General Design Principles for DuPont Engineering Polymers - Module

This is not always easy, because appreciable motor
power is only needed during acceleration of the mass.
Once the operational speed has been reached, only the
friction needs to be overcome, for which a very low
power is sufficient. The motor is now practically
idling, and may get into an unstable state (e.g., with
series-connected collector motors).
Examples of suitable drives for this type of rotationwelding
machines are:
• Repulsion motors, based on the principle of adjustable
brushes. Single-phase 0.5 kW motors operating
at about 4,000 rpm are generally adequate. A
disadvantage of this kind of motor is the difficulty
of fine speed control.
• Thyristor controlled three-phase or single-phase
squirrel cage motors. The control unit must enable
speed to be adjusted independently for the load,
which is not always the case.
• D.C. shunt motors with armature voltage adjustment.
These are very suitable. Control unit costs
are very modest, so that the overall cost remains
reasonable. The speed can be kept constant enough
without using a tacho-generator and the control
range is more than sufficient.
Experimental welding machines, or production
machines used for parts of different diameters, must
be fitted with one of these types of motor.
For machines used only for joining one particular
component, a variable-speed drive is not absolutely
essential, although of course very useful. If the
machine has a fixed-speed drive, then it is better to
start operating at a rather higher speed than is strictly
necessary. This builds up a little extra energy, so that
proper welds will still be made even when the joints
fit together badly because of excessive molding
tolerances. Of course, more material will be melted
than is strictly necessary.
Compressed air motors or turbines are occasionally
used to drive the machines, but they are more expensive,
both in initial investment and in running costs,
than electric motors, and do not present any advantage.
Jigs (Holding Devices)
These can be subdivided depending on whether:
• the parts are gripped by a jig which is already
rotating as the spindle descends; or
• the parts must be placed in the jig when the spindle
is stationary.
In the first case, the cycle time is shorter, and this
solution is therefore preferred whenever possible. The
following types of jigs are suitable:
• A tooth crown as in Figure 11.16 will grip the
plastic part, as the spindle descends, and cause it to
85
rotate with it. If the teeth are designed properly, and
the piston moves fast enough, the unavoidable
toothmarks made in the plastic can be kept small
and clean. The cutting edges of the teeth must be
really sharp. The teeth are not generally ground, but
the crown must be hardened, especially on production
machines.
• The dimensions indicated in Figure 11.17 are
intended to be approximate; dimensions should be
matched to the diameter of the part. With very thinwalled
parts, it is better to reduce the distance
between the teeth to ensure that enough pressure is
exerted on the joint.
• With larger or more complicated jigs it is better to
design the tooth crown as a separate part which can
be changed if necessary.
Figure 11.16 Jig tooth crown
Figure 11.17 Suggested tooth dimensions
30°
1.2
~4–8 ~3–6
(mm)
• Figure 11.18 shows two typical weld sections with
their corresponding tooth crowns and jigs.
• If the joints have no protruding bead, the bottom
holder a, must fit closely, so as to prevent the part
from expanding (especially if the wall is thin). The
top of the plastic part, b, should if possible have a
rounded bead, to make it easier for the teeth c to grip.
With inertia-type machines, an outer ring d is often
necessary to center the part accurately, especially if
there is too much play between the bottom plastic
part and its holder, or if the piston rod guides are
worn.