General Design Principles for DuPont Engineering Polymers - Module

simultaneously items having different sizes and Figure 11.71 Principle of linear welding machine
shapes. They must, however, be arranged in the
vibrating jig symmetrically in order to obtain the
same surface pressure on all joints, as shown in
Figure 11.70B.
e e
b
c) Linear welding
Parts which, for reasons of shape or size, do not fit
into an angular jig may be welded by means of
c
d
linear vibrations. This method is especially appropriate
for large size non-circular parts above a
length of 100–150 mm. It is, however, also possible
to weld several parts simultaneously provided
a
they can be fitted into the vibrating plates. rods d. The lower jig slides in two ball bearing rails
allowing free lengthwise motion. The upper jig is
pressed down by four pneumatic operated levers e.
Figure 11.69 Location of motion center
It is essential to synchronize mechanically the motions
Y
of the latter in order to obtain a perfect parallelism of
the parts to be welded.
At the end of the weld cycle, motion transmission is
disengaged whereupon both parts are brought into the
final position and pressure is maintained for a short
time to allow freezing of the melted resin.
The same basic device is used for an angular welding
machine as indicated in Figure 11.72. In this case,
vibrations are transmitted to the upper and lower jigs a
A B
rotating on ball bearings. The upper jig is mounted
directly onto the piston rod b to provide pressure.
Theoretically, the same weld result could be obtained
Figure 11.70 Simultaneous welding of multiple parts
with one part stationary and the other vibrating at
twice the frequency. Experience has proven, however,
that this method is unsatisfactory for various reasons.
As illustrated in Figures 11.71 and 11.72, the considerable
acceleration and deceleration forces cancel out,
provided that the weight of the upper jig plus the
plastic part is equal to the weight of the lower jig plus
the plastic part. (In the case of angular welding the
two moments of inertia must be identical to provide
equal and opposite inertia forces.)
X
A
X
Y
Typical Arrangements for Producing
Vibrations
Although vibrations can be generated by means of
alternating current magnets, all available machines so
far have been equipped with mechanical vibration
generators.
Figure 11.71 shows schematically the function of a
linear welding machine as was first perfected by
DuPont.
The vibrations are generated by two eccentrics a
rotating around center b and transmitted to jigs c by
B
112
Figure 11.72 Principle of angular welding machine
a
b