General Design Principles for DuPont Engineering Polymers - Module

Welding Double Joints
The simultaneous welding of two joints, e.g. in the
carburettor float in Figure 11.33, requires special
processes and greater care. Practical experience has
shown that it is impossible to get good results if the
two halves are gripped and driven by tooth crowns.
Recesses or ribs must always be provided. It is best if
the machine has facilities for adjusting the respective
heights of the inner and outer jig faces, so that the
weld pressure can be distributed over both joints as
required.
In these cases the moment of inertia and the welding
pressure must be calculated for the sum of the surfaces.
The speed, on the other hand, should be chosen
as a function of the smaller diameter.
Figure 11.33 shows a double-joint float, with appropriate
jigs and small ribs for driving the parts. After
welding, the spindle does not travel all the way up, so
that the next part can be inserted into the jig at rest;
only then is the flyweight engaged and accelerated to
its operating speed.
The dimensions of the plastic parts should preferably
be such that the inner joint begins to weld first, i.e.,
when there is still an air-gap of about 0.2–0.3 mm on
the outer joint (see Figure 11.34).
Welding double joints becomes more difficult as the
ratio of the two diameters increases. Although, in
practice, parts with an external diameter of 50 mm and
an internal diameter of 10 mm have been joined, these
are exceptions.
Designs like this should only be undertaken with very
great care and after expert advice.
In order to avoid all risks, it is better to follow the
procedure shown in Figure 11.35. Here the double
Figure 11.33 Welding double joints
93
joint has been divided into two single ones, which can
be welded one after the other and which pose no
problem. This solution enables the parts to be gripped
with tooth crowns in the normal way, automation is
easier, and the total cost is very little more than for
one double joint, while avoiding long-winded and
expensive preliminary testing.
Figure 11.34 Design of double joints
Figure 11.35 Double joint split-up in 2 single joints
Welding Reinforced and Dissimilar
Plastics
Reinforced plastics can generally be welded just as
easily as unreinforced ones. If the filler reduces the
coefficient of friction, the weld pressure may sometimes
have to be increased so as to reduce the effective
weld time.
The weld strength of reinforced plastics is generally
lower because the fibers on the surface do not weld
together. This is not usually evident in practice,
because the joint is not usually the weakest part. If
necessary, the weld profile can be enlarged somewhat.