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gaston

Unexpected Cura slicing

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I am in progress to design a casing for an electronic device but my fist tries in the Cura slicer do not produce the expected slicing results.

The casing currently has a bottom and 4 sides as well as 4 cylindrical (well not really cylinders but that does not matter for this purpose) elevations for the screws holding the PCB.

The bottom and sides have been designed with 1.5mm thickness. The whole thing was build in a CAD programm, exported in STL (mm untits).

When I import the casing in Cura and set the layer thickness to 0.06mm, shell thickness to 0.5 (as the default 0.8 may interfere ith the 1.5 over all structure thicknes) with fill density of 100%.

The problem is that with a layer thickness of 0.06 and a case botom thickness of 1.5mm I would expect to see 25 layers (1.5/0.06=25) for the bottom. In Cura however the slicer only uses 21 layers (platform adhesion has been set to none for this measurement). If I set the adhesion to Raft again, the bottom takes 25 layers including the raft (coincidence?).

So I am wondering why Cura reduces the bottom by 4 layers (0.24mm).

Loading the same file in to Netfabb and slicing it renders 26 layers for the bottom so only 1 layer in excess which may be due to rounding.

In both applications the STL file was just loaded and sliced, no conversion or other modification done.

It looks like Cura counts adhesion layers toward the object even if no adhesion layer is selected. And even if selected this should not be counted for the bottom of th eobject thickness IMHO. But this may also be a coincidence and the problem having a different source.

In many applications this may not matter but for empty structures with "thin" bottoms like casings it does.

P.S.: Just to be honest, I have to admit that I am very new to 3D printing, even still waiting for my printer to arrive but having prepared myself with the topic for several months.

 

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Quick calculation gives that you most likely still have the default 0.3mm bottom layer this gives 1 (0.3mm) + 20 ((1.5 - 0.3) / 0.06mm) = 21 layers as you saw.

Note setting first layer height to 0.06m also requires a perfect calibration of the print bed for the first layer to stick well, with a first layer of 0.3mm this is much less a problem.

 

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Since you are new to 3D printing I recommend you stay between .1 and .2mm layer heights and keep the first layer at .3mm. At least until you've printed a few things successfully. I think you will get better quality at .2mm at 30mm/sec print speed than .06mm at 100mm/sec print speed and both will take the same amount of time to print.

Also the shell should always be a multiple of the nozzle width (.4mm) as if you do shell of .5 with nozzle .4 then it will overextrude by 25% and each pass will be .5mm apart. Whereas with .8mm (or 1.2 or 1.6) shell it will work fine with a 1.5 width actual wall (it will probalby do .4mm pass on the inner and outer surface of the wall followed by diagonal infill in the remaining .7mm gap - or it might do a second shell with just the right of extrusion to fill the gap - not sure which but it should work properly).

I've printed 20mm wide parts with shell of 40mm and it works just fine (does "circular" infill instead of diagonal infill).

 

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Wow that was quick ! Many thanks for your valuable answers.

After posting th etopic yesterday I noticed I forgot to mention the Cura varsion (14.07) but could not add it as my topics still need to be approved. But version seems not to matter in this case anyway.

@Mikk36 & Dirkels:

You are right ! The problem is the first layer. As a newbie one may expect that I did oversee that detail, but this is not the case. But for some reason I read it, still as noob, as 0.03instead of 0.3mm. As this would have been smaller than the normal layer I did ignore it as it would only add an additional layer.

Thanks to pointing me to my mistake.

@gr5

Many thanks for your advice !

Regarding the layer thikness I did take the default when switching to expert mode in Cura for the time being. I was looking for a thicker layer later for the actual print as the case does not require that vertical acuracy.

The strange thing is that I tried to reproduce this by deleting the current_profile.ini, now switching to expert mode comes up with 0.1mm instead of 0.06 while I am quite sure I did not modify this at any time. Also the adhesion type now is "None" while it was "raft" when I started. And this was not changed by me for sure. Possibly Cura comes with a current_profile.ini which differs from th edeafult one generated by itself.

This brings me to one thing I am missing in Cura: The possibility to reset the current expert profile to any of the predefined quickprint profiles. This would allow newbies to get some quick insight into the settings if they want to (like me) and have god starting points for the first expert settings. But possibly its just me missing it, not Cura missing the feature.

Regarding the shell widht you are of course also right with the noozle size impact. While I was starting from the point (probably right) that the thickness itself would not matter in this case with 100% fill, I have to admit that I would most probably have missed the importance of the noozle size for other prints.

Good to know that shell thickness and object volume thickness are handled properly.

I will for sure follow your advice for my first prints.

 

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This brings me to one thing I am missing in Cura: The possibility to reset the current expert profile to any of the predefined quickprint profiles. This would allow newbies to get some quick insight into the settings if they want to (like me) and have god starting points for the first expert settings. But possibly its just me missing it, not Cura missing the feature.

 

You can load a profile from a previously sliced file. So you could slice something random using the quick print profiles, save it somewhere, switch to the expert mode and then load the settings via File -> Load profile from GCode

 

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Also the shell should always be a multiple of the nozzle width (.4mm) as if you do shell of .5 with nozzle .4 then it will overextrude by 25% and each pass will be .5mm apart. Whereas with .8mm (or 1.2 or 1.6) shell it will work fine with a 1.5 width actual wall (it will probalby do .4mm pass on the inner and outer surface of the wall followed by diagonal infill in the remaining .7mm gap - or it might do a second shell with just the right of extrusion to fill the gap - not sure which but it should work properly).

 

Actually the UM nozzle can print 0.5mm (or even 0.8) lines just fine. You'll lose some external quality, but the lines are put down perfectly. This due the flat area on the nozzle. That's where the whole "shell thickness" setting comes from, it's fine to have it at a not-multiple-of-nozzle-size.

 

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it's fine to have it at a not-multiple-of-nozzle-size.

 

well - mostly. And I've played with this. But for a newbie I reccomend you stick to integral multiples. For example if you print .5 shell with .4 nozzle you are sort-of overextruding by 25% in the sense that the feeder has to work harder and there is more pressure in the nozzle. If you are near the limit of what the printer can do this will push it over the edge. It takes more force to squeeze out that filament in the thin crack at the tip of the nozzle.

But, yes, it usually works quite well and is *very* useful to muck around with when you need to print text or thin walls.

 

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You can load a profile from a previously sliced file. So you could slice something random using the quick print profiles, save it somewhere, switch to the expert mode and then load the settings via File -> Load profile from GCode

 

Yes that is a great feature allowing to review previous print profiles, but I see it "only" as a workaround. But at least it is possible to do it this way, assuming the full profile is stored in the gcode file.

I am a friend of clean solutions, and in that world the best would be to have it implemented in the application. So one would always have the latest profile if evere it would be changed for some reason. Also this should not require much code to be implemented as most bits are there already. - Just a suggestion

 

Actually the UM nozzle can print 0.5mm (or even 0.8) lines just fine. You'll lose some external quality, but the lines are put down perfectly. This due the flat area on the nozzle. That's where the whole "shell thickness" setting comes from, it's fine to have it at a not-multiple-of-nozzle-size.

 

Not sure if I will ever require this but, if so I will for sure remember your words :) So if I get this right the nozzle opening is 0.4mm, if one chooses a shell that is not a multiple of that size but larger than it it will result in one path to have to be printed in a 0.4-0.8 width which is achieved with over extruding and the overextruded material flows under the noozles flat end achieving the larger width. So the theoretical limit of this is the width of the noozles flat end (not speaking of quality here).

 

well - mostly. And I've played with this. But for a newbie I reccomend you stick to integral multiples. For example if you print .5 shell with .4 nozzle you are sort-of overextruding by 25% in the sense that the feeder has to work harder and there is more pressure in the nozzle. If you are near the limit of what the printer can do this will push it over the edge. It takes more force to squeeze out that filament in the thin crack at the tip of the nozzle.

But, yes, it usually works quite well and is *very* useful to muck around with when you need to print text or thin walls.

 

No worries, I understood from your initial post that this could work but that it would not be th ebest choice for me to go that way if it can be avoided. :)

P.S.: As said before, I am still waiting for my printer to arrive But I am convinced already that I did the right choice, seeing the speed and valuable information provided in this forum from the community and Ultimaker Staff. This thread could have ended in post #2 (well #3 for the "Thanks") with something like "This is probbaly a typical noob error, you forgot the initial layer thikness". No, not only got I this information with a high detail, I also got many valuable additional tips around the topic.

So let me thank every poster in this thread for your valuable help, which is really appreciated ! Unfortunately this forum lacks banner smileys like "Thanks", "Great Job",... I would use them all :)

 

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