General Design Principles for DuPont Engineering Polymers - Module

Joint Strength versus Specific Weld
Pressure
As already mentioned, the appropriate specific weld
pressure should be determined for each plastic material
by trials. For Delrin ® 500 for instance, it was
found to be about 3.3 MPa as plotted on the curve in
Figure 11.79. It appears that too high a pressure
reduces joint strength as well as too low a pressure.
Figure 11.79 Specific weld pressure influences joint
strength
Burst
pressure
2 3
Specific weld pressure
4
All grades of Delrin ® are suitable for vibration welding.
Delrin ® 500 F gives the best results, whereas
Delrin ® 100 is somewhat inferior. Weld joints on parts
in Delrin ® 100 are usually the weakest area due to the
high elongation of this resin. This was also the case
for the test vessel shown in Figure 11.77. The same
part molded in Delrin ® glass filled resin does not
break at the joint but in a corner, because of the lower
elongation. It must also be kept in mind that colored
compositions have a lower weld strength than the
same grade in natural color. This applies to all polymers.
Pigment loadings have a slight adverse effect on
properties. Even though the average strength values
differ somewhat from one grade to another, it is
surprising to notice that the upper limit of about
14 MPa tensile strength is the same for most grades.
Vibration welding is equally suitable for all grades of
Zytel ® nylon resins. It allows many new and attractive
applications for which no other weld procedure would
be applicable. The automotive industry in particular
requires various non-circular vessels and containers in
the cooling circuit as well as for emission control filters.
No special care has to be taken concerning water
absorption before welding, provided that the parts are
stored at a relative humidity no higher than 50%.
Butt joints of parts in unreinforced polyamide are
usually stronger than the part itself. Fillers and glass
fibers reduce joint strength depending on their quality.
Thus 30% glass fibers cause a reduction of up to 50%
in strength. Parts in this resin must be designed very
carefully.
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Design Examples
Figure 11.80 shows a typical centrifugal pump design
with an angular welded spiral housing in Delrin ® .
Figure 11.80
Figure 11.81 shows an automotive tank in Zytel ®
nylon 66 resin. The joint is provided with a flash trap
to avoid any post deflashing operation.
Figure 11.81