Overhang blobs.

I've gotten my Prusa i3 about as tuned as possible and it make great prints with one exception: overhangs. I haven't been able to find the magic formula that works for this. Things like the cube test come out as close to perfect as you could hope for.



But overhangs look like shit.



Both were printed with 1.75 PLA (Inland black) using cura with identical settings except infill (0%/20%). 195°C/50°C, 50/150 mm/s print/move, and 1.5 mm retraction at 90 mm/sec.

Printer is a chinese Geeetech-ish I3 clone with MK8 extruder with fan.

Do you just have to accept this with overhangs? Any ideas?

You can tweak a little and maybe get it to look a little better, but the part you printed was not designed for FDM printing or if it was, the designer didn't understand the limitations of the process. FDM can't print unsupported curves into thin air. You have to provide more support by reducing the overhang angle (measured from vertical) by stretching the object in the Z axis, by using the support material option in the slicer which usually leaves scars on the print surface, or by editing the design to reduce that overhang to something more printable.

Cheer up- if you tried to print that part with ABS it would be worse.

Edited 1 time(s). Last edit at 05/04/2016 06:56AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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MK8 extruder with fan.

Is that a print cooling fan or just the fan on the extruder? PLA needs a well directed print cooling fan for good overhang performance. Overhangs also tend to improve with wider extrusion. I'd expect it also to be better with smaller layer heights (because there is more overlap of the outer extrusion per layer, and a smaller percentage hanging out in space), but this doesn't always seem to work out. I'm guessing there is a sweet-spot for layer-height on overhangs of different angles.

DD is right of course, that model looks like more than the 45 to 60 degrees of maximum overhang that is recommended, so it may not be possible to get 'great' results with it.

You have some interesting diagonal lines on your test-cube.

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JamesK

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MK8 extruder with fan.

Is that a print cooling fan or just the fan on the extruder? PLA needs a well directed print cooling fan for good overhang performance. Overhangs also tend to improve with wider extrusion. I'd expect it also to be better with smaller layer heights (because there is more overlap of the outer extrusion per layer, and a smaller percentage hanging out in space), but this doesn't always seem to work out. I'm guessing there is a sweet-spot for layer-height on overhangs of different angles.

I have both. I've tried it with and without print cooling with no discernable difference. I do like the print width idea though. I pretty much leave that alone. Maybe I'll play with it.

BTW, the workpiece is a queen from a chess set on thingiverse: [www.thingiverse.com]

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JamesK
You have some interesting diagonal lines on your test-cube.

I've wondered about that before. I may try and do the math, but I suspect that it is a function of the rotation of the extruder gear or idler. What else would be so periodic, but not tied to any axis.

Edited 1 time(s). Last edit at 05/04/2016 11:07AM by hoxsiew.

Thingiverse is full of things that are unprintable. The first clue is absence of a photo of the printed object. Even if there's a photo, it's typically shot to hide flaws. They may also do some post printing clean-up before shooting the picture (which you can do, too).

It isn't cheating to do some post-print clean-up. It also isn't cheating to break a part into multiple pieces to avoid the need for support material and glue them together after printing. FDM can print a lot of things, but it can't print all of them well. Parts that are designed for casting or injection molding don't always print well. Recognizing FDM's limitations and designing accordingly takes experience with both design and printing.

Edited 1 time(s). Last edit at 05/04/2016 11:27AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]