INDUCTION WELDING

Induction welding is a form of welding that uses electromagnetic induction to heat the workpiece. The welding apparatus contains an induction coil that is energised with a radio-frequency electric current. This generates a high-frequency electromagnetic field that acts on either an electrically conductive or a ferromagnetic workpiece. In an electrically conductive workpiece, such as steel, the main heating effect is resistive heating, which is due to magnetically induced currents called eddy currents. In a ferromagnetic workpiece, such as plastic doped with ceramic particles, the heating is caused mainly by hysteresis as the magnetic component of the electromagnetic field repeatedly distorts the crystalline structure of the ferromagnetic material. In practice, most materials undergo a combination of these two effects.

As suggested above, nonmagnetic materials such as plastics can be induction-welded by implanting them with metallic or ferromagnetic compounds, called susceptors, that absorb the electromagnetic energy from the induction coil, become hot, and lose their heat energy to the surrounding material by thermal conduction.

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 , 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.

Induction welding is a form of welding that uses electromagnetic induction to heat the workpiece. The welding apparatus contains an induction coil that is energised with a radio-frequency electric current. This generates a high-frequency electromagnetic field that acts on either an electrically conductive or a ferromagnetic workpiece. In an electrically conductive workpiece, such as steel, the main heating effect is resistive heating, which is due to magnetically induced currents called eddy currents. In a ferromagnetic workpiece, such as plastic doped with ceramic particles, the heating is caused mainly by hysteresis as the magnetic component of the electromagnetic field repeatedly distorts the crystalline structure of the ferromagnetic material. In practice, most materials undergo a combination of these two effects.

As suggested above, nonmagnetic materials such as plastics can be induction-welded by implanting them with metallic or ferromagnetic compounds, called susceptors, that absorb the electromagnetic energy from the induction coil, become hot, and lose their heat energy to the surrounding material by thermal conduction.