Summary

ultrasonic welding is the way to splice this stuff...

Instructions

[A quick update:
The handle STL should not be used because it presses against the back end of the "sonotrode" assembly. This pressure has a similar effect to putting your finger on the cone of an audio speaker. The finger absorbs energy and muffles the sound vibration. Instead, hold or secure the probe around its middle.

Learning as I go here... Ultrasonics is one complex field... wish i had a better math background... shoulda stayed in school..]

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I was not happy with the usual filament splicers. They are not easy to build and results are not great. But there was something in the back of my mind...

Ideally, to splice filament you wanna heat only a small section without melting anything beyond the splice, while the heating energy should be added quickly, and the heat source removed instantly. Also, it would be nice if the splice didn't need any trimming or further work. Anyone who's tried it knows what I'm talking about.

Then it occurred to me that industries have been using ultrasonics to weld plastics (and other stuff) for over 50 years. It offers the above advantages and more.

But where the heck does one find an ultrasonic welder? It turns out that some common household things might melt plastic if they are hacked correctly. Ultrasonic cleaners, ultrasonic fogger / mister / humidifiers.. These devices work at various cycles per second (hertz). Welding supposedly needs a particular range, commonly 15-70 Khz for plastics.

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It so happens that most ultrasonic "cleaners" operate in this range. Pictured is a hand held, battery powered "Ultrasonic Stain Cleaner" I bought at Harbor Freight several years ago. #96474.

Took it apart and hooked it up to a 0-18v 3Amp bench power supply. (It will eat the 4 AA batteries within a minute because it draws over an amp. No wonder (almost) nobody sells them anymore.)

When everything's right the weld takes about half a second.
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In an experiment, I held a piece of plastic against the piezo transducer (little round disk) of an ultrasonic Humidifier and it melted. They work at higher frequency but show much promise.

The large (~2" dia), bare transducer from a tank type parts cleaner was removed and tested but it shattered into pieces before it melted anything. Ultrasonic transducers are somewhat delicate. Ceramic types are very brittle.... They need solid support. Some must have a heat sink. The vibrations made this one self destruct. See the "exploded view". It was originally encased in epoxy and evidently needed to be...

In particular, the thin metallic coatings needed to solder connection wires to the crystalline transducer peels off very easily. Then it is useless. (Electroless deposition of silver is not difficult but is a lot of work. Better to preserve the original soldering surface).

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The photos show a simple little Masonite jig to position and hold the filament ends together. Vibrations cause everything to move so spring clamps (not shown) are needed.

The end of the transducer "horn" has a 1.75mm diameter half-moon notch filed into it. That notch prevents it from melting a flat in the joint. Theoretically with the right horn and "anvil" shapes, a perfect round joint will be the result.

There is a jig STL to cut filament ends at 60 degrees (or is it 30 degrees? whichever is the sharper angle...) for an angled lap joint. Not the ideal joint, but it works well enough. A flat lap would be better according to what I've read, but it's not easy to do..

Also included is the STL for a handle to hold the transducer instead of leaving it mounted in the original plastic body. The circuit board (frequency generator) is laid bare in the photo. Box is in the background.

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The welded joint is very strong. Actually, it is stronger than filament since filament polymer strings are oriented along its axis during extrusion, and it breaks easily cross-wise. With welding, the plastic is homogenous. There are no weak directions. See the examples of crossed filament breaks. The joint doesn't break.

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There's a whole lot to this, and I could write pages of info I've recently learned and discovered, but I'd rather people ask questions if something needs expansion. Otherwise find some suitable ultrasonic device or appliance and hack along with me. I hope someone does because it took me 5 days to get this far and I'm running out of steam.

Ultrasonic filament splicer