If I had.... (a daydream)

a daydream

If I had a well equipped workspace, an operating reprap and buckets of energy I would experiment with a nozzle of brass which attaches to a metal which is a poor heat conductor. The metal with the higher expansion coef screwing into the other.. but anyway, why not just heat the nozzle (say an extended bighead style with 5-10 mm melting void) and link to a low conductor which will perhaps if necessary link to PTFE/PEEK

anyone tried this? It seems obvious to me so I must be missing something??


Is it perhaps because of the friction?

Edited 1 time(s). Last edit at 07/10/2010 03:24PM by AgeingHippy.

There is no such thing like a low heat conducting metal. Stainless steel is about the worst metallic heat conductor 20 W/(m*K) available. For comparison, copper has 230 W/(m*K)

If you want a poor heat conductor, you need plastics (PTFE = 0.24 W/(m*K)) or some mineral (glass (0.8 W/(m*K)), plaster (0.4 W/(m*K)), concrete (1 W/(m*K)), etc.).

All values mentioned vary somewhat by material alloying/composition, of course.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

OK

so using an extended brass nozzle with a 'melt chamber', a stainless barrel attached to the plastic of your choice. Surely that would beat the copper/plastic join? I imagine it should not be too hard to machine?

Somehow the problems with extruders seem to be the hot/cold join. ??

Nophead had done long, thin stainless barrels; read his blog for details. It takes a lot of Q to keep one side hot and one side cold; he used very thin SS and heat sink plus fans.

I've built a copper nozzle, aluminum heater block, SS threaded rod heater, that I intend to mount to heatsinks, ala nophead... I expect to have trouble with it, but we'll see in a few weeks.

Al...

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[araspitfire.blogspot.com]

So even though stainless is about 11 times worse heat conductor than brass it is still too good a conductor. Is that the gist?

That's the gist... SS is 100 times better heat conductor than PTFE.

Al...

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[araspitfire.blogspot.com]

If your still interested, I covered a few of the basics of heat conduction here.

It may be worth pointing out that due to its poor thermal conduction PTFE has a very steep thermal gradient. In other words, even just a few millimetres of PTFE separating two conducting metals may be just as effective as a long stainless steel thin walled pipe.

I can tell you from first hand experience that wrapping just a few turns of PTFE (teflon) tape around the end of a pipe and then inserting it into the flared end of another pipe can act as a very effective insulation.

AgeingHippy Wrote:
-------------------------------------------------------
> a daydream
>
> If I had a well equipped workspace, an operating
> reprap and buckets of energy I would experiment
> with a nozzle of brass which attaches to a metal
> which is a poor heat conductor. The metal with the
> higher expansion coef screwing into the other..
> but anyway, why not just heat the nozzle (say an
> extended bighead style with 5-10 mm melting void)
> and link to a low conductor which will perhaps if
> necessary link to PTFE/PEEK
>
> anyone tried this? It seems obvious to me so I
> must be missing something??
>
>
> Is it perhaps because of the friction?

Friction or fiction?

Seemed obvious to me too...... I hate when somebody tells me I can't.



Never give up on a dream.


.4064 mm nozzle, 55mm/sec.

The melting void idea didn't work. I ended up using a crude representation of Nopheads superstruder tube drilled out of a plain old 1/4” bolt. I turned the the heat chamber/nozzle out of piece of 5/8” Home Depot aluminum rod. 3' for 6 bucks. Home made heat sink and a little fan and voila, no muss, no fuss, no plastic, no jams..... No replacement parts.

Seems to me that Ageing Hippy is thinking about the upper end of the brass ferrule being screwed into say a tapped stainless tube that also has a thread on the outside which would screw into the PTFE. Would this not reduce the heat into the PTFE to prevent it from failing???

Hey cluso

That is exactly what I am thinking about.

I think from the responses is that the heat conducting properties of the stainless is still too high.

Probably what would improve matters would be a PTFE washer of say 2 mm thickness between the stainless and brass. Pwerhaps even PTFE tape around the thread where the stainless and brass join to minimise the contact further....

will have to wait till I have a workspace.

Perhaps ultimately the stainless would ultimately heat up to the same temp as the brass.

Guess what we really need is a ceramice threaded ferrule to go over the brass thread near the top. If a ceramic ferrule were possible how would we hold the ferrule into it? I guess epoxy would melt or even catch fire??? What uses ceramic with a 3+mm hole in it???

Edit:
Answer to ceramics... Ceramic tubes come in 3.18mm (1/8") ID and 6" lengths which can be cut to length. Gluing can be done with ceramic cement. It can be milled (tapped). [thermometricscorp.stores.yahoo.net] (doesn't have 1/8" ID but has good info)

Edited 1 time(s). Last edit at 07/14/2010 11:06AM by cluso99.

I wonder how accurate the inner and outer diameters are on those ceramic rods. I am envisaging a rod with a hole sized to fit the filament (1/8" would be good) enclosed within a ceramic rod whose ID matches the OD of the rod holding the filament. It says that these rods come with one end sealed, so you could machine a heater block that fitted within the larger rod and hold it in with the smaller rod that fits the filament. A 0.5mm hole in the closed end could act as the extruder nozzle.
If you used the multi-holed rods, you could even pass coolant through the holes to ensure the upper end was kept cool for as small a melt zone as possible. Controlling the melt zone so that it was within the ceramic tube should make for a reliable extruder.
Any idea how much they charge for these rods?

Greg: I think the idea is that the brass ferule which is 3mm OD is either remade with 3-32 (IIRC that was the tap they come with for standoffs) so that it would screw into the ceramic. That is providing we can drill the ceramic out as the standoffs have a solid between the two taps. Otherwise we have to glue the brass into the ceramic with ceramic cement. The brass rod is currently 3mm and the ceramic tubes are 3.18mm so this should work fine. The tubes seem to come in 6" lengths but can be ordered cut smaller, which is required because the tubes are not blocked at one end and you can have either. I am unsure if we can cut them or they will break. The cost appreas to be about $5 each. This may just solve our extruder problem with the PTFE breakage.

I am awaiting some replies from the manufacturers.

I have had good results using a stainless steel tube as the thermal transition. I described it here: [builders.reprap.org]. Please note that the TO-220 heater part of that design did not work very well. I have since gone with an aluminum body type resistor like nophead has been using.

I found a stainless tube which is barely thick enough to put a 10-32 thread on. This means that the thickness of the stainless at the bottoms of the threads is very thin. In fact if you look in the tube, you can see the threads where the die has deformed the tubing. This holds down the heat conductivity.

This means that you can screw any sort of head with a 10-32 thread onto the tubing.

fdavies

I just bought some glass tubing and I want to experiment with using my welding torch to shrink the tip and make the entire hot section from one piece of glass with nichrome wrapped around it. Might even be able to embed bare nichrome directly in the glass so that no electrical insulation is needed. Hopefully results in a few weeks (life keeps getting real busy around here.

very helpful indeed.