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Join Date: Dec 2018
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Sonotrodes for an Old Ultrasonic Welding Machine

12/21/2018 11:16 AM

I managed to rebuild an old ultrasonic welding machine. It is a Radyne FW 1/13 (200-250 volts, 50 cycles, 1 phase).

I didn't know anything about radio-frequency welding before, but I've done some research and I've managed to weld a few plastic sheets together with varying degrees of success.

The head of the machine has a 31cm long part made from brass with screws that is meant to hold other brass beams/bars that will come into contact with the material to be welded. I made some of these bars myself. My research tells me these are called "sonotrodes", but I'm not 100% sure.

I've read that manufacturing these sonotrodes is something that needs to be done very carefully and precisely. That the shape, format and length of these bars is critical to the correct working of the machine. This scared me. The machine is still working okay, are things really this critical?

The objective is to weld semi-transparent vinyl sheets together with an almost invisible seam. I sharpened one of the sonotrodes like the blade of a knife and this is what I'm doing: I place two sheets one on top of another under that "knife" and I carefully remove the excess material during the welding process (if I let the process finish, I'm no longer able to separate the excess material, I just need to be careful not to burn my fingers). After that I open the two welded sheets and I use the same machine with a wider sonotrode to flatten the seam.

This method works reasonably well, but the seams are fragile. I assume there is an ideal sonotrode length to achieve a better result and I'd like some help with this.

Thanks in advance.

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#1

Re: Sonotrodes for an Old Ultrasonic Welding Machine

12/21/2018 1:07 PM

Ultrasonic devices usually rely on resonance to develop full power. That is why the sonotrodes dimensions are critical. You could probably tune your sonotrodes by monitoring the transducer drive signal on an oscilloscope. The signal will increase and peak as you approach and achieve resonance.

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#2

Re: Sonotrodes for an Old Ultrasonic Welding Machine

12/21/2018 1:29 PM
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#3

Re: Sonotrodes for an Old Ultrasonic Welding Machine

12/22/2018 7:17 AM

I have worked on ultrasonic welders in the past that were used to weld plastic together to close up end seam of material.

The down pressure on the horn against an anvil on the other side of the material was the most critical factor to developing a good weld versus a really weak one.

The more the pressure was set the higher the signal from the controller became. They had a 2" cylinder with 80 psi of air pressure on it. The main problem I fixed to begin with was the cylinder was leaking by and you could all most push it up by hand.

The horn was damaged (what the book call the tool on the end of the sonotrodes) and we machined a like one from standard steel with no real difference in performance (but it did match the original one in basic size and shape)

How are you developing down pressure on your horn?

Do you have an anvil behind it that has roughly the same contact area as the horn?

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#4

Re: Sonotrodes for an Old Ultrasonic Welding Machine

12/24/2018 10:56 AM

Hi, thank you for the answers.

The machine is very old (almost as much as I am ). Unfortunately oscilloscopes are not something I know how to use.

The pressure is applied with a pedal, I have nothing to measure that pressure. But looking at the way the material is crushed, I believe it may be enough.

One of the main problems is my ignorance of the terms. Most places I’ve found say that the sonotrode and the horn are the same thing (including the links that Mr. SolarEagle has posted). Others (like Mr. khelectrical above) say that the sonotrode and the horn are different things.

Looking at my machine again, if the 31cm brass part with screws is a sonotrode, then the other interchangeable brass bars I have are the horns?

If this is the case, then I did not change the sonotrode, only the horns, and maybe that’s the reason the machine was not damaged. For example, I cut one of the horns in two and both halves are now welding better than the original did.

The anvil of my machine is a flat metal table that cannot be separated from the machine itself. I applied a polyester sheet on it and on top of that, in the area where the horns press against the material to be welded, I glued a self-adhesive teflon tape, because I was told it reduces the glossy look left on the material by the welding process.

So the material to be welded is pressed between the horns (the interchangeable brass bars) and the teflon tape. Should I replace the teflon tape with another metal bar to reflect the resonance back to the material? Like a secondary anvil? Are the dimensions of this new anvil also critical? Should it also be made of brass, like the horns?

Happy Holidays everyone, if that's the case.

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