General Design Principles for DuPont Engineering Polymers - Module

In all plastics, glass fibers or fillers reduce tensile
elongation, so that stress concentrations are very
harmful. Designers pay far too little attention to this
fact.
Occasionally one is also faced with the problem of
joining plastics of different types, with different
melting points. The greater the difference between the
melting points, the more difficult welding will be, and
one cannot call such a joint a true weld, as it is merely
a mechanical adhesion of the surfaces. The strength of
the joint will be low. It may even be necessary to have
special joint profiles and work with very high weld
pressures.
In practice there are very few such applications, and in
all these cases the parts are not subjected to stresses.
Typical applications are oil-level gauges and transparent
polycarbonate spy-holes welded into holders of
Delrin ® .
The following test results should give some idea of the
possibilities of joining Delrin ® to other plastics.
The float of Delrin ® shown in Figure 11.13 has a
burst pressure of about 4 MPa. If a cap of some other
material is welded onto a body of Delrin ® , the burst
pressures are as follows:
Zytel ® 101 (nylon resin) 0.15–0.7 MPa
Polycarbonate 1.2–1.9 MPa
Acrylic resin 2.2–2.4 MPa
ABS 1.2–1.6 MPa
It must be remembered that, in all these cases, the
weld forms the weakest point.
Spin Welding Soft Plastics and
Elastomers
The softer the plastic, with a few exceptions (e.g.,
fluoropolymers), the higher the coefficient of friction.
Spin welding therefore becomes increasingly difficult
with soft plastics, for the following three reasons:
• The deceleration produced by a high coefficient of
friction is so great that the flyweight is unable to
produce heat by friction. Much of the energy is
absorbed in the deformation of the component,
without any relative motion occurring between the
joint faces. If the amount of kinetic energy is
increased, one is more likely to damage the parts
than to improve welding conditions.
It is sometimes possible to solve this problem by
spraying a lubricant onto the joint faces (e.g. a
silicone mold release). This reduces the coefficient
of friction very considerably at first, so that the
usual rotation takes place. The specific pressure is,
however, so high that the lubricant is rapidly
squeezed out, the friction increases, and the material
melts.
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• For soft plastics having a very low coefficient of
friction a very much higher specific pressure is
needed to produce sufficient heat by friction in a
short time. Most components cannot stand such a
high axial pressure without being permanently
deformed, and there is to date no reliable way of
making satisfactory joints between these materials
by spin welding.
• Soft plastic parts are difficult to retain and cannot
easily be driven. Transmission of the high torque
frequently poses an insoluble problem, particularly
since it is scarcely possible to use tooth crowns.
To sum up, it can be said that marginal cases of this
sort should be approached only with extreme caution,
and that preliminary experimental work is unavoidable.
Figures 10.36–10.38 show only a few selected
examples out of the great number of possibilities in
this field.
Examples of Commercial and Experimental
Spin Welding Machines
Figure 11.36 Commercial spinwelding machine. This
machine is driven by a compressed air
motor which allows the speed to be
adjusted within broad limits. The specific
model shown in the photograph is
equipped with an 8-position turntable and
jigs for welding a spherical container.