1.75mm filament force on bowden hotend

Hello fellow reprappers,

I am new to this forum and this is my first post.

I have searched exstensively on this sight and on google, but could not find a decent answer regarding my problem. Apologies if this has been answered properly, a link to the answer would be appreciated!

I have a 1.75mm MK8 extruder connected to a bowden hotend. The PTFE tubing is about 120mm long and I can freely move the ABS filament through the tubing. My problem is that the filament does not flow freely though the bowden hotend.
The temerature reads back 230 degrees celcius and when I feed the filament through the bowden by hand it takes a great deal of force to get the filament to extrude from the hotend. This is my first custom home made printer and I have never seen filament
flowing freely through the bowden hotend. I could not find a single video showing how this is supposed to happen, the closest I got was with calibration videos where only the top of the filament is shown (and not the melted end by the hotend).
This seems to go pretty fast, about 1mm every second, maybe a little less than that.

My question is, how much force should it take to get the filament to melt and come out the hotend on a bowden extruder BY HAND? I would expect it to take almost not effert, and that gravity should to most of the work if the hotend is at 230 degrees celcius.

I have cleaned the bowden and hotend twice allready, and the filament runs through the heatsink and heatbreak with almost no resistance. I doubt this is the problem, as it seems to be assembled correctly.

It may be that it is working correctly and I am just not patient enough, but when I force the filament thought the heated bowden it goes very slowly, and it feels to me that more filament is going into the bowden compared to how much is coming out.
Another question, is the hotend supposed to' fill up' before normal extruding will occur? The two times I have cleaned the hotend it was completely filled with melted (and stalled) ABS filament.

Any input would be appreciated!
Thanks

Edited 1 time(s). Last edit at 05/22/2016 06:00AM by v1talogy.

One massive problem with bowden systems is that the tube is not really suitable for the filament.
Take the ones for 3mm filament as an example, most are 4mm in diameter, some go even higher to use the cheap 10mm connectors.
If there is a lot of play it means the filament can start to twist inside the tube resulting in a lot of force required to move it.
IMHO the best option is to abandone the PTFE tube in favour of a normal (stiff) air hose that has an inner diameter just enough to get the filament through.
Before I abandoned the bowden completely I tried several things to address the problems of retraction, friction and reliable printing - I gave up and really would never go back to a bowden system unless using a Delta printer.

230°C are at the lower end for ABS 240-250 are much more likely to produce good results. In either case ABS will not flow from gravity alone, you can and will see this with PLA and some other materials. It is usually unwanted and called ooze, since it is uncontrollable and usually only the material within the melting zone will ooze out, but the filament will never feed itself deeper into the hotend from gravity alone (ok, maybe on jupiter).
Chances are your problem is either heat related, or the hotend doesn't work correctly. The Extruder and the Bowden tube are very unlikely to be a problem, except if the extruder is just not working correctly (missing idler pressure, blunt bolt teeth etc.) or the bowden is damaged (sharp kink or similar).
How much force it takes to push by hand is a very unreliable indicator, it is very uncontrollable and not sustained. A better measure is the power the extruder stepper needs to push the filament. You can get that information from the reference voltage of the stepper driver you have to use to make it work.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

Thank you for the prompt replies gentleman,

I will move the temperature up to 250 degrees celcius and see what happens. I am using pulolo drivers on a RAMPS 1.4 board and it was usually set to 0.4V. I have even moved it up to 0.7V, for the NEMA 17 stepper motor but it still skipped steps. It actully did not move the filament at all. It really struggled to get the filament through the tubing, but when i tried it by hand it used NO force at all, it slided through very easily with almost no effort.

I put that problem aside and just tried to get the filament through the bowden with little ease. I will then move to the tubing problem, and as you suggested Downunder25m I will see if I can use a different material to guide the filament.

One step at a time I guess.

Does the extruder transport the filament with force if no bowden tube is attached? Maybe you have a mechanical problem/obstruction in the extruder?

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

Hello Srek,

No, the extruder moves the fiament with no problem at all. I believe it has to do with the resistance in the tubing as Downunder mentioned.

And you were right about the temerature! I set it to 250 degrees celcius and it worked MUCH better, still at a slow rate but much improvement. Thanks.

I will do more testing and post my findings on here once I am satisfied with the information I can provide.

What inner diameter does the tube have? Does your filament have consistent diameter? The extruder will happily move filament that is out of specs, but the bowden is more discerning.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]

I am not sure of the diameter of the tubing, and I do not have a vernier/caliper to measure it accurately either.

If I move the filament, which is NOT very consistent (Verbatim ABS 1.75mm), through the tubing there is little resistance. I think I may have described the problem in my previous post incorrectly.

The problem I am having, and which I *think* I may have solved now, is the angle the tubing is bent during the process of printing. When the Z-axis is a its home position the tubing is relatively straight and the extruder can push the filament
through the tubing. It is when the Z-axis move upwards that the tubing gets bent and only then is the extruder not able to push the filament throught the tubing. I only realized recently that this is my big problem.
I trust that the bowden hotend is OK and that it should be able to deal with this inconsistencies of the filament thickness. Just to mention, I have previously tried to up the temperature (250 degrees celsius) but it did not work. I think this is becuase the hotend was
allready blocked by the time I tried that. With a clean hotend and 250 degrees celcius it seemed to work as intented (I have nothing to compare it with but my intuition), as I mentioned in my previous post.

For now what I have done is raise the 'extuder driver', if you will, further up on the printer (by about 200mm). This enables the bend radius on the tubing to decrease greatly, even when the Z-axis is at its highest point.
At the moment I am waiting for some glue to dry, so that I can test this theory. I have held it there by hand and it seemed to have worked. The proof is in the pudding, as they say.

Thanks for the prompt replies and suggenstions/questions!

Edited 1 time(s). Last edit at 05/22/2016 02:54PM by v1talogy.

I didn't think of the radius, it can easily be the problem. I usually tried to use a radius of >15cm when i used 1.75mm bowden setups.

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[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]