General Design Principles for DuPont Engineering Polymers - Module

Vibration welding is not a competitor to pure
spinwelding. For all circular parts which do not
require a determined position to each other,
spinwelding is still the cheapest and fastest assembly
technique.
Design Considerations for Vibration
Welded Parts
Parts which are intended to be assembled by vibration
welding must be designed correctly to avoid rejects
and failures. Perfect fitting of the joint area is essential.
The first step is to choose an adequate joint giving the
required strength and tightness. It should be decided at
this stage of development whether flash traps or
means to cover or conceal the joint are necessary.
It is essential to support the joint flange all around the
part in order to maintain equal pressure over the whole
weld area.
If, as shown in Figure 11.85, the jig cannot fulfill this
requirement due to an interruption, weak spots or
leakage can be expected.
Thin ribs, however, are permissible, provided their
thickness does not exceed appr. 80% of the wall
section (see Figure 11.86).
Special care must be taken to make sure vibrations are
transmitted from the jig to the part with as little power
loss as possible. Such loss may occur from too much
clearance in the jig or because the part is held too far
away from the joint.
Circular parts without protruding features allowing a
tight grip must be provided with ribs as shown in
Figure 11.75A.
With parts having relatively thin walls or which are
molded in soft materials, vibrations should be transmitted
to the part as near to the joint area as possible.
For non-circular parts this is often only possible with
a design similar to that shown in Figure 11.76B,
regardless of whether it is a linear or angular weld.
Some materials which have a high coefficient of
friction, as for instance elastomers, require an initial
surface lubrification before they can be satisfactorily
vibrated and welded.
The amount of melt produced during the vibration
cycle is in direct relation to the surface flatness. Stiff
parts, especially in glass filled resins, may not be
flattened out completely by the weld pressure and so
require longer vibration cycles to achieve good joints.
When designing and molding such parts, it should
therefore be kept in mind that the total assembly time
depends partially on joint levelness which in turn can
often be improved with appropriate design.
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Figure 11.85 Bad joint design
Figure 11.86 Ribs in vibration welded parts
L = 0.8 T
Figure 11.87 Vibration welding machines
A) Commercial linear and angular welding machine.
Manufacturer: Branson Sonic Power Company, Eagle
Road, Danbury, Connecticut 06810, USA. Technical
centers around the world.