General Design Principles for DuPont Engineering Polymers - Module

Figure 11.82. A linear welded motor bicycle petrol
tank in Zytel ® . The groove in the joint collects flash,
then a PVC profile is snapped over the flange. This is
one solution which effectively hides the whole weld
joint.
Figure 11.82
Figure 11.83a shows an angular welded, square
shaped gasoline filter housing in Zytel ® . The joint is
provided with a groove to retain the thin walls in the
jigs, thus preventing them from collapsing during the
welding operation.
Figure 11.83b shows an angular welded container in
Zytel ® . Body and cover house connections must be
oriented in the given position. A classic spinweld joint
with an external flash trap is used for this vibration
welding technique.
Figure 11.83
a b
117
Figure 11.84 shows a rubber diaphragm assemblies
can also be welded by angular vibrations. Steps must
be taken, however, to prevent the upper part from
transmitting vibrations directly to the rubber. This can
be achieved by means of a very thin nylon washer
onto the diaphragm, the use of graphite powder or a
drop of oil. The solenoid valve in Zytel ® glass fiber
reinforced nylon resin shown here has a burst pressure
of 8–9 MPa. A significant advantage over self tapping
screw assemblies lies in the fact that a welded body
remains tight up to the burst pressure.
Figure 11.84
Comparison with other Welding
Techniques
Vibration welding is by no means a rival to ultrasonic
welding although in some cases they may compete.
The solenoid valve shown in Figure 11.84 can for
instance easily be welded ultrasonically. However, the
high frequency can cause the thin metal spring to
break, in which case the whole housing must be
scrapped. Sometimes a complicated part shape does
not allow the welding horn to come close enough to
the joint. In addition gas and air tight ultrasonic joints
require close tolerances which cannot always be
achieved.
Thin wall vessels such as pocket lighters can never be
provided with a large enough joint to reach the
required burst pressure. It would therefore be unwise
to weld them on a vibration machine. Here ultrasonic
welding is the preferred technique.
Vibration welding can be considered in many applications
as a rival to hot plate welding against which it
offers some considerable advantages:
• much shorter overall cycle;
• lower sensitivity to warpage, as the relatively high
weld pressure flattens the parts out;
• since the molten resin is not exposed to air, the
procedure is also suitable for all polyamide grades.