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