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madoverlord

Overhang testing - results and a couple of suggestions

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I decided to do some tests to see how well Cura and my Ultimaker could handle overhangs, using a variant of a calibration model I found on Thingiverse (http://www.thingiverse.com/apps/customizer/run?thing_id=58218&code=aede3fe580c5b5328a88c5749fe73508)

My best results were at 210c where I was able to get perfect overhangs -- both unsupported and bridged -- at 69 degrees with .1mm layers @ 50mm/sec. I set the wall thickness to 3mm and Cura nicely obliged me by laying down the wall from the inside out, which no doubt helped.

Past that point I started to get imperfections on the underside and upward curling on the unsupported spans, but given those limitations I got up to 80 degrees, which is much further than I'd expected. Any advice on how to tweak things to improve on this would be appreciated.

However, that is not the main reason for this post. In the process of doing these tests, two possible improvements to Cura occurred to me.

1) Almost every time I start a print when the hot end needs to warm up, the initial adhesion is bad for the first 30 seconds or so. It might be interesting to have an option that would have Cura lay out a pattern in a corner away from the main print job to get past this critical time period. In the case of this particular model, I needed brim because it had little support area, so the first few rings of the brim would mess up, but then it would "catch".

Something as simple as being able to specify extra rings of brim/border might do the trick, though sometimes the filament drags across the middle

2) Adhesion seems better at higher temperatures. Consider an option to specify a different temperature for the first layer. Usually this would be higher, but in the case of models with a lot of contact with the bed, you might want it lower to make removal easier.

 

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You may want to adjust your start gcode to extrude a little more plastic in the home position to avoid the problem with it taking a while to prime the head. I changed the default 3mm extrusion to 6mm to help with this. (I also recommend reducing the speed on that line to F60, to ensure you don't overload the head).

The skirt setting is also intended to provide this extra preliminary extrusion. You don't get that if you have the brim enabled, but then the exact quality of the first layers of brim is less important anyway.

Other slicers do allow for different first layer temperatures. Some people swear by it, but personally I don't see the need. I just print on blue tape, rubbed down with alcohol, and get extremely good adhesion at pretty much all temperatures.

 

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Adhesion seems better at higher temperatures. Consider an option to specify a different temperature for the first layer.

 

I always set the cura temp to 0 and set the temp with the ulticontroller (or cura). I usually make the first layer hotter at 240C and then when almost done with first layer I lower the temp. No need for this to be done automatically.

Having the temp set to 0 is nice cause I can start the print as the temp is passing 235 and by the time the home is finished and the brim or skirt are started the temp is close enough to 240 to start.

You can get steeper angles (more than 80 degrees possibly) with thinner layers. You can't make the nozzle hole any bigger (it's .4mm) but the thinner the layers, the less z changes for a sideways movement of up to .4. So if layer height is .1 you should be able to get a slope of 1/4 or arctan(1/4) = 76 degrees from vertical. layer height of .05 in theory gives you atan(1/8) which is 83 degrees from vertical.

Also it's important to keep the part cool (more than 5 seconds per layer and fan on) if you want good overhang results.

 

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Thanks for the tips about alcohol and the gcode, I'll try them.

My experience testing layer heights is that 0.1mm is better than both 0.2 and 0.6. My preliminary hypothesis is that the 0.2 layers have more thermal mass, so they tend to have worse quality under the overhang, and the 0.6 layers cool too fast and tend to curl up.

 

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1) Almost every time I start a print when the hot end needs to warm up, the initial adhesion is bad for the first 30 seconds or so. It might be interesting to have an option that would have Cura lay out a pattern in a corner away from the main print job to get past this critical time period. In the case of this particular model, I needed brim because it had little support area, so the first few rings of the brim would mess up, but then it would "catch".

I'm using an Ulticontroller. As the indicated temperature is approaching the set temperature I turn the extrusion wheel by hand. I see the effect that this has on the indicated temperature, and feel the variation of resistance from the filament going through the nozzle. Worth a try.

 

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UM1 or UM2? On UM2 you can set the temp by going to MATERIAL menu and changing the default temp for PLA and than saving that back to the PLA profile. Or every time you hit PRINT and choose something to print you can then go to the TUNE menu and change the temperature there. You can continue to change things in the TUNE menu while it is printing such as bed temp, nozzle temp, fan, print speed. Also it will not start printing while you are in the TUNE menu but once it starts you can go back to the TUNE menu and stay there for the whole print.

 

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