Internal structures affecting external perimeter of print, any ideas why?
 
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Rick Lohr
(@rick-lohr)
Trusted Member
Internal structures affecting external perimeter of print, any ideas why?

Best Answer by Rick Lohr:

Well, I found the answer and it turns out to be really easy.  There is a switch that tells the slicer to do the outer perimeter first.  I turned that on, and the outer skin front printed with no hint of inner structures on it.  The back wall still had a hint but it turned out that wall was only three rows thick, so I widened it a bit and all is good.  My rule of thumb will be to keep inner structures at least 2 mm from outer walls.  I did change from 2 to 3 perimeters and I'm going to always use at least that too.

Posted : 22/02/2024 7:06 pm
Robin
(@robin)
Prominent Member
RE: Internal structures affecting external perimeter of print, any ideas why?

Because things are complicated, physics and stuff... There are quite a few rabbit holes to go down, and some don't go anywhere at all (like https://help.prusa3d.com/article/the-benchy-hull-line_124745 ).

Generally it is really really difficult to find and mitigate the cause of all those artefacts and distortions appearing on 3d prints. Increasing the number of perimeters and decreasing the print speed in general and/or the speed for the perimeters often helps.

If at first you don't succeed, skydiving is not for you.

Posted : 23/02/2024 10:53 am
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Zappes
(@zappes)
Reputable Member
RE: Internal structures affecting external perimeter of print, any ideas why?

Just to give an extremely oversimplified, yet easily understandable explanation: The plastic shrinks, and it depends on the external structures just how much it can shrink. Think of the external perimeter as a vacuum bag - when you extract the air, the contours of the stuff in the bag will be visible.

My models on Printables
Posted : 23/02/2024 11:23 am
Rick Lohr
(@rick-lohr)
Trusted Member
Topic starter answered:
RE: Internal structures affecting external perimeter of print, any ideas why?

Thanks, that gives me something to work with.

Posted by: @robin

Because things are complicated, physics and stuff... There are quite a few rabbit holes to go down, and some don't go anywhere at all (like https://help.prusa3d.com/article/the-benchy-hull-line_124745 ).

Generally it is really really difficult to find and mitigate the cause of all those artefacts and distortions appearing on 3d prints. Increasing the number of perimeters and decreasing the print speed in general and/or the speed for the perimeters often helps.

 

Posted : 23/02/2024 2:00 pm
Rick Lohr
(@rick-lohr)
Trusted Member
Topic starter answered:
RE: Internal structures affecting external perimeter of print, any ideas why?

I must be dense because I don't follow that at all, lol.

Posted by: @zappes

Just to give an extremely oversimplified, yet easily understandable explanation: The plastic shrinks, and it depends on the external structures just how much it can shrink. Think of the external perimeter as a vacuum bag - when you extract the air, the contours of the stuff in the bag will be visible.

 

Posted : 23/02/2024 2:01 pm
Zappes
(@zappes)
Reputable Member
RE: Internal structures affecting external perimeter of print, any ideas why?
Posted by: @rick-lohr

I must be dense because I don't follow that at all, lol.

According to my wife, failing to follow my ramblings is not an indication of density. 😀

What I meant to say: When the plastic cools down, it shrinks a bit. That's just basic physics. Now, when a piece of plastic that has a massive inner structure behind it shrinks, it can't go farther than that inner structure. In places where nothing is behind the plastic, though, the plastic will shrink a little more. It's like pulling a really tight skin over some structured object - you will be able to see the structure below.

My models on Printables
Posted : 23/02/2024 4:07 pm
Rick Lohr
(@rick-lohr)
Trusted Member
Topic starter answered:
RE: Internal structures affecting external perimeter of print, any ideas why?

Thanks for the clarification.  Now I know why I didn't understand.  My artifacts are actually indentations, not raised areas.

Posted : 23/02/2024 6:00 pm
Rick Lohr
(@rick-lohr)
Trusted Member
Topic starter answered:
RE: Internal structures affecting external perimeter of print, any ideas why?

Well, I found the answer and it turns out to be really easy.  There is a switch that tells the slicer to do the outer perimeter first.  I turned that on, and the outer skin front printed with no hint of inner structures on it.  The back wall still had a hint but it turned out that wall was only three rows thick, so I widened it a bit and all is good.  My rule of thumb will be to keep inner structures at least 2 mm from outer walls.  I did change from 2 to 3 perimeters and I'm going to always use at least that too.

Posted : 25/02/2024 5:01 pm
Rick Lohr
(@rick-lohr)
Trusted Member
Topic starter answered:
RE: Internal structures affecting external perimeter of print, any ideas why?

Posted : 25/02/2024 5:06 pm
Rick Lohr
(@rick-lohr)
Trusted Member
Topic starter answered:
RE: Internal structures affecting external perimeter of print, any ideas why?

I looked at a print I did of this same object a year ago, and it was clean.  What could have changed that would do this?

Posted : 29/02/2024 2:41 pm
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