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Bad walls (gaps) when layer time changes (UM3E)

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we recently came across a problem with bad walls, even on very simple prints like a calibration cube.

After some testing I figured out that it has to do with the layer time. If - at a certan layer- a surface becomes to print somewere in the model, every wall in the model gets a defect at that layer.

The problem can be reproduced printing a calibration cube with a visible "Z" letter on the top. The buttom surface of the "Z" increases the layer time and the problem occures.

I also made a very small model to demonstrate the problem (see picture below, printed with default 0.15mm profile).

It is just a small cube (15x15x10mm) with two surfaces in the middle.

I have never seen such an issue on my other printers at home (mostly DIY kits from china) - so at the moment I'm stuck, can someone explain to me what is going on?


Printed on Ultimaker 3 Extended with Ultimaker PLA white, Cura default profile (Normal - 0.15mm layer height)

Here is a picture of the same issure when printing a calibration cube:




Edited by Guest

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This is caused by drastic changes in layer cooling time, due to changes in layer-area to print. You will notice this especially on small objects, or on objects where the area to print suddenly changes from very large to very small. Exactly like the objects you show.

A solution is to place a dummy cube next to the model, and hollow it out at appropriate heights, so that the total cooling time per layer is almost identical for each layer. The simplest method is to put a copy of your model into the dummy cube, and subtract it. However, make sure the dummy has a bottom surface layer covering its total area, so it sticks well to the glass plate, otherwise it may be knocked over. Or manually design the dummy.

Printing slow and cool also helps minimising this effect, but this alone can not prevent it.

I often need to use this method, due to the layer area of my models suddenly changing from ca. 10cm^2 (=1000mm^2) to only a few mm^2.

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The simplest method is to put a copy of your model into the dummy cube, and subtract it.

Can you explain how to do that? You create a seperate model with your 3D modeling tool or can I do that just using Cura?

How comes this problem only occures (atleast so far) on our Ultimaker. In the following picture I placed the calibration cube printed on the UM3E next to the same cube printed on a Flsun Cube (0.1mm layer height, Janbex PLA). Is this a problem of the Ultiamer PLA, does it shrink so much during cooldown?


Edited by Guest

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