Ultrasonic Welding: The Need for Speed Control
by Tom Kirkland, Tributek
A couple of years ago, demonstrations were set up at a seminar on ultrasonic plastics welding to illustrate the effects of contamination and energy director damage on an ultrasonic weld and to demonstrate the capability of a particular piece of equipment to detect that something went wrong.
The first demonstration, set up prior to the seminar, was intended to show the effects of contamination, so silicone grease was smeared on the energy director. The machine welded through the grease on several parts as though it was not there, and none of the monitored parameters on the machine varied enough from an uncontaminated part to even detect the presence of the grease. All assemblies were leak-tested and found to be good. Those present were astonished.
The second demonstration, involving damaged energy directors, again set up prior to the seminar, worked more or less as expected. That is to say, there was a noticeable difference in the amount of energy used to make the joint, though the difference was smaller than expected, and the only parts that failed the leak test were those in which the energy director was actually interrupted for more than 2/3 of its height at some point in the part perimeter. Simply sanding the top 1/3 off of the energy director all the way around did not reliably produce bad assemblies. Again, those present were astonished.
Neither of these two demonstrations was used at that seminar, because there was no way to explain what was happening. Both of these demonstrations had been done at many seminars in the past and up until that day, the results had always been the same bad parts. So the basic question every engineer asked was, What was new about these demonstrations that could possibly explain why they did not work as expected?
The only readily apparent answer is that this was the first attempt at these demonstrations using the newest generation of servo-driven ultrasonic welders.
If these newer servo-driven systems are, in fact, much more immune to the effects of contamination and energy director damage, would this not be a game-changing breakthrough along the lines of the development of pneumatically operated ultrasonic presses in the 1960s, triggering by force (dynamic trigger) in the 1970s, weld by (collapse) distance in the 1980s and closed-loop digital amplitude control in the 1990s? And, if this is the case, why would servo-driven machines have a process stability advantage over even the most sophisticated of pneumatic machines?
Speed Control Not Really Understood
In the 1980s and 1990s, as the use of computer-controlled ultrasonic welders spread, users expressed frustration that the new weld-by-distance units solved many problems but, in some cases, could not make the ultrasonic welding process completely stable. Ultrasonic welding still was considered a black art by many, and solving process problems by simple knob twisting or trying everything was, more often than wed care to admit, the norm.
There were many applications issues with energy director welding that could be solved by the use of hydraulic speed control attachments or sophisticated pneumatic control systems. In retrospect, they were partial solutions to problems that were only beginning to be understood. All of the approaches available in that era to deliver the clamp force to ultrasonic welds attempted to address the question of appropriate joining velocity, but because they all ultimately relied on the actions of compressible gases, they were never able to force the process to come into control.
The servo-driven welders of that time were very expensive and of limited capability. Joining velocity could be programmed, but triggering usually was done by head position; and head position was monitored through the servo motor and ball screw assembly itself. Rigid-mount boosters were, to the plastic welding world, either a lab curiosity or something that was seen as practical only in the ultrasonic metal welding world.
Come forward to the early 2010s, and rigid-mount boosters are in use in more and more applications. The advances in control and reduction in cost for servo drive systems make them a much more viable alternative for high-precision or high-reliability ultrasonic welding. In addition, in the most advanced machines, triggering and distance measurement are separate from the servo motor functions.
The new generation of ultrasonic servo-driven welders has the best of both worlds: a true force-trigger system independent of the servo drive; precise position measurement, also independent of the servo drive; rigid-mount boosters, allowing for precise control of the actual horn face position and movement; and the ability to program the joining velocity most appropriate to the application and deliver it precisely. Further, these systems typically have a more rigid structure than standard welders, which both minimizes deflection (therefore better controlling parallelism in tooling) and better focuses the ultrasonic energy to the workpiece.
Why it Works
The reason controlling velocity is so critical in ultrasonic welding is that the heat is generated by a process involving intermolecular friction, in which clamp force is a direct input to the process. All other things being equal, if clamp force increases by 20 percent, then the rate of temperature rise in the joint will increase by 20 percent. The problem is that during a weld, all other things are not equal. The cross-sectional area of the joint is increasing exponentially as the weld progresses. The temperature of the molten material and material in the immediate vicinity is constantly changing, resulting in constant changes in the flow properties of the material. Ultimate joint strength depends upon attaining an appropriate temperature in an appropriate volume of material in a way that does not result in excessive molecular orientation parallel to the joint, and in a way in which residual stresses are minimized.
To ask a clamp system using compressible gases to precisely control a process with so many variables was the equivalent of sending a ship on an ocean voyage with a map and a compass from a box of cereal. It might get there eventually, but you wouldnt want to count on it.
What a servo system actually does for an ultrasonic weld is to take control of the process away from the plastic. Almost none of the variation in the welding process comes into it through the equipment, but almost all comes in through the plastic parts. In a programmed-velocity process, if the material happens to be more densely packed in one part than in the previous part, the extra clamp force required to maintain programmed velocity will result in additional heating, which promotes easier flow, which helps to resolve any issues. If surface contamination inhibits friction, the additional clamp force required to maintain programmed velocity will provide a plowing effect that will help to move the contamination out of the way while it increases the heat and flow rate of the material. If the energy director is under-filled, the resulting reduction in force required to maintain velocity actually will avoid overheating the lesser amount of material available. It Took Fifty Years to Figure This Out?
Why was this not discovered 20 or 30 years ago? Because we were taught (and subsequently we taught) ultrasonic welding in the language of the 1960s. We talked of the three main variables in welding being time, amplitude and clamp force, because with early equipment those where the three things that could be adjusted. While that is a good, basic analogy of the process, it doesnt yield a lot of understanding about what it takes to actually make the process of welding fundamentally better. The industry focused on building better and better pneumatic machines, but never really understood the importance of programmed velocity. As in all good science stories, the next great breakthrough was waiting for someone to stumble upon it.
The fact that servo-driven machines on the market from the 1980s to the early 2000s cost a lot more and didnt weld appreciably better than pneumatic machines of the time didnt help get us there any more quickly. The failure of these machines to gain popularity seemed to confirm some of the old thinking that the springs in the trigger assembly and the compressible booster o-rings provided some compliance that was needed to maintain horn contact with the part as the process progressed. As it turns out, that talk was merely an honest desire to see a weakness in the welders built up until the 1980s as a strength. If enough people say something over a long enough period of time, it can start to sound like the truth.
Situations such as the demonstrations mentioned at the beginning of this article abruptly changed ideas about how to best control the process of making a great ultrasonic weld. Those situations also gave advanced warning that servo-driven ultrasonic welders soon would earn a place at the top of the process-capability food chain.
The people who build ultrasonic machines today are building a spectrum of machines from pneumatic machines, both simple and advanced, to servo-driven machines that can produce a level of consistency in welding that can be astonishing to someone who has been around for many years. Not every application will benefit from the most advanced equipment, but for a high-value critical assembly that has been troublesome in the past, todays servo-driven ultrasonic machines could prove that you do, in fact, have a need for speed control.
Tom Kirkland has been working with ultrasonic plastics assembly for over 25 years, first as a user, then in sales, marketing and engineering of plastics welding equipment and accessories. He has given over 1,000 seminars on plastics assembly and is the author of numerous technical papers and magazine articles on the subject. Kirkland is the owner of www.tributek.biz, which supplies replacement parts for plastics assembly machines and consulting on plastics assembly issues.