Materials for engineering, 3rd Edition - (Malestrom)

Organic polymeric materials 177
Friction welding
Friction welding can be achieved by several techniques; the simplest is the
spin welding of two thermoplastics at relative speeds of up to 20 m s –1 under
pressures of between 80 and 150 kPa. Welds of high quality may be produced
in a few seconds, although residual stresses may be generated. Tubes and
hollow sections can be welded satisfactorily and, since the process can be
carried out in liquids, it is also a useful method of encapsulation of liquids.
Relative movement of the components by vibration in linear oscillation
may also be employed. This method of friction welding is widely used in the
automotive manufacturing industry to produce large, complex joints. A
development of this principle is ultrasonic welding, in which the parts to be
joined are held together under pressure while mechanical vibrations
perpendicular to the area of contact are applied by means of a piezo-electric
transducer at frequencies in the range 20–40 kHz. As the energy output of
these devices is limited, the size of possible weld is much smaller than that
in normal vibration welding and tooling is expensive, but the method is wellsuited
to mass production and finds wide use in industry in the assembly of
domestic products. No heat is required, and joint strengths approaching 100%
of that of the parent materials are readily achieved.
External heating methods
Hot tool welding employs an electrically heated flat plate which is sandwiched
between the two pieces to be joined. When a temperature of 180–230°C
(depending on the particular polymer) has been achieved, the plate is withdrawn
and the surfaces are pressed together under a specified stress for sufficient
time for a joint to be made. For items of large cross-section, such as large
pipes, this time can be quite protracted (several tens of minutes is common),
but very strong joints can be produced, with strengths at least 90% of the
parent material. It is essential that the surfaces to be joined are clean, for a
successful weld to be achieved.
A variant of this approach is the use of an ‘electrofusion connector’, for
joining plastic piping. It consists of a coiled electric heating element embedded
near the inside surface of a specially constructed joint made of the same
plastic as the pipes to be joined. The joint is assembled, a current passed and
the joint fuses with the pipe material. Rapid weld times can be achieved,
although the joint strength may be impaired by the presence of the heating
element.
Hot gas welding heats a filler rod and the edges of the workpiece to be
welded by means of a stream of hot gas from a welding gun. Compressed air
is normally employed, but if the polymer can undergo degradation by oxidation,
then a nitrogen stream is used. Temperatures of 200–300°C are achieved and