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Very small circles are filled (UM2)

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I have a problem with prints of small tubular structures. The tubes dimensions are between 0.4 to 2 mm in diameter.

They are oriented vertically on the buildplate. Laying the structure in horizontally is not an option. Below a diameter of 2 mm the tubes are clogged. You can see that the printer is printing circles where the outer diameter should be, but the filament fills the inside of the tube. Due to this problem, there is too much filament at these points. You can see this problem in the first picture . The tubes have the following diameters from left to right:

0,4mm 0,6mm 0,8mm 1,0mm 1,5mm and 2,0mm

The used settings are:

Layer Height : 0,075mm

Shell thickness: 0,8mm

Retraction: enabled

Fill density: 100%

Print Speed: 50mm/s

FanSpeed: above 0,5mm 100%

Material: ABS

Nozzle: Ultimaker 2; 0,4mm

Material flow: 107% (same problem with 100%)

Printhead temperature: 260°C

Bed temperature: 90°C


Is this a problem with the print settings, or is the slicing process the problem? The problem with the filament getting pushed into the circle and closing it only comes up with small diameters. There is also a problem with every print that the dimensions of the CAD model and the final print do not match.

I attached a picture of the sliced structure in Cura. You can see Cura showing the circles with small squares. These squares are overlapping to the inner side of the circle. The smaller the circle, the more they overlap.

Does anyone have an improvement, comment, or idea, how we can optimize this print?




Picture 1:


Picture 2:



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All bores printed with an FFF-printer come out smaller than in the model due to the shrink of the plastic when cooling down. You have to adjust the model for this shrink (as it is also done in e.g. injection moulding). How much you have to adjust depends on the type of plastic you're printing and the settings like e.g. print temperature. I suggest the try and error method for that.


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And that shrinkage seems to be especially bad on circles as there is nothing stopping it from collapsing in on itself. The plastic behaves a little bit like a rubber band and the effect appears more dramatic on smaller circles since X amount of shrinkage makes a bigger impact on a small circle than the same amount of shrinkage on a big circle.

The circle looks like a bunch of overlapping squares because the circle gets divided up into a bunch of straight lines. The firmware doesn't support arcs (yet) so it's the only way to get a circle.


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What printer are you using? Are you using Marlin firmware? Do you know the jerk setting? I recommend printing much slower - print at the jerk setting speed. For Ultimaker's that would be 20mm/sec. I'm no 100% sure this will solve all your problems but it will greatly help.

There are 3 factors that cause vertical holes to be smaller than desired. The first one is shrinking and for abs is about 6%. The second factor is the "rubber band" effect that Robert partly described. The 3rd one is qty of points on a circle since CAD software inscribes the polygon *inside* the circle instead of outside of it. However the 3rd factor doesn't seem to be much of an issue for you. The fix is to increase the hole size in CAD. Try increasing all of these holes in CAD by about .5mm plus an additional 5% (or 3% for PLA). Do not increase *outside* dimensions also by these same amounts - typically you can increase the scale of the model in X,Y by 5% but not Z (3% for PLA).

Printing with thicker layers should also improve the quality of these holes. I would try .2mm just for an experiment. Also if you print at 20mm/sec you can print much cooler - try 230C for ABS. You will get slightly less of factor#2 (rubber band effect).


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Thank you for the replies.

I also noticed that Cura uses 15% infill overlap so that the outline and infill connect properly. This could lead to too much material next to the bores.

Another factor is the slicing software. I used Simplify3D as a slicer with the exact same settings I used in Cura. The holes with diameters smaller than 1.5mm were still plugged, but the over extrusion (too much material) did not occur. So there might be the possibility to solve this issue in the slicer. One approach is a strech algorithm. I havnt looked into it in detail, did anyone already try that?



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