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In the previous Design Guide we looked at relative motion and the movement of parts in the friction welding process.

In this Design Guide we con-tinue the theme of joining moulded parts by welding, this time with a look at induction welding – sometimes known as electromagnetic welding.

Induction welding differs from other plastics welding processes in that it requires a bonding agent or material that is inserted at the joint interface. This is because induction weld-ing demands a magnetically ac-tive material, while conventional thermoplastics are not magneti-cally active.

The bonding agent can be a metal foil, fabrication or stamp-

Introducing induction weldingDesigner’s notes

Principle of the induction welding process.Source: After Stanelco

Extending our focus on joining moulded parts with welding, this week we look at electromagnetic welding

Preferred joint types. Source: After Stanelco

■ Induction welding requires a joint insert of magnetically active material■ The process can be used with irregular or very large joints■ Dissimilar materials can be joined■ Jigs must be made of non-conductive material■ Joints can be remelted for disassembly

ing but it usually takes the form of a specially compounded ther-moplastic material containing ferromagnetic particles, usually iron or stainless steel. The ther-moplastic base should be the same as, or highly compatible with, the parts to be joined.

When such a material is ex-posed to a magnetic field of rapidly alternating polarity, the ferromagnetic particles attempt at an atomic level to align with the polarity of the field. This atomic motion creates heat which is conducted to the ther-moplastic matrix.

The matrix reaches fusion tem-perature in as little as one or two seconds for small joints. Very large assemblies may take from 10 to 30 seconds for fusion.

Induction welding machines use an induction generator to

convert a standard electrical supply to a frequency in the range 2-10MHz, with a power of 1-5kW. Induction heaters de-signed for metals operate at much lower frequencies and are not suitable for plastics.

The high-frequency alternat-ing current is supplied to a wa-ter-cooled copper induction coil that surrounds the joint, and it is this coil that radiates the magnetic field. The field energy obeys the inverse square law, so coils should conform as closely as possible to the joint.

The compounded bonding agent comes in a variety of forms. Tape, strip and extruded profiles can be placed in joints of relatively simple shape, while complex joints can be handled with stamped or injec-tion moulded preforms that

is usually formulated specifical-ly for each application, to pro-duce an optimum combination of heating and melt bonding.

Induction welded joints cost more than other types of weld because of the need for an in-serted preform but they have a number of advantages. Irregu-lar and large joints can be welded, dissimilar materials can be joined, thermal and me-chanical stress is low, and part tolerances need not be ex-tremely precise.

Should it be necessary for repair or dismantling, induc-tion welded joints can be bro-ken by applying the magnetic field again; only the material containing ferromagnetic par-ticles will melt.Clive Maier, Econology

conform exactly to the joint shape.

To start the welding process, the parts are assembled with an insert of bonding agent trapped in the joint. Workholding jigs used to locate the parts in the welding machine must be made of non-conductive mate-rial to avoid interfering with the magnetic field.

The parts are pressed to-gether under moderate pres-sure – typically less than 0.5MPa – and are exposed to the magnetic field which melts the thermoplastic bonding agent. The pressure forces the molten agent material through-out the joint where it contacts and melts the joint surfaces, and bonds to them on cooling.

Joint configurations are de-signed to confine the molten bonding agent. Tongue and groove, shear and step joints are the usual forms, although plain and grooved flat joints can be used for large applications such as panels.

Because the type of thermo-plastic material in the bonding agent can be varied in the com-pounding process, induction welding can be used to join most plastics to themselves and can also be used for a wide range of dissimilar thermoplas-tics. Indeed, the bonding agent

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