic species, such as carbon black. Rubber does not absorb microwaves very strongly until it is formulated with carbon black. Other materials that absorb microwaves inlcude some iron oxides, ferrites, and electrically-conducting polymers, such as polyaniline salts (emeraldine).
The process offers considerable promise as a means for continuous processing of manufactured parts. However, a major concern is heating uniformity due to oven configurations (multimodal applicators). Single-mode, resonant applicators can produce uniform fields but are limited in the sizes of parts that can be heated. In addition, the behavior of some polymers toward the field can cause concerns. Polar polymers that melt will absorb more strongly when molten (above Tg, probably), and absorption increases as viscosity decreases, so they exhibit the process of "runaway" heating. Nonpolar polymers do not exhibit this phenomenon. On the other hand, adhesives, such as epoxies, can be heated by microwaves because of polar groups in the hardener component. As the adhesive cures and the chemical bonds become less mobile, the ability to absorb microwave energy decreases as well, so the process is self-limiting.
Friction Stir Welding of PlasticsWhile conventional friction processes involve rubbing the parts together, a new developmental process involves rubbing a third body against the parts to be joined. The third body can be a blade or a rotating mandrel. This process is currently used mainly for metals, but the potential for plastics is clear.
A crude analogy to this process is observed sometimes when cutting a piece of thermoplastic with a sabre saw. If conditions are right, the oscillating motion of the blade melts the polymer on either side of the blade and it flows together behind the sawblade. Of course, no pressure is applied and material is being removed by the sawblade, but the concept is similar.
Induction Welding
Induction heating is observed when conductive materials are subjected to a radio frequency (RF) field. RF energy induces eddy currents in the conductive material, and heating occurs primarily by I2R heating. In materials with permeabilities greater than one (nickel and iron, for example), dipole heating augments the eddy currents up to the material's Curie point, but eddy current heating is the major mechanism. As the frequency of the RF field increases, e.g. from 50 Hz to 10 MHz, the eddy currents are generated, increasingly, in the outer layers of the conductor (skin effect).
For application to plastics which are neither conductive nor with high permeabilities, RF fields are generally applied to tapes, rods, or gaskets of thermoplastic that are filled with iron oxide or metal particles (6). These tapes, rods, or gaskets are placed in the bond line and, on application of a suitable RF field, the particles cause the thermoplastic to melt and create a weld. Higher frequencies are required to heat the small metal or metal oxide particles that are mixed into most induction welding implants. The field is often applied to the workpieces by a coil made from copper tubing which is connected to the induction generator. Coil design and fixturing are critical factors in induction heating.
Resistive Implant WeldingResistive implant welding is observed when an electrically conducting element is heated in a bond line by application of a current (see figure). The region around the element melts and welds are created by the application of pressure. Sometimes, it is beneficial to include additional polymer in the bond line to provide for better melt flow and filling of gaps in the bond line.Power supplies range from simple variable voltage transformers to programmable units that operate in either the AC or DC mode. The resistive element can be any material that conducts current, including metal wires and braids, and carbon-based elements, such as tapes, ropes and sheets. This process has normally been applied to larger structures and to those that do not involve a closed-loop weld joint.
Implant welding has been applied to complicated joints in automotive applications such as vehicle bumpers and panels, joints in plastic pipe, containers and medical devices, such as blood oxygenators. Implant heating processes are reasonably fast, from seconds to minutes, depending on the application, and the processes can be used to join most thermoplastic-based materials. Implant materials should be compatible with the intended application, since they remain in the bond line.
