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What is spin welding of thermoplastic materials?


There are two types of spin welding: direct drive and inertia. Both techniques are used for welding circular cross-section parts and involve rotating one component and forcing it against the other component which is kept stationary. This creates frictional heat in the region of contact and causes the materials to melt, welding the two components together.

Direct drive spin welding

Frictional heat, produced by a combination of rotational velocity, weld force and weld time, must be sufficient to melt and flow the plastic at the weld interface. In this technique, rotation is kept constant until melting occurs at which point it is stopped. A preset cooling time then begins during which axial force is increased slightly to consolidate the weld.

Welding is fast, taking around 3-5 seconds. On older equipment it is difficult to ensure that the welded parts end up with precise rotational alignment. Modern machines with servo-motor technology can orientate the parts within 1 degree relative to each other.

The automotive and domestic appliance industries are major users of direct drive spin welding. Applications include mass produced under-bonnet components such as thermostat housings, and domestic water cistern floats.

Inertia spin welding

In this technique, sufficient heat to melt and flow the plastic at the weld interface is generated by a combination of weld time, weld force and kinetic energy - the latter being governed by the mass of the rotating workpiece head and its rotational velocity.

The process begins with rotation being brought to a maximum.  At this point, drive (usually a pneumatic motor), is turned off and the two components are immediately brought together under an applied force. Friction between the components to be joined causes the workpiece head to stop rotating. The kinetic energy transferred into the materials creates the heat to make the weld. The components remain under an axial load during the cooling period. Welding takes about the same time as the direct drive technique.

The amount of kinetic energy is varied by changing the mass of the workpiece head or its maximum rotational speed. In practice, it is the head mass which is usually varied to change weld energy input.

The automotive and domestic appliance industries use spin welding for a large range of components including polyethylene floats, aerosol bottles, transmission shafts and PVC pipes and fittings.

As with older direct drive welding machines, it is difficult to ensure that the welded parts end up with precise rotational alignment.

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