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Rough shell

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Hi everyone!


My UM2 works 24/24h since April and I had few differents problems but really easy to resolve thanks to all the ressources of the community!


But now I've a different problem since one week, the print is good until the 7 or 8 layers (100 micron) and after it start to have a weird shell





I've also try the underextrusion test and here is the result




After the writings, we can see that there is an extrusion problem.


my parameters are normal:


layer: 100 microns

speed: 50mm/s

temp: 210°C


I also did some test with differents speed/temperature but nothing change.


I saw that after 8 layers, the lines were thinner and I thought about two probable causes to my problem:


1) problem with the Z axe, after 8 layers, it move down to much, like 120 micron or 140 micron instead of 100 micron


2) Underextrusion but I've tried everything to solve a "normal" underextrusion and nothing change!


My last test was to increase the flow (normally it doesn't solve an unerextrusion problem right?)




The object is perfect at 180% of flow and also at 120%-140% in other try


I gave to you all the things that I've and tell me if you want something else but I tried almost everything and nothing change :)


Thank you for your help!


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After the writings, we can see that there is an extrusion problem.


That's normal and expected for this test. PASS! It takes a while for the extruder pressure to build back up after slowing down for the "writings". This test passed. It is meant for 230C. Did you do this test at 210C?

The CUBE however looks like a FAIL!

Was the cube at these settings?


layer: 100 microns

speed: 50mm/s

temp: 210°C


Those seem okay. That's the absolute maximum speed I would print at that layer height and temperature. You are about half of the absolute limit of what the machine can do at this cold temperature.


It might be that this particular color of PLA needs a higher temperature. Did you change colors recently?


Also check your feeder to see if there is something inside letting it slip. Check the teeth of the feeder for bits of plastic inside the teeth.


Is this PLA harder or softer? Maybe it slips a little more in the feeder?


Another thing that may cause underextrusion on the cube: if your infill speed is slower than your shell speed. This seems very unlikely as most people do it the other way around.


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You said you did some different temp/speed tests. However, if you look to the bottom of the speed test object, you can see that the surface is rough for 3 (and 4 , but less). So at 3 cubic mm per second you have a too... higher temperature. It's higher because as you increase the speed the phenomenon disappears (if it should have been lower, it should have degraded with increasing the speed.

Now, in your test cube, 50 mm/s, at 0.1 mm, means 50*0.4*0.1=2 cubic mm per second. At this speed the temperature is much too higher and the surface would look like it is under-extruded, while it is actually just smashed hot plastic.


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