General Design Principles for DuPont Engineering Polymers - Module

For the example illustrated, F is about 3 cm2 and the
mean weld diameter d = 60 mm. Starting at 3 cm2 on
the left-hand scale, therefore we proceed towards the
right to meet the line which corresponds to a diameter
of 60 (Point 1), and then proceed vertically upwards.
A convenient diameter and associated length of
flywheel (see Figure 11.28) are chosen. But the
diameter should always be greater than the length, so
as to keep the total length of the rotating flywheel as
small as possible. In the example illustrated, a diameter
of approximately 84 mm has been chosen, giving
a length of 80 mm (Point 2).
The nomogram is based on a peripheral speed of
10 m/sec, which gives about 3,200 rpm in this example
(60 mm diameter). A higher speed can be
chosen, say 4000 rpm, which corresponds to Point 3.
The tool dimensions obtained by moving upwards
from this point will of course be smaller than before.
In this example we have Point 4, which corresponds to
a diameter of 78 mm and a length of 70 mm.
Moving towards the right from the point corresponding
to 3 cm2 , the corresponding welding force required
is read off from the right-hand scale; in this case,
about 1500 N.
This nomogram considers only the external dimensions
of the tools, and ignores the fact that they are not
solid; but the jig to some extent compensates for this,
and the values given by the nomogram are accurate
enough.
Figure 11.27 Welding parameters example
θ d
F
θ d
F
90
Figure 11.28 Flywheel size example
θ D
Motor Power
In addition to their many other advantages, inertia
tools require only a very low driving power.
In a fully or semiautomatic machine, the entire cycle
lasts between 1 and 2 seconds, so that the motor has
sufficient time to accelerate the flyweight up to its
operating speed. During welding the kinetic energy of
the tool is so quickly converted into heat that considerable
power is generated.
For example, if the two tools considered in the nomogram
of Figure 11.26 are stopped in 0.05 sec, they
will produce about 3 kW during this time. If a period
of 1 second is available for accelerating again for the
next welding cycle, a rating of only 150 W would
theoretically be required.
0.5 kW motors are sufficient to weld most of the parts
encountered in practice.
We have already mentioned that it is highly desirable
to be able to vary the speed. With production machinery
which always welds identical parts, the speed can
be adjusted by changing the belt pulleys.
P
Quality Control of Welded Parts
To ensure uniform quality, the joint profiles should
first be checked on a profile projector to see that they
fit accurately. Bad misfits and excessive variations in
diameter (due to molding tolerances) cause difficulties
in welding and poor quality welds. Correctly dimensioned
joint profiles and carefully molded parts will
render systematic checking at a later stage superfluous.