Input to solve issue with "strings"

Hi everybody

Anyone who has good ideas for a solution to get rid of this mess of strings show in the video below. Its a general problem the printer has with ooze from the hot end.
I am out of ideas to find the solution to this problem…

Please see the youtube video by following the link below:

[www.youtube.com]

I am using Slicer and MacPronterface.

Settings in Slicer are as follow:
Layer height. 0,3 mm
Speed:
perimeter: 20 mm/s
infill: 40 mm/s
Bridges: 10 mm/s
Travel: 80 mm/s
Temp: 185 C
Temp bed: 65 C
Retraction length: 2 mm
Lift z: 0,3 mm
Retraction speed 10 mm/s
Extra length on restart: -0,1 mm
Min travel after retraction: 2 mm
Whipe while retracting: On

Nozzle diameter: 0,5 mm

Any ideas are welcome, thanks in advance...

Make sure the filament isnt slipping or getting stuck; also increase your retraction speed by a lot. I don't think lift z and extra length are helping much, so try disabling those too.

Hi AlexY

Thank you for the tips.
How do I find out if the filament is slipping or gets stuck? When I feel on the filament, it is moving simultaneously with the movings of the extruder gear.
I have tried to increase the speed of retraction but if I go beyond 10 mm/s the stepper motor will skip steps. I have tried to adjust the Pololu stepper driver without any luck. Regardless of pot meters position on the Pololu the stepper motor won't manage speeds beyond 10 mm/s!

It´s a temperature problem too. For bridges you choose lower temps, but to get rid of strings you choose higher temps.
2mm retraction is on the low side. I´ve got best results with 4.5mm and 80mm/s retraction speed. Also z-lift helped a lot, but increases wear on my M5 lead screws, so I cancelled this.

I also found, that a 0.5 nozzle always oozes, but a 0.4 nozzle is flawless.
-Olaf

Quote
o_lampe
It´s a temperature problem too. For bridges you choose lower temps, but to get rid of strings you choose higher temps.

I found that when printing PLA on my Ormerod, stringing was worse at higher temps. But the OP is already printing at quite a low temperature (185C), so I didn't suggest reducing it.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Right dc42,
I also expected an reply about the fast retraction speed. People may fear to strip their filament. I try to mimic my own behavior, when working with a hot glue gun:

If you want to rip the glue string apart, move the gun away fast and wide. ( 80mm/s and 4.5mm )

And for me this works better with higher temps = lower viscosity of the filament ( 210°C PLA ) in combination with high speed "G0" settings.
-Olaf

I have a feeling that stringing may also be affected by print cooling. Stringing on my delta, which doesn't yet have any print cooling, is much worse than on my Ormerod, in which the hot end fan is also used as a print cooling fan. However, the delta also uses a higher print temperature, in order to allow higher printing speed.

I agree about using a fast retraction speed.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for your comments.

It seems like everyone agrees that the retraction speed is to low and maybee I need to increase the temperature a bit.
My problem is though that if I increase the retraction speed above 10 mm/s the stepper loses a lot of steps during retraction and the print becomes a mess. I even reduced extruder acceleration a lot just to reach 10 mm/s without losing steps (as you can see in the video in the first post).
So a retraction speed of 80 mm/s or so seems impossible for me unfortunately...

hmm.. Is your idler tension absurdly high on the filament? How hard is it to turn the extruder gear by hand? If its easy then I say your extruder stepper drivers aren't at their max power yet. Also I heard somewhere that stepper drivers can also cause skipping from over-powering the motors, but i'm not sure about this.

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Master Tinkerer

It is pretty easy to turn the extruder gear by hand. Not much tension on the filament.
Yes, I probably need to measure the stepper current again.

Quote
Stensig
Yes, I probably need to measure the stepper current again.

Or get yourself some modern electronics that lets you set the stepper current in software, instead of messing around with pots and multimeters.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

@dc42
I can imagine a low temp scenario, where the filament is still a bit "grainy" and therefor ripping it apart is easy.
Your fan cooling argument points in the same direction.
But such low temp would cause stress to the extruder.
A higher temp, where the filament is like warm honey, also makes it easy to rip it apart ( and suck it back into the nozzle ), but at much lower stress for the extruder.
Also the fusion of the filament with the printed part is better.

It´s a balancing act, where 2-3° can make a huge difference.

@Stensig
You definitely have to get your extruder going. Could you add a picture of your extruder?
How much pressure do you put on the idler gear? Too much or too less won´t work.
Is the idler turning or is it broken?
Did you tighten the grub screws of the gear?
Is the gear in line with the filament?
-Olaf

Have you tried taking the extruder stepper off, running it, and trying to stop it with your hand? If it's easy to make the motor stall then the torque is probably too low and there's either something wrong with your current value, the driver itself, or the motor.

Also, 185º does seem a bit low even for PLA.

Edited 1 time(s). Last edit at 09/04/2015 09:33PM by AlexY.

You've mentioned lowering the extruder acceleration. I'd put that back to 2000mm/s2. There's another setting: retraction acceleration. Also set this to 2000mm/s2. I've managed to get up to 65mm/s retraction speed. At first I had a retraction acc. of 3000mm/s2 and the stepper motor stalled beyond 30mm/s speed. At 2000mm/s2 it was much improved. I use a bowden setup and had 5mm retraction length set up, after setting the acceleration to 2000mm/s2 I could lower it down to 3.5mm. The shorter the retraction the less time spent on one place (the head doesn't move during retraction).

What stepper motor and what torque do you use? You may be able to replace it with a heavier and stronger motor. Make sure the potmeter on the step stick is set properly. Is it direct drive or a wades extruder with gears?

Also, there's a stepstick decay hack for both A4988 and 8825 versions. I've recently replaced a non hacked 8825 with a 'pre hacked' A4988 and it runs much smoother at slow speeds. Since you print really slow, this may have an impact too.

Also increase your non printing moves to 125mm/s, the less time spent moving the better. More time moving means more time leaking. Set print speeds to 30mm/s, it's a good starting point.

Another thing to keep in mind is even though the hot end indicates a certain temperature, it may be that it is off by a certain amount. Use a thermo couple + dvm for a more reliable measurement. See if the hot end is indeed 185 degrees and not 215 degrees or 160 degrees instead for example. It may be necessary to choose a different thermistor lookup table in the firmware or switch to a thermistor that's known by the firmware (which means it includes a lookup table). Alternatively you can switch to a thermo couple + thermo couple board instead.

Hi all
thank you for all your helpfull inputs.
I found out that the stepper motor I use for the extruder has a coil resistance of 63 ohms, so I assume that it is designed for a much higher voltage than 2-4 volts as often recommended. Its a nema 17 with a 40 mm body, but no label with any information on it.
The same applies for my y-axis stepper motor so I will try to exchange these two motor with the two I use for z-axis who has a resistance of 1,7 ohm.
I found in a data sheet that these motors has a holding torque of 3.6 kg.cm. will this be sufficient?

3.6 kg-cm should be fine for both the y-axis and the extruder.