General Design Principles for DuPont Engineering Polymers - Module

3. Vibrations, generated either by a gear box or by
an electric magnet, are transmitted to the jigs and
through them to the joint surfaces. The motions of
the two parts take place in opposite directions,
thus creating a relative velocity at the weld
surfaces. As a result of friction, temperature rises
immediately, reaching the melting point of the
plastic, usually in less than a second.
4. After a pre-set time, an electrical control device
stops the vibrations whilst pressure on the joint is
maintained. Simultaneously the parts are brought
into the correct position relative to each other.
5. Pressure is maintained for a few seconds to allow
the melt to freeze. Then the jigs open and the
welded parts are ejected.
Basic Principles
The various weld methods for joining parts in thermoplastic
materials differ essentially in the way heat is
built up on the joint surface.
The presently known procedures can be split into two
basically different groups:
1. The heat required to reach the melting temperature
is supplied by an outside source. This is the case
with hot plate welding, induction welding and hot
air welding.
2. The necessary heat is generated directly on the
joint surfaces by means of friction. The best
known methods using this procedure are
spinwelding and ultrasonic welding. They have
the obvious advantage that the melted resin is
never exposed to open air, in this way preventing
decomposition or oxidation which, for some
plastics, must be avoided.
Spinwelding, however, is limited to circular
shaped parts which, in addition, do not require
positioning. If the two items are to be joined in an
exact position relative to each other spinwelding
equipment becomes quite costly because there
are no simple mechanical means to fulfill this
requirement.
Vibration welding belongs to the second group since it
produces heat by means of friction on the two joint
surfaces. Unlike the spinwelding procedure, vibration
welding is not limited to circular parts. It can be
applied to almost any shape provided that the parts are
designed to permit free vibrations within a given
amplitude.
Definition of Motion Center
The center around which two parts vibrate can be
located:
a) inside the joint area
b) outside the joint area
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c) at an infinite distance, in which case the motion
becomes linear
Based on this, two distinct variations can be defined:
Angular and linear welding.
a) Motion center inside the joint area
All parts having a perfectly circular weld joint
would logically vibrate around their own center as
shown in Figure 11.68A. Such parts can be
provided with a V-shaped weld joint as described
in chapter “Circular Parts.” All parts having a
non-circular shape must of course be provided
with flat joint surfaces. If the weld area has an
irregular shape, as for instance shown in Figure
11.68B, it can still vibrate around an internal
center. The latter would, however, be chosen in a
place which produces the least possible difference
in circumferential velocity.
From experimentation it has been found that if the
ratio of X/Y exceeds ~1.5, the motion center must
be placed outside the joint.
Figure 11.68 Weld joint shapes
A B
X
Y
X = max. distance to center
Y = min. distance to center
Parts having a rectangular weld area similar to
that shown in Figure 11.69A can also vibrate
around their own center provided the above
mentioned ratio is not over ~1.5 to 1.0.
With a shape like that shown in Figure 11.69B,
the motion center would have to be located
externally in order to obtain similar weld velocities
all over the joint.
b) Motion center outside the joint area
In cases where the above described conditions
are not fulfilled, parts must be placed far enough
from the motion center to obtain again a ratio
of X/Y