Print issue on overhangs

[tauceti82]

Hi guys,

I designed a replacement chair leg and printed it out multiple times. I made it quite stable by using there settings:
A1 mini, textured plate 0.4 nozzle
PLA+
0.20 Strength profile and changed the some parameters to the following
8 wall loops
honeycomb 40% infill
support enabled
outer brim only
Bambu 1.9.7.52

printed the object on the side.

But each time I got this ugly rough lines at the overhangs at the same position. I enabled support but for the model but no support was generated for these regions but that is ok as in my 8 year 3d printing experience I know that no supports are needed here as the angle is not that steep.
I also did fixing the model in bambulab and reprinted but the results were the same.

But the result is strange as when I did a prototype print with 0.6 nozzle and 0.42mm layer height and less infill with 2 walls the surface was smooth
Do you know what the reason might be?

BTW: I don’t had any of those problems with other models. So for me it seems like either some kind of model issue or print settings. Of course I did the printer calibration and A1 maintenance regularly.
Thanks!
failed-print

[Chris1974]

Overhangs that are shalower than 45 degrees will never come out pretty… It is just a property of FDM 3D printing. You are basically ptinting in mid air…

You can improve the apprearance by changing the design so the overhang is less than 45 degrees. Or re-orient the part on the buildplate so the overhang is less…

If that is not possible you can reduce the layer hight but it will increase print time. Using adaptive layers might give you the best chance.
Adding support is another option, but it will still not be pretty.

Good luck.

[tauceti82]

Thanks for the reply. Just checked the the angle and it is exactly at 45 degrees.

[Chris1974]

Can you share the model? It looks a lot less, but that can be the picture…

If it is truly 45 deg then you might have a cooling issue. Try to increase cooling or position the overhang exactly in fromt of the cooling fans.

Oh wait… you use an A1 mini I see now… No aux part cooling fan. Increae the part cooling on the print head if you can.

[formulate]

looks like heat buildup! increase cooling, lower temps, or reduce number of walls. less walls with thinner layer heights can give comparable strength, depending on the situation.

[annin_miru336]

try playing with speed and temerature

[tauceti82]

Here is the model:
chairleg.3mf (160.5 KB)
Would be nice if you can look at it.

[Chris1974]

Thanks for the model. I checked the angle in Fusion360 and according to that the angle is 33 deg. Much smaller than 45… I suspect that is the issue.

Looking in the preview in the slicer I can see the next layers are almost on the edge of the previous layer… They have nothing to hold on to which causes the sagging…

Change the angle if you can…

[print.in.3d]

Try a line width of 0.6 mm for (at least) outer walls and reduce the line height to 0.12mm to increase the overlapping.

[MalcTheOracle]

This prints without issue, it is more than just the angle, it is how it is formed.

That bottom left portion (where the green starts) prints perfectly.

I will take a quick look.

[Chris1974]

Yours is round, that’s why…

[MalcTheOracle]

The problem is the bottom few layers were the angle is so steep so quikcly that the lines being added have nothing underneath them.

As you can see form the image above, this is your model and the blue lines indicate overhangs. This is not an issue if there is at least something below them that even barely touches them, but in your case, you can see a gap between the blue overhand wall and the yellow inner walls, this will be filled in within the same layer, but, the problem has already occurred.

Due to this issue, walls above that look fine on the slice preview are unsupported in real life and they are the cause of the sag.

As you can see in my example, the provided a couple up, the overhang is steep, but, the increasing layer build up maintains contact on each layer.

Illustrated below.

You can see the build-up of overhang walls are still connected to the inner walls even when it is at is steepest.

The trick isn’t the obvious angle, it is how you create (if possible) other angles to play with.

I have experimented a bit and there are not any obvious tricks within the slicer to fix what is a design issue.

I tried using a PETG interface layer but, that doesn’t get below the overhang walls. I hanged the layer height to adaptive in several styles, each have the same issue.

The fix is make the curved surfaces uniformly curved. The inside and outside curves you have included are not 90º of a cylinder which is the common way of achieving the transition you have, yours appears to be ellipses with elongated start and ends on the inner curve (face the centre of the build plate).

For strength, you should consider having a straight angle begin the transition, move to a circle portion and end on a straight angle. The straight angles need only be for 2mm at most. This will remove the overhang walls issue and increase part strength as a benefit.

The inside of the angle bracket on the first plate should be a true curve, currently it is made up of several flat pieces. This would also increase part strength.

Your peg holes do not require supports, the circles will hold themselves up and the hex hole has sufficient bridging going in the correct direction to form a decent surface.

[tauceti82]

Thank you very much for your analysis @MalcTheOracle and @Chris1974.
I modeled the chair leg after the original which is in wood and already modified a bit to make it thicker for more strength. I modeled it in Shap3r and the curves are round in the model. Unfortunately in the free Shap3r version I can only export in low poly STL so that is why the round curves got split into multiple flat regions.
Also I read that the chamfers I used are 45degrees by default. Thanks for the analysis that they seem to be at 35. also you are right @MalcTheOracle regarding the support. I printed without them and you really do not need any for the connector holes. Strange that bambulab slicer generated them there but on the other hand no support for the overhangs. I will adapt the angles and hope that the final model will still fit as a replacement part for the broken original wood leg.
I also printed the model with 0.6 nozzle and with big layer height where I didn’t have the issue. Quite strange as I thought the smaller the layer height the lesser the overhang size.
Nevertheless thanks again for your help!