Design Guide - Solvay Plastics

Figure 89: Lap Shear Joint Configuration
Figure 90: Shear Joint Configuration
Depth
of Weld
0.2 mm (0.008 in.)
30° - 45°
Interference
The machine used for these experiments was a
Vinton Hydroweld Vibration welding machine. This
machine operates at a nominal frequency of 240 Hz.
The specimens used were 102 mm (4 in.) long, by
25 mm (1 in.) wide, by 6 mm (0.25 in.) thick, and they
were welded in a 25 by 13 mm (1 by 0.5 in.) lap shear
configuration as shown in Figure 89.
This method was very effective and welds as strong as
the parent material were easily obtained. This technique
proved relatively insensitive to welding conditions, giving
good results at weld times as short as 0.60 seconds and
pressures as low as 2.20 MPa (320 psig). Best results
were obtained using specimens containing a normal
(1.8%) amount of moisture.
Spin Welding
The spin welding method uses frictional heat to join two
cylindrical or spherical mating parts. While one half is
held stationary in a nest fixture, the mating part is spun
rapidly against it. Friction at the interface raises the
temperature of the material until melting occurs. When
the spinning action is stopped, the parts are held under
pressure until cooled. Obviously, this welding method
is limited to parts with circular geometry at the joint.
The advantage obtained through spin welding is the
increased dispersion of material at the joint compared
to other techniques. This allows for the development of
hermetic seals and reduces the tolerance requirements
on the joint geometry.
Spin welded joints using Amodel ® PPA should
incorporate a design known as a shear joint like that
shown in Figure 90. Details known as flash traps may be
incorporated to channel the localized flow of the material
to one side of the part depending on requirements.
The spin welding machine used for these experiments
was a Mechasonic KLN Omega machine, model SPN-
063. Typically the important parameters for this welding
method are angular speed, in revolutions per minute,
normal force, and welding time. Because this specific
machine was an inertia- type, the energy available for
spinning the sample was limited to that stored in the
flywheel. Instead of adjustments per se to speed, the
energy stored in the flywheel was controlled. More
modern spin welding machines allow for the control of
speed, force, time, and in some cases, angular location.
The specimens used for the spin weld testing were
injection molded cups with an interference joint design.
Excellent weld strength was obtained using this method.
Because the welding condition settings were machinespecific,
they are not generally useful in setting a starting
point. Rather, it was observed that as forge pressure
and angular velocity were increased, weld strength
increased up to a maximum and then decreased when
an excessive amount of either pressure or velocity were
applied. The values for these parameters are strictly
dependent on the part geometry and therefore they
are not noted here. The explanation of the observed
phenomena is that when a weld was made with too high
a forge pressure, the spinning motion was stopped too
rapidly and not enough polymer melted and flowed to
create a sound joint. In the other extreme, a high angular
velocity and a low forge pressure, the top part essentially
sat on top of the bottom part and freely rotated without
being forced into the interference fit. Thus, to assure a
good joint, a welding condition must be found so both
melting and forging occurs.
Moisture content did not significantly affect weld strength.
Ultrasonic Welding
In the ultrasonic welding of thermoplastic materials,
high frequency (10 to 40 KHz) mechanical vibrations
are transmitted through one of the mating parts to the
joint interface while the other half of the assembly is held
stationary. The combination of friction and an applied
82 Amodel ® PPA Design Guide
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