General Design Principles for DuPont Engineering Polymers - Module

Machines for Inertia Welding
The principle of the inertia welding machine is so
simple that it is possible to build one with very little
investment.
If the machine is mainly used for joining one particular
pair of components, it will not generally require to
have facilities for varying the speed. If this should
prove necessary, it can be done by changing the belt
pulley.
Except for the welding head, the machine shown in
Figure 11.12 is entirely built from commercially
available parts. It consists basically of the compressed
air cylinder a, which supports the piston rod at both
ends and also the control valve b. The bottom end of
the piston rod carries the welding head c (see Figure
11.13), driven by the motor d via the flat belt e. The
machine also incorporates a compressed air unit f with
reducing valve, filter and lubricating equipment.
The welding head shown in Figure 11.13 (designed
by DuPont) consists of a continuously rotating belt
pulley a, which carries the coupling lining b. In the
drawing, the piston rod is at the top of its stroke and
the movement of rotation is transmitted via the
coupling to the flywheel c.
As the spindle descends, the coupling disengages and
the tooth crown grips the top of the float, shown as an
example.
Figure 11.12 Inertia welding machine
d
e
f
a
c
b
83
Figure 11.13 Inertia welding machine head
a
b
c
d
If it is impossible to grip the part with a tooth crown,
and it has to be fitted into the top jig by hand (as in
Figure 11.06, for example), an extra control will be
necessary. The piston will have to pause on the
upstroke just before the coupling engages, to enable
the parts to be inserted. This can be managed in
various ways. For example, one can buy compressed
air cylinders fitted with such a device. A pulse passes
from the travelling piston directly to a Reed switch on
the outside.
So that the parts may be taken out conveniently, the
piston stroke must generally be about 1.2 times the
length of the entire finished welded part. Long parts
require considerable piston strokes, which is impractical
and expensive. Figure 11.14 shows a typical
example—a fire-extinguisher—for which a piston
stroke 1.2 times the length of the part would normally
have been needed.
However, there are various ways of circumventing
this:
• The bottom holder a, can be fitted with a device for
clamping and centering, so that it can easily be
released by hand and taken out sideways.
• Two holders are fitted, a and b, which can swivel
through 180° about the axis X-X by means of a