Why 3mm for the Feedstock Filament

OK Dumb Ass question time.

3mm has been selected for the feed filament diameter.

I have no real opinion either way on this but am quite curious as to how the choice of 3mm instead of something else say 5mm was arrived at???

Is there something artful and clever about 3mm or was it just a convenient figure ??

I's just gots ta know.

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

I'm not sure. I don't think there was any particularly compelling reason for it. In May of 2005, they were doing experiments using hand-made feedstock. There's a mention here [blog.reprap.org] of 4mm rod. Then 2mm here: [blog.reprap.org] . The first mention I can find of 3mm is this: [blog.reprap.org] .

Edited 1 time(s). Last edit at 01/26/2008 01:00PM by Steve DeGroof.

It's pretty much one of the standards used for plastic welding equipment the others being bigger, this means they is some availability for different plastics quite cheaply (but not CAPA ) also conversion to 3mm filament is easier due to there being companies setup to do this.

We will try to go the granule feed as this is the cheapest for raw material.

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Ian
[www.bitsfrombytes.com]

Aha all is explained.

That sounds sensible to me, I thought there may be some clever polymer/physical science behind the choice. But there is even cleverer fiscal science at play. Grin

The notes and experiments on extruding polymer chip are interesting. I think this one will take a clever bit of lathe work to get the differing diameters into the extrusion screw (Compresing/Feed, Degassing, Metering & Extruding).

I found a book in Foyles last I was in there on Polymer science and skimmed the section on extrusion out of curiosity. (Funds were limited or I would have bought it, but had already spent too much on books I didn't want to put back)

Looking at the extruder design and reading the feedback on experiements with higher temperature thermoplastics, the PTFE barrel in the extruder looks to be turning out to be a weak point in the design particularly at the higher temperatures.

PTFE is'nt terribly strong and I would guess as the temperature/pressure is increased it would become weaker.

Do you think there is some mileage in replacing the PTFE barrel with a PTFE barrel that has been filled with something to help with toughness and dimensional stability. ie glass fiber or carbon.

These people seem to do a lot of plastics in various sizes and even smaller quantities. I found them whilst looking for Delrin suppliers. The more common you can buy on line the less common they will get for you.

More of note is that they claim to offer Virign PTFE, Glass Filled PTFE and more interestingly Carbon Filled PTFE.

[www.theplasticshop.co.uk]

Thoughts for what they are worth

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

I don't think there is a problem with the short term strength of PTFE, it is the slow creep that causes it to lose the nozzle. Hopefully that is fixed by adding a pipe clip. Possibly the fillers would do the same job but a bit more exotic.

The main limitation is the top temperature. PTFE, J-B Weld and the thermistor are all rated to about 300C. Thermosteel looks like a high temperature replacement for the J-B Weld and / or BBQ paint. I will be evaluating them in a long term trial soon.

The thermistor can be replaced with a thermocouple to get up to 800C. Zach has done a PCB to handle one.

All we are left with is finding a high temp replacement for the PTFE. It's a tall order because it has to stand a high temparture, be quite strong in elongation and also be a very good thermal insulator if the extruder body is CAPA.

At Victor's suggestion I am evaluating soap stone for this role, but it may require a higher temperature extruder body as well.

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[www.hydraraptor.blogspot.com]

Thoughts

Instead of using a large chunk of insulator what about using a much thinner thermal break made out of something less plastic like. Followed immediately by an aluminum barrel with turned fins on it to act as a heat sink.

The figures I have seen for PTFE seem to indicate around 260 Deg C ish, 300 and more is getting awfully close to the temp it will be in it's most runny (although not much it has relatively high viscosity). It would be interesting to somehow get hold of a graph of it's mechanical strengths & viscosity versus temperature.

Givent that the actual equivalent of the temperature of the PTFE in contact with the threads of the heated barrel has to take into acount the pressure the threads are exerting on the polymer I am not entirely surprised that it creeps.

Have you ever tried the experiment with embedding a wire or thread into an ice cube. This is done by standing the cube on top of a bottle or some such and placing a thread or thinnish wire across the cube with weights on either end.

If left for some time the thread slowly creeps into the ice cube with the ice refreezing over the top.

If you wait until the thread is in the middle you can pick it up and see that the thread is embedded in the cube.

If you wait long enough the thread will work it's way through to the bottom side.

The force/pressure exerted by the thread is sufficient to make the ice do a state change to liquid (the same way ice skates work funnily enough).

I think the higher temperatures folk are trying are close enough to a point on the temperature/state curve that in conjunction with the pressure the plastic is becoming more plastic ie weaker and therefore the barrel creeps.

For higher temperature work I can't help but feel that a redesign of the barrel would be a good move. Maybe some kind of cool actively cooled zone is required or a replacement of some of the materials.

As ever I think the original design was excellent for what it was originally designed for ie Polymorph.

As ever for what they are worth.

cheers

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

Been musing on High temperature/pressure extrusion.

Stainless steel is a not particularly great conductor of heat. Arguably not as good an insulator as PTFE but certainly much better than brass.

How about replacing the Brass Acorn Nut with a Stainless steel one (Less heat sinking) Although drilling the stainless one could be painful.

Then replacing the PTFE Barrel with an Aluminum one joined to the Brass barrel with a Stainless steel studding connector (sort of elongated Nut). Inside the Studding connector joining the two threaded rod/tubes (Brass & Aluminum) place a thick washer of some suitable insulating material creating a thermal break between the two Dissimilar metals.

So 1 third of the stainless steel studding connector is filled with the Brass heated barrel, ! Third filled with a thermal break material and the final third is the Aluminum Cool Barrel.

If you fin the Aluminum barrel with some aluminum washers and nuts you could arguably then screw it back into the PTFE barrel rather than replace the PTFE altogether.

Net effect is a higher working temperature hot zone (Less heat sinking due to Stainless being a poor conductor of heat, a cool zone to protect the extruder mechanism. But a slightly longer Barrel. No or less creep. greater capacity to cope with increased pressure of longer durations

Don't know if these ideas are any good to anyone.

Cheers

aka47

Edited 1 time(s). Last edit at 02/02/2008 01:16PM by Andy Kirby.

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Necessity hopefully becomes the absentee parent of successfully invented children.

i have definitely tried drilling stainless steel nuts... its nearly impossible. one of the reasons aluminum was chosen was because of its easy of fabrication. that and it conducts heat from the barrel really well.

Back to the subject of 3mm feed stock, I now have:-

3mm HDPE actually 3.2mm
3mm CAPA actually 2.95mm
3mm ABS actually 2.75mm

It seems there is a very big tolerance on stock filament. I made my extruder 3.3mm to handle the HDPE without jambing so the ABS is very loose in it. I don't know if it will work like that or not. I suppose the danger is that the molten plastic will back up and leak out the top.

I may need three different extruder barrels, I should find out soon.

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[www.hydraraptor.blogspot.com]

Zach

Yes I think you have a point S-Steel is a pain to work best not, even leaving the acorn nut alone there may be some mileage in the thermal break and cool zone in the barrel route.

Nophead

An idea, if you need/want it.

If you go with a S Steel studding connector, thermal break washer and aluminum cooler barrel.

You could make your Brass and Aluminum parts large enough for the larger diameter feedstock but have a thermal break washer that had a hole to suit the diameter of the feedstock.

Tada one size barrel three sizes of thermal break washer. Three relatively closely matched sizes of feedstock.

Change the thermal break washer for the intended run.

(You might not need this at all depending on how your experiments turn out)

cheers

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

Alternatively

One perhaps wonders if there is some mileage in having a thicker barrel drilled out to say 6mm with an insert-able sleeve to reduce the diameter from 6mm to size of the feedstock.

The acorn nut would actually cope with what ever liner was used as the hole is central and the shape of the dome end stops whatever diameter is used even up to say 6mm...

5mm might be better drilled though an M8 although M10 barrel with M6 hole makes better use of standard rod and stud sizes.

For thicker stud sizes maybe a switch to aluminum for all the barrel (with or without the thermal break is an option) Its certainly easier to work and should be strong enough.

I guess it depends on what tooling you have to hand.

cheers

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.

... what could help handle different diameters of filamet with only one setup is an entry-sleeve made from a heat-resistant resine.

It should be possible to tightening or loosening the entry hole by compressing the resine-tube with a hollow screw, as in some plumbing-devices, where you simply insert the tubes in the resin and compress it until it's tight enough.

Then lubricating the resine or filament with vaseline for lesser friction is helpfull too ...

Viktor

If you think about it for a minute you will realise that force required to thrust the plastic filament into the heated extruder barrel goes up proportionally to the area of the filament cross-section or as the square of it's radius. Make the diameter of the filament much bigger and you will soon be requiring a much larger gearmotor. As it is, the Mk II will extrude adequately to print at up to 14 mm/sec print head speed. That's far faster than most people's 3D positioning systems will move. Make the extruder heavier and the positioning system has to have a lot more power to throw it around above the xy printing surface.

Interesting.

Is this perhaps why the choice of 3mm feedstock diameter then ??

aka47

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Necessity hopefully becomes the absentee parent of successfully invented children.