Friday, March 09, 2007


BBQ paint fails

Just a note in passing. While I was checking out Tommelise so that it will be ready for printing tests this weekend now that the PC-side software to let it print real things is operational I noticed that the insulation on the extruder barrel heating coil had gone white. That means that we're looking at the glass insulation left after both the paint and much of the silicone insulation has boiled away.

I did a careful check and noticed as well that in places it was no longer stuck to the BBQ paint on the extruder barrel. While the heating coil was loose, it was not standing away from the barrel. I used only 175 mm of #32 nichrome 80 on this barrel in a single strand, so the loops in the coil are well away from each other.

Just to note I've had about 14 hours of hot operation on the this barrel to date. When I talk about "hot" operation, I'm talking about measured extruder barrel temperatures in the vicinity of 200-205 degrees C. I've had about the same number of hours of operation on my old test barrel which has been operated at about 140 degrees C. until very recently when I started trying to extrude ordinary solder. The BBQ on that one is still as good visually as the day I built it.

What that means is that if you use this approach with CAPA don't expect to encounter BBQ paint failure like I have.

Right now I'm in the process of putting another six coats of BBQ paint on the barrel to tack down the heating coil again. It's a sunny day out in my shady little patio where I'm doing that and that's going well.

I have another thought for a product to tack down the nichrome; there is a commercial epoxy product called JB Weld:
the spec sheet says it is stable up to 500 degrees and electrically insulating. I'm not sure about it's thermal conductivity but most epoxy products are decent in that regard. The FAQ page also suggests that it can be thinned down to pourabilty with lacquer thinner. Just my $0.02
Interesting! I'm going to chase some of that up and try it out. It's about time I made another extruder barrel. -)
what is your estimated cost to build your reprap or a repstrap or...???
LOL! I've spent several thousand dollars on this quest so far. Of course, I didn't have a design or even a clue when I started. I also didn't have any tools or any idea what tools I needed.

I've estimated and still believe that the parts for my second Tommelise will be around $150-175. I believe, as well, that the tools to build one would be cost about $100-200 in addition to that. If you had somebody replicate all of your Tommelise parts I think you could get the price of the tools down to well below that to perhaps about $50-75.
Forrest, what happens when all the paint leaves? If you can cook it off and the unit survives (assumption since you repaint), why did you need paint in the first place. Is there some kind of anodization that would suffice to protect the extruder barrel?
It's not a matter of protecting the barrel. It's more keeping the nichrome 80 wire stuck to the barrel so that you get a good thermal connection. If you lose that thermal connection the nichrome overheats and eventually fails.
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