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Really strange observation with my MP Select Mini V2 printing PLA. I've printed a dual-fan shroud so I can have a dedicated part-cooling fan. I've noticed that when I turn it on, the extruder starts stringing everywhere. Little tiny spider-web wisps. No amount of temperature change will make them go away (printing at 200 °C). But if I simply turn off the part-cooling fan, I get a perfect print. This doesn't make sense to me - isn't the part-cooling fan supposed to reduce stringing by cooling the filament faster?

More info: Using Ultimaker Cura 3.4.1, retraction enabled at 3.5 mm and 40 mm/s, which works great when the part-cooling fan is OFF. But when I turn it on without changing any other settings, I get stringing everywhere.

Please see picture below. The part on the left was printed using the part-cooling fan and the one on the right was printed without the part-cooling fan. No other changes. The part-cooling fan is pictured in upper left (bottom fan).

Two printed fan shrouds

Why does my part-cooling fan cause stringing?

Note that I print at 0.175 mm layer height, or "Normal" for the MP Mini. The cooling fan can be controlled but if I turn it completely off then the cold end part of the nozzle won't get actively cooled. Using Cura I can slowly increase the fan over the first several levels as well, but I'm worried that if the cold end doesn't stay cold it will get clogged up with melted filament. I will try some different fan speed settings and report back - currently running at 100 % all the time.


I went back to the stock shroud with single fan to remove variables. The stock shroud has a small vent at the bottom where air blows over the part, but not much. I had the same problem - I had to put a piece of tape over the blow hole to get the part to print without strings. I also couldn't get the PLA to stick to the bed easily when the blow hole was open. I'm not using the heated bed in any cases - I've found I can print most PLA without heating the bed at all. I'm beginning to think maybe this PLA is just extremely temperature sensitive. The brand is 3D Solutech Printer Filament Real Black.

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    $\begingroup$ Are you experiencing problems that the part cooling fan solves? If you get a perfect print with it turned off, what feature does it provide when on? $\endgroup$ – fred_dot_u Sep 10 '18 at 23:34
  • $\begingroup$ @fred_dot_u Originally I was hoping to improve the bridging capability by forcing more air over the extruded PLA to cool it faster. I just can't figure out why more air moving over the nozzle makes strings when the whole point of part-cooling I thought was to improve bridging and decrease stringing, not increase it? $\endgroup$ – Josh Painter Sep 10 '18 at 23:36
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Prior to a move the print cooling fan causes the filament to cool on the outside while the nozzle is still hot, when it then moves it causes a strings to form that will be cooled instantly. This means that the cooling you have is too much and should be reduced.

This printer has a single fan to cool the cold-end and the print through a slot with the same fan. As this fan can be scheduled, it would be recommended to create a dual fan on the carriage where you have a dedicated fan cooling the cold end at a fixed voltage, while the print is cooled by a separate fan that can be scheduled through slicer settings.

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    $\begingroup$ Too much cooling can't make strings unless there's already oozing material to make them out of. $\endgroup$ – R.. GitHub STOP HELPING ICE Sep 16 '19 at 15:13
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The fans are allowing the strings to harden instead of break. Maybe you should use more retraction or wipe before crossing perimeters to reduce material ooze during moves.

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  • $\begingroup$ This is the answer. OP had insufficient or no retraction and was depending on the tension on molten strings to break them and let them spring back. $\endgroup$ – R.. GitHub STOP HELPING ICE Sep 16 '19 at 11:53

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