Plastics at TWI - an update

TWI Bulletin, September/October 1992

Maria joined the Plastics Joining Department in September 1989, after completing an MPhil at Bath University on matrix modification of glass fibre reinforced composites. She obtained her first degree, in Materials Technology, at Coventry (Lanchester) Polytechnic.

Since she came to TWI she has become widely involved in welding gas and water pipes, ultrasonic welding, including development of a magnetostrictive transducer, and adhesive bonding.

Recently, she has become chairperson for the medical industry team at TWI which pursues technology transfer in this area.

Maria has continued to be actively involved with committees of the British Composite Society, and the Institute of Materials. With both societies she has successfully promoted activities for younger scientists.

Since last reviewed in the pages of Bulletin the work of the plastics team at Abington has grown in diversity. Both the department's equipment range and the scope of industries to which its work appeals have broadened. Maria Girardi takes up the story.

Welding of plastics is confined to thermoplastics since only these materials can be melted or softened by heat. The various welding methods available today are classified according to the type of heating - thermal, friction or electromagnetic. Figure 1 lists the alternative methods applicable to thermoplastic materials, and this article, briefly reviews the joining techniques currently available at TWI.

Plastics welding techniques

Heated tool welding

Heated tool welding, more commonly called hot plate welding, is perhaps the simplest welding technique used with plastics. It is popular both in mass production, e.g. window frames, and for large structures such as pipelines.

A heated plate is clamped between the surfaces to be joined until they soften. The plate is then withdrawn and the surfaces are brought together under a controlled pressure for a specific period. The fused surfaces are allowed to cool, forming a joint which normally has at least 90% of the strength of the parent material.

Two types of commercially available Bielomatik hot plate machine are currently used at TWI for experimental purposes ( Fig.2). One has a small hot plate of 150 X 150mm with dual heating plate facility, the other a large hot plate of 250 X 250mm.

Butt fusion is a variation of hot-plate welding. The technique is widely used in the gas and water distribution, sewage and effluent handling, building, agriculture and chemical industries. The equipment is simple and easy to use on site ( Fig.3). It comprises a frame to hold the pipes and to apply the joining pressure (hydraulically), a planing cutter to trim the pipe ends prior to joining and the hot plate. The equipment has been developed to allow automatic operation which is controlled by a microprocessor, and data-logging facilities for inspection purposes.

Hot plate welding large components with wall thicknesses above 25mm currently presents particular challenges. To investigate this topic further, TWI has designed and built a test-bed machine at Abington. The machine, now commissioned, has an electrically-heated hot plate and hydraulically applied axial force in common with commercial equipment, but these parameters can be varied over wide ranges. The maximum hot-plate temperature is 650°C and force is adjustable from 1 to 150 kN, so that all current thermoplastic materials, including high-temperature composites, can be welded in sizes up to the dimensions of the hot plate (650 X 650mm).

Typical areas of current research include joining large diameter (up to 630mm) polyethylene pipes, and advanced thermoplastic composite parts for aerospace use. Commercial large-part hot-plate machines are expensive, so the machine also provides a facility for member companies to evaluate their applications in detail before making an investment.

Although considerable progress has been made in understanding the hot-plate process, the factors that influence weld quality and the weldability of different materials are poorly documented. TWI has attempted to gain a better understanding of material behaviour during the welding process and has conducted or been involved in the following research programmes:

• Influence of melt viscosity on bead formation and weld strength;
• Effect of crystallinity in butt fusion weld joints;
• Improved productivity;
• Effect of defects in butt fusion weld joints;
• Finite element analysis of the welding process.

Hot gas welding

In hot gas welding filler material is fed into a prepared joint and a stream of hot gas (generally air) is used to heat filler and parent material, see Fig.4. The equipment can be manual or semi-automatic. Normally, the filler material is a solid rod which is forced under light pressure into the joint. The filler rod is not melted in the gas stream, but is sufficiently softened to allow it to fuse to the parent material.

The filler rod can be circular in section, but recently triangular section rods have been used as they allow multi-run welds and fillet welds to be made more easily. In some cases, particularly where semi-automatic or automatic welding of large assemblies, e.g. pipe flanges to pipes, is required, a variant of the technique called extrusion welding is used. In this case, instead of a solid rod, the filler material is extruded into the joint.

Very little process research work has been carried out in this area. The only recent work in the UK was by Turner and Atkinson ^[l] on repair of car bumpers by hot air welding. The aim was to find a repair method that performed satisfactorily and was cheaper than replacement of damaged bumpers.

One of the major problems of hot gas welding is the lack of a universal standard in the UK. As a result TWI has become involved in defining a European standard for certification of hot air welding personnel with the help of the UK industry and is currently setting up a certification scheme at TWI for hot air welding inspectors.

Ultrasonic welding

Ultrasonic welding uses high frequency mechanical vibrations, ie. ultrasonic vibrations, as a source of energy to join thermoplastic materials. Heat is generated by a combination of surface and intermolecular friction. The ability to weld a plastics component is governed by the design of the welding equipment, the physical properties of the material to be welded, the design of the component and the welding parameters.

Joint design influences the quality of ultrasonic welds. The most important factors are a loose fit and the provision of an energy director. A slip fit is essential since the welding process depends on movement between the two parts as well as friction and high pressure. The energy director is a small triangular ridge, typically 0.25-0.8mm high, depending on the material.

This concentrates the applied power to provide rapid melting of the material contained in the director. Molten material from the energy director flows across the joint interface and fuses with the two components to form a weld. Figure 5a illustrates a simple butt joint modified with an energy director. A shear joint ( Fig.5b) provides a small initial contact area and then a controlled interference along the joint as the parts collapse together.

Ultrasonic energy can also be used to produce joints that do not involve welding. Plastics can be staked or riveted to other plastics or metals, e.g. in disposable razor blades, or threaded metal inserts can be pushed into plastics using ultrasonics to soften the plastics material.

Ultrasonic welding is one of the most poorly understood of the processes available, so most of the difficulties of production or material weldability cannot be solved other than by trial and error. At TWI, research has been conducted to make key measurements on ultrasonic welding equipment in operation to gain a better understanding of how the process works. Instrumentation has been developed to allow transmission power and amplitude to be monitored continuously during welding of various materials.

A further major development has been design, construction and demonstration of a new type of energy transducer, ^[2] based on magnetostrictive material recently made available by Johnson Matthey Rare Earth Products. This should enable higher powered and more easy to set up welding equipment to be manufactured, and should be of great benefit, for example, in the automotive components supply industry.

Friction welding

Heat is produced in friction welding through mechanical rubbing of two surfaces in contact under an applied axial load. The technique is ideally suited for joining thermoplastics because the frictional heat developed at the joint line is sufficient to cause rapid surface melting without significantly raising the temperature in regions away from the rubbing surfaces. Since plastics have a relatively low thermal conductivity and melting point, compared with most common metals, short welding times (generally 0.5-3.0sec) may be used to give joints with properties that approach the parent material strength. Types of friction welding are spin, rotational and vibration welding.

Spin welding is known to have been used in Germany prior to, and during, the Second World War for joining PVC pipes. Subsequently, use of friction welding on thermoplastics did not develop and interest was only renewed following major developments in the process for joining metals throughout the 1950s. The potential of spin welding for joining thermoplastics received increased recognition in the early 1960s with information given by Cheney and Ebeling ^[3] who described successful applications including manufacture of pressurised plastics bottles. In general, rotational speeds are 1-20 m/sec, friction pressures 0.8-1.5 bar, and forge pressures 1-3 bar.

Vibration welding, also known as linear friction welding, was first described in the mid-1970s and has since become increasingly used for welding various mass-produced thermoplastic components. In vibration welding, movement consists of linear oscillations. The vibration is typically 100-240Hz at 1-5mm amplitude, while pressures are similar to those used in rotary friction welding.

Research at TWI has included experimental work to compare the performance of spin and linear vibration welding with two injection moulded thermoplastic materials, ABS and MDPE. Overall, friction welding was excellent for both materials, however spin welding was found to be better as a fast production joining method for circular objects, while the linear vibration process was better suited to welding larger components.

A new vibration welding machine has recently been installed in the Plastics joining laboratory ( Fig.6). The novelty of the machine is that it has an inbuilt dual force cycle. Research by Potente and Kaiser, ^[4] revealed that an increase in the mechanical characteristic values of the weld can be achieved through a process controlled welding pressure profile (with a high welding pressure at the start of vibration and a relatively low welding pressure at the end of vibration and into the cooling phase).

The high welding pressure serves to generate an even layer of melt over the whole of the joining surface within a relatively short time, while the low welding pressure maintains the thickest possible layer of melt. Consequently, addition of this facility has achieved maximum weld strength for a shorter weld cycle.

Implant welding

Implant welding is based on the principle of trapping a metal insert between two parts to be joined and then heating the insert by induction or resistance heating so that the plastics material around the implant fuses to form a joint.

With induction heating a high frequency electromagnetic field (2-30MHz) is used to induce an electric current in the metal implant and resistively heat it.

A development of this technique uses ferromagnetic particles (usually iron oxide) dispersed in a thermoplastic matrix which is preplaced along the joint in the form of a paste or tape.

In resistance implant welding, conductive wire is trapped between the components and directly resistance heated by an electric current as high as 150A.

Implant welding is a simple process which has been applied to complicated joints in large components such as vehicle bumpers, electrically driven vehicles and sailing dinghy hulls.

Welding times are short - up to 20sec for the largest components. The process can be used to join almost all thermoplastics, but the insert limits the joint strength.

A modification of resistance heating implant welding is widely used for joining (>25mm diameter) polyethylene pipes ( Fig.7). In this process, called Electrofusion, the ends of the pipes to be joined are inserted into a fitting with a heating element moulded in. As before, an electric current is used to make the joints. Both the resistance and induction welding techniques have proven to be effective as joining methods for thermoplastic composites. These processes provide a minimum amount of fibre displacement in the joint area when welding composite materials compared with all other fusion joining techniques.

Radio frequency welding

Novel welding techniques

New or improved joining methods are continuously being sought at TWI. As a result techniques using focused infrared, microwaves and induction welding for joining dissimilar materials have been developed.

Focused infrared

Microwave

Microwave heating of plastics is both quick and efficient, and industry is becoming increasingly aware of its potential especially in joining thermoplastic composites.

Preliminary investigations at TWI have been conducted using a 2.45 GHz 800W domestic microwave oven. Welding trials have begun using intrinsically conducting polymers placed at the joint line instead of a metal implant as in resistance welding, eliminating the mechanical stress concentrations that can occur while in service.

Joining dissimilar materials

PCM welding involves surface pretreatment of one component with a thermoplastic coating. A conventional plastics welding technique, induction welding for example, can then be used to join the plastic coated component to either a thermoplastic component or to a second coated component.

The technique has potential, for example in the automotive and aerospace industries, for joining metallic fittings such as hinges and handles to thermoplastic panels.

Current core research

The CRP projects on welding of plastics to be conducted over the next three years are:

• Weldability of recycled thermoplastic materials

  Strong emphasis is being placed on recycling plastics because of the difficulties that have arisen in disposing of materials because of environmental considerations. Currently very little is known about the weldability of recycled materials, so work is being conducted to identify suitable welding techniques for different types and quality of recycled plastics, i.e. PVC, PP and PS.
  

  
  
• Novel methods for joining plastics composites

  The feasibility of using novel welding techniques described above, e.g. microwave and infrared is being determined for joining thermoplastic composites. Benefits will arise from the greater design freedom offered, which will allow for cost effective production methods, reducing overall manufacturing costs.
  

  
  
• Large scale joining techniques for plastics

  In the past, welding of plastics has been limited to relatively small components. However, there is an increasing need to join thicker sections, e.g. 800mm diameter pipe. Since process parameters for welding thick sections have never been extensively derived but have been based on extrapolation from smaller components, joints have been known to be rejected and have had to be rewelded. So it is aimed to optimise welding parameters for hot plate welding of large diameter polyethylene pipe, In addition, work will be carried out to demonstrate the feasibility of vibration welding thick sections of polyethylene and polypropylene. The aim is to improve productivity compared with that of hot plate welding, which is relatively slow.

  Welding of flexible liners will also be considered. The objective is to develop equipment and welding procedures for hot gas or hot wedge welding for manufacturing large structures from flexible plastics sheeting.
  

  
  
• PCM welding technique

  Work has been defined to determine the mechanism of the PCM technique (described previously) so that the process can be optimised in terms of joint mechanical properties and durability to a level at which the techniques can be considered for production.

References