Small overhangs curl UP

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Small overhangs curl UP

Postby Sigmoid » 2014-Nov-Thu-11-Nov

Hey,

So far I'm extremely satisfied with the surface finish of printed objects, the only problem is that I haven't been able to complete any due to small overhangs curling up. Note, it's NOT the bottom of the print that's curling, and it's not ABS, it's PLA. So this curling is not that curling. ;)

When printing overhangs, it looks as if there was too much plastic extruded, and the whole thing curves upward more and more with each pass. At first the hot-end pushes it back down, but after a while the plastic hardens, and as the hot-end tries to print the next layer, it hits the curled overhang - which, acting as a lever, rips the entire print off the bed - and the print is pretty much ruined from that point on.

I've read two explanations...
One explanation is that it's due to insufficient cooling. A suggested solution was to reduce printing temperature - I'm printing at 185C, I don't think it goes much lower than that... The other was to improve cooling - I set a minimum layer duration of 15 seconds, with cold wait, which improved the situation, but didn't wholly get rid of it - the print still failed, only later. I'll try printing the fan shroud, and see if it helps.
The other explanation was that it's due to over-extrusion. I have only read this once... It certainly does LOOK like over-extrusion, but regular areas that aren't overhangs print quite beautifully.

Anyone had this problem and solved it successfully?

(The print I experienced this on is the Ultimaker robot, which is the "demo" print in Cura. It fails where the body of the robot begins, with a sudden overhang.)
ultirobot_problem.jpg
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Small overhangs curl UP

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Re: Small overhangs curl UP

Postby RetireeJay » 2014-Nov-Thu-12-Nov

Are you trying to print this puppy standing up (oriented like in your picture)?

It's impossible, at least without "support" turned on. Look at the "fingers" on the hands. If you are printing it standing up, then the bottom tips of those "fingers" are not supported by, or attached to anything at all until upper layers get printed later.

Most of the pictures of the robot on Thingiverse seem to have the robot printed lying on its back. Even then, either the arms must be lying on the bed or else you must be printing with "support" turned on.

A truism of 3D printing is that you can only print very small overhangs. You can do sloping sides up to about 45 degrees away from vertical. You can also do "bridging" where you connect two pillars with a horizontal bridge. But without "support" you can't do horizontal overhangs or isolated objects like those robot fingers.
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Re: Small overhangs curl UP

Postby Sigmoid » 2014-Nov-Thu-12-Nov

RetireeJay wrote:Are you trying to print this puppy standing up (oriented like in your picture)?

It's impossible, at least without "support" turned on. Look at the "fingers" on the hands. If you are printing it standing up, then the bottom tips of those "fingers" are not supported by, or attached to anything at all until upper layers get printed later.

Most of the pictures of the robot on Thingiverse seem to have the robot printed lying on its back. Even then, either the arms must be lying on the bed or else you must be printing with "support" turned on.

A truism of 3D printing is that you can only print very small overhangs. You can do sloping sides up to about 45 degrees away from vertical. You can also do "bridging" where you connect two pillars with a horizontal bridge. But without "support" you can't do horizontal overhangs or isolated objects like those robot fingers.


Support is taken care of. :) The model that comes with Cura includes small struts that prop up the finger. As I said, it's the Cura demo print. When you first open Cura, this thing is on the build plate, ready to be your first print.

Also, lack of support result in overhangs drooping, not curling up. There is a very little, acceptable amount of drooping on the marked points, which I think will get even better with the fan shroud.

When I turned on support generation, it didn't generate any supports, as it didn't find them necessary. I think supports wouldn't really help with this problem anyway.
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Re: Small overhangs curl UP

Postby Mooselake » 2014-Nov-Mon-20-Nov

It's from the extrusion stretching while printing, then pulling back as it cools. I had it trying to print a model of a human inner ear a couple years ago for Mrs. Moose (she's an audiologist), and it took some digging to figure it out. There's an old discussion on the reprap.org forum here. Or google "3d printing stretch curl" (minus the quotes).

It's related to the width of the printed filament, the narrower the extrusion the more it's stretched (by extruding slower relative to head movement) to be smaller relative to the nozzle orifice size. Change your printed extrusion width so it's wider, if Cura gives you that option (sorry, don't remember if it's in Cura or not; it is in slic3r). If not, you may be able to fake it by specifying a somewhat larger nozzle so it'll automatically pick a bigger width.

Kirk
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Re: Small overhangs curl UP

Postby Sigmoid » 2014-Nov-Sat-22-Nov

Mooselake wrote:It's from the extrusion stretching while printing, then pulling back as it cools. I had it trying to print a model of a human inner ear a couple years ago for Mrs. Moose (she's an audiologist), and it took some digging to figure it out. There's an old discussion on the reprap.org forum here. Or google "3d printing stretch curl" (minus the quotes).

It's related to the width of the printed filament, the narrower the extrusion the more it's stretched (by extruding slower relative to head movement) to be smaller relative to the nozzle orifice size. Change your printed extrusion width so it's wider, if Cura gives you that option (sorry, don't remember if it's in Cura or not; it is in slic3r). If not, you may be able to fake it by specifying a somewhat larger nozzle so it'll automatically pick a bigger width.

Kirk

Thanks! That's extremely interesting! :D

I'll try specifying a larger extrusion width and see how it handles. :)
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