What things should I consider to allow for optimum overhang printing performance?

Currently, the best my Huxley can do is about 55deg off vertical before things start to look bad. The hardware is using PLA with a 0.3mm nozzle and a large full-area cooling fan.

Are there any variables I should look over in Slic3r to give me the best results when printing large overhangs (i.e. speed, cooling, layer height, perimeters, etc.)?

Update: After many ugly attempts at printing overhangs, I decided to print a blower attachment for my hot end. Results were less then perfect even when using this overpowered 40mm by 20mm fan. I reverted back to the large full-area cooling fan.

What would help a lot is if Slic3r would pause for a moment between each layer of a small object, move a few mm away, then move back again after the layer had time to cool. I noticed when printing small features of any object, the last dozen or so layers remain in a gelatinous state from the perpetual heat being applied from above. Changing the overall minimum feed rate in Slic3r's "cooling" section does not seem to help.
Sure, I could have Slic3r print something else on the other side of the build plate, but then I'll get unwanted ooze and artifacts.

Are there any Slic3r tricks or Slic3r alternatives I could try?

"Changing the overall minimum feed rate in Slic3r's "cooling" section does not seem to help."

Make sure you also set the parameter "slow down if layer print time is below 0 seconds" to a longer time, perhaps 20 seconds or more. If you leave that at zero then the overall minimum feed rate will have no effect.

The idea is that if a layer is estimated to take less than the time you define it will slow down the feed rate enough to make it last that long. The 'minimum feed rate' parameter places a limit on how much it can slow down. If there isn't any slowing down to start with then that parameter will do nothing at all.

Lowering the temperature may also help.(Lowering the extrusion width?)

Note that moving the nozzle away doesnt actually help much, the heat conduction down to the model is small due to the printed thread having a really small cross-section. This is why there is a `slowdown-before-layer-time` instead, the potential for messing up the surface of the print isnt worth it. (Note that that setting isnt specific enough in the documentation.. Slow down by how much? I'd expect, clamp the layer time to that time? But that doesnt seem to be what is see happening..)

lister6520 Wrote:
-------------------------------------------------------
> Make sure you also set the parameter "slow down
> if layer print time is below 0 seconds" to a
> longer time, perhaps 20 seconds or more. If you
> leave that at zero then the overall minimum feed
> rate will have no effect.

Whenever I would try this feature, I would stick with the defaults: "slow if layer print time is below 30 seconds... ... Min feed rate 10mm/s."

Perhaps I should try a new set of values. What settings do some of you use that you know yield the best results?

Physics_Dude Wrote:
>
> Perhaps I should try a new set of values. What
> settings do some of you use that you know yield
> the best results?

I use 20 seconds minimum layer time. I don't know about the minimum feed rate though as I use Skeinforge and there doesn't seem to be the equivalent of that (or else I just cannot find it). In any case I think you should go lower than 10mm/s for the minimum, perhaps even 1mm/s. At 10mm/s a 10x10 cube with two walls will take just 8 seconds to print, not quite enough for the PLA to solidify. Keep in mind that the minimum feed rate takes priority over the minimum layer time, so even though you set minimum layer time to 30 seconds the minimum feed rate of 10mm/s will cause the layer to be printed in 8 seconds, thereby disregarding the minimum layer time.

Note that the 20 seconds I use is for ABS. For PLA I would think a longer time would be needed.since it remains quite fluid even down to 90 celcius, so will need more time to cool.

Is the feed rate actually slowing down as it should when you activate the slow down feature? If not then maybe the slicer isn't working properly or there is some other confliciting parameter elsewhere that is preventing it from slowing down.

You could manually introduce 'orbits' into the last layers, that is have the extruder move away from the object just to waste some time without extruding but that will introduce other problems as it is never a good idea to have the extruder stationary for any length of time, especially with PLA.

I find that printing two of something gives each part a little more time to cool, and it keeps the extruder busy so that it doesn't drool. You could also introduce a simple "time killer" part with a known layer print time that you print along side your real part.

Update: Using the cooling settings of: "Slow down if print time is less then 30 seconds with a min feed rate of 5mm/s" I got acceptable results:

http://imgur.com/a/suhzC

That was a curved transition from zero to a near 90deg overhang. This one piece of a scan took just under two hours to print (0.15mm layers with 3 perimeters and 13% honey comb infill). The final, full piece, which will be about the size of your thumb, will take an estimated 8 hours with these same settings at 0.1mm layer heights.

I am currently printing the same piece seen above at 0.1mm layers with a "Slow down if less then 20 seconds and a min feed rate of 5mm/s". I also dropped the nozzle temp by 4C and set Slic3r to only infill every three layers. Results are looking promising, but man is it slow. The cooling minimums are actually applying to the layers where no infill is present (and frankly where I don't need cooling), causing an alternating slow-slow-faster-slow-slow-faster pattern on the perimeters where the "faster" perimeters occur at levels of infill. I'm not sure if I should consider this good or bad on Slic3r's part.

Edited 1 time(s). Last edit at 10/14/2013 08:31PM by Physics_Dude.

LoboCNC Wrote:
-------------------------------------------------------
> I find that printing two of something gives each
> part a little more time to cool, and it keeps the
> extruder busy so that it doesn't drool. You could
> also introduce a simple "time killer" part with a
> known layer print time that you print along side
> your real part.

I'm using a ~400mm Bowden extruder tube which, even with retraction, still oozes a lot.

Update again: well, it took over two hours, but the latest of the overhang test finished.

http://imgur.com/a/pqrzz

Slic3r settings were 0.1mm layers, 3 perimeters, 13% infill, infill every 3 (seems to have rounded to 2) layers, and slow if layer print time is below 20 seconds but don't drop below 5mm/s. I also dropped nozzle temperature from 204C to 200C.

Overhang quality dropped from the previous test and holes developed both in the overhang and on a very shallow curve at the top of the head. I suspect either more perimeters are needed or I need to disable "infill every n layers".

Video update: I went for all or nothing and started printing a larger, slightly altered, version of the test object/head seen prior with other stuff printing next to it on the bed. When it came to the overhang, something cringe worthy happened. The end of the overhang curled up a significant degree and caused the bed to flex down to it's lower limit whenever the nozzle passed overhead. This was severely messing up the model's dimensions, so I took a pen torch to the curled up end progressively caressed it back down. All while the print was running (Printing a 12+ hour print from SD, no pause buttons)

You'll see the issue at about 33 seconds in: http://youtu.be/05lM_eRs38c
0.1mm layers, 10% infill, 5 perimeters.

Would this curling (I'm using PLA) be result of the piece facing and being too close to the fan? Is it even avoidable without using support?
I ask because Slic3r's support feature is a bit messed up for me at the moment (Bug #1082 - Slic3r 0.9.10b). Otherwise I'd probably restart and use it here.

Nonetheless, It looks like I have a lot of post-print refining left for me to do tomorrow.