General Design Principles for DuPont Engineering Polymers - Module

The holder a, will have equal and opposing teeth, and
when the plastic part is gripped no damage is caused.
Ring faces b inside and outside the coupling will
transmit the welding pressure to the part, so that the
teeth, in fact, transmit only the torque. The number of
teeth should be kept small to reduce the danger of
their tips breaking off.
These tips should not be too sharp; the teeth should
terminate in a tiny face c 0.3–0.5 mm.
This solution is also suitable for the pivot tools
described before, which do not rotate as fast as inertia
machines. With the high peripheral speed of inertia
machines, it is more difficult to ensure that the teeth
engage cleanly.
Figure 11.21 Couplings with interlocking teeth
a
b c
15°
Cast Resin Couplings
In certain cases it is also possible to drive or grip the
parts by means of elastomer jigs. Synthetic resins are
cast directly into the holding device, the plastic parts
forming the other portion of the mold, so as to get the
right-shaped surface.
Since the maximum torque which can be transmitted
in this manner is low, and the permissible pressure per
unit area is low too, this method is only worth considering
for parts having relatively large surfaces.
Conical parts are the most suited to this type of jig
(see Figure 11.22), because a greater torque can be
transmitted for a given welding pressure.
When this type of jig is used with an inertia machine
and the plastic part has to be accelerated to its welding
speed, there is bound to be a certain amount of slip;
this can cause excessive heating of the surface.
It is therefore extremely important to select a casting
resin of the right hardness; this has to be determined
experimentally. Figure 11.22 shows, in essence, how
the cast elastomer a, also has to be anchored to the
metal parts by bolts, undercuts or slots. The recesses b
are machined out afterwards, because contact here
should be avoided.
87
Making cast resin grips requires a lot of experience
and suitable equipment. The initial costs of this
method are therefore considerable and it has not found
many practical applications.
It may however be economically worth considering
for machines with turntables which need several
holders.
Figure 11.22 Cast resin coupling
a
a
Joint Profiles
If welded joints are to be tight and strong, some
attention must be paid to the joint profiles. The
strength of the weld should be at least as great as that
of its two component parts, so that the area of the weld
face must be about 2–2.5 times the cross-section of the
wall.
V-profiles, used for many years now, have proved far
the best; Figure 11.23 shows two typical examples.
The joint profile in Figure 11.23a is suitable for parts
having equal internal diameters, which can be provided
with external shoulders for the purpose of
driving or gripping. (For example, cylindrical containers
or pressure vessels which have to be made in two
parts on account of their length).
The profile in Figure 11.23b is particularly suitable
for the welding-on of bases or caps (for instance, on
butane gas lighter cartridges, fire extinguishers, or
aerosol bottles).
b
b