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Nozzle under pressure?

Posted by sungod3k 
Nozzle under pressure?
October 15, 2013 01:12AM
Hey printer commanders,

I just made my bowden extruder work a minimum reliable level, now it doesn't clog after each 5cm of filament anymore.

What I´m observing now is that when I push out 5mm filament after the command is done there is at least a string of 3-5cm of extruded plastic slowly dripping out, as if there is to much pressure during the initial push and this is the afterglow.

Its hard to find that into search terms, any idea where or how to solve this?

Edited 1 time(s). Last edit at 10/15/2013 02:25AM by sungod3k.
Re: Nozzle under pressure?
October 15, 2013 04:46AM
During printing, and after a printing move, you quickly retract (run the filament backwards) a certain amount to relieve the pressure in the nozzle. When you start a printing move again, you quickly move the filament back to the same length and position as before. I have no experience with bowden, but I understand it can take some fine tuning both in retraction speed and length in your slicing software to get good prints in various situations.

When idling, a hot nozzle will always drip a little.

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Re: Nozzle under pressure?
October 15, 2013 02:11PM
I dont have the feeling that this is a retraction issue. I think retraction is for the dripping and precise control of what comes out of the nozzle. But this seems some more basic to me.
Re: Nozzle under pressure?
October 15, 2013 02:39PM
Does retracting 2 or 3mm of filament after extruding solve the dripping issue? As Nudel had said, an idle hot nozzle will always drip. Are you using PLA or ABS? PLA tends to ooze and drip much more. How about your temperature? Does decreasing the extruder temp by 5 degrees reduce the problem? If none of the above, I would suspect that the nozzle is partially clogged and increasing the time necessary to extrude the length specifed.
Re: Nozzle under pressure?
October 15, 2013 07:34PM
I found using a bowden setup with pla you really need to fine tune the temp, you really want it just right so you can extrude it, Iv tried 1-3 mm retraction, 1 or 2mm works best for me, i also think it depends on the hotend design as well. the speed is also set around 30 for me in slicer, which seems quite fast. for some reason faster than 100mm's extrusion rate.

you should be able to hand feed the plastic, and it should be able to be fed at a consistent rate,

you wanna isolate the issue, maybe taking a video or something as well, or pics to post up here so we can see this drip you or ooze ur having

Edited 1 time(s). Last edit at 10/15/2013 07:35PM by nechaus.
Re: Nozzle under pressure?
October 16, 2013 02:09AM
The bowden hot end oozes a lot more than a direct drive. This is a function of the inherent springiness of the system. When you advance the extruder gear what happens is you are putting tension into your bowden cable. What you are describing as the 'afterglow' is the release of this extra tension applied to the cable. It acts as a spring which continues to push filament through after you finish turning the gear.

The fact is that extrusion rate is proportional to pressure and not the amount of filament you push in. As long as you maintain a constant pressure on the molten plastic inside the hot end you should get a reliable extrusion. Retraction is designed to relieve pressure as well as "unspring" the tense bowden cable. On my ultimaker I observe an 'afterglow' extrusion as you describe, but while printing there are no strings or any symptoms of bad quality because I retract quite a lot after each extrusion (7 mm). The point being that without anything in front of the nozzle it will ooze and drip but when squishing plastic into a hard surface (the print or the print bed) you don't see this 'excess extrusion' because the pressure you are dealing with inside the head is much greater and in fact self-regulating.


Don't solve it just print.
Re: Nozzle under pressure?
October 16, 2013 04:07AM
How interesting.

I've been trying to find references to the principles underlying 3D printing and I had been wondering about how you stop the plastic at a precise point. This is the first time I have come across the idea of retracting the filament.

Wouldn't it be nice if all the design principles were included in a single document.

Re: Nozzle under pressure?
October 21, 2013 03:38AM
The flexible filament is a good point. and i found a possible blockage in my extruder construction. I will have to do some more test because i have some other trouble which makes it hard to separate different symtoms.
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