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Using an FDM printer and PLA or ABS, without adding support material. What modifications can I make to improve how steep an overhang my printer can print before it starts having problems?

The obvious first answer is to add a cooling fan, increasing the airflow over the freshly-extruded material ensuring it solidifies. What other things can improve it?

Does lowing the temperature help? Raising it?

Does speeding up or slowing down the print head help?

Does increasing/decreasing the extrusion diameter, or layer height help?

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The two most important things you can do are:

  • Provide adequate cooling to solidify the plastic quickly
  • Minimize layer height

Cooling is really obvious. You need the plastic to solidify before it has a chance to sag. PLA in particular has to shed a lot of heat before it is fully solid. A fan and air guide setup using a "squirrel-cage" radial blower around the nozzle is optimal. A little 30mm or 40mm axial fan will not provide optimal performance.

Low layer height when slicing is less obvious, but is extremely effective. When you use thinner layers, two things happen:

  • There is less melted plastic per pass and a higher surface area to volume ratio, so the fresh material cools faster.
  • A larger percentage of each strand in the overhang is supported by the previous strand. If you do 0.2mm thick by 0.4mm wide, half of each strand is unsupported. But if you do 0.1mm thick by 0.4mm wide, only a quarter of each strand is unsupported.

When you combine these two effects, it is possible to exceed 70 degree overhangs with good surface quality.

Another lesser factor is printing shells/perimeters inside-out rather than outside-in. This helps anchor the outermost strand a little better as the overhang is built. This is pretty minor though.

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  • $\begingroup$ The layer height explanation helps a lot, thank you! $\endgroup$ – Martin Carney Mar 4 '16 at 20:19
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There's an answer here that holds some of the same concepts. Regarding your questions:

  • Does lowering the temperature help? Raising it? : Yes, lowering the temperature can help. I've found that it is best to stay closer to the lower end of the material's melting point and just a bit above the point. Not only does this help with potential over extrusion, but also shortens the time it takes for the material to cool (refer to the link above). However, this could cause clogging if your temperature is too low. Keep an eye on your drive gear to see if there is too much friction while at lower temperatures. Increasing may keep the drive gear from "eating" your filament.
  • Does speeding up or slowing down the print head help? : I prefer to print slower, most of the time, to allow the material to cool a bit more to avoid curling/warping (I primarily print with ABS, so it matters more). You might be able to give and take between temperature and speed. Consider if your nozzle is cooler and your speed is up, bridging gaps might yield the same results as if you proportionately swap these two values. This concept may only matter if you are in a pinch to get the part done. Again, I prefer slowing my machine down as it allows current/previous layers to cool more before continuing. This can be especially helpful with overhangs when paired with lowering your nozzle temperature.
  • Does increasing/decreasing the extrusion diameter, or layer height help - I assume that extrusion diameter equals layer height (not difference in nozzle diameter, aka swapping nozzles). I'm not completely sure, but I think this depends on the part as well as slicing engine settings. For me, MakerWare is pretty good about proportionately adjusting extrusion steps with layer height, so I see an equal change in the width of the extrusion. I would think that in general, a larger layer height would yield a larger extrusion width. This would be helpful when printing overhangs, but may not be helpful when printing bridges (a larger strand will retain heat longer than a smaller one).

Hopefully this helps, please comment if you need more information/clarification.

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  • $\begingroup$ @jwpat7 good catch, fixed now $\endgroup$ – tbm0115 Mar 4 '16 at 17:49
  • $\begingroup$ By extrusion diameter, I do not mean layer height, nor even necessarily changing the nozzle out for a different size. In Slic3r, you can specify the width of a "line" of extruded material. It can be wider or narrower than the nozzle's actual opening, with different results. In another question, I asked if I could print 0.6mm extrusion with a 1.2mm nozzle. I tried it out and it's possible, but it's prone to problems. $\endgroup$ – Martin Carney Mar 4 '16 at 20:17
  • $\begingroup$ @MartinCarney That's a very cool feature! I'll need to check if MakerWare has a similar parameter. What sort of problems have you encountered? $\endgroup$ – tbm0115 Mar 4 '16 at 22:54
  • $\begingroup$ Well, I only tried a few prints, and never measured them to check precision, but there were problems in print quality that are hard to describe. See my question on the subject. Basically, your layer height to extrusion diameter ratio must still be "good" if you want to have any success extruding smaller than your nozzle diameter. $\endgroup$ – Martin Carney Mar 7 '16 at 19:18

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