Monday, December 22, 2008


Roast-in-a-bag Duck...


The hair dryer works remarkably well as a means of creating a constant warm environment for building with ABS. This prevents both curling and delamination. But it can be a bit hit-or-miss, and is also noisy and uses a silly amount of power (1.4 kW according to its rating plate, or - to put it another way - over 20 times as much as the whole of the rest of the RepRap machine...).

Forrest had a brilliant idea: use an oven roasting bag to enclose the build in a bubble of warm air. So, from a suggestion by Nophead, I built a miniature fan convector (above). It's an old processor heatsink and fan with two 10-ohm pieces of nichrome wire woven in among the heatsink fins. They're in parallel, and at 12v will give about 30 W. As an alternative you could obviously just bolt a fat 4.7-ohm resistor on the back.

Then I phoned Christine and Sally (w. and d.; they were out buying things for some sort of festival that they claim is imminent; I think they're trying to pull the financial wool over my eyes...) and asked them also to buy oven bags. I bolted a sheet of balsa (previously used to build on) just under the extruder mechanics and duck-taped the bag to it and to the build base.

I moved the machine about to make sure there was enough slack to accommodate a build. This was a bit tricky, as too much and folds would have got in the way.

The cream object to the right of the picture is a thermometer. With both the extruder on and the mini fan convector the temperature in the bag stayed pretty constant in the top sixties.

I ran a test build:

Another part of the newest granule extruder. Holding a rule against its base and squinting at the light in any gaps reveals it to be as flat as a pancake in Flatland.

Clearly the bag arrangement could be considerably tidied-up with the addition of some concertina folds and springs and so on. I don't know what polymer they use, but the extrude head (internal temperature 240 oC) came to rest against it several times and didn't cause any damage at all.

A rather good side effect was that the slightly acrid smell of molten ABS was completely enclosed and thus eliminated. Even when I opened the bag at the end no smell emerged, implying that the volatiles had condensed inside.

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This is so cool! What with the milling, software and firmware work I'm doing on T2 right now I wasn't going to be able to try out the oven bag idea for another month or so. Now I don't have to speculate about whether it is going to work all right because Adrian already proved that it does. Synergy is a wonderful thing! :-)

Also, I read that the oven bag material is some sort of high temperature nylon. It's supposed to be usable at 400F (240C) which tells me that it can probably handle another 50-100 degrees F above that.

That is GREAT news about getting rid of the plastic odor from ABS. That was one of the big problems that I saw with kids running a Reprap in their bedroom.

I've been thinking about putting an acoustic enclosure around T2 when I use it to do milling.
If you wrapped the whole reprap in a cooking bag, would the temperature weaken the structural parts of the running reprap machine?
That bag looks rather awkward, particularly to set up between runs - might it not be easier to just build an insulated cabinet around the whole machine? Maybe something along the lines of a double wall of plywood or fibreboard filled with a few centimetres of expanding foam?
If you build a box around the Darwin the whole Darwin has to be designed to work well at 60C.

All you have to do is put a slight positive air pressure inside the bag and the problems go away.

A clipped binder ring around the edge of the xy working surface makes getting into and out of the bag trivial.
Yes - the motors and electronics probably wouldn't be happy at 67 C. Plus the bigger the volume, the more power you need to maintain the temperature.

I think it should be pretty easy to set up a neat compact enclosure system as Forrest suggests.
Yes, maybe the heatsink fan combo could double as a pump which adds a bit of positive pressure to the bag. Add some toggle clamps around the edges, and you're good to go!

This will actually make the build process much faster and easier for me, as right now I have to have such good bed adhesion to counteract the warping that I end up having to use a hammer and chisel to remove the parts from the build. With a turkey bag I might be able to go back to just snapping the parts off the bed.
Yes. I am posting blog comments on Christmas Day. Sad. Or what?

I thought of using the fan to give a slight overpressure to inflate the bag and to keep it clear of the build, but I reckoned that the addition of new cold air would cause the setup to fail to reach temperature. That would have been the case - it was stable just under seventy when fully enclosed with the fan and extruder both pumping out heat.

But more watts in the heater would obviously solve that if that was the way one wanted to go.
If you have a pretty tight seal between the working surface you won't be putting much new air into the bubble to keep that positive pressure. Mind, adding a new power resistor or two wouldn't hurt.

Merry Christmas, Adrian. :-)
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