Tomorrow's solutions today - new plastics joining methods

TWI Bulletin, July/August 1997

Mike Troughton joined TWI's Advanced Materials and Process Department in 1993. His work involves development of plastics welding techniques and the structural integrity of plastics welds.

Mike Troughton reports on five new welding techniques for plastics which are currently being developed at TWI.

Plastics welding techniques have been around for over 50 years; both hot gas welding and spin welding were first reported in the 1930s. Later, as applications became more diverse and more demanding, other processes, such as high frequency welding, ultrasonic welding and hot bar welding were developed. Since then, the total number of welding techniques for plastics has risen to twenty, five of which are still under various stages of development at TWI: laser welding, microwave welding, focused infrared welding, forced mixed extrusion welding and friction stir welding.

Laser Welding

Although laser welding of plastics was first demonstrated in the 1970s, and lasers have been used for a number of years for cutting plastics, it is only relatively recently that they have been considered for welding on an industrial scale. The technique involves generation of an intense beam of radiation, usually in the infrared or visible areas of the electromagnetic spectrum, for melting plastic parts at the joint (either in a lap or butt configuration).

The most common types of laser used in industry are the CO₂ laser and the Nd:YAG laser. The CO₂ laser uses a gas mixture to produce laser energy with a wavelength of 10.6µm, normally in a continuous mode. For this wavelength, the laser beam is transmitted through air and is reflected and focused using mirrors and lenses. CO₂ laser energy generally couples well with plastics, which has made such lasers ideal for cutting these materials. However, the tendency for the laser to vaporise the plastic, due to its high power densities, necessitates considerably lower powers for welding applications.

Work at TWI ^[1-3] has shown that low powered CO₂ lasers can join thin (<0.2mm) plastic sheets and films, in lap joint and cut/seal configurations, at speeds of up to 500 m/min. This is over ten times faster than existing methods. An example of a CO₂ laser weld in polyethylene film is given in Figure 1.

The Nd:YAG laser uses a solid crystal rod, excited by flashlamps, to produce the laser energy with a wavelength of 1.06µm. For this wavelength, the beam can be transmitted through a fibreoptic system, which is advantageous for complex robotic manipulation and remote access applications. Work at TWI ^[1-2] has also shown that Nd:YAG lasers have potential for joining plastics in the thickness range 0.2-2.0mm in the lap and butt joint configurations. However, weld speeds are much slower (<1 m/min).

Laser welding is a fast, clean non-contact process and is expected to find applications in the packaging and medical industries.

Microwave Welding

Microwave welding involves placing a microwave susceptible material, such as a metal or conducting polymer, between two plastic components and then exposing the assembly to microwave energy (see Fig.2). This is typically carried out in a multimode cavity (like a domestic microwave oven). Since most thermoplastics are, to a first order approximation, transparent to microwave radiation, heat is only generated in the implant material, which in turn heats and subsequently melts the surrounding plastic parts. By applying pressure to the parts a weld forms as the material cools. Weld times can be in the order of a few seconds and are a function of input power density and electrical properties of the implant.

The main advantage of microwave welding over other techniques is that joints with a three dimensional geometry can be welded just as easily as a linear joint, since a volume, rather than an area, is irradiated. This also means that assemblies comprising many separate components, which previously had to be welded in multiple operations, can be microwave welded in a single operation. Commercially, benefits of this new technique will be in terms of a reduction in capital cost of equipment, particularly for welding complex components. It is also possible to weld plastic parts with moulded-in metal inserts, provided that there are no exposed metal surfaces from which discharges can initiate.

Design and construction of a prototype 2.2kW microwave welding machine was undertaken by TWI in 1993 ^[4] . This machine is now available to carry out feasibility studies for Industrial Member companies who are interested in developing this new technology.

It is expected that applications for microwave welding of plastics will be found in the automotive and domestics appliance industries.

Focused Infrared Welding

For infrared welding of plastics two different approaches have been used, both based around the principle of hot plate welding. One is to use tungsten filament line heaters as the heat source; the other is to use an electrically heated ceramic plate. Both systems involve bringing the two plastic parts to be joined in close proximity to the infrared source for sufficient time for the parts to become molten. Then withdrawing the source and pushing the parts together to form a weld.

The system developed by TWI ^{[3], [5]} uses a single tungsten filament, which is placed in a linear parabolic mirror. The focused infrared beam produced from this lamp is split into two, using an angled copper mirror and directed to the welding surfaces to be joined (Fig.3). Infrared welding has a number of advantages over other techniques, such as hot plate welding: weld times are reduced, the joints are free from contamination (since it is a non-contact process), and low modulus materials can be welded (since there is little or no shear of the parts during heating).

The infrared heat absorption characteristics of plastics are generally colour dependant and it is important to select an infrared source with an output wavelength characteristic which is suitable for the material being welded. Long wavelength infrared (3.3-7.2µm) is the easiest to generate but the least efficient; short wavelength infrared (0.9-1.8µm) is more difficult to generate but is more efficient.

Applications for infrared welding are expected in areas such as the joining of pipes and uPVC window frames, and in many other areas where hot plate welding is currently used. It is also expected to find extensive use in the medical industry, where high production rates of welded seals are often required in flexible materials.

Forced Mixed Extrusion Welding

Forced mixed extrusion welding (see Fig.4), which was patented by TWI in 1995 ^[6] was actually developed in Ukraine as a means of welding thick section (>10mm) plastic components with a parallel-sided joint configuration. Like conventional extrusion welding it involves the preheating of parts to be joined by hot gas, followed by the extrusion of molten plastic into the joint. However, unlike conventional extrusion welding, the joint is parallel-sided which means that the welds made are free from the distortion sometimes caused during cooling of V-shaped preparations. Another feature of this technique is a heated plate, which is inserted into the parallel-sided joint ahead of the extrusion point and serves to further preheat the surfaces to be welded. The third, and most important, feature of forced mixed extrusion welding is the extrusion point itself, which is a hollow rotating metallic rod whose diameter is slightly larger than the gap between the parts to be joined. This rod removes softened material from the parts to be joined.

Weld speed is a function of how rapidly material can be extruded; welds in 10mm thick polyethylene can be made at a rate of 2 m/min.

Potential applications for forced mixed extrusion welding include fabrication of chemical containment vessels, thick walled plastics pipes and plastics lined pipes.

Friction Stir Welding

Friction stir welding was invented at TWI in 1991 ^[7] as a means of welding aluminium alloys and has since been successfully applied to plastics ^[8] . As its name suggests, it is a friction process. However, unlike conventional friction welding processes which rely upon relative motion between the two parts to be welded, friction stir welding involves driving a rotating or reciprocating tool along the joint-line between two fixed component. Frictional contact of the moving tool with the plastic causes the material at the joint to melt and then solidify to form a weld once the tool has passed (Fig.5).

Since, in friction stir welding, the parts to be joined are fixed, this makes it an ideal process for continuous joining of sheet or plate.

Summary

As the number and complexity of components made from plastics increases so too does the need for welding techniques which are faster, more flexible, more cost effective and which produce welds of better quality. TWI is at the forefront of developing new and existing welding technology to satisfy any application. It is anticipated that the current list of plastics welding techniques available at TWI will increase further over the coming years.

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