Steep Overhangs - advice and help?

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Trnquill
Posts: 66
Joined: Fri Oct 05, 2012 10:01 am

Re: Steep Overhangs - advice and help?

Post by Trnquill »

stratogavster wrote:I'm certain my machine could never print that as seen from the holes previously displayed, though in theory they are less challenging than this floating horizontal feature.
No, quite the contrary! The shape in the blue parts is quite trivial: 45 degree overhang + 5mm (?) bridge. You should be able to do 45 degree overhangs no problem and with no height limit. And bridges should also be possible quite easily, at least when they are short.

You part on the other hand has overhang that goes gradually from 0 to 90 degrees. There's virtually no bridging. So, in order for your part to success the printer should be able to produce near horizontal overhangs which is not possible without some support material.

I would say the blue part in the previous post is quite easy (well, on edge of what is possible because of the 45 degree overhang but still very doable) but your part is impossible in that orientation without support.

stratogavster
Posts: 120
Joined: Tue Jul 17, 2012 9:04 pm
Location: United Kingdom

Post by stratogavster »

The reason I thought they could be less challenging than an 'open expanse' is that there is at least some 'approach' inherent within the fact i'm printing a 'circle' and it's not a complete drop off to the bed. I'm considering tuning the profiles further to experiment with layer height, skin and loop/fill/perimeter strategy.

Though it has been pointed out that a dirty or worn hot-end tip could be having an effect on my results.

My next course of action is to address the hot-end tip for blockage or damage, upgrade to v1.5 and then recalibrate. Hopefully that will address mismatch i'm getting compared to others (shown via printing prepared geode)

stratogavster
Posts: 120
Joined: Tue Jul 17, 2012 9:04 pm
Location: United Kingdom

Post by stratogavster »

gfelixstals dirty nozzle investigation has been furthered by me over in High Quality Prints!!

http://www.felixprinters.com/forum/view ... 1117#p1117

superhornet59
Posts: 60
Joined: Wed Apr 03, 2013 1:58 am

Post by superhornet59 »

I'd like to chime in to this dinosaur thread with some experiences of my own.

First of all, I should tell you right off the bat I use Cura as my slicer, and I love the duplicate outlines feature. It's not quite a skin, but it's close enough. It just goes over the outer perimeters twice at half-layer-height.

Cooling

If you can find an aftermarket fan like a Delta and pump out more CFM's, you'll be in for a treat!

Why it works: PLA hardens almost immediately upon cooling, it has seemingly very little heat capacity compared to ABS. Cranking your fan up to 100% will help for sure.


Fine Layer Height

Why this works: The Square-Cube Law.
Image

For those who don't know, this means that when a 3D object is scaled by a factor of x, its surface area scales by x^2, and its volume by x^3.
Imagine (like in satoer's diagram) the way filament is represented as a circular cross section. While the real extrusion isn't exactly circular, it does follow the square cube law just as well. For a 2D cross section, this means is that if you use 1/2 of the layer height, the extrusion cross section will have 1/2 of the circumference, but 1/4 of the area (and therefore, cross sectional mass). The previous layer will have only 1/2 as much contact area, but will only have to support 1/4 of the weight, which reduces sagging greatly.

This square-cube effect also greatly increases heat outflux from the extruded filament because of the improved surface area to mass ratio, so it hardens quicker. It also reduces the amount of heat energy transferred back into the part, meaning the previous layer doesn't get as soft, and is therefore better able to hold up the new layer.

Printing at 50 microns (yes that means 25 micron skins!) I have been able to achieve unbelievable overhangs compared to my 250 micron prints.


Print Infill First

Why this works:
Most slicers have the following order of print: Inner Loops > External Loops > Infill.
I fundamentally disagree with this default setting, even outside of the context of overhangs. As it cools, infill will contract and deform your beautiful outer perimeters. I would rather put this stuff down first, have it cool/shrink, then lay the external perimeter down the way it is intended to remain.

In the context of overhangs, this can push your overhang making to the next level. If your wall thickness is large enough, it may not matter, but for thinner walled parts with very steep (+65 degree) overhangs, this will make a difference. Most slicers by default put the inner loops before the outer perimeters, which makes sense. That way the new overhanging perimeter is not only supported by part of the previous layer, but also by the side of the current layer. These inner loops, however, can be softened by the radiant heat from the extruder. In the case of a thin walled part, there may not be enough of them to soak up the heat from the previous layer either. Of course the plastics we use do not have amazing heat conduction, but that does not mean we can disregard these factors entirely. I have found putting the infill down first to give great results for overhangs, and even small ledges (not bridges... ledges!)


These three factors have proven to be an excellent way to improve steep overhangs. It should go without saying that you need to become familiar with which temperatures and speeds give you good results first, and ensure that you have your filament packing factor set correctly. I for one used to have issues with under-extrusion which weakened inter-loop bonding and therefore overhangs.


Finally, I'd encourage everyone to try printing this object: http://www.thingiverse.com/thing:100934 and see where you stand. If you look in the image gallery, the maker of the 'thing' successfully printed out the 70 degree bar... WOW!

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