General Design Principles for DuPont Engineering Polymers - Module

Other Ultrasonic Joining Techniques
a) Ultrasonic Heading
Ultrasonic equipment can be used for heading or
staking to tightly join parts of DuPont engineering
plastics to parts of dissimilar materials, usually metal.
A stud on the plastic part protrudes through a hole in
the second part. A specifically contoured horn contacts
and melts the top of the stud and forms a rivetlike
head. This produces a tight joint because there is
no elastic recovery as occurs with cold heading.
Suggested horn and part design are shown in Figure
11.61. The volume of displaced plastic equals the
cavity of the horn. Many variations of the design are
possible to fit particular applications.
Where possible, an undercut radius at the root of the
stud and a radius on the hole of the part to be attached
should be included. This increases the strength and
toughness of the headed assembly. A thinner head
profile than that shown is not suggested.
Figure 11.61 Ultrasonic heading
Metal or plastic part
Plastic part
0.25 radius
D
1.6 D
Heading horn
(replaceable tip)
0.5 D
2 D
0.5 D
b) Stud Welding
Ultrasonic stud welding, a variation of the “shear
joint” technique, can be used to join plastic parts at a
single point or numerous locations.
In many applications requiring permanent assembly, a
continuous weld is not required. Frequently, the size
and complexity of the parts seriously limits attachment
points or weld location. With dissimilar materials,
this type of assembly is generally accomplished
by either cold heading, ultrasonic heading or by the
use of rivets or screws. When similar plastics are used,
ultrasonic stud welding can perform the function with
greater ease and economy. The power requirement is
low, because of the small weld area, and the welding
cycle is short, almost always less than half a second.
Among the many applications where ultrasonic stud
welding might be used are clock frames, timers,
electromechanical devices, electrical connectors and
pump impellers.
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Figure 11.62 shows the basic stud weld joint before,
during, and after welding. Welding occurs along the
circumference of the stud. The strength of the weld is
a function of the stud diameter and the depth of the
weld. Maximum strength in tension is achieved when
the depth of the weld equals half the diameter. In this
case, the weld is stronger than the stud.
Figure 11.62 Ultrasonic stud welding
C B
D
A
Before welding During welding
Stud-weld
completed
Dimension A 0.25 to 0.4 mm for D up to 13 mm
Dimension B Depth of weld B = 0.5 D for maximum strength
(stud to break before failure)
Dimension C 0.4 mm minimum lead-in
Dimension D Stud diameter
The radial interference, A, must be uniform and
should generally be 0.25 to 0.4 mm for studs having a
diameter of 13 mm or less. Tests show that greater
interference does not increase joint strength but does
increase weld time.
For example, studs with a diameter of 5 mm with
0.4 mm interference require four times the weld cycles
of studs with 0.25 mm interference welded to the same
depth. The hole should be at sufficient distance from
the edge to prevent breakout.
In the joint, the recess can be on the end of the stud or
in the mouth of the hole, as shown in several of the
examples. When using the latter, a small chamfern can
be used for rapid alignment.
To reduce stress concentration during welding and in
use, an ample fillet radius should be incorporated at
the base of the stud. Recessing the fillet below the
surface serves as a flash trap which allows flush
contact of the parts.
Other ways in which the stud weld can be used are
illustrated in Figure 11.63. A third piece of a dissimilar
material can be locked in place as in view A. View
B shows separate molded rivets in lieu of metal selftapping
screws or rivets which, unlike metal fasteners,
produce a relatively stress-free assembly.