TWI Knowledge Summary

Induction Welding of thermoplastics

by Felicity Chipperfield

The process

Induction welding describes welding techniques where heating is generated by an induction field. The two most commonly encountered mechanisms by which heat can be generated by an induction field are eddy current heating and heating due to hysteresis losses.

Induction welding is similar to resistive implant welding, in that an electrically conducting implant is required at the joint line. A work coil, which is connected to a high frequency power supply, is then 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 the implant. For induction welding by eddy current generation, electric currents are induced in the implant and when these are sufficiently high to heat the conducting material, the surrounding thermoplastic parts melt or soften. If pressure is applied to the joint a weld will form.

Induction welding can be very fast; weld times may be a few seconds. Applications include sealing plastic coated metal caps to plastic bottles and welding metal grilles to the front of loudspeaker units. In both of these cases, the implant has been a part of the item being welded. One of the features governing the efficiency of induction welding is the magnetic permeability of the implant. If the implant has high relative permeability, i.e. is ferromagnetic, then heating may be very rapid.

Induction welding via eddy current generation has also found use in joining advanced thermoplastic composites. The fact that many carbon fibre reinforced composites conduct AC and DC electric current is probably due to percolation ^[1] , although several other theories exist. It is therefore possible to heat many carbon fibre reinforced thermoplastic composites using an induction field to produce a weld, and several research groups have harnessed this effect.

Recently, a new form of induction welding by eddy current has been developed. Originally developed by Metcal and now owned by Uponor this new concept revolves around an implant material that is able to 'switch off' at certain temperatures. The implant is ferromagnetic (has high relative permeability) until it reaches a certain temperature (the Curie point) above which it becomes paramagnetic and loses its ferromagnetic properties. This results in a very large reduction in heating effect by induction. As the implant cools back through the Curie point, the implant becomes ferromagnetic once more and heating recommences. In this way it is possible to stabilise the weld temperature around the implant. It is also possible to alter the characteristic Curie point by varying the composition of the alloy comprising the implant. Field trials on polyethylene pipes have demonstrated that this system enables the temperature in the joint to be controlled and hence the chance that welds are defective due to overheating is dramatically reduced.

Induction welding by eddy current heating is generally a geometry dependent technique because complete circuits are required to allow eddy currents to flow in the implant. For this reason, long thin linear joints are difficult to induction weld using eddy current generation. Where components contain circular symmetry or complete circuital implant paths this technique is often the most efficient. If the thermoplastic component to be joined is electrically conductive, due to filler content for example, the development of induction welding may be difficult due to problems with controlling the distribution of eddy currents.

EMA (electromagnetic) welding

EMA weld has been developed by one company (EMAbond) and is a process which harnesses hysteresis loss in ferromagnetic materials as the main heating mechanism. All ferromagnetic materials exhibit some hysteresis loss due to a number of mechanisms which operate on the scale of ferromagnetic domains or smaller. Losses due to hysteresis are generally frequency dependent for any material but whose magnitude may be calculated by the area enclosed in a B vs H hysteresis loop for the material (see Fig.1), where B is the induction field (tesla) and H the magnetic field strength (A/m).

Fig.1. Graph showing B the induction field against H the magnetic field strength for iron. The magnetisation curve is the full line (OS) and the hysteresis loop is the broken line. The energy dissipated in one circuit of the hysteresis loop equals the area enclosed by the loop.

EMAbond market implants, which are a composite of thermoplastic with particulate ferromagnetic filler, based on iron. This implant is trapped between the two parts to be welded and excited by the dynamic magnetic field generated by a work coil placed in close proximity to the implant. Since the hysteresis loss mechanism is dependent on frequency, the power supply required to excite the implant has to supply current in this peak efficiency region which is usually between 2 and 10MHz.

Unlike induction heating by eddy currents, EMA weld is able to produce long linear welds in a few seconds. Applications have included the welding of automotive and domestic appliance components. EMA weld is able to join most types of thermoplastic except those which are electrically conductive because of screening effects. However, the tape consumable is only produced in a standard range of thermoplastics and consumables outside this range can be prohibitively expensive to produce.

More information

You can use the Weldasearch literature database to supplement what you find in JoinIT.

[1] D Stauffer and A A Harony, Introduction to perculation therory, 2nd Edition, Taylor and Francis, 1992.

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