General Design Principles for DuPont Engineering Polymers - Module

Figure 11.18 Typical weld sections
d
b
1–2 mm
a
c
• The bottom half of the plastic part can be fitted with
an identical tooth crown (see also Figures 11.13 and
11.20) to prevent its rotating. With the Venturi tube
shown in Figure 11.19, its side part is used for
retention. Obviously this makes automatic insertion
very difficult, if not impossible. The lower part is
about 200 mm long, which in itself would make
automation too complicated. This is a good example
of what was said before about the minimum length
of piston stroke. Since the total length of the welded
parts is about 300 mm, the piston stroke would have
to be about 350 mm; a machine like this would be
Figure 11.19 Part with Venturi tube
s
86
impractical and expensive; and the rotating flywheel
on the long piston-rod would be very dangerous.
This problem could be avoided by using a turntable,
but this would not be very practical either, because
the parts are so long.
• The arrangement suggested in the drawing shows a
holder a, which embraces one half of the part only,
the other being held by a pneumatic device b. This
enables the piston stroke to be kept short, and the
parts are easily inserted and removed. In addition, the
joints are supported around their entire circumference.
• Frequently the tooth crown cannot be sited immediately
above the weld; e.g., with the float shown in
Figure 11.20 this is impossible for technical reasons.
In such cases the length L, i.e., the distance
between weld and tooth crown, must be in proportion
to the wall thickness, so that the high torque and
the welding pressure can be taken up without any
appreciable deformation. This will of course also
apply to the bottom plastic part.
• Selection of the joint profile and of the jig is often
governed by the wall thickness.
Figure 11.20 Part with Venturi tube
Couplings with Interlocking Teeth
Instead of a tooth crown which has to be pressed into
the plastic in order to transmit the torque, toothed
couplings are occasionally used, and matching teeth
are molded into the plastic part; they may either
protrude or be recessed (as in Figure 11.21), whichever
is more convenient.
L