Small Dots Added to Model in Slic3r
 
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Cosmic Shan
(@cosmic-shan)
Active Member
Small Dots Added to Model in Slic3r

Hi everyone, 

I'm quite new to 3d printing (about 2 weeks new xP), so I'm not exactly sure what's happening to my model once I slice it (see photo). It appears that the dots are added at layer 40/46 (not sure if that helps) were it goes from two perimeter lines to one. Is this just for adherence of the next layer, or is it something that I can change in the Printer Settings tab?

Thanks in advance and happy holidays!

Best Answer by Neophyl:

Slightly tricky one this.  Looks simple at first sight but its not.  The simplest thing that gets rid of all the issues in the walls immediately is to turn off 'Detect Thin walls' in the Print Settings>Layers & Perimeters>Quality section.  While it sometimes helps, in as many cases it can actually cause issues like it is doing here. 
Turning that off cleared up almost everything straight away without having to change any other settings.
The only issue is the top layer doesn't print as its too thin for the extrusion width.  SO unless you can change the model to make the top of the cone thicker that option is out the window.

OK back to having Detect thin wall turned=ON to pick up that top section.  So now if I go into Print Settings>Advanced and change the External Perimeters and perimeters values from 0.45 to 0.4 (the nozzle size, cant realistically go below that).  It then picks up the top walls but the artifacts in the lower section re-appear. (must be in at least Advanced mode to see the settings).  So that option is also out.

Basically I can't find a combination that will correctly slice that model.  Its not the model either, as its is fully manifold and has nothing wrong with it (did a fill check down to vertice level in Blender).  Basically unless you can make it thicker at the top I think you are out of luck, at least with Prusa Slicer.  The top rim is only 0.3mm thick so printing it with a 0.4mm nozzle is always going to be tricky.  You could swap to a smaller nozzle like 0.25.  Easier to actually model something that is printable though.   I was using the latest 2.3 rc2 software too.

 

DO note that there is another problem, though one that is easy to work around, that at the layer height of 0.15 with the normal first layer height of 0.2 that the best it can get is 6.95 (of the 7mm height of the model).  There are 2 really easy options to get the 7mm height and another that is almost as easy but is not my preferred option.  
The 2 easy options are to either change the layer height to 0.2, this allows a perfect division to get to 7mm, OR to change the first layer height from 0.2 to 0.25.  Once again that will add up to 7mm.

The slightly less easy option is to use the variable layer height feature and make the top layers thinner so that a multiple of the thinner layers can add up to 7mm.  I find though that the surface finish becomes noticeable different when you change layer heights.  Depending on your intended use this may or may not be acceptable.  Personally I would go for a first layer of 0.25.  You will be hard pressed to see the difference on just that first layer I think.

Posted : 25/12/2020 9:00 pm
Neophyl
(@neophyl)
Famed Member
RE: Small Dots Added to Model in Slic3r

Its a side effect of the slicing algorithm, a perfect circle when saved as a stl is no longer a prefect circle (or cone in this case).  As it gets converted to triangles.  Where the triangles meet the thickness is ever so slightly different.  If it falls to one side or the other of the slicers rather black and white process it causes the issue you are seeing.  The same can happen on corners but it happens most noticeably on curved perimeters.

Its highly model dependent.  You can tweak your extrusion widths slightly to try and get rid of them but often making one areas better will make another worse.

If you save your project (file>Save Project as) and then take resulting .3mf file, zip it up into an archive and attach it to a post here we can take a look at it and play around with the settings to see if we can help.  The project MUST be zipped up as the forum does not allow most file types.  It does however allow zip files.

Posted : 25/12/2020 10:53 pm
Cosmic Shan
(@cosmic-shan)
Active Member
Topic starter answered:
RE: Small Dots Added to Model in Slic3r

@neophyl I've attached the .3mf file here. 

Circle 1.5cm

 

Posted : 25/12/2020 11:50 pm
Neophyl
(@neophyl)
Famed Member
RE: Small Dots Added to Model in Slic3r

Slightly tricky one this.  Looks simple at first sight but its not.  The simplest thing that gets rid of all the issues in the walls immediately is to turn off 'Detect Thin walls' in the Print Settings>Layers & Perimeters>Quality section.  While it sometimes helps, in as many cases it can actually cause issues like it is doing here. 
Turning that off cleared up almost everything straight away without having to change any other settings.
The only issue is the top layer doesn't print as its too thin for the extrusion width.  SO unless you can change the model to make the top of the cone thicker that option is out the window.

OK back to having Detect thin wall turned=ON to pick up that top section.  So now if I go into Print Settings>Advanced and change the External Perimeters and perimeters values from 0.45 to 0.4 (the nozzle size, cant realistically go below that).  It then picks up the top walls but the artifacts in the lower section re-appear. (must be in at least Advanced mode to see the settings).  So that option is also out.

Basically I can't find a combination that will correctly slice that model.  Its not the model either, as its is fully manifold and has nothing wrong with it (did a fill check down to vertice level in Blender).  Basically unless you can make it thicker at the top I think you are out of luck, at least with Prusa Slicer.  The top rim is only 0.3mm thick so printing it with a 0.4mm nozzle is always going to be tricky.  You could swap to a smaller nozzle like 0.25.  Easier to actually model something that is printable though.   I was using the latest 2.3 rc2 software too.

 

DO note that there is another problem, though one that is easy to work around, that at the layer height of 0.15 with the normal first layer height of 0.2 that the best it can get is 6.95 (of the 7mm height of the model).  There are 2 really easy options to get the 7mm height and another that is almost as easy but is not my preferred option.  
The 2 easy options are to either change the layer height to 0.2, this allows a perfect division to get to 7mm, OR to change the first layer height from 0.2 to 0.25.  Once again that will add up to 7mm.

The slightly less easy option is to use the variable layer height feature and make the top layers thinner so that a multiple of the thinner layers can add up to 7mm.  I find though that the surface finish becomes noticeable different when you change layer heights.  Depending on your intended use this may or may not be acceptable.  Personally I would go for a first layer of 0.25.  You will be hard pressed to see the difference on just that first layer I think.

Posted : 26/12/2020 1:23 am
Cosmic Shan
(@cosmic-shan)
Active Member
Topic starter answered:
RE: Small Dots Added to Model in Slic3r

The reason I've made the top most layer 0.4 mm and have Detect Thin Walls ON is because I want to use the printed shape to cut clay (anything thicker will just squish the clay down rather than make a cut (which is also why I've used a chamfer rather than just a straight model--the tapered edge helps with the cutting)). I will go back into the model and change the top of the cone to 0.4 mm rather than 0.3 mm though. That may help things.

Having the printed model height EXACTLY at 7.0 mm is not a large concern, but I will change the settings based on your advice to keep all my cutters a uniform height.

I think ultimately the small artifacts won't affect the clay cutting, but it does help with the learning curve of 3D modeling and printing to know what's going on.

I really appreciate your help, and I will definitely be looking into a smaller nozzle to see if that will help with having Detect Thin Walls ON and negate the artifacts. 

Posted : 26/12/2020 3:51 am
Cosmic Shan
(@cosmic-shan)
Active Member
Topic starter answered:
RE: Small Dots Added to Model in Slic3r

@neophyl

I just made another model which is more of an arch shape, and used the same tools and measurements as I did to make the cone, but I ended up with zero artifacts even with the same print settings. Arch [Stretch]

Not sure what the difference is.

Posted : 26/12/2020 4:27 am
Neophyl
(@neophyl)
Famed Member
RE: Small Dots Added to Model in Slic3r

Your wall thickness at the top on that model is above 0.4mm though so the thin wall artifacts should not show up as it's not trying to cope with a massively thinner area than it can print.  A quick and dirty check shows the thickness varies between 0.4 and 0.44 at different points around the top.  It slices ok even without thin walls on.  Generally I have thin walls turned off, unless I find I have some missing walls after a slice due to them being too thin.  In which case I will also usually make the missing wall thicker in the model rather than turn on detect thin walls.  After all even with detect thin walls active it cant actually print them properly when the thickness is less than the nozzle diameter anyway.  

 

Posted : 26/12/2020 12:20 pm
Neophyl
(@neophyl)
Famed Member
RE: Small Dots Added to Model in Slic3r

Run out of edit time, here is your original circle

Posted : 26/12/2020 12:27 pm
Cosmic Shan
(@cosmic-shan)
Active Member
Topic starter answered:
RE: Small Dots Added to Model in Slic3r

@neophyl

I changed the model of the cone to reflect a 0.4mm width at the top of the cone and all artifacts have disappeared!

Again, I really appreciate the help. 🙂

Posted : 26/12/2020 4:37 pm
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