Inside overhang problems

[13brv3]

Greetings,

I struggled with the title of this post to try to make it descriptive, since I wasn't really sure what to call this. I have an M2, and have started using it a lot more than I did in the past. I upgraded it, and I'm printing almost exclusively PETG now. On a few prints, I've had issues with the filament not adhering to the model when printing an inside diameter that's reducing in size.

For example, if I print a hollow sphere that's 2" in diameter, the bottom prints fine, but the top doesn't. When the filament is being wrapped around the outside, it seems to have a better chance to stick to the model, but when printing on the inside diameter at the top of the model, the filament loses grip, and pulls away. To save time, I lowered the model below the table, and have been testing by just printing the top cm or so of the model. The pics show the inside of the top of the sphere.

I have tried everything I can think of to improve this, and only two things have helped. The first is reducing the layer height. If I go from 0.2mm to 0.1mm, it helps a lot, but doesn't completely solve the issue. I also tried something really outside the box as a test, which was to tell S3D that my 0.35mm nozzle was 025mm. That also helped a lot, but I'm sure isn't a good solution.

I know the sphere test is a real challenge, since the top translates to a near horz overhang. I guess I'm wondering what setting you'd expect to help this succeed? I'm hoping there's some less obvious setting that I haven't though to test yet, since I've tried most everything I can think of. I just wish I could tell the program to overlap the last layer by a certain amount.

Thanks for any suggestions.

Cheers,
Rusty

[CompoundCarl]

Lowering the layer height will definitely help. I would also try decreasing the outline underspeed (to print the outer most perimeter even slower) and perhaps also try adding extra fan cooling, since that will help the plastic cool in place quicker and generally helps with overhangs like this.

[13brv3]

Thanks for the suggestions. I did a really quick test using 10% for the outline underspeed instead of the typical 50% setting. I kept the layer height at 0.2mm for comparison to my original prints, and there was a difference. I think there were probably about as many strands that didn't stick, but at the slower speed, they just don't get dragged far from where they were supposed to be. Later I'll try a test with the slower outline speed and lower layer height as well.

The fan is at 100%, and the fan shroud is removed, so adding additional fan will require a temporary setup with another fan. I still worry that there just isn't enough overlap to allow the overhanging layer to adhere to the lower layer.

Thanks,
Rusty