Monday, February 26, 2007


Printing circuitry in the low-rent district

I'm always inspired by Adrian's groundbreaking work. I guess I must see things with different eyes, though. I read his blog entry on working with low temperature eutectic alloys in the a little while ago. It seemed to me he'd gone to heroic lengths to make what was, for practical purposes, a piece of 3 mm solder. Mind, being Wood's metal it's solder that will melt in a hot cup of coffee, but it's solder all the same.

His saying that he's wanted a filament that he could run through a Mk 2 FDM extruder put me on the hunt, I guess.

You see, I know that my low thermal inertia 2 amp extruder barrel is hitting somewhere around 190-210 degrees C (374-410 degrees F). I keep several diameters of solder. In regular acid core lead I keep heavy (1.27 mm) and fine (0.9 mm). Recently, I've been experimenting with a RoHS (no lead) compliant acid core solder in a fine grade.

Knowing how hot my extruder barrel gets I began to wonder whether I could get it to melt in my extruder barrel. Problem was my extruder barrel is 3 mm while my solder is 1.27 mm. As well, if I got it wrong it would be likely that I would jam the barrel.

Then I got to thinking about the old 1 amp prototype extruder barrel that I'd built and tested back in late December and early January. Naah, it'd never get warm enough. On the other hand if it jammed I hadn't really lost anything.

So, one thing leading to another I cut off a couple of inches of the heavy grade Sn63/Pb37 solder and folded it into a plug and stuffed it in the test extruder barrel which I'd locked into my vice and fired it up. I had a short length of 3 mm HPP to use as a piston so I was good to go. The test extruder had a plug of either ABS or HPP in the end from the last time I used it, so after the barrel heated up I inserted the HPP filament and began to feed it into the cold end (~70-80 degrees C) of the extruder barrel with a pair of pliers so that I wouldn't risk burning myself however slightly.

After the plastic plug melted and began to clear as I fed the HPP filament into the extruder I began to see droplets of solder coming out with the HPP as you can see here.

After a few seconds of transition between plastic and solder the rest of the plug came out in a rush and fell into the HDPE sherbet container I'd thoughtfully placed under the extruder barrel to keep things off of the carpet. The HDPE surface that they hit was relatively rough and the acid core solder adhered to them slightly.

Here was what was important though. The specific heat of metals is, on average, about 10% of that of organic materials like plastic. What that means is that dropping small amounts of something like solder on a plastic surface usually doesn't melt the plastic surface significantly in spite of the fact that the molten solder is far hotter than the melting point of the plastic. The plastic simply absorbs the heat like a sponge and chills the solder instantly.

Here is a closeup of one of the solder splashes that I peeled off of the HDPE surface laid on the tempered glass xy working plane of Tommelise.

I got my micrometer out and measured it at a very consistent 0.1 mm in thickness. Basically, I had a very nice piece of tin-lead foil for my troubles.

I repeated the experiment with the RoHS solder.

It behaved very differently. It came out in tiny beads with the HPP and in a few cases coated or partially coated the HPP. The two long threads of solder shown here at the center and left of the pic are actually electrically conductive wires made of RoHS solder. RoHS is going to take some more thinking about.

I guess I'll have to head down to the hardware store and see if I can find some hard copper tubing that can seat 1.27 mm acid core solder. I wonder if I can talk Adrian into printing me up a Mk 2 parts kit that can handle 1.27 mm solder. It would be interesting to see if I could print an HDPE blank and then print the traces for a Tommelise microcontroller board in solder on it. :-D

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