TWI Knowledge Summary

Resistive implant welding technique

Resistive implant welding involves trapping an electrically conducting insert between the two parts to be joined, and then resistively heating the insert by passing an electric current through it. The inserts are frequently made of metal in wire, braid or mesh form and the electric current is generally DC or low frequency AC. As the implant heats due to resistive losses, the surrounding thermoplastic softens and melts and, with the aid of an applied pressure, forms a weld.

Resistive implant welding is a simple technique, which can be applied to almost any thermoplastic. Weld times can be relatively short, typically under 60 seconds for most applications.

As with any implant welding technique there is a consumable, which must be taken into account in any calculation of cost. However, the rest of the equipment is relatively simple, comprising a power supply and pressure application system.

The major application for resistive implant welding is in the joining of thermoplastic pipes. In this application, the process is generally called 'Electrofusion' and involves the use of a specially designed coupler containing an integral heating coil.

Resistive implant welding has also been used for the joining of carbon fibre advanced composite materials, where the implant is a single ply of the carbon fibre reinforced composite material. In this case, an electric current is passed down the carbon fibres, which heat due to ohmic losses.

Other applications for this welding technique include the joining of sailing dinghy hulls, and the joining of two-part plastic car bumpers.

One extra consideration for the exploitation of this technology, particularly in the automotive industry, is that it requires an electrically conducting implant, which may cause problems with recycling.