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avh

sudden retraction when printing the first layer - solved

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I'm having trouble printing the first layer. I just got my UM2, I've updated cura and the firmware, leveled the bed.

During the first layer the feeder seems to suddenly retract the material a few mm, resulting in an incomplete print, and a mess. It is as if the feeder retracts because there is too much pressure in the nozzle. Is that even possible?

I tried printing with retraction turned off, but that did not help. Lowering the material flow to 50% helped, but that does not seem the right thing to do...

Has anyone seen this?

 

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If the pressure in the head gets too high. the extruder can no longer push the filament, and instead when it tries to turn, it will click back a notch - much as happens in the later stages of the automated filament load process.

This can happen whenever the print requires too much pressure. That might be because you are printing fast, or with very thick layers, or at too low of a temperature. It can also happen on the first layer if the nozzle is too close to the bed; there is no where for the plastic to get out quickly, so the pressure builds up.

What layer height are you using for your print, and what is the first-layer height set to? And what speed?

 

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If this is only happening on the first layer than I wouldn't worry about it. It just means you are slightly closer to the bed than desired.

If this is happening on subsequent layers then you need to print either slower or hotter or with thinner layers. What is your: layer height, print speed, temperature. And are you printing PLA? For PLA cut the speeds in half from the table here:

http://umforum.ultimaker.com/index.php?/topic/3418-um2-extrusion-rates/

 

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I am having very similar problems, but the extruder is not clicking, it is doing full retractions. Also, it doesn't appear from the quality of the print that there is an excess of material being laid down anyway.

I initially unchecked the "enable retraction" setting in Cura. Then I changed expert settings in Cura to min travel = 1000, min extrusion = 1000. In spite of this, I got the EXACT same result as the first time. The third test was to also change retraction settings on the machine directly to speed = 0, length = 0. In spite of this additional step, I got the EXACT same result again. The machine just retracts every few lines, regardless of ANY settings.

Any assistance here would be GREATLY appreciated.

Thanks in advance!

photo-1.JPG

 

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I'm 90% sure this is slipped extruder steps. If you look at the extruder and look at the tiny little set screw on it, it should be turning forwards very slowly as it prints and then *suddenly* spin backwards quickly quite a bit - maybe half a rotation. Not sure exactly how much. This is caused by missed steps in the extruder motor.

If the probem *only* happens on your first layer then your build plate is a little too close to your nozzle - relevel a tiny tiny bit lower(like half the width of paper).

If the problem occurrs on every layer then you are printing too cold, too thick, or too fast. Here is a chart that shows the maximum speed you can expect for various printing speeds, layer heights and temperatures:

http://umforum.ultimaker.com/index.php?/topic/4127-um2-extrusion-rates-revisited/

 

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gr5, thank you for the response.

This is an interesting theory. Are you saying that if the stepper motor misses enough steps (say, because the filament is not moving through the nozzle as fast as the extruder is trying to push it) that it will eventually move quickly in reverse about half a revolution, basically taking the shortest and easiest path to the point in the shaft rotation that the gcode is telling it to go to? Not knowing much about stepper motors and such, this sounds plausible.

But here's the rub... I just printed several other prints (robot, coffin's cube) with none of the retracting (or step-skipping) happening. And I have not changed the level of the BP at all, or changed any other settings.

Thoughts?

 

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gr5, thank you for the response.

This is an interesting theory. Are you saying that if the stepper motor misses enough steps (say, because the filament is not moving through the nozzle as fast as the extruder is trying to push it) that it will eventually move quickly in reverse about half a revolution, basically taking the shortest and easiest path to the point in the shaft rotation that the gcode is telling it to go to? Not knowing much about stepper motors and such, this sounds plausible.

But here's the rub... I just printed several other prints (robot, coffin's cube) with none of the retracting (or step-skipping) happening. And I have not changed the level of the BP at all, or changed any other settings.

Thoughts?

 

A partial Clog maybe?

 

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Well it looks like you were 100% right, gr5. The BP was too close to the nozzle. I haven't completely fine tuned this, but have been getting much better results by adjusting the BP downward a bit. I have been printing at 220deg, and when I accidentally let it run at 210deg, I noticed the same symptom...skipped steps. Once I heated back up to 220deg, no more skipping. Sensitive little thing.

I also took note of how far the shaft counter-rotates when skipping...it appears to be 1/8 rotation or less. And the reason I wasn't seeing the problem on the other prints was because they had a relatively small footprint as compared to my rectangular test model. There wasn't enough travel distance for the pressure to build up.

Thank you so much for the help!! I feel like I am getting somewhere now. And thank you Takei for weighing in as well. Glad to report I haven't suffered a clog yet, knock wood!

 

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Think of it as similar to the difference between static and dynamic friction. If you are in a car and hit the brakes too hard you skid and don't slow down as fast as if you hit the breaks *almost* as hard but the wheels still turn. Once you start to slip - you switch from static friction (good grip on the road) to dynamic friction (not as good).

The pressure on the filament from the extruder is huge - 5kg. On the order of 1000 psi in the print head. When the stepper finally slips, it gets up momentum and doesn't just skip one step backwards but many steps because it's like a spring let go. After the 1/8 turn the pressure is now low enough that even though there is some momentum the magnetism in the stepper can stop it from rotating any further.

There is no feedback with steppers. If they slip, the controlling program (Marlin) doesn't know. The steppers don't know either. They just keep on stepping normally once the slip is over.

 

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