General Design Principles for DuPont Engineering Polymers - Module

d) At the end of the welding period, the cross-slide
moves back again to release the part a1, which
immediately begins to rotate.
e) The motor is switched off (or the spindle
disengaged).
f) Pressure must be kept up by means of the springloaded
point for a short time, the duration of
which will depend on the solidification properties
of the particular plastic, before the parts can be
taken out.
Figure 11.01 Pivot welding on a lathe
b a a1 c
e d
This sequence is often made simpler by not removing
the stop e at the end of the welding cycle, but by
merely disengaging or switching off. Since, however,
the moving masses in the machine are generally fairly
considerable, they will not decelerate fast enough, and
the weld surfaces will be subjected to shear stresses
during solidification, often resulting in either lowstrength
or leaking joints.
In general, the narrower the melting temperature range
of the plastic, the more quickly does the relative
velocity of the two parts have to be reduced to zero; in
other words, either the fixed partner must be rapidly
accelerated, or else the rotating partner must be
quickly stopped.
Using a lathe for spinwelding is not really a production
method, but it can be used sometimes for prototypes
or pre-production runs. It is, however, a very
good way of welding caps and threaded nipples onto
the end of long tubes. For this purpose the tailstock is
replaced by a spring-loaded jig which grips the tube
and at the same time exerts pressure on it; although
the lathe needs to be fitted with a clutch and a quickacting
brake, because a long tube cannot be released
and allowed to spin.
Pivot Welding on Drilling Machines
Components up to 60 mm in diameter can very easily
be welded on table-type drilling machines with
special-purpose tools. This is the most suitable method
for pre-production runs, hand-machined prototypes, or
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repair jobs. The process can be made fully automatic,
but this is not sufficiently economical to be worthwhile.
Some practice is needed to obtain uniform
welds, because the welding times and pressures are
influenced by the human factor.
The tool shown in Figure 11.02 has an interchangeable
tooth crown a whose diameter must match that of
the plastic part. With a set of three or four such
crowns it is possible to weld parts ranging from about
12 to 60 mm in diameter.
Figure 11.02 Pivot welding on drilling machines
b
The pressure of the point can be adjusted, by the
knurled nut b, to suit the joint surface. The tightness
and strength of the weld will depend on the pressure,
and the correct pressure must be determined by
experiment.
To make a weld, the drill spindle is lowered slowly
until the tooth crown is still a few millimeters above
the plastic part (see Figure 11.03a). Contact should
then be made sharply, to prevent the teeth from
shaving off the material, and so that the part starts
rotating immediately. In the form shown in Figure
11.03b, the pressure should be kept as constant as
possible until a uniform flash appears. Then the tooth
crown is pulled up as sharply as possible (see Figure
11.03c) until the teeth disengage, but with the point
still pressed against the part until the plastic has
hardened sufficiently.
a