Problems printing simple cylinder geometry using PLA

[Spakis 0]

Hi everyone ,

I have been trying to print what should ha[/media]ve been a very simple object using PLA on Ultimaker 2+

I keep getting the problem seen in the pictures on the base of the smaller cylinder where it touches the large cylinder.

I am using the 0.25mm nozzle (as the dimensions of the object are very small) and using Cura 2.3 as a slicer.

Any ideas on what causes this problem and how to fix it ?

Edited May 16, 2017 by Guest

[gr5 2,210]

edit: in retrospect my answer below was not applicable - geert_2 answer is much better.

Try making the base twice as thick (try scaling it in cura 2X but in Z only - uncheck the lock).  This will tell you if something is wrong with the hardware or the slicer.  

I think something is wrong with your z screw in that it stops moving for several layers.  It gets stuck.  I think it's just a coincidence that it happens to happen at the top of the wider cylinder.

If I'm right then try cleaning the z screw.  With power off push the bed way down.  Clean the grease and dirt out of the z screw.  Consider using WD-40 (for cleaning only!  not for lubrant so remove it all after).  And then lubricate with the tiniest amount of grease.  In fact if you only clean a few inches of the z screw then you don't need to add any grease.

Edited May 17, 2017 by Guest

[Spakis 0]

Thanks for the advice. I did try printing a version scaled x2 in the z-axis and the print came out perfect. I guess this means it is not a problem with the z-screw ?

Looking at the printing process closely, the deformity seen in the pictures at the base of the thinner cylinder seems to be caused by material already deposited sticking to the nozzle as it passes again. But in the scaled print there was no such effect...

Any ideas ?

[geert_2 558]

This seems to be a very tiny model? If so, excess heat might also contribute to the deformation, I think.

When I need to print very small models, and when there is a sudden change in surface area to print (from a big surface to a very small), I also see deformations. This seems to have to do with temperature: different cooling times between layers do show up as deformations. And not enough cooling time in very small details shows up as blobs and overextrusion, similar to the one you have here.

I am guessing now, but if you print the base at high speed and high temp, and then the printer has to slow down considerably to print the tiny part (due to the minimum layer time setting), you may have too much pressure in the nozzle, and too much heat accumulated, before the system has time to adjust to the new lower speed and heat requirements? Could that be the technical cause?

Anyway, I reduce this effect by printing a dummy block (e.g. a tower of 15mm x 15mm) next to tiny models, to give the printed layers enough time to cool down, before putting on the next layer. And I print as slow and as cool as possible.

If you want to get really fancy, you could give the dummy tower the inverse shape of the real model: make a cube, put the original in it, subtract the original from the cube, so the dummy cube now has a hollow in the shape of the original. Then fill the bottom of the cube with a thin layer of 0.5mm, for good bed adhesion. So, now each layer area is the sum of the cube and original model, and it always has the exact same printing surface and cooling time (except for the bottom layer, but good adhesion is more important). This method may pay off for very small models.

If there is a little bit of overextrusion, or junk accumulated on the nozzle's outside, or you are printing rather fast and hot, or if the material flow setting is higher than default, that worsens the situation in my experience.

This does not exclude other possible causes, such as Z-problems, of course.

[XYZDesignPro 70]

It appears you are using Concentric for your Top/Bottom pattern. If so, on such a small part, the build up of material at the nozzle can be excessive. Scaling up the part 2X masks the problem because the root diameter of the smaller cylinder, the ("Pin") is twice as large. Try using Lines as the Top/Bottom pattern.

Just a thought . . .

[geert_2 558]

These photos show what I mean (see my post above). This is a tiny cone: 20mm wide, 20mm high, printed in PLA. With default settings, deformation suddenly occurs when the layers don't get enough cooling anymore.

When printing at a lower temp, and with a dummy tower next to it, this deformation occurs much later. But you still can't avoid it.

[gr5 2,210]

Oh!  yes - I forgot about that effect.  Listen to @geert_2.  Just print two of these parts at the same time.  Try to space them left-right such that when the tower of the left one is being printed the right one is right under the fan.  With 3 towers you will do even better.

It could also be partly the speed thing. When it switches to printing faster (higher pressure in nozzle) to slower you get a bit of overextrusion for a few seconds. But I like the other answer better that geert_2 suggests.

Edited May 17, 2017 by Guest