General Design Principles for DuPont Engineering Polymers - Module

11—Assembly
Techniques,
Category II Welding,
Adhesive Bonding
Spin Welding
Introduction
Rotation welding is the ideal method for making
strong and tight joints between any thermoplastic parts
which have symmetry of rotation. Engineers faced
with the choice of either the ultrasonic or the spinwelding
process will unhesitatingly prefer the latter, in
view of the following advantages which it presents:
• The investment required for identical production is
lower with spinwelding than with ultrasonics. There
are no special difficulties in construction the machinery
from ordinary commercial machine parts,
either wholly or partly in one’s own workshop.
• The process is based on physical principles which
can be universally understood and mastered. Once
the tools and the welding conditions have been
chosen correctly, results can be optimized merely
by varying one single factor, namely the speed.
• The cost of electrical control equipment is modest,
even for fully automatic welding.
• There is much greater freedom in the design of the
parts, and no need to worry about projecting edges,
studs or ribs breaking off. Molded in metal parts
cannot work loose and damage any pre-assembled
mechanical elements. Nor is it essential for the
distribution of mass in the parts to be symmetrical
or uniform, as is the case with ultrasonic welding.
If the relative position of the two components matters,
then an ultrasonic or vibration welding process must
be used.
But, in practice, there are often cases in which this is
essential only because the component has been badly
designed. Parts should, as far as possible, be designed
in such a way that positioning of the two components
relative to each other is unnecessary.
Basic Principles
In spinwelding, as the name implies, the heat required
for welding is produced by a rotating motion, simultaneously
combined with pressure, and therefore the
process is suitable only for circular parts. It is of
course immaterial which of the two halves is held
fixed and which is rotated. If the components are of
different lengths, it is better to rotate the shorter one,
to keep down the length of the moving masses.
In making a selection from the methods and equipment
described in detail below, the decisive factors
77
are the geometry of the components, the anticipated
output, and the possible amount of capital investment.
Because of the relatively small number of mechanical
components needed, the equipment can sometimes be
constructed by the user himself. In this way, serious
defects in the welding process can often be pinpointed,
some examples of which will be described later.
Practical Methods
The most commonly used methods can be divided
roughly into two groups as follows:
Pivot Welding
During welding the device holding the rotating part is
engaged with the driving shaft, the two parts being at
the same time pressed together. After completion of
the welding cycle, the rotating jig is disengaged from
the shaft, but the pressure is kept up for a short time,
depending on the type of plastic.
Inertia Welding
The energy required for welding is first stored up in a
flywheel, which is accelerated up to the required
speed; this flywheel also carries the jig and one of the
plastic parts. Then the parts are forced together under
high pressure, at which point the kinetic energy of the
flywheel is converted into heat by friction, and it
comes to a stop. In practice this method has proved
the more suitable one, and will therefore be described
in more detail.
Pivot Welding
Pivot Welding on a Lathe
Easily the simplest, but also the most cumbersome
welding method in this group, pivot welding can be
carried out on any suitable sized lathe. Figure 11.01
illustrates the setup.
One of the parts to be welded, a, is clamped by b,
which may be an ordinary chuck, a self-locking chuck,
a compressed air device, or any other suitable device,
so long as it grips the part firmly, centers and drives it.
The spring-loaded counterpoint c in the tailstock must
be capable of applying the required pressure, and
should be able to recoil 5–10 mm. The cross-slide d
should also, if possible, be equipped with a lever. The
plastic part a1 should have some sort of projecting rib,
edge, etc., so that the stop e can prevent it from
rotating.
The actual welding will then proceed as follows:
a) The part a is fixed into the clamp, and then its
companion piece a1 is placed in position, where it
is kept under pressure by the spring-loaded point.
b) The cross-slide d travels forward, so that the stop
e is brought below one of the projections on a1.
c) The spindle is engaged or the motor switched on.