Friction welding techniques

The process
Friction welding refers to a range of techniques that rely on a relative motion between the thermoplastic parts to be joined, while a force is applied between them, to cause the material at the interface to heat and melt. The vibration is then stopped and the parts are aligned, and held together under pressure until a solid bond is formed. Such bonds are permanent, and have a strength approaching that of the parent material.

The main welding parameters associated with friction welding are speed or frequency friction pressure, forge pressure, displacement and duration. These parameters are generally interdependent and for any application a set of weld property optimisation trials would be undertaken prior to mass production of the welded thermoplastic component.

Almost any thermoplastic material can be friction welded, including filled, structural foamed, crystalline, and amorphous materials. There is the possibility of welding dissimilar thermoplastics and alloys, and successful joints can be made with, for instance, PS and ABS, PMMA and PC, and PPO/PA and PA.

 

Linear Vibration Welding
The principle behind linear vibration welding is that the parts to be joined are brought into contact, under pressure, before being moved so that the joint areas rub together with a linear reciprocating motion.

The parts are vibrated through a relatively small displacement, commonly referred to as the amplitude, of typically between 1.0 and 1.8 mm, for a frequency of vibration of 200Hz (high frequency), or 2-4 mm at 100Hz (low frequency), in the plane of the joint.

Industrial applications tend to be based around joints that are too long to be ultrasonically welded (i.e. greater than around 200 mm) and hot plate welding would typically take many minutes to perform.

Automotive Two-part bumper, fuel tanks, fuel pumps, expansion vessels, instrument panels, air channels, parcel shelves, inner door panels, hermetic sealing of a length of air ducting to the internal surface of a dash-board
Consumer Spectacle frames, typewriter cover
Industrial Filter housings, motor saw housings, heating valves, air induction ducting

 

Orbital Friction Welding
In orbital welding each point on the surface of one part orbits a different point on the face of the other part. The orbit is of constant rotational speed and is identical for all points on the joint surface. This motion is stopped after sufficient material is melted and the thermoplastic then solidifies to form a weld.

Orbital welding is a relatively new technique, and tends to fill the size gap between benchtop ultrasonic units and linear vibration welders, and most applications tend to be for automotive components.

 

Spin Welding
Spin welding requires a relative rotational motion between the parts to be joined, which always have a circular joint area.

Possible configurations for spin welding thermoplastics

The technique can involve relatively simple pieces of equipment such as lathes or drilling machines. A lathe would produce a constant speed during the frictional heating stage (continuous drive friction welding) and a drilling machine would produce a reducing speed characteristic during the frictional heating stage (inertia friction welding). In practice, purpose built machines are generally employed for spin welding to provide greater control and they may be of either the continuous drive or inertia type.

Spin welding has been exploited for applications as diverse as the manufacture of polyethylene floats, aerosol bottles, transmission shafts and PVC pipes and fittings. Apart from being a fast technique, another particular advantage is that welds can be formed beneath the surface of a liquid.

 

Angular Friction Welding
The principle behind angular friction welding is similar to linear vibration except that the motion is angular. The components to be welded are pre-assembled and vibrated in an angular motion through a few degrees. When the weld cycle is complete the component parts are returned to the pre-welded position ensuring good alignment. The angular friction welding process is used for circular components where upper and lower component alignment is critical, and equipment tends to be bespoke.

 

*Information courtesy of The Welding Institute World Center for Materials Joining Technology; UK