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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

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