Good results with overhang on fan wheel?

Dear community,
I have successfully bolted together my Mendel and gone through half the calibration process with excellent results. Very happy! The half millimeter thin wall object prints perfect and I can even print the hollow cube with a perfect top surface.

On to the next challenge:
To print a model fan wheel. The profile of the fan blades is upside down aircraft wing looking and so the incline starts at maybe 30 degs and increases toward the top of the object. This profile involves printing into thin air and this seems to be extremely difficult for this object. I have tried various layer thicknesses and the problems increase with thinner layers. They are worst at the lower layers where the fan blade angle is lowest. Toward the top of the blades where the incline gets steeper, the problems almost go away.

Problem description:
The overhang on the back of the blade is printed in thin air and has a tendency not to stay at its z layer, but rather making the back of the fan blade bend upwards. The next pass of the nozzle presses down the curled up parts of the layer (adhesion to the Kapton tape bed is no problem here - this is higher up) and increases this error, layer by layer. With 0.1 mm layers, the situation is compounded to the point that the nozzle gets caught on the object and the print fails completely due to missed steps. With thicker layers, it appears that the bending/curling tendency is smaller and when the angle of inclination gets big enough, toward the end/top of the blade, the print goes back to normal with ok (not perfect) quality. The fan blades have an ok looking top surface, but the back looks like filament has been drooping. It has not. It's just the result of the nozzle pressing down the curled parts and finally fixing them in that position with successful layers above. The wing looking fan blade profile, as seen from the side, is of course not what it should be with such a back side.
I have tried increasing the perimeter thickness (to get rid of infill in the fan blades (they are about 2mm thick in the problem layers) and add something better to cling to for the midair print pass) and reducing the temperature to 225 (ABS). I added a 120mm fan with Slic3rs automatic control and slowed down bridge prints to minimum. The fan seems to have no effect on this phenomenon, although I note that people mention this as a critical success factor. I also increased the "stretch" parameter a notch. Adding support does not prevent the curling, however, and so has no positive effect. The combined effect of the mentioned actions have moved things forward from a glob of plastic to a half decent fan wheel. However, I would like to attain a "perfect" print at high resolution for my modeling purposes without having to resort to spackle and filing... Or too much of it, at least.

What settings are you using for quality prints of this difficulty? Is it at all possible?

Experimenting randomly is extremely time consuming (especially with ABS and its higher temperatures, I think), so any pointers are greatly welcome.

Thanks,
/Chris

Strange, I thought my signature would be added to the post? Here it is anyway:
Signature:
Beginner using:
RepRap Mendel from RepRapPro
0,5 mm nozzle with bowden extruder
1,75 mm filament
Software:
Pronterface v July 2014
Slic3r v1.1.7

Edited 1 time(s). Last edit at 08/28/2014 09:15AM by cahlfors.

do you have a link, picture or STL of what you are trying to print?

I can't manage to upload any pictures, but it's a simple axial fan. What do you guys do to succeed with overhangs?

Thanks!
/Chris

Quote
cahlfors
I can't manage to upload any pictures, but it's a simple axial fan. What do you guys do to succeed with overhangs?

Thanks!
/Chris

We really need to see what you are trying to print in order to help. Did you design this yourself, or did you download an STL file from somewhere? If you have an STL file, can you provide it here in place of photos? Providing the STL file and/or photos of your print issue are the best way to get help here.

Aha thanks!
Well, uploading seems to work today. Last time I tried, it would not accept anything.

It would be nice to know what people do to print overhangs successfully and what parameters that control this.

Thanks!
/Chris

In case this helps someone else, here is what I did:
1 Replaced the nozzle with 0.3 mm. This did not help the overhang problem per se, but since it got worse with thinner layers, it might have had some effect.
2 Changed the design to incorporate vertical supports. The Slic3r supports don't adhere to the wings and so they are free to curl upwards. By designing supports into the model, they are now an integral part of the structure and keep things down. Also, Slic3r now treats this as a bridge and turns on the cooling fan, whatever good that does.

The result at 0.1 mm layer thickness can be seen below (underside). Before I was totally unable to get below 0.2 mm. I will now give 0.05 a try - gotta test the limits!

The supports can be easily removed with flush cutting snippers, as can be seen to the right in the picture.

/Chris