Corner Of Print Has Rounding-off Issue

Hi everyone, I'm new to reprap and just got my machine on monday, a Prusa i3 from Sunhokey.com. All acrylic frame, bowden extruder.

I got it working well printing squares and solid blocks without problems, after some tuning. Interestingly, I don't actually know the nozzle diameter! Their instructions are lacking in clarity and seem to claim to be a 0.3mm nozzle and thats what I've been using. If thats not what it is, I must have compensated with other settings because there aren't any gaps on solid infill. Extruding into open air (possibly too quickly, causing swelling?) and measuring the strand that comes out measures to be 0.48mm or so. I neglected to measure before I filled the nozzle with filament so I'm going to have to try heating the nozzle and sticking a digital caliper in there to measure it.

Anyway my prints were going okay so I tried a 10mm x 10mm x 50mm tower, with 25% honeycomb fill. I'm running repetier host and slic3r, tip 205C and 60C heated bed on kapton, 0.2mm layers. After about halfway through the tower, the back right corner started rounding off and getting worse and worse as it went. See the attached image. The tower finished but it developed some bad defects on the right and rear sides. I can see the nozzle pass over the open space and attempt to squirt filament into midair, only to wipe it off on the back side edge of the perimeter.

I thought maybe the fan was blowing on it and causing it to screw up, so I dropped the fan from 100% to 50% and the problem got way worse. The short tower was the same print but I cancelled it since it was clearly going very poorly.

I then did a third tower, back to 100% fan and tried turning the heated bed off after the print started, thinking maybe there was some weird heat buildup screwing it up. It came out almost identical to the first tower.

My only remaining ideas; maybe the honeycomb infill is screwing it up? Maybe the extruder is having a problem retracting? I'm running it a little hot it seems but I was having a lot of extruder skipping and I increased it incrementally until that went away.

I've searched the pictorial print troubleshooting on the Reprap wiki and couldn't find anything like this happening.

Anyway I hope someone can give me a pointer in the right direction so I can stop spinning my wheels on unhelpful tests. Thanks for the help!

- Matt

When ive seen this on my own smaller items. Its usually because its not cooling quickly enough.
I bet that corner gets the nozzle on it twice so stays hotter and sags.

Try slowing down the print and more cooling.
Or alternatively print more than one so the print has better chance to cool by moving to the other object before getting back to that hot spot.

Gordon

You could be right. The machine has demonstrated very poor bridging up to now. I tried to print a hollow Maker Faire robot model, with 3 perimeters, and it failed to even properly do the bridging at the bottom of the torso between the legs, a span of only a few mm. I i think the fan and dinky little cowling they provided needs to be adjusted and improved.

Thanks for the tip, I will experiment today and see if I can't get some better cooling out of that fan.

Try using a desk fan over the whole print bed first.
Ive been reading that these cowls are too precise. They only cool at the point of extrusion.
If you think about it you want the material to continue to cool after the nozzle has passed as well.

Gordon

Thanks Gordon, it worked flawlessly with a small desk fan aimed across the bed during printing. I went on to experiment with bridges and some other shapes, with mixed results. I need to try and get slic3r to be better about doing overhanging and mini-bridges in perimeters slower, and with the fan running, so they don't droop and cause a perpetual screwup afterwards.

Thanks again for the help. I'm having a good time working on getting this thing running well.