General Design Principles for DuPont Engineering Polymers - Module

If one part is only vibrated at twice the frequency, the
acceleration and deceleration forces are four times
higher and would have to be compensated for by
means of an additional and adjustable device. The
whole gear box would therefore be much heavier and
more expensive for a machine having the same
capacity. In addition, it has been shown empirically
that it is easier to obtain a good, tight joint if both
parts are vibrating.
Figure 11.73
W
W
Y = 0.635 W
Y
Y
1 revolution
2 Y = 1.27 W
2 Y
W = maximum velocity of each part
Y = average velocity of each part
2 W
Welding Conditions
In order to reach the melting point of the material, two
parts must be pressed together and vibrated at a
certain frequency and amplitude. These conditions can
be defined as a PV value, where P is the specific joint
pressure in MPa and V the surface velocity in m/s.
The two eccentrics generate a sinusoidal velocity
curve as shown in Figure 11.73. Since they move in
opposite directions, the maximum relative velocity
of one part against the other is 2 W. The resulting
relative velocity is therefore 1.27 times the maximum
value W.
Example: A machine welding acetal according to
Figure 11.71 has an eccentric distance f of 3 mm and
runs at a speed of 5000 rpm. The circumferential
velocity is therefore:
V = f × π × n = 0.003 m × π × 5000 = 0.78 m/s
60
This equals the maximum velocity W in Figure 11.73.
The average relative velocity of one part against the
other would then be:
1.27 × 0.78 = 1 m/s
113
At a specific joint pressure of 3 MPa, the resulting PV
value becomes:
3 × 1 = 3 MPa × m/s
As the generated heat is also a function of the coefficient
of friction, the above PV value must be related to
the materials being welded. Glass-reinforced polyamide
for instance has been welded successfully at a
PV value of 1.3. It would therefore appear that a
machine which is supposed to weld various materials
and part sizes should be provided with adjustable
pressure, speed and amplitude. Once the best working
conditions are determined for a given part, the production
machine would, however, not require any adjustments,
except for the pressure.
Weld time is a product of velocity, pressure and
amplitude. Experience has shown, however, that
above a certain pressure, joint strength tends to
decrease, possibly due to squeezing out of the molten
resin. On the other hand, there are certain limits with
regard to the resulting mechanical load on the gear
box. Thus, doubling the speed produces four times
higher acceleration forces of the vibrating masses.
Extensive tests have proven that a frequency of about
100 Hz is very convenient for small and medium size
parts whereas larger, heavy parts are welded at a
frequency of 70–80 Hz.
However, successful joints for big parts have also
been designed, using frequencies up to 250 Hz, see
also Figure 11.76D.
On linear machines, the distance of the two eccentrics
(f in Figure 11.71) should be adjusted in order to
obtain a relative motion of about 0.9 × joint width, as
shown in Figure 11.74.
The specific surface pressure giving the highest joint
strength must be determined by testing. As a basic rule
it can be said that a machine should be capable of
producing approximately 4 MPa of pressure on the
surface to be welded.
Figure 11.74 Relative motion—joint width
W
0.9 W