General Design Principles for DuPont Engineering Polymers - Module

Inertia Welding
By far the simplest method of spinwelding, and the
most widespread, is the inertia method. This requires
minimum mechanical and electrical equipment, while
producing reliable and uniform welds.
The basic principle is that a rotating mass is brought
up to the proper speed and then released. The spindle
is then lowered to press the plastic parts together, and
all the kinetic energy contained in the mass is converted
into heat by friction at the weld face.
The simplest practical application of this method
involves specially built tools which can be fitted into
ordinary bench drills. Figure 11.08 shows a typical
arrangement. The mass a can rotate freely on the shaft
b, which drives it only through the friction of the ball
bearings and the grease packing.
Figure 11.08 Inertia welding using ordinary bench drills
As soon as the speed of the mass has reached that of
the spindle, the latter is forced down and the tooth
crown c grips the top plastic part d and makes it rotate
too. The high specific pressure on the weld interfaces
acts as a brake on the mass and quickly brings the
temperature of the plastic up to melting point.
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Once again, pressure must be kept on for a short
period, depending on the particular type of plastic.
The tool illustrated in Figure 11.08 has no mechanical
coupling, so that a certain period of time (which
depends on the moment of inertia and the speed of the
spindle) must elapse before the mass has attained the
necessary speed for the next welding operation, and
with larger tools or an automatic machine this would
be too long. Moreover, there is a danger—especially
when operating by hand—that the next welding cycle
will be commenced before the mass has quite reached
its proper speed, resulting in a poor quality weld. The
tool shown in Figure 11.08 should therefore only be
used for parts below a certain size (60–80 mm in
diameter).
Since small components can also be welded with
flywheels if high speeds are used, very small tools
(30–50 mm in diameter) are sometimes constructed
which will fit straight into the drill chuck. Figure
11.09 shows such an arrangement, for welding plugs.
Since speeds as high as 8,000 to 10,000 rpm are
needed, a pivot tool like that in Figure 11.02 is
sometimes preferred.
Figure 11.09 Inertia welding for small components
For tools with diameters over 60–80 mm, or where a
rapid welding cycle is required, a mechanical coupling
like in Figure 11.10 is best. Here the mass a can move
up and down the shaft b. When idling, the springs c
force the mass down so that it engages with the shaft
via the cone coupling d. The mass then takes only an
instant to get up to its working speed.
As soon as the spindle is lowered and the tooth crown
grips the plastic, the mass moves upwards and disengages
(see Figure 11.10a). But since the pressure of