Persistent defect when printing round-edged base of a model

[Bubble 1]

Slicer:  Cura 5.1.1

Printer: Heavily moded Ender-6

 

Hi everyone,

 

I'm struggling to get a decent print of this simple model with two round-edges on the base:

 

 

 

The rounded edges have a radius of 10mm so the vertical transition and overhangs don't seem very steep to me (or am I wrong about this?)

 

The problem in a nutshell: instead than getting the expected curved surface, the first 20-30 layers look flat much like a straight bevel

(sorry about the quality of the pictures, the model is small and black so hard to photograph):

 

 

 

 

I don't think it's an under-extrusion issue (the pics make it look like that, I know), but all my other prints come out smooth and nice enough (including one with vertical curves and domed curves). Yet this specific geometry just gets messed up somehow and I can't figure out why that is 🧐

 

After tinkering with this for about a week and trying numerous different settings (inc. resolution, wall thickness, print speeds, and even slightly modified geometries), I'm almost certain that this issue is caused by some slicer setting(s) that somehow modifies the mesh to create this particular defect.

 

Here are the original stl and Cura project file below for reference

round_base_10mm.3mfround_base_10mm.3mf

round_base_10mm.stl

 

Does anyone have any suggestions on what settings might improve/solve this defect?Insert

 

 

 

 

Edited December 9, 2022 by Bubble

[ahoeben 1,991]

Try printing it standing up...

 

It looks like this is simply caused by overhangs not printing well without support. But even supports won't be a magical fix; sometimes you just have to design for 3d printing, or adjust the printing orientation to get the best result.

Edited December 10, 2022 by ahoeben

[Bubble 1]

1 hour ago, ahoeben said:

Try printing it standing up...

 

It looks like this is simply caused by overhangs not printing well without support. But even supports won't be a magical fix; sometimes you just have to design for 3d printing, or adjust the printing orientation to get the best result.

 

Thanks @ahoeben, but that's not the case. I say this with confidence because as part of the testing process, I printed the exact same model in so-called 'vase mode' (aka 'spiralize outer contour'), and it printed very well without supports. In other words, the defect I was talking about was totally gone and the edges came out nice and round throughout. This mode is of course useless for printing this particular model as it only creates the outer shell, but it does clearly indicates that the overhangs are printable without supports and further strengthens my theory that the printing geometry created by Cura is the source of the problem in this case.

 

Edit: I should probably clarify that the part I'm discussing here is just a very simplified version of the part I'm actually aiming to print. The reason I use this simplified model is to isolate the issue and hopefully make it easier to pin down and solve. Hence, while it would probably make sense to print this simplified model standing up and circumvent the problem as you suggested, that won't work with the actual part I have in mind.

Edited December 10, 2022 by Bubble

[GregValiant 1,358]

"...this issue is caused by some slicer setting(s) that somehow modifies the mesh to create this particular defect."

 

So far as I've been able to tell Cura can't do anything to the mesh itself.  In this case the outer walls of the first 5 layers have no overlap.  It isn't until layer 6 that the new outer wall overlaps the outer wall of layer 5.  That means the outer walls of the radii near the build plate are going to sag.  TAN(OverhangAngle) * Layer Height = Step Width and when the step width is greater than the Line Width then there is no contact between the Outer Wall extrusion and the layer below.  It is going to stick a bit to the inner wall that was laid down previously on the same layer, but gravity is going to win.

Looking closely at the last image you can see that starting with about layer 7-8 then outer wall stays where it was put.  No more overhang there.

 

Having your "Outer Wall Inset" at 0.2 isn't hurting anything but isn't helping either.

Arc Welder will not work with these radii as they are in the wrong plane.

 

These are PETG on a stock Ender 3 Pro.  The inside of the radii look good to me.  I printed this one with support so there is some scuffing from the exacto knife where I removed the support.

 

The top print was without supports, and I got that divot/chamfer look.  It's because the "overhang" of the first 5 layers have no "overlap" with the layer below and so those outboard extrusions sag and give the impression of the divot/chamfer.  The plastic is there - it just didn't stay where it was put.

[Bubble 1]

Quote

So far as I've been able to tell Cura can't do anything to the mesh itself.  In this case the outer walls of the first 5 layers have no overlap...

 

Wow! @GregValiant, that is such a brilliant reply and a great analysis! I'm very grateful for the time and effort you put into all this! 🙏

 

Seems like first in order is an apology to @ahoeben. Turns out you were right about the overhangs/supports and I was wrong. Sorry. Thinking about it again with the help of @GregValiant explanation, it looks like the 'vase mode' worked despite having no supports because there is just a single lined outer wall printed and therefore little weight to cause the extrusion to droop due to gravity like in the case of the thicker and heavier model.

 

@GregValiant, going back to your explanation, I did notice the lack of overlap with the outer walls in the initial layers, but couldn't (and still can't) find a setting in Cura to fix this. I hoped changing the outer wall thickness and/or printing from 'inside out' will improve things but that obviously didn't work. 

 

So the main question now is are supports absolutely necessary for this geometry to print well or are there perhaps setting(s) that can be tweaked in Cura to rectify the missing outer wall overlap in the initial layer?

 

Also, if supports are indeed necessary, what kind of supports would be best suited to give the model enough structural integrity to maintain the curve and yet cause the least amount of damage to the outer surface when removed? 

 

[btw, when I spoke about Cura and mesh geometry, I was probably talking too loosely. I didn't mean that Cura literally changes the mesh, but was only trying to refer to the way Cura translates the mesh into gcode movement commands. I hope this makes sense.]

 

Thanks again @GregValiant for the very helpful reply and explanation!

 

 

[GregValiant 1,358]

There isn't a fix.  It is the nature of FDM that "layers" are involved.  It gets better as the layer height approaches zero, but who the heck would print something that took 100 years.  One solution could be a dual extruder setup.  For one thing you can get by with no air gap between the support and the model since using a dissimilar material means the support wouldn't bond to the model.  When the radius on the base of any model is tangent to the build plate then the support must be some distance from the model - because "Layer Height" is involved.  So the outside wall of the second layer on a big radius is almost never supported.  There is often a small mar on the surface because of that.  It is also something that you might notice but that 99% of people would never notice.

 

When I print something that has a small contact patch on the bed I move it up 1mm so the support goes under the model.  That way the little patch is on support and the failure rate is a lot lower.  If you were to do that then the support would grow properly and envelop the radius and the situation with that radius would go away.  With a flat bottom like you have there - it isn't really an option as you would end up with ugly marks from the support removal on the flat area.

 

"Adaptive Layer Height" is an option.  The lower the layer height then the closer together the "steps" and that directly plays in to how the overhangs print and how well they self-support.

 

I played around with "Support Flow" and settled on 90% for both the Support and the Support Interface.  If support has to grow on the model then I make the gap between the model and the interface (the floor gap) 2X layer height.  The extra gap allows the support interface plastic to cool just a tad more and it doesn't stick near as well so it comes off really clean.

I always start out with Normal supports and only when I'm sure they aren't getting the job done do I move to Tree supports.  A caveat to that is when the model is such that using Tree supports would allow me to use "touching buildplate" instead of "everywhere".

[Bubble 1]

On 12/11/2022 at 3:28 AM, GregValiant said:

There isn't a fix.  It is the nature of FDM that "layers" are involved.  It gets better as the layer height approaches zero, but who the heck would print something that took 100 years.  One solution could be a dual extruder setup.  For one thing you can get by with no air gap between the support and the model since using a dissimilar material means the support wouldn't bond to the model.  When the radius on the base of any model is tangent to the build plate then the support must be some distance from the model - because "Layer Height" is involved.  So the outside wall of the second layer on a big radius is almost never supported.  There is often a small mar on the surface because of that.  It is also something that you might notice but that 99% of people would never notice.

 

When I print something that has a small contact patch on the bed I move it up 1mm so the support goes under the model.  That way the little patch is on support and the failure rate is a lot lower.  If you were to do that then the support would grow properly and envelop the radius and the situation with that radius would go away.  With a flat bottom like you have there - it isn't really an option as you would end up with ugly marks from the support removal on the flat area.

 

"Adaptive Layer Height" is an option.  The lower the layer height then the closer together the "steps" and that directly plays in to how the overhangs print and how well they self-support.

 

I played around with "Support Flow" and settled on 90% for both the Support and the Support Interface.  If support has to grow on the model then I make the gap between the model and the interface (the floor gap) 2X layer height.  The extra gap allows the support interface plastic to cool just a tad more and it doesn't stick near as well so it comes off really clean.

I always start out with Normal supports and only when I'm sure they aren't getting the job done do I move to Tree supports.  A caveat to that is when the model is such that using Tree supports would allow me to use "touching buildplate" instead of "everywhere".

 

Thanks @GregValiant, looks like I'm going to use supports and see how goes 🙂