Friction welding is a technique in which the heat needed to melt the thermoplastic material is generated by pressing one of the parts to be joined against the other and rapidly vibrating it through a displacement.
The heat generated by the resulting friction melts the material at the interface in 2-3 seconds. 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 rotational speed, 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.
The principle behind 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. The displacement, commonly referred to as the amplitude, is typically between 0.07 and 1.8 mm, with 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 linear joints which are sufficiently long such that ultrasonic welding cannot be easily accomplished (approximately 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, type writer cover |
| Industrial | Filter housings, motor saw housings, heating valves, air induction ducting |
Orbital welding is another frictional technique for joining thermoplastic parts, where each point on the surface orbits a different point on the face of the stationary 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 the first welding units are being marketed as devices to fill the size gap between benchtop ultrasonic units and linear vibration welders. This means that items such as medium sized automotive components (fluid reservoirs etc.) may be joined using this process.
The principle behind spin welding is similar to that of linear friction welding. The joint areas are always circular and the motion is direct rotational.
Direct rotational friction welding (spin welding) of thermoplastics involves 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.
The principle behind angular friction welding is similar to linear vibration except that the motion is angular.
In spin welding it is not possible to determine where one half of the component will end up in relation to the other half when welded. In angular welding the components 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.
Copyright 2001, TWI Ltd
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