Radio Frequency Plastics Welding - The State of the Art
Steve Ottaway, Electrotechnical Engineering
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.
We were 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 supplies.
Feedback from customers over several years has indicated very clearly that the most advantageous changes would be,
- Improved cycle by cycle repeatability
- Simplified set up
- In-line Quality Control (QC)
- Provision of data capture
- Reduced downtime
The above are in no particular order, but all are important to customers that we have spoken to.
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 parameters.
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.
This 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).
The 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) 'learning' procedure.
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 the material.
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.
On 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 primary concern.
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.
Arc 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.
A selection of operation and diagnostic screen captures from a production system.