General Design Principles for DuPont Engineering Polymers - Module

Figure 11.23 Joint profiles
t 0.1 t
15°
0.6 t
t 0.6 t (min. 1 mm)
0.4 t
15°
0.05 t
0.6 t
0.8 t 0.8 t
1.8 t
a b
(mm)
The wall thickness dimensions given are only suggestions;
the structure of the parts must of course also be
taken into consideration. But the area of the joint face
should never be reduced. Plastics which have a high
coefficient of friction tend to be self-locking if the
angle of inclination is too small, preventing the tooth
crown from rotating and causing it to mill off material.
Angles of less than 15° should therefore be employed
only with the greatest care.
For profiles like that in Figure 11.23a, a certain
amount of play should be provided for, before welding,
between the surfaces at right angles to the axis of
the part. This ensures that the entire pressure is first
exerted on the inclined faces, which account almost
entirely for the strength of the joint.
It is impossible to prevent softened melt from oozing
out of these joints and forming flash, which is often a
nuisance and has to be removed afterwards. If the
welded vessels contain moving mechanical parts,
loose crumbs of melt inside could endanger their
correct functioning and cannot therefore be allowed.
Figures 10.24a-d show four suggested joint profiles,
all of which have grooves to take up the flash.
The simple groove flash trap shown in Figure 11.24a
will not cover up the melt but will prevent it from
protruding outside the external diameter of the part;
this is often sufficient. The overlapping lip with small
gap, shown in Figure 11.24b, is common.
Figure 11.24c shows flash traps so arranged that they
are closed when welding is complete. Figure 11.24d
shows a lip with a slight overlap on the inside, which
seals the groove completely and prevents any melt
from oozing out. The external lip will meet the
opposite edge when the weld is complete.
1.8 t
0.5 t
5°
0.4 t
15°
t
5°
0.2 t
30°
0.5 t
1.5 t
88
The type of weld profile shown in Figure 11.23b can
also be given an edge which projects to the same
extent as the top of the container.
Figure 11.25 shows such a design, used occasionally
for butane refill cartridges. Generally an open groove
is good enough. A thin undercut lip a, can also be
used, so that the flash trap becomes entirely closed. Of
course, a lip like this can be provided on the outside
too, but it demands more complicated tooling for the
ejector mechanism and should not therefore be used
unless absolutely essential.
Figure 11.24 Joint profiles with flash traps
a b
c d
Figure 11.25 Joint with prevented outside protrusion
0.8 T
0.3 T
T
a