Plastics Decorating - January/February 2008 - (Page 14) ASSEMBLY UPDATE SSEMBLY UPDATE Ultrasonic Welding - Which Frequency? By Tom Kirkland A little research reveals that ultrasonic plastic welding machines can be purchased in a variety of frequencies, including 15, 20, 27, 30, 35, 40, 50, 60, or 70 KHz. As if this doesn’t create enough confusion, even more frequencies most likely are available. If two potential suppliers recommend different frequencies, which one is providing the best information? If an ultrasonic welder of one frequency already is in place, is there a need to buy another of a different frequency? Does it really matter? The Right Tool for the Job 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 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 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 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. 14 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 continued on page 16
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