Help with quality in printing recess

[ptchan5]

Not sure if this is the right place to post this, and I'm new to 3D printing and the forum so please go easy.

I am designing a part that basically has a recess on the bottom. Think of a standard tea cup type shape that normally has just a rim that touches the ground and the center section is recessed into the bottom of the cup. When I print this I do get a recess, but the recess is not that pretty because as you would expect, the roof of the recess has no support. So when the printer goes to string across the opening it lays down long unsupported threads. My problem is not so much the sagging that you'd expect to happen, but the roughness of the unsupported threads.

The design can't be flipped 180 degrees because I have something sitting on top of that "roof".

How does one design this so that the "roof" section of the recess is smooth or at least nice enough to be considered a finished part instead of poorly printed piece. I tried little support extrusion pillars with the thought that the printer would tie the cross threads to them, but that didn't happen. Came out exactly the same. If I make something too solid it will be difficult to remove.

How would you experts do this?

Thanks in advance,
Newbie (Peter)

Attached .stl file for review.

[curious aardvark]

well thoeretically that's a bridgeable space. I've bridged a 55mm gap with pretty decent results.
Depends on your printer and slicer.

But the best way to design a space like that is to make it a dome.

Looks like an opencasd design so that should be pretty easy (to make the recess remove the top of a sphere from bottom cylinder rather than another cylinder). Also add $fn=100;
to the top of your script to make it rounder.

Alternatively, reduce the facet number to 12 for the bottom cylinder, stand it on its side and slope the top so that it finishes smaller than it starts. That should print without any issues.
'course if it's not an openscad desig that won't make much sense :-) But it'd still work :-)

And supports should work okay, they just have to be tall enough.
If your filament is sagging between the supports then it's not printing cool or fast enough for the bridging. Which is down to your slicer settings.

So the standard questions:
what machine, what slicer, what material and what settings ?
Succesful 3d printing requires a close attention to fine detail. The more detail you give us, the better chance we've got of sorting the problem out :-)

spots something and has closer look at the model....
There's a noticeable gap between the top of the first cylinder and the start of the second.
That won't effect the bridging, but it will effect how the top part connects to the base. And how the rest of the model will print. Generally it'll look crap as it's all shifted a layer of 2 downward.

dodgypuck.jpg

[ptchan5]

well thoeretically that's a bridgeable space. I've bridged a 55mm gap with pretty decent results.
Depends on your printer and slicer.

But the best way to design a space like that is to make it a dome.

Looks like an opencasd design so that should be pretty easy (to make the recess remove the top of a sphere from bottom cylinder rather than another cylinder). Also add $fn=100;
to the top of your script to make it rounder.

Alternatively, reduce the facet number to 12 for the bottom cylinder, stand it on its side and slope the top so that it finishes smaller than it starts. That should print without any issues.
'course if it's not an openscad desig that won't make much sense :-) But it'd still work :-)

And supports should work okay, they just have to be tall enough.
If your filament is sagging between the supports then it's not printing cool or fast enough for the bridging. Which is down to your slicer settings.

So the standard questions:
what machine, what slicer, what material and what settings ?
Succesful 3d printing requires a close attention to fine detail. The more detail you give us, the better chance we've got of sorting the problem out :-)

spots something and has closer look at the model....
There's a noticeable gap between the top of the first cylinder and the start of the second.
That won't effect the bridging, but it will effect how the top part connects to the base. And how the rest of the model will print. Generally it'll look crap as it's all shifted a layer of 2 downward.

dodgypuck.jpg

Thanks for the quick reply!

Just noticed that gap myself and yes, it made for a crappy adhesion between the pucks! hahaha.

Here are the specs:
Machine: lulzbot mini
filament: PLA 3mm (gizmo)
slicer: Cura (drawn in TinkerCAD).

Didn't quite get the dome explanation. But if you think the supports should work, then maybe I should go back to that. It's not sagging, but I noticed that some filaments were not connecting across the spans and caused a few loops to dangle. Not pretty looking.

Thanks,
Peter

[Mjolinor]

Why not just let Cura support it when sliced. It generally does a reasonable job and there are some "expert settings" you can mess with.

[ptchan5]

Why not just let Cura support it when sliced. It generally does a reasonable job and there are some "expert settings" you can mess with.

Good idea. I'll try and mess with the expert settings and see if I can't get a support to work through Cura.

Thanks,
Peter

[curious aardvark]

It's not sagging, but I noticed that some filaments were not connecting across the spans and caused a few loops to dangle. Not pretty looking.

perfectly normal.
That's why they invented sanding paper :-)

[curious aardvark]

dome explanation.

Treat the entire model as a solid and hollow out the bottom with a sphere squashed upwards slightly.
This will create a hollow space that is much easier to print than a straight bridged gap.

[ptchan5]

dome explanation.

Treat the entire model as a solid and hollow out the bottom with a sphere squashed upwards slightly.
This will create a hollow space that is much easier to print than a straight bridged gap.

OMG, that makes so much sense to me now. Ok, let me try that and see how that works. Thanks everyone for your comments, this is really encouraging.

Peter

[curious aardvark]

A lot of designing for 3d printing is lateral thinking :-)
And the slightly warped way I look at the world, lol