Nozzle running into large prints

I just printer my first large (for me) print and noticed that the nozzle sounded like it was running into the dry layers below. Eventually the print unstuck and was ruined. When I say large I mean with all dimensions over 100mm. I've calibrated my x,y, and z steps basically to perfection using a 20mm test cube and calipers. I'm currently calibrating my e-steps percisely using a 95% infill test. I am curious why this scraping only presents itself on large prints and usually on the transverses (fast travel movements). Also, there is no visible damage caused by the scraping besides the fact that my prints come unstuck 75% the way through. Have any of you had this issue or an idea as to what may be causing it? Thanks

I've had this issue a few times. In my case it has happened for 3 different (or a combination of) reasons:

1) my bed was not level with the X-axis. I think this was the result of auto-homing with plastic still on the nozzle and causing a crash between the nozzle and bed. This isn't "that" big of a problem on our printers because the nuts will work themselves out of the traps on the X-axis, but if only one side (as happened to me) comes out, your X-axis is no longer level with the bed. Solution: adjust Z motors independently until X axis is level with bed (a digital caliper helps here)

2) my nozzle was set too low, and the nozzle was pressing the bed surface artificially low (compressing the spring-loaded corners) I discovered this after noticing I barely had any material extruding for the first layer or two. The corners of the bed were pinched down and significantly lower than the center of the bed. Solution: level bed manually with a thin gauge/paper in at least all four corners and center.

3) outside temperature dropped (my printer is in my unheated garage), and the print started to peel away from the bed. I believe this to be due to a drastic temperature difference between ambient and the extruded material (although I use primarily PLA). Solution: increased bed temp by 10*, switched from hairspray to elmer's purple disappearing glue, disabled print cooling fan for first 3 layers.

Edit: also, a quick way to determine if #2 is your issue is to set your z-offset in your slicing software for a test print. Adjust it in .1mm increments and see if it helps. If so, adjust your z endstop and/or level your bed and you'll be good to go.

Also, I find that using a large layer height (0.2mm) prevents this for me. If I'm printing with a resolution of 0.02mm then this can occur.

Thanks for all the replies. I still have the issue even with layer heights from 0.1 to 0.3 mm. I think I have found the cause though. The actual glass im using for my bed seems warped in the middle. It droops down about 1mm in the middle. So when my edges and corners are leveled perfectly there is a 1mm gap in the middle. I would then level the middle, causing my edges and corners to be excessively high. For small prints this wasn't a big deal because they were mostly in just the middle. However, I think for large prints the height deviation is causing the nozzle to run into the edges of the prints. I got my glass from a hardware store. Where did you guys get your glass? Would getting a $35 piece of borosilicate glass solve the problem? Any sources for 300mm x 300mm borosilicate glass? Amazon was currently unavailable. Thanks for the tips!

This is probably a hit and miss thing, but I sourced my glass from a very cheap photo frame. I just ripped out the glass and threw the rest of frame away. Worked for me, but I would guess the quality would vary and you end up with the same problem.

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Originally Posted by pjfossee

Thanks for all the replies. I still have the issue even with layer heights from 0.1 to 0.3 mm. I think I have found the cause though. The actual glass im using for my bed seems warped in the middle. It droops down about 1mm in the middle. So when my edges and corners are leveled perfectly there is a 1mm gap in the middle. I would then level the middle, causing my edges and corners to be excessively high. For small prints this wasn't a big deal because they were mostly in just the middle. However, I think for large prints the height deviation is causing the nozzle to run into the edges of the prints. I got my glass from a hardware store. Where did you guys get your glass? Would getting a $35 piece of borosilicate glass solve the problem? Any sources for 300mm x 300mm borosilicate glass? Amazon was currently unavailable. Thanks for the tips!

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I got my glass from a hardware store too. The thicker the glass the better, but I didn't go super thick. I had a problem with my glass being bowed downward in the middle. It was instantly solved by doing two things.

1. Flipping the glass.
2. Removing some/most of the binder clips from two sides of the bed/glass.

I tried both at once so I'm not sure which one solved the problem, or if it was the combination of the two. It is easy to try it and easy to reverse it if it does not help :)