Blemished outer layer when printing inside structures

[alfonsvh 12]

Hi,

 

Very new S5 Pro Bundle user. I designed a box with some structures on the inside.

 

When printing however, the first part of the box looks okay-ish (although not perfect), but once the internal 'triangles (see picture above) start printing, the walls (even those not right behind the triangles) suddenly get blemishes. 

 

Does anyone have any ideas on what might cause this / suggestions on how to fix?

 

3mf file attached.

 

Thanks!

 

UMS5_Box.3mf

Edited August 25, 2022 by alfonsvh

[MariMakes 203]

Hey @alfonsvh

 

This is a tricky model to print. Luckily we've introduced new settings in 5.0 and higher that give you the opportunity to tune these thin walled models. You can learn more about them here: https://support.ultimaker.com/hc/en-us/articles/4792077687068-What-new-print-settings-are-introduced-to-Cura-5-

 

As you can see here, your printer needs to switch between two line thicknesses quite abrubtly. This change results in the underextrusion you see on your printer. 


I have two suggestions to resolve it. If you significantly increase the Wall Transition Lenght, you see there is a softer transition between the wall widths.

Or what I think is going to help best is to define the outer wall line widht to match the thickness of your other wall lines, in this case that's 0.5 mm. 

I really think these tips will make a difference!
Good luck with printing. 

[alfonsvh 12]

Thanks a lot! Your analysis makes sense; I'll experiment a bit with the settings you mentioned and will post results later!

[alfonsvh 12]

On 8/29/2022 at 1:24 PM, MariMakes said:

Or what I think is going to help best is to define the outer wall line widht to match the thickness of your other wall lines, in this case that's 0.5 mm.

 

Ok, I adjusted the general line width parameter (not only outer line width - I don't see a reason why the other line widths can't be 0.5 mm as well, and in my head it seems more consistent that way 😄)

 

Unfortunately no great improvements there...

UMS5_Box-Adjusted settings.3mf

[gr5 2,224]

First note that in PREVIEW mode you can see that the outer wall is always printed clockwise (maybe not relevant).  So it prints okay at the corner and as it speeds up in the middle it underextrudes.  It slows down at the corner and is okay and then speeds up and underextrudes again.  The outer wall is printed at 34mm/sec.

 

Just before it prints the outer wall it is printing the triangular infill portion (on the layer below) but it's called "top/bottom" which is printing at about half the speed at 22mm/sec.

 

I don't know - it's not super obvious but I have 2 theories

 

1) Speed changes.  Anytime you speed up you will get underextrusion for a few seconds.  So you are printing at 22mm/sec for those triangles and then 34mm/sec for the walls.  I'd make all the printing speeds the same at 35mm/sec which is a reasonably slow speed where quality should be excellent.  Type "speed" above the settings and you'll just see settings related to speed.  Set all printing speeds to 35mm/sec most especially top/bottom.  Make sure travel speed is still fast at 150mm/sec.  I'd leave initial layer speed at 20mm/sec.

 

Also I would disable jerk control and acceleration control.  This will increase ringing (vertical lines near the corners of your print) but reduce underextrusion between the corners.  What happens with accel and jerk control is it prints the corners much slower (it has to slow down to about 20mm/sec on the corners or slower with jerk control on) so they get a bit over extruded and farther from the corners it prints faster and you can get a little underextruded.  But the flip side is you will get more ringing which you may hate.  So maybe try initially with these off and if the underextrusion is fixed then try turning these 2 settings back on.

 

2) printing over air.  When you are printing the triangle corners and when it's printing the inside "outer wall" as shown in the photo below, it's printing over "nothing" and so there is nothing to keep the filament from squirting out and lowering the pressure in your nozzle thus underextruding when it goes to print the outer "outer wall". To be clear the gray in the image below is the layer below.  The green is initially printing the next layer above. If possible I'd change the slope of that corner piece to 45 degrees so the overhang isn't so bad.  The more vertical the slope, the more you are printing on the layer below.  The more horizontal the slope, the more you are printing over air.

 

 

 

 

And finally, when experimenting you don't want to spend 4 hours printing to get the answer to "did this change in cura help?".  So I'd make an identical box but only 2cm high and maybe only 4 cm on a side.  Or you can simply set the Z value to a negative value (click on part, click on the move icon that shows the xyz arrows, type a number into z box) to sink the part mostly below the print bed so it only prints the top third of your print (to do this you also have to uncheck a setting in "preferences" menu then "configure cura" then uncheck "automatically drop...").  That way you can experiment with things like speed and jerk and acceleration and not have to wait 4 house (and waste 9 meters of filament) to find out if your changes worked.

[GregValiant 1,344]

This is with all print speeds at 75 except the outer-wall speed which was 50 (for finish look) on my cheesy printer.  Bouncing the speeds (and/or line widths) up and down causes the flow rate to constantly change.  As @gr5 says, that is never a good thing.

What appear to be some swirlies in this image are actually lighting effects from the lamp I had too close.  You can see the marks at the triangles that @MariMakes was talking about.  I didn't try to do anything about those.  The print is with some moist PLA that I keep around for these sorts of tests.

 

 

[alfonsvh 12]

Thanks a lot for your input!

 

I'll experiment with the speed settings like you suggested and will keep you updated!

[MariMakes 203]

I've also done some quick printjobs.

I only changed the Outer Wall Line Width and enabled Alternate Walls. 

I did not get rid of the underextrusion completely, but I believe the biggest improvement comes from slowing down the printjob by picking a lower layerheight. Those profiles are more optimized for part quality over speed. 

During printing I also noticed that the nozzle was moving over the wall during traveling. You can consider changing the combing mode to Within Infill, that way your nozzle avoids the walls. But I haven't checked that one yet.

 

I was wondering... Is it an option to change/improve the model?  What's the intent for the model?
Having a thicker wall, one that can be divded by 0.4 mm, like 0.8, 1.2, 1.6 or 2.0, would significantly improve the printability of this model and would result in not having to change the Outer Wall Line Width.

 

UMS3_AlternateWallBox.3mf

[alfonsvh 12]

Thanks all. 
 

I designed a slightly adjusted model (but this was before I read the suggestion about the 'ideal' wall thickness so I didn't include that) and printed it in white PLA instead of Though PLA.

 

The result  are much better now! Although to be honest I don't know exactly which parameter change fixed the problems 😄

 

The only slight frustration left is that some of the layer changes happen near the cylindrical protrusions rather than in one of the corners and I can't seem to fix this by playing with the Z seam alignment parameter. But that's not really important.

 

Thanks again for everyone's help & suggestions. I learned a lot 🙂 

 

UMS5_Box-Design v2.3mf