I tried to print this:

And this happened:

Looks like something to do with cooling, perhaps. The end of it either warped up, or it shrank. It’s as if that end got cooler and shrank, but is this really the case? Did it warp because it came unstuck, or did it come unstuck because it warped?

both. they’re related! tbh anything like that with a tiny contact area vs the overall size I’d probably print with a brim just for peace of mind and to avoid a potential reprint (and I’ve got a considerably better print surface than you!)

A brim would help? I tried a brim but it kept not coming out right. The brim went over the edge of the print area and it got all mangled up.

There may also be an issue with the accuracy at the extremes of X. As the hot end approaches either side, the belt is pulling it from an angle, so in theory each step of the motor is pulling less belt. I certainly had a lot of failures at the top left corner of this piece where it was just a wobbly mess.

I bought some 3dlac. I’ll see if it helps.

yeah a brim would help loads there! Almost mandatory with a print like that IMO. When you’re trying to print something very large wrt to the bed size a brim is tricker, looks like you’ve got a tiny bit of leeway from the pic though? You should be adjust the brim so it’s smaller than the default if that helps. Even just a couple of mm brim either side should help massively as it will basically triple the contact area!

Not sure about the X accuracy issue but in the past when I’ve had prints fail on particular parts of the bed it’s been due to the bed level being way out of whack in that area!

Have you ever had any decent prints? There is something fundamentally wrong with your set up, I just don’t know what!

I’m checking the bed levelling every time, multiple times, paper and feeler gauge. I don’t think that’s it. Unless it’s not tracking properly somehow. On this printer the bed itself moves for the Y axis.

I did try a brim but it screwed up for another reason – it hit the edge of the bed.

Have you ever had any decent prints?

Yes we got a pretty good cube last time but I don’t think I got a pic.

I would always print that short of thing with a brim, as above. You have very little surface area in touch with the bed and it will pop off easily.

Was it a model you made or found online? If you made it, you can add your own brim to the model and put it internally, rather than externally – just make a feature the same height as your intended layer height to the shape you want.

But saying that, you have an issue with your printer settings in the slicer software if it lets you create a brim that falls off the edge of the build plate as you mention above.
Are you using Cura? It (and all other slicers i have used) will not let you place an object on the build plate that will not fit. If you put a part on that just fits, but your print settings will push it outside the boundary, then it won’t let you slice.

Check your build volume and how this compares to the settings for your printer in the slicer.

Have you tried something on the build plate to help with adhesion yet? Ah, I see you bought some 3dLac – you should just need a thin misting of it.
Are you printing with a nice and slow first layer?

The lines in the z-axis suggests you have a few potential things wrong with the set-up. Not all of these but you should check everything.

The printer could be not mechanically sound. – you mentioned adjusting the eccentric nuts previously. Did you do this for X and Y? Have you had a general check around the machine for loose parts, tight parts etc? Do the carriages run well in all directions?
Z-axis – check your z-axis isn’t binding. The screws that hold the brass lead screw nuts in place (I think there’s only one on the Ender) shouldn’t be really tight. If too tight then the rollers on either side and the z- axis screw will bind. The motion should be controlled by the rails, but enabled by the lead screw.
You have an extrusion issue – clogged nozzle or extruder gear slipping.

Regarding accuracy in the corners, I don’t understand your statement about moving at an angle and the motor pulling less belt.
The X and Y axis are independent of each other. The motion can sometimes look strange when x and y are moving at the same time, but they are not related and there is no change in distance moved for a given number of motor revolutions in either direction, with increasing distance from the bed centre.

My printer is rubbish at large base stuff, it always lifts at the extremes, tall thin stuff it has no problem with. Always assumed it was a cooling thing.

Molgrips – not sure why you are constantly having issues I got an Ender3x (a three with a glass bed) for Christmas so am no expert – but have had far less issues than you. The glass bed meant I needed to push the bed temp up. Then I tried to print some odd stuff and found I needed a raft to stop warping – since then I just print everything with a raft unless I really don’t want one (15 mins to print the raft is quicker than failing and restarting the job). Unless you have something wrong in the mechanics I wonder if you are still using the weird PLA+ filament? I’m itching to move beyond basic PLA but want to be sure I’ve got everything right first.