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How to print a smooth bottom surface ? (PLA, UM2)

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I received my new UM2 printer. I notice a huge quality (smoothness) difference between the bottom surfaces and the top surfaces. The bottom surfaces are rigged and not smooth (I tried different layer heights and shell thickness) as shown on the picture.


However the top surface print rather smooth.

What parameters control the smoothness of the botoom surfaces ?




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I do it while the printer is doing the first few layers as well, it saves time messing about calibrating it and also saves times when you get jams at the start as it often just works through them. because you if you get bad nozzle jams after calibration you will have to do it again anyways and fall into a circle of calibrating and pulling out the broken filament then trying to clean the nozzle and it can waste a lot of time. On the fly you can just turn the screws until they are close to the nozzle at the start. Little mess ups are often negligable at the start or if they bother you just get some tweezers and pull away the stuf that has messed up when the print head is not in the way. Im glad to hear im not the only one doing this.

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The parameters that control it are bed levelling; nozzle to bed distance, 1st layer print speed ,“glues” and to some extent bed and extruder temp.

Irrespective of the model print speed you need to print the 1st layer slower; at least 30mm/s, a lot of people use 20mm/s., I think some even go to 15mm/s

“Glues” – I disagree with the statement above abut not using them. People use a variety of substances, including nothing. I print on glass and use hairspray because I have found that for me it improves adhesion but I think it is a very personal thing. You will find lots of posts on the forum on this subject.

If you are struggling a bit with adhesion then increasing the bed and or extruder temp. for the 1st layer can help. Unlike some other slicers base Cura does not provide this option, (there is a plugin, “attweakZ” or something like that that may allow you to do this) so I think a lot of people on the Ultimaker side maybe do not use this technique. Although I think the printer jog dial may let you do it and of course you can put it into the gcode but that would be PIA for every print.

But, assuming your bed is level then to me the major factor is nozzle to bed distance. Of course you can change the screws with the printer not operating but the advantage of doing it in situ during a print is that you can see the effect and it is normally quite a quick process. But IMHO if you want to get it really good you have to do it electronically. As it happens yesterday I retuned my nozzle distance. I did 5 prints with 0.02mm difference in distance between each one. They were all good but one was the best. I defy anyone to manually change the distance with that precision using the screws, let alone changing the height exactly the same on both sides so you do not screw the bed levelling.

Manually in situ is fine; you cannot argue with what you can see but then again it may not be the best you can achieve. The paintwork on a new Ford will look great until you park a new Bentley beside it.




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