The Right Tool for the Job
A medical device manufacturer purchased a 20 KHz ultrasonic welding machine (just like the other ten machines it owned) and shipped it to a supplier with instructions to have tooling built in order to weld a certain small electrical assembly. The tooling was built to fit the machine and the assembly in question, and production was scheduled to commence. When the machine was finally adjusted to produce acceptable parts, a reduction booster of 0.4 gain was installed, the weld time was less than 70 milliseconds, the clamp force was approximately 150 Newtons (requiring air pressure in the cylinder of less than 0.5 bar), and the force trigger setting was set at its lowest possible setting. It was like hunting rabbits with an elephant gun.

When this fact was pointed out, the supplier felt rather powerless stating, “This is what they sent us to use.” Two years later, a task force was formed to address the erratic quality coming from this process. After many meetings, the members of the task force sourced another supplier who finally told them that they did not have the right tool for the job.

Heating Rate
The heating rate in ultrasonic welding is the result of the combined effects of frequency, amplitude, and clamp force. In most cases, the ultrasonic welding process is a race between destruction of the joint detail by the combined effects of clamp force and amplitude and the heat that is generated by these two effects. Restated, the plastic must get hot enough to weld before the joint detail is pounded flat.

In the heating rate equation, clamp force and frequency appear as multipliers. Frequency is usually fixed for a given machine, so let’s come back to that. The heating rate in plastic varies directly and in proportion to the clamp force applied. Apply more clamp force, and the heating rate increases in direct proportion to the change.

In the same equation, however, the heating rate varies with the square of the amplitude. Increase the amplitude, and the heating rate increases dramatically.

An outworking of the laws of physics results in an inversely proportional relationship between the frequency of an ultrasonic welder and its output amplitude. There is not a lot of variation from manufacturer to manufacturer on this. Even if there is a difference at the output of the converter/transducer among machines of the same frequency, the physics of sonotrode/horn design will result in similar amplitude capability at the end of the tool.

Using the highest amplitude available that yields consistently acceptable results, minimal part damage, and long sonotrode/horn life usually is desirable. Utilizing a machine of a different frequency basically allows for tailoring the range of available amplitude to the characteristics of the assembly, since the change in frequency in one direction has much less effect than the change in amplitude in the other.

About Plastics
An important consideration in the ultrasonic welding process will be the material. Softer materials simply do not carry sound as well as harder materials and will require more amplitude from the tool to get a usable amount of amplitude to the joint. Materials with higher melt temperatures also will require more amplitude to get up to weld temperature before the joint detail is gone. Choosing a machine that is lower in frequency and therefore higher in amplitude is often advisable with soft or high temperature materials.

Stiffer materials may be damaged by high amplitude, and may heat so quickly that the process becomes uncontrollable. Welding too quickly also can result in weak welds. Therefore, choosing a machine of higher frequency can address these issues.

Tool Design Limitations
The laws of physics that govern sonotrode/horn design are related to wavelength. Most of the factors that reduce acoustic performance have to do with transverse dimensions; that is, dimensions perpendicular to the direction of amplitude. If a tool has a longer wavelength (lower frequency), it can have larger transverse dimensions.

Another consequence of this factor is that a given tool face size will, in effect, be smaller relative to wavelength at lower frequencies. So all other things being equal, a lower frequency tool will be simpler and potentially more durable than a higher frequency tool doing the same application.

The Machines
High frequency welders typically run small tools - making small, delicate parts with great precision. They typically have small, light slides driven by small air cylinders. Low frequency welders typically run large tools at high amplitudes, making larger parts made of softer materials. They typically have large, heavy slides driven by larger air cylinders.

The Rest of the Story
Going back to the beginning of the article, the answer to the story about the medical device manufacturer who didn’t have the right tool for the job was to switch frequencies to 40 KHz.

The assembly itself was quite small - a cylinder split lengthwise that was smaller in diameter than a pencil and somewhere around 30 millimeters in length. The weld was delicate because the nominal wall was quite thin and there were internal components that were easy to damage.

The 20 KHz welder was the same make and model the manufacturer used throughout its plant. It was capable of about 2000 Newtons of clamp force, and the minimum trigger force setting was supposed to be about 170 Newtons. At less than 70 milliseconds of weld time, the tool had not yet run up to full amplitude. Given this fact and the reduction booster, it is clear that the assembly did not need much amplitude to weld.

With a 40 KHz machine, available amplitude at the converter/transducer dropped about in half, and a 1.5 booster was used. Weld time came up to over 650 milliseconds - a much more controllable situation. The welder was capable of about 400 Newtons of clamp force, so it was still very controllable at about 130 Newtons, and a force trigger setting of about 80 Newtons worked very well.

Since changing welders, the company has made millions of acceptable parts with very few quality issues. It’s amazing how much better the results are when you use the right tool for the job.

Tom Kirkland has over twenty years of experience as both a user and supplier of plastics assembly equipment and tooling. He has been active in several professional societies, is a past president of the Ultrasonic Industry Association, and is well versed in a wide variety of plastic welding/joining processes. Tom has conducted over a thousand plastics assembly training sessions, conference presentations, and talks in many countries. He is currently a consultant in plastic welding, and is the proprietor of www.tributek.biz, a supplier of parts and supplies for plastic welding.