Frequency Plastics Welding - The State of the Art
Plastics' welding has been with us for a great many years and sadly the
basic system design philosophy hasn't changed appreciably since its introduction
as a manufacturing process.
We at Electrotechnical Engineering Ltd have
been working very hard over the last couple of years or so to produce a welding
system that addresses the many shortcomings of traditional systems.
a pioneering company in the introduction of solid-state 50W power amplifiers manufactured
by our parent company Comdel, Inc. to industrial processes and since the completion
of the basic work required achieving that, we have been looking at ways to fully
exploit the enormous benefits made possible by the inclusion of high spec power
Feedback from customers over several years has indicated very
clearly that the most advantageous changes would be,
cycle by cycle repeatability
2. Simplified set up
3. In-line Quality Control
4. Provision of data capture
5. Reduced downtime
above are in no particular order, but all are important to customers that we have
We began by considering the definition of a 'good' weld. To paraphrase
several months work, a good weld may be characterised by depth of sink, power
and time. There are of course many other parameters that can, and do, affect weld
quality, but they all have an effect upon or are affected by, these three key
The next point is to determine how these three parameters may
be monitored, measured or controlled. We have developed a system to measure the
depth of sink in real time as the weld progresses, this allows us to select the
desired final material thickness and then weld until that condition is achieved.
technique means that no weld time is set, just the desired depth of sink and output
power level. The PLC system determines when the weld is completed and disables
the RF power, a major benefit of this approach is that the tool temperature is
compensated for as an inherent part of the weld process; if the tool temperature
increases the time taken for the plastic to sink is shortened, because we have
no predetermined weld time the weld simply ends sooner, the reverse is also true
if the tool temperature decreases.
The RF amplifiers internal control and
metering circuitry monitors the RF voltage and current in both the forward and
reflected direction, via two independent wattmeter multipliers produces accurate
measurement of forward and reflected RF power. The same signals are used within
our control software to determine the power into the load; since we also measure
the time we are able to calculate the energy into the weld. The accuracy of the
amplifier metering and control system is such that we are able to set the RF power
in 1Watt increments if required (the standard display is in 10Watt increments).
same data that is captured for control purposes is also available for QC purposes
and may be readily downloaded to a PC or network. The data is presented in a form
suitable for use with Microsoft Excel spreadsheets making graphs, trending, production
rates, reject rates etc all easily accessible.
The final area of concern
has been setting the machine up for a new product. This is an area that we have
given a lot of effort; our solution is a simple 3 or 4 step (dependent upon materials)
The operator simply touches the "learn mode"
button and then follows the on-screen instructions, the process consists of nothing
more technical than placing material in the press, opening and closing the press
and removing the material. During this simple process the software 'learns' about
Upon completion of the 'learn' procedure all that is required
is to enter a value for the required depth of sink and RF power. All values recorded
during the set-up stage may be saved (typically against a product name or number)
for single button set-up anytime in the future.
The system also records
number of products per hour, day, week, month, year as required, these counters
can be resettable or non-resettable as required.
The measurement of depth
of sink allows us to detect lay-up faults, missing material, extra material etc.
We also measure the platen 'levelling' and display four bar graphs on a set up
screen permitting press levelling to 0.001". This is a dynamic system and
is active throughout operation of the machine, should the platen become unlevelled
the system will detect it and display a message to warn the operator.
our open C presses we use the depth of sink measurement system to determine any
obstruction to the press movement, this provides an additional level of operator
safety in that the system responds extremely quickly and reverses the press a
long time before traditional pneumatic systems. We still use all of the more common
techniques to prevent entrapment since operator safety has to be any manufacturers
All of our systems incorporate system status monitoring;
all switches etc are monitored for illegal status (for example a switch that appears
to be open and closed at the same time). Once again should a problem be detected
the machine is stopped and the operator made aware by means of an error message.
detection is achieved by two independent systems, one electronic the other software.
The two compliment each other to provide dependable protection; should one system
fail the other provides protection and a message is displayed alerting the operator
to failure of the one system.
The power may be ramped up from any start
level to any final level and if necessary can be ramped down from the final level
to any lower level. The power profile can be tailored to match the requirement
of any application.
In summary, our current system can be used with confidence.
The control and monitoring system will provide a very high degree of repeatability,
consistent weld performance, set up is virtually automatic, data is stored for
each weld and every product set-up may be stored for later use. The system even
monitors itself looking for internal problems and faults, it monitors the machine
looking for operator errors and will detect even subtle lay-up errors.
selection of operation and diagnostic screen captures from a production system.