Polymer Technology Group, TWI Ltd
Paper presented at the I Mech E seminar, 'The Joining of Plastics and Polymer Composite Materials', held on 24 October 2002 at TWI and published in The Joining of Plastics and Polymer Composite Materials.
The majority of everyday products contain a joint of some description, with even small items such as mobile phones incorporating over 100 joints. Polymers and composites can be joined by welding, adhesive bonding and mechanical fastening. The choice of process is affected by the material to be joined, the joint configuration, the strength of joint required, the level of seal required, the process cost and speed, and the production quantity. Welding processes rely on heat at the joint to melt the adjacent polymer, with a weld forming on cooling. Therefore, welding is only an option with thermoplastic polymers and composites, whilst thermoset systems must rely on adhesives bonding or mechanical fastening to join them.
There are around 20 different methods of welding plastics, which can be split into three general classifications:
- Techniques employing an external heat source
- Techniques where heat is generated by mechanical movement
- Techniques which directly employ electromagnetism.
A selection of these techniques are also applicable to polymer composites. However, since composites gain their superior properties from the reinforcement, the weld is inevitably the weak point in the system as the fibre will bediscontinuous across the joint interface. All the welding processes are governed by the parameters: time, temperature and pressure. In order to achieve high quality welds, careful optimisation of the welding parameters is required for each application.
This paper provides an overview of the available welding processes and considers the factors affecting the choice of process. Examples of industrial applications are given throughout.
Techniques employing an external heat source
Hot plate welding
In hot plate welding, the parts to be welded are held in fixtures, which press them against either side of a heated platen. Once the parts are sufficiently molten, the platen is removed. The components are then pressed together and held until they are cooled. Hot plate welding is a versatile technique with equipment available for factory use or in a portable form suitable for on-site use. The heated platen is generally flat, but three-dimensional joint profiles can be achieved with the design of a more complex plate geometry. Hot plate welding has the drawback of being a relatively slow process with weld times ranging from 10 seconds for small components to 1 hour for parts with a large joint area. However, short-term weld strengths equal to that of the parent material can be achieved. Hot plate welding can be performed on composite components, using higher welding forces, but high temperature resins such as PEEK have a tendency to stick to the hot plate during heating.
The most important application of hot plate welding is in the joining of thermoplastic water, gas and effluent pipes where it is often referred to as butt fusion welding. It is also used by the automotive industry in the manufacture of fluid reservoirs and in the welding of PVC door and window frames.
Hot bar welding
Hot bar welding is a technique for the sealing of films. A heated metal bar applies pressure to the films, softens the plastic at the joint and forms a weld. The equipment consists of one or two electrically heated bars, one of which is hinged for the insertion and removal of the films. The film is placed on the base bar, the upper bar is brought down and pressure is applied either mechanically by the operator, or pneumatically. After the required weld time the bar is lifted to release the joined films. The bars are often PTFE coated to prevent molten polymer from adhering to them.
Hot bar welding is a rapid process. Typical weld times for a 100µm thick sheet are in the range from 1-3 seconds. A reduction in welding time can be achieved by the use of two heated bars, one on either side of the films to be joined. The necessity for the heat to conduct through the film to the joint, imposes a restriction on the thickness of material that may be welded, of approximately 1mm.
Hot bar welding is widely used in industry, mainly in the joining of thermoplastic films having a thickness of less than 0.5mm. It could be used to weld thin composites and has the potential to tack prepregs. It is used extensively within the packaging industry for producing plastic bags.
Impulse welding is an advanced form of hot bar welding in which both the heating and cooling regimes are controlled whilst the joint is still under pressure.
Hot gas welding
Hot gas welding of thermoplastics is a manual welding technique, analogous to oxygas welding of metals. In the hot gas gun, a stream of gas (typically air) passes over an electrically heated element and emerges from a nozzle. The stream of heated gas is directed towards the joint between two thermoplastic parts, where it melts or softens the polymer and a filler rod. A weld is formed by the fusing together of the thermoplastic parts and the filler rod, which is composed of the same polymer type as the parts. The main advantage of hot gas welding is that the equipment is easily portable. However, it is a slow process and the weld quality depends on the skill of the operator. Typical applications include chemical storage vessels and pipework.
Extrusion welding is similar to hot gas welding except that the filler material is separately heated in the barrel of a hand-held screw extruder. The molten material is then extruded through a PTFE die into the joint. The joint is pre-heated using a hot gas gun mounted on the extruder barrel. It is preferable to hot gas welding when thicker sections are to be joined. Extrusion welding could be used for composites although the weld strength would be that of the filler rod and not the composite.
Flash free welding
Flash free welding refers to techniques for butt joining thermoplastic parts (sheets, pipes or rods) without the generation of weld flash. The parts to be joined are butted together and fixed in place to prevent axial movement during the welding cycle. The parts are then constrained laterally using heated metal parts, which would be bars for joining sheets or a collar in the case of pipes and rod.
As the metal parts are heated to above the melting point of the thermoplastic, the material at the joint softens and melts. However, it is totally constrained and a melt pressure is built up due to thermal expansion. For pipes, the molten material is prevented from extruding into the bore by using an inflatable bladder, which is expanded at the joint-line prior to welding. After a predetermined time, related to the thickness of the thermoplastic part, the heat supply is switched off and the joint is allowed to cool.
Flash free welding machines are currently only commercially available for joining thermoplastic pipes, where the smooth bore at the joint-line is a major advantage for high purity applications, such as in the food or pharmaceutical industries.
Techniques where heat is generated by mechanical movement
Vibration welding, also known as linear friction welding is one of a number of welding techniques in which the heat is generated by the mechanical movement of the components to be joined. It is an established industrial technique with commercially available equipment. The two parts to be joined are brought into contact under an applied load. One part is constrained whilst the other undergoes a rapid linear reciprocating motion in the plane of the joint. The heat generated by the friction at the two surfaces creates local melting. Subsequently, the vibration stops, the parts are aligned and the joint is cooled under pressure to consolidate the weld.
Vibration welding may be performed on almost any thermoplastic material. The joint alignment is consistently good and joint strengths approaching that of the parent material. Vibration welding is feasible for composites but a higher force is required and fibre damage at the joint can be a problem. It is most suited to the welding of linear joints that are too long for the practical application of ultrasonic welding (eg. >200mm) and for which hot plate welding would prove too slow (vibration welding being four times faster than hot plate welding for large weld areas). The major drawbacks are the high capital cost of equipment and the difficulty in dealing with three-dimensional joints.
Vibration welding is used extensively by the automotive industry in the manufacture of components such as car bumpers, air intake manifolds, fuel pumps, instrument panels, parcel shelves, inner door panels and for the hermetic sealing of air ducting to the internal surface of the dash board. Other applications include spectacle frames, typewriter covers, filter housings, motor saw housings and heating valves.
Spin welding is a variation of friction welding which uses a rotational motion. Consequently this technique is only applicable to circular joint areas. Spin welding can involve relatively simple pieces of equipment such as lathes ordrilling machines. A lathe would produce a constant speed during the frictional heating stage (continuous drive friction welding), and a drilling machine would produce a reducing speed characteristic during the frictional heating stage(inertia friction welding). In practice, purpose built machines are generally employed for spin welding in order to provide greater control, and they may be of either the continuous drive or inertia type. Spin welding has been exploited in applications as diverse as ball cocks, aerosol bottles, transmission shafts and PVC pipes and fittings.
In orbital welding, unlike spin welding, each point on the surface orbits a different point on the face of the stationary part. The orbit is of constant rotational speed and is identical for all points on the joint surface. This motion is stopped after sufficient material is melted and the thermoplastic then solidifies to form a weld.
Ultrasonic welding involves the use of high frequency mechanical sound energy to soften or melt the thermoplastic at the joint line. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations via the welding horn, usually at a frequency of 20 or 40kHz. The heating effect of the ultrasound varies with the degree of crystallinity of the material being welded. The ability to weld a component successfully is governed by the design of the equipment, the mechanical properties of the material to be welded and the design of the components. Ultrasonic welding is a fast process (weld times are typically less than one second) and can easily be automated. Consequently, it is ideally suited to welding components in mass production. However, the joint and horn design are critical and there is a restriction of approximately 250mm on the length of weld possible.
Ultrasonic welding is widely used in the automotive, appliance, medical, textile, packaging, toy and electronic markets. It is suitable for composite materials, but highly filled materials can result in weak joints. Examples of ultrasonically welded components include vacuum cleaners, automotive light fixtures, audio and videocassettes, blister packs, juice pouches and toys.
Friction stir welding
Friction stir welding was invented at TWI in 1991, primarily as a means of welding aluminium alloys, but it has subsequently been found to be applicable to the welding of thermoplastics. 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 components. 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.
Since in friction stir welding, the parts to be joined are fixed, it makes an ideal process for continuous joining of sheet or plate. Applications are anticipated in the fabrication of tanks and vessels with thick sections. Joints with good mechanical properties have been demonstrated in materials such as PMMA, polyethylene and polypropylene with weld speeds up to 200mm/min. This process is under continuing development at TWI.
Techniques which directly employ electromagnetism
Resistive implant welding
In resistive implant welding, heat is generated through the introduction of an electrically conductive implant at the joint, through which a high electric current is passed. Implants commonly consist of copper mesh or carbon fibres. The implant heats up due to resistive losses, softening the surrounding plastic. The application of pressure at the joint fuses the two parts together and on cooling, a weld is formed.
The implant remains at the weld, which rules out the use of resistive implant welding for some applications and it has the disadvantage of requiring a consumable implant. It is a useful for the joining of composites, where the implant could be manufactured from the prepreg thus forming a homogenous joint. Resistive implant welding is particularly suited to the joining and repair of gas and water pipes since it can be performed in the field. In the pipe joining industry it is referred to as electrofusion welding.
Induction welding is similar to resistive implant welding in that a conductive implant is required. However, in this case the heat is generated by an induction field set up either by eddy currents or due to hysterisis losses. A work coil connected to a power supply is placed in close proximity to the joint. As electric current at high frequency passes through the work coil, a dynamic magnetic field is generated whose flux links to the implant. As the implant heats up, the surrounding thermoplastic softens and melts. If pressure is applied to the joint, a weld forms as the joint cools. Induction welding is ideal for attaching metallised tops to plastic bottles. It is also applicable to a range of composite materials.
High frequency welding
High Frequency (HF) welding, also known as Radio Frequency (RF) welding or Dielectric welding, is a method of joining thin sheets of polar thermoplastic material together. It uses high frequency (13 to 100 MHz) electromagnetic energy to fuse together the materials. A rapidly alternating electric field is set up between two metal welding bars. The electric field causes the polar molecules found in some thermoplastics to oscillate and orient themselves with respect to the field. The energy generated by this process causes a temperature increase resulting in the melting of the materials. Combined with the pressure applied by the clamping of the welding bars, this causes a weld to be formed.
RF welding relies on the vibration and orientation of charged molecules within the polymer chain to generate heat, consequently its use is restricted to plastics containing polar molecules. Polyvinylchloride (PVC) and polyurethanes are the most common thermoplastics to be welded by the RF process. It is possible to RF weld other polymers including nylon, PET, EVA and some ABS resins, but special conditions are required, for example nylon and PET are weldable if preheated welding bars are used in addition to the RF power. Due to the impending restrictions in the use of PVC, a special grade of polyolefin has been developed which also has the capability to be RF welded.
A wide range of products is manufactured using high frequency welding. Examples include ring binders and stationary wallets, inflatable items such as beach balls and life jackets, large items including tents and lorry covers, blood bags and colostomy bags for the medical industry and automotive components such as air bags and sun visors.
Laser welding was first demonstrated on thermoplastics in the 1970s, but has only recently found a place in industrial scale situations. The technique, suitable for joining both sheet film and moulded thermoplastics, uses a laser beam to melt the plastic in the joint region. The laser generates an intense beam of radiation (usually in the infra red area of the electromagnetic spectrum) which is focused onto the material to be joined. This excites a resonant frequency in the molecule, resulting in heating of the surrounding material. Two forms of laser welding exist; CO 2 laser welding and transmission laser welding. CO 2 laser radiation is readily absorbed by plastics, allowing joints to be made at high speeds, but limiting the depth of penetration of the beam, restricting the technique to film applications. The radiation produced by Nd:YAG and diode lasers is less readily absorbed by plastics, but these lasers are suitable for performing transmission laser welding. In this operation, it is necessary for one of the plastics to be transmissive to laserlight and the other to absorb the laser energy, to ensure that the beam focuses on the joint region. Alternatively, an opaque surface coating may be applied at the joint, to weld two transmissive plastics. Transmission laser welding is capable of welding thicker parts than CO 2 welding, and since the heat affected zone is confined to the joint region no marking of the outer surfaces occurs.
Laser welding is a high volume production process with the advantage of creating no vibrations and generating minimal weld flash. The technique relies on the initial outlay for a laser system, however, the benefits of a laser system include; a controllable beam power, reducing the risk of distortion or damage to components; precise focusing of the laser beam allowing accurate joints to be formed; and a non contact process which is both clean and hygienic. Laser welding may be performed in a single-shot or continuous manner, but the materials to be joined require clamping. Weld speeds depend on polymer absorption. It is possible to create joints in plastics over 1mm thick (with transmission laser welding) at up to at least 20m/min whilst rates of up to 750 m/min are achievable in the CO 2 laser welding of films. Transmission laser welding could be used for composite materials although the transmission properties of those containing carbon fibres are probably too low to make the process viable.
Clearweld ® is a novel development of the transmission laser welding process. A patent has been applied for by TWI. The carbon black absorber commonly used is replaced by a colourless dye thus expanding the applicability of the technique to clear plastics. The dye is either printed/painted onto one surface of the joint, encompassed into the bulk plastic, or produced in the form of a film that can be inserted into the joint. It absorbs infrared laser light allowing an almost invisible weld to be produced between materials that are required to be clear or have a predetermined colour. The process is especially suitable where the appearance of a product is important. In the case of fabrics joining, positioning of the dye at the joint restricts melting to the interface rather than through the full thickness of the joint as occurs in other welding methods for fabrics. Consequently, flexible seams are produced making the process suitable for the joining of fabrics for clothing applications.
Laser welding has proved to be especially effective in the welding of thermoplastic films in a lap joint configuration. The speeds attainable with laser welding make it especially suitable for use in the packaging industry, whilst biomedical applications exploit the cleanliness of the process. The Clearweld ® process extends its applicability to circumstances where final appearance is important. Applications in the areas of food packaging, medical devices and packaging, electronic displays and fabrics are being developed.
Two different approaches to infrared welding have emerged, both based around the principle of hot plate welding. One is to use tungsten filament line heaters as the heat source; the other, which is commercially available, is to use an electrically heated metal 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, withdrawing the source, and then pushing the parts together to form a weld.
Infrared welding has a number of advantages over 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 shearing of the parts during heating).
The current application for infrared welding is in the joining of plastic pipes, but it has the potential to be used in many areas where hot plate welding is currently used and has been demonstrated on composites.
The possibility of using microwaves to weld thermoplastics has existed since the development of the magnetron in the 1940s. In 1993 TWI built a research facility to explore the feasibility of exploiting such an operation. The modified multimode cavity, similar in nature to a microwave oven, operates at a frequency of 2.45 GHz and has the capability to apply pressure to a joint.
Most thermoplastics do not experience a temperature rise when irradiated by microwaves. However, the insertion of a microwave susceptible implant at the joint line allows local heating to take place. If the joint is subjected simultaneously to microwaves and an applied pressure, melting of the surrounding plastic results and a weld is formed. Suitable implants include metals, carbon or a conducting polymer. The particular advantage of microwave welding over other forms of welding is its capability to irradiate the entire component and consequently produce complex three-dimensional joints. Welds are typically created in less than one minute.
The technique is still in the development stages and as such there are currently no reported industrial applications. However, it is anticipated that microwave welding may prove to be suitable for joining automotive under-body components and domestic appliance parts.