General Design Principles for DuPont Engineering Polymers - Module

variable in most applications; 0.3 to 0.5 seconds are
usually sufficient for most applications unless an
internal load tends to disassemble the welded parts,
such as a coil-spring compressed prior to welding.
c) Amplitude of Vibrations
The physical amplitude of vibrations applied to the
parts being welded is an important process variable.
High amplitude of vibration of appr. 0.10 to 0.15 mm
peak-to-peak is necessary to achieve efficient and
rapid energy input into DuPont engineering plastics.
Because the basic transducer delivers its power at high
force and low amplitude, the amplitude must be
stepped up before reaching the tool tip. The horn
design usually includes amplitude transformation
inherent in tapering or stepping its profile down to a
small diameter. Where the part geometry requires a
large or complex tip shape, this amplification may not
be possible in the horn. In this case, amplification can
be conveniently achieved in most commercial systems
by use of an intermediate tuned section called a
booster horn. Boosters up to 2.5:1 amplification are
commercially available. Negative boosters to 0.4:1 for
horns having too high an amplitude for a given
application are also available. Boosters which provide
a 2:1 or 2.5:1 amplification are typically required,
except for small parts which permit the use of high
gain horns.
Increasing amplitude improves weld quality in parts
designed with shear joints. With butt type joints, weld
quality is increased and weld time is reduced with
increasing amplitude.
d) Pressure
Weld pressure provides the static force necessary to
“couple” the welding horn to the plastic parts so that
vibrations may be introduced into them. This same
static load insures that parts are held together as the
melted material in the joint solidifies during the
“hold” portion of the welding cycle.
Determination of optimum pressure is essential for
good welding. If the pressure is too low, the equipment
is inefficient leading to unnecessarily long weld
cycles. If it is too high in relation to the horn tip
amplitude, it can overload and stall the horn and
dampen the vibrations.
The overall amplitude gain provided by the booster
and the horn is analogous to the load matching provided
by the gear ratio between an automobile engine
and its rear wheels. In ultrasonic welding, low pressure
is required with high amplitude and high pressure
is required with low amplitude.
This is shown in the graph in Figure 11.59. It is a plot
of weld efficiency vs. weld pressure for three levels of
amplitude obtained by use of the boosters indicated.
There are several methods for measuring welding
efficiency.
104
Figure 11.59 Welding efficiency vs. amplitude and
pressure
Welding efficiency
Booster
2:1
Weld pressure
Booster 1.5:1
No Booster
These are fully described in the next chapter. In
addition to showing the relationship of amplitude and
pressure, another very important effect is shown. As
amplitude increases, the range of acceptable pressure
decreases. Therefore, finding the optimum pressure is
most critical when the amplitude is high.
Guide to Equipment Operation
Proper operation of welding equipment is critical to
the success of ultrasonic welding. The following
comments are suggested as a guide to the use of
ultrasonic welding machines with parts of DuPont
engineering plastics.
a) Initial Equipment Setup
Horn Installation
The transducer, welding horn, and booster horn (if
needed) must be tightly bolted together to insure
efficient transmission of vibrations from the transducer
to the parts. The end surfaces of the transducer output
and horns are usually flat to within several microns.
However, to insure efficient coupling heavy silicone
grease or a thin brass or copper washer cur from 0.05
or 0.08 mm shin stock is used between horns. Long
spanner wrenches are used to tighten horns. Care must
be exercised when tightening horns, so as not to turn
the transducer output end. Such turning may pull the
electrical leads from the transducer.
After installation of the horns, some welders require
manual tuning of the electronic power supply. Small,
but important adjustments to the frequency of power
supply are made to exactly match its frequency to that
of the horns. Some welders accomplish this tuning
automatically. The operations manual for a particular
welder will indicate required tuning procedures, if
any. This procedure must be repeated each time a horn
or booster is changed.
If the amplitude of vibration of a horn is not known, it
may be determined quite simply with either a microscope
or a dial indicator. A booster should not be used
if only the amplitude of the welding horn is to be
determined. A 100 × microscope with a calibrated