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Head moving down mid print, melting gap through print - UM2 /w pictures

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Hi,

Wondering if someone had the same problem as me.

I've been trying to print a simple 3h print for a couple of times and it now seems that the head moves down at 90% of the print and melts and pushes through my print.

I've attached some photos. The gap/crevice is about 5 mm deep and about the width of the nozzle. I haven't witnessed it during the print but can only conclude from what I see afterwards that the print head at some point moves down, melts and knocks over the print. The last 2 prints are almost identical.

I've looked at the g-code and searched for any wrong z-axes movement but couldn't find any. I've tried a couple g-code viewers and they all render perfectly.

I've updated the UM's firmware and the result is exactly the same.

Is this a hardware error, g-code interpretation error ? Anyone else experienced this? And if so how did you fixed it ? :)

Cheers

IMG_7298.thumb.JPG.0e41922dbd05d29d73bca6f17d1cc9ca.JPG

IMG_7299.thumb.JPG.4bbd33057f3ddfbe745702f9841a5de4.JPG

IMG_7300.thumb.JPG.b392a5b3403f4e7b1bd9504ffee2b1b3.JPG

IMG_7301.thumb.JPG.93ff300076d8db99df7357c430ef886c.JPG

IMG_7298.thumb.JPG.0e41922dbd05d29d73bca6f17d1cc9ca.JPG

IMG_7299.thumb.JPG.4bbd33057f3ddfbe745702f9841a5de4.JPG

IMG_7300.thumb.JPG.b392a5b3403f4e7b1bd9504ffee2b1b3.JPG

IMG_7301.thumb.JPG.93ff300076d8db99df7357c430ef886c.JPG

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Probably the first layer it's too close to the bed. When you used the calibration card it barely fell it or it scrached the nozzle? Turn the 3 bed whells a bit (the same amount) counterclockwise to give a bit of gap. The. The nozzle won't push the filament and it won't build around the nozzle since there's a point it burns and falls over the print leaving that brown goo.

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I noticed that parts with big overhangs tend to curl up. The printer has to print part of those layers "in the air", so it has no good support. These overhanging edges can curl up quite a lot, even one or two millimeter.

What may happen next, is that the print head brutally collides with these ridges, and knocks the print over. I have seen it happen in a testprint.

Could this be the cause?

If you have a compressor, you could try blowing cool air on the surface, to make it cool faster and get less curling up.

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The overhangs are in the model but not quite big. And yes it curls up, I know that this can cause collisions. For this particular model I printed without support since the overhangs are quite small and the head stays within the printed area. I saw that when it leaves the model to print the support outside , that when it moves back it can knock over the print.

But the position where the head strikes the print is way above any overhang edges, so that shouldn't pose any problems. The rest of the model is printed just perfect up to the point where it did what it did.

I can attach the model for someone to test as well if you'd like.

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