filament feed towards extruder: 2mm inside diameter or larger?



  • I am rebuilding my printer. This printer had it's electronics below the build area and stored the 1.75mm filament on top in a closed box with desiccant. ~40cm of 2mm I.D. PTFE tubing ran from that box towards the direct-drive extruder. I never had a single issue with this setup.

    Now that I moved to a Duet board which is far more compact than what I had I end up with a largely unused space below the build platform. I am considering making a nice 'drawer' and moving the filament rolls there. Then I do not have to climb on a chair to exchange the filament roll anymore.

    This would mean that the feedlines towards the extruders increase in length, say 120-150cm.

    Would it be best to use the regular 2mm I.D. PTFE tubing, or would 3mm I.D. PTFE tubing provide less resistance? I have a lathe and some PTFE stock, making an adapter to narrow the tube before it enters the extruder is not an issue.
    For ABS, ASA and PC I don't think it matters that much, but I also use TPU98A a lot which is quite soft and stretchy.



  • If your filament is within specifications, 2mm tubing should be fine.

    My printer has a modified Polybox which sits on the table next to it. From the box is one tube, which is probably about 700mm long which goes to the first of a pair of push fit connectors mounted at the top of the frame. From the second push fit connector, I have another 700mm of tubing which goes to the extruder.

    The pair of push fit connectors are just screwed back-to-back into a small PLA mount which is bolted to the frame's upper extrusions. This lets me open the feed line when loading filament.

    I have printed semiflex filament with this setup and had no problems. You just need to make sure that there are no tight bends in the tubes.

    I don't think the internal diameter of the tube makes much difference. If the tube curves, the filament will run against the side of the tube, regardless of the tube's internal diameter.



  • I'd go with 3mm if this is just a guide tube to the extruder and not a bowden tube from the extruder to the hotend. Reason being that I've recently had to change my own guide tube from 2mm to 3mm due to some filaments that had a rough texture to them being overly constricted causing under extrusion. Stone fill or wood fill for instance, but also just regular PLA with a bronze appearance to it had a very rough feel. Filaments that are more stiff and brittle like stone or wood fill don't like to be bent either. The extra internal space allows for more unrestricted movement.



  • Well, there you go; two completely different opinions!

    🤣



  • And even worse, both have valid arguments 😂

    2mm would be the easiest; no adapters needed, just one run from Bondtech BMG to filament roll.
    I use name-brand filaments, mostly PC, ASA, ABS and TPU. I may want to add a light PP for drone canopies; seems to work well for that purpose. And I would love to add a dissolvable/breakaway support material such as E3D scaffold; that is the sole reason to go dual extruder (E3D Chimera) this time. Oh man, I hate removing support with TPU.
    I suppose all those filaments fall in the 'within specifications' category.

    I am not too interested in wood/bronzefill/stonefill filaments. Maybe in the future some of that 98%-stainless-steel-powder filament that needs sintering, but the price of that stuff is not funny.

    Would 3mm tubing hurt? Would it make feeding filament to the extruder harder, for example?



  • @DaBit Presumably the tubes are to keep the filament dry. If so, I guess another question you have to ask is how effective are they in doing that job? Unless you blow dry air down them then maybe not all that good. So minimising the volume of air inside the tubes is probably best. Which means using the smallest diameter that you can get away with. Alternatively put a small fan in your filament box so that it's under a slight pressure but then you might need something other than desiccant to keep the air dry.



  • The closed box and feeder tube are used to keep the filament dry, and also important: to keep dust away. The 3D printer is in my garage, not the cleanest place on earth.

    I expect that most of the airflow into and out of the closed filament box is due to temperature changes. At night the environment cools, air in the box contracts, pressure equalisation happens through the feeder tube and air is sucked in. At day temperature rises, air in the box expands, well, you get it. Back of napkin: Assuming (p.V)/T=constant, a temperature difference of 10C and a box volume of 8 liters that's about 270cc of air in/out the box every day containing around 2.5 milligrams of water at 50% RH and 20C.

    Not sure if a tight fitting feeder tube vs a losse feeder tube would make a difference.

    In the 'old situation' a few grams of silicagel desiccant with indicator in the box lasted several weeks to months, depending on how often the box was opened to exchange the filament.



  • @DaBit I was just thinking along the lines that when the filament is on the reel, it's really only the outer layer or two that are exposed to the air. But inside the tube, all of the filament is surrounded by air. That's a relatively high surface area (depending on the length of the tube). So the bigger the volume of the tube, the bigger the volume of free air which could absorb moisture which could then get transferred to the filament. But on the other hand, that air would be static so maybe any ambient moisture wouldn't find it's way too far up the tube from the open end? Dunno - just kind of thinking out loud.

    The reason that I'm taking an interest is that I'll eventually be moving my printer into my garage so I'll be faced with a similar problem. But I have 5 reels of filament that have to be loaded whenever I print something (one downside of using a mixing hot end), and it's a bit of a pain to load and unload so I'm a bit lax at removing all the reels when the printer is not it use. For my usage case, it might be better to build a semi sealed dustproof cubicle around the entire printer and put a de-humidifier inside that cubicle.



  • Like Phaedrux I also use a 3mm feed tube but just so it fits all the std fitting it is still 4mm Od so that actually makes it more flexible. As it is the feed to the extruder then IMHO the less friction whilst still guiding the filament is a good thing.

    Doug



  • @Dougal1957 ..........and the lowest friction of all is to not use a feeder tube which is what I do (but of course, that doesn't keep the filament dry).☺



  • @deckingman said in [filament feed towards extruder: 2mm inside diameter or larger?]

    @DaBit I was just thinking along the lines that when the filament is on the reel, it's really only the outer layer or two that are exposed to the air. But inside the tube, all of the filament is surrounded by air. That's a relatively high surface area (depending on the length of the tube).

    I don't think it is only the outer few layers of the reel; air can enter from any side and the filament is loose enough

    But on the other hand, that air would be static so maybe any ambient moisture wouldn't find it's way too far up the tube from the open end? Dunno - just kind of thinking out loud.

    Thin layers or air are quite sticky. That's also where clothing and cell foam get it's insulating properties from.
    But just like you I am not so sure about 1.25mm of spacing, though.

    For my usage case, it might be better to build a semi sealed dustproof cubicle around the entire printer and put a de-humidifier inside that cubicle.

    That would be best I think. Moisture is one thing, dust and UV-light is another.
    I could do the same; my printer is a closed frame ( I used to have chamber heating, but I am dropping that feature since I rarely use it). Just not airtight. But that is nothing a bit of silicone and closed cell foam can't solve. Hmm...
    Here is an older picture: http://www.icecoldcomputing.com/directlink/3dprinter/3dprinter_smurf.jpg

    @Dougal1957 said in filament feed towards extruder: 2mm inside diameter or larger?:

    Like Phaedrux I also use a 3mm feed tube but just so it fits all the std fitting it is still 4mm Od so that actually makes it more flexible.

    I did not even check the O.D. of 3mm tube. If it is indeed 4mm it is a no-brainer.

    @deckingman said in [filament feed towards extruder: 2mm inside diameter or larger?]> @Dougal1957 ..........and the lowest friction of all is to not use a feeder tube which is what I do (but of course, that doesn't keep the filament dry).☺

    I used to do that, until I had prints with a terrible surface that turned out to be moisture problems. Since I placed the filament in a box with a piece of PTFE tubing guiding the filament I never had moisture issues again, and as a bonus I never had a jam or tangled filament on the roll either.



  • Followup: I did a test with some Chinese translucent PTFE tubing to run the filament from the bottom of the printer all the way to the extruders, but that was borderline at best.

    With PLA it takes quite some force to push the filament from the bottom to the extruder, up to the point where it starts kinking. Then I tried routing the PTFE lines inside some 6mm OD polyamid pneumatic tubing to prevent small bend radiuses and possible flattening of the tubes, made no difference.
    Well, the Chinese translucent tubing is probably not the best a man can get, but quality tubing will not do an order of magnitude better.

    I ordered some 3mm I.D. PTFE tubing, hope that helps.


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