"Perfect" heater design?

I am trying to make a RepStrap. I have a 3-axis machines working with Linux EMC, now all I need is the Extruder.

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The thing is - I don't have any power tool with me except a crappy drill. No drill press, no drill, no lathe, limited hand tools. I am not planning to acquire them either. But being living in China right now, it's affordable to have parts professionally machined.

I got a M6 brass barrel/nozzle (made according to the RepRap controller v2 instruction), and I was wrapping some PTFE wrapped nichrome wire (At least I was told it's PTFE wrapped, strangely I couldn't find any other kind of insulated nichrome wire here), I attached a thermistor to the barrel. They are held to the barrel with Kapton tape.

I fired them up with the extruder controller, watched the temperature reaching 100C, and then -- smokes! The nichrome wire shorted and burned.

So I am back to square one.

--- My asks ---

I want to avoid using cement, plus the copper jacket thing looks hard for me to make. I have really low confident in my mechanical skill.

And assume I could just have parts fabricated from the machine shop, what would makes a good heater which is simple and reliable?

[hydraraptor.blogspot.com]
Is Nophead's thermal block design sounds real good to me, and I can reuse my barrel. Or is it too good to be true? What kinds of pitfalls should I be watching?

On the other hand...

For attaching the M6 brass barrel to the toolhead...I am planning to make the toolhead out of a 5mm thick AL base, and just screw the barrel through it.

The AL base will be screwed to the Z platform which is also made of AL, I think that would create a huge thermal heat capacity and should create a sharp temperature gradient. Though I am not sure if the heat loss too quick to render it unusable. Am I on the path to failure?

If you have kapton tape, you can use that to insulate the nichrome. Wind kapton round the barrel, then carefully wind the nichrome, then more kapton, then more nichrome (if you need to wind more).
Make sure that none of the nichrome coils are touching.

I've build a couple of 'heating blocks' using wirewound resistors - and they work well. They're a bit harder to make, but they work.

You need some heavy resistors (I used 7W 12Ohm wirewound resistors). Just make sure they can withstand the 250C temperatures they will get to.

Check out Nopheads or my blog for more details.
[renoirsrants.blogspot.com]

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Reprapping blog and other rants: [renoirsrants.blogspot.com]
My Reprap: [sites.google.com]

there's insulated constantan wire available, afaik the lacquer will withstand the temperature, but it will get brittle, that is no problem I guess since it's fixed in JB Weld or firecement and thus will not move that much...
But that needs to be verified first since it expands when heated and not just a little

Resistors a far more elegant, but quite hard to find around here (well at least those which are small enough and still withstand the the temperatures )

'sid

I was sourcing for the resistors...

However, the enamel resistor I have found around doesn't have axial connection, but comes with a base instead...

I am also looking at aluminium case Wirewound resistor, however they are rated at 200 degree C. I don't really understand what it means...will it disintegrate when reaching that temperature? shorts and catches fire? or just inaccurate resistance value?

I can bear with the last one because we don't really care about the resistance anyway.

Does anyone know?

I have also use AL case resistors above their temperature rating: [hydraraptor.blogspot.com]

A little smoke as the markings burnt off but no other problems, YMMV.

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[www.hydraraptor.blogspot.com]

I think brass is too good a thermal conductor to connect directly to the heatsink. A stainless steel tube works but brass will need a thermal insulator.

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[www.hydraraptor.blogspot.com]

Looks like SS as thermal barrier + enamel resistor heater seems to be promising.

After reading a reader comment on Nophead's blog about it's AL Shell resistor exploding...I decided to go with Enamel resistor.

I have spent a long time on Taobao.com (Think it as better-than-eBay in China) but couldn't find a enamel resistors with axial output, luckily the RS-China carries similar enamel resistors part (Part NO 159-360). Yah!

I am going to order the SS rod and thermal block from the machine shop, let see how it goes

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The problem with the info on the Wiki/blog is that it often have some assumption - like certain power tools (or more important it assumes no access to certain tools / material)

But when I have no tools at all, I have access to all tools by paying the machine shop, but then the instruction is not cost-optimized for ordering from the machine shop. I usually have a hard time in following instruction, determining if is a workaround because some tools are assumed not accessible, or it's a must-have design feature.

Anyway, kudos to everyone. I found that in this community, it's full of smart and nice people. There are a few times when I read on a forum thread and think that "What a stupid idea, it's going to be a flamebait", there are replies that proves that it's actually not stupid, but indeed an interesting idea.

Documentations/implementation has been improved a lot over the time. There are rooms for improvement but we should be getting there in a foreseeable future.

Have you seen this design [builders.reprap.org] ?, looks pretty simple to make.

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[www.hydraraptor.blogspot.com]

Oh yes! The article just came in time!

Oh the resistor from RS and the thermal blocks finally came back.

I spent a few hours to hook them up. No drill, no cement, just assemblying and hey, it works!!

I got my very first filament extruded! Albeit the motor can't get the plastic so I could only extrude like 5mm/sec.

The top of the SS pipe is even cool to touch, a little bit fuzzy warm but won't get hotter than my laptop. I think that's because my whole toolhead assembly is made of AL, a very big heatsink.

It took like 3 minutes to reach 150 degree, then another 5 minutes to reach 250 degree though. May be 6.8 ohm is a bit too high? Or my heatsink is too "strong."

Thanks everyone. I would put some photos very soon.

With a little adjustment to the motor...it's extruding like 50mm/s!

The "heatsink" is getting hotter now after 30mins at 250 degree, but still comfortable to touch.

I think I should go back to work on the program again, before I used up all my plastic stock! It's too much fun watching it extrude.

I havn't looked at skeinforge and AoI yet, oh..also the M-Apps. Yeah! I'm over-joyed.

http://picasaweb.google.com/sam0737/HRepStrap

Here are the photos of my extruder, thanks for everyone inspiration!

Looks nice! Have you had any problems with lateral forces on the filament causing it to bunch up before it goes into the heater barrel? That seems to be the main problem I have with my extruder.

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[www.binaryconstruct.com]

I don't think so. Do you mean because the filament going through the glass transition temperature so it becomes very hard to push in midway?

I don't have this problem in my test (yet). I believe my temperature graident is very steep (given that it's comfortable to touch on the top while the bottom is extruding, even after an hour)...effectively the plastic are either melt, or solid, but not in between.

I had some issues with the grips of the motor. The motor is like 20rpm I guess.

So I bought a new one, similar size (so I could re-use the mount!) but at 6rpm. Though the resistance is 10x higher (the old one is 5ohm, the new one is 50ohm), but with the new one it never skip a bit!

At max speed it could extrude at 12mm^3 (if my math is correct...), I hope it could go a little bit faster (may be I could just buy another gear head), but so far it's very satisfactory!

In particular, I didn't have any bug on my motor mount made by the workshop, the spacing were just fitted, so does the heater. And I didn't run into any nozzle blockage problem (yet?).

Here are the pictures: [picasaweb.google.com]

It prints! it prints!!!

Though the temperature requires much more tweaking...I found that the sweet spot is really small.
And the raft does curl quite a bit.

But given that it's the first print, I would say it's a success.

[picasaweb.google.com]

My heater seems to be losing heat too fast, for one layer it has to be spend more than 30 seconds to re-heat to the target temperature. I am trying to solve this with easy way: buy a resistor with lower value, and in fact I ordered another pack from RS.

For the sake of completeness, I am attaching the drawing.