Fan not working on overhangs

[Humungus]

Fan doesn't seem to switch on when working on an overhang. (Detect Bridging Parameters is on, bridge fan speed is set 100%). This can result (and often does with me) in curling overhangs and ugly prints.

The issue seems to exist both on overhangs with support and without, and with support generation turned off.

STL to test on: https://www.dropbox.com/s/tkwa6nb04so8f0y/overhangs.stl

There are two configs, one with no supports (1), one with supports under the longer overhang (2):
https://www.dropbox.com/s/ltbjfabfo2490jn/config.ini
https://www.dropbox.com/s/6x38z6c49bfebyj/config2.ini

Version of Slic3r is from git, commit 2ca0d46

OS: Win 7 SP1

Also worth mentioning that on actual bridges the fan switches on fine.

[alranel]

Actually, fan is enabled on overhangs. However, the supplied model does not fall in the definition of "overhangs" which is being used by Slic3r. I think we should start having multiple thresholds for detecting overhangs:

• one for enabling fan on perimeters (and possibly using a slower speed, Add print speed setting for overhang #328)
• one for using bridge flow and speed
• one for generating support material

[DrLex0]

I have also noticed that enabling the fan even on the slightest of overhangs can make all the difference between a perfect print and an ugly deformed brown-baked surface due to the nozzle rubbing against curled-up material. For this reason I tend to always enable the fan, even on ABS prints. However, as I noticed yesterday, this affects layer adhesion and can cause large models to crack. The only workarounds I can think of at the moment, are to manually hack the generated GCode or sit next to the printer with a blower and manually administer airflow at the right moments. Neither of these options are appealing :)
Hence it would be nice if Slic3r could ensure the fan is only active (and obviously, spun up sufficiently in advance) whenever a layer segment is not fully supported by underlying material according to some threshold.

[lordofhyphens]

@DrLex0 That would generally fall under bridging flow/speed I think for the threshold.

[curiouspl2]

+1 for this one. for ABS prints global cooling causes very poor layer adhesion, still overhangs (including partial overhangs) print much better and curl up much less when cooled.