PETG First Layer issues (burn marks)
When printing PETG I get black marks in the top left corner. It appears that it is always the top left corner.
I tried printing a full print sheet of 1 layer PLA to test. And that worked out fine.
I found out that PETG needs a slightly higher Z offset than pretty much every other material. I tried a z offset of 0.1mm in prusa slicer (which was not enough saw the same behavior). Also this is only printer / nozzle setup. I did not find a way to set this in the filament profile. I am not sure if this is the correct way to solve this issue. Especially, because it only seems to be happening in the top left corner and only with PETG. Maybe if I raise the offset enough to fix this it will be too high everywhere else. Could this be an assembly issue?
Best Answer by miroslav.h4:
This is actually caused by the contact of the cable cover on the pad with the extruder air duct when probing the pad. The evidence is the shiny trace on the left next to the layer on the pad in the second picture, which is caused by the nozzle moving over the next measuring point of the pad during measurement because the pushed-off pad rises and touches the nozzle. All you need to do is replace the screw that holds the terminal cover on the pad (the middle one) with a screw with a smaller head. I also encountered this, but I solved it by redesigning the terminal cover on the board and the extruder air duct to get more clearance between the parts, just to be safe.
RE: PETG First Layer issues (burn marks)
This doesn't seem like a burn mark to me. Could you zip the .3mf with the model and upload it here?
RE: PETG First Layer issues (burn marks)
Could it be just an area where the first layer becomes very thin for some reason? I.e. the print head gets too close to the bed? There is also what looks like a vertical scratch mark just to the left of this area.
Any chance there is a bit of debris under the print sheet, or a dent in the sheet? Or it could be that the Z probing has gone wrong in that area: It is known that in some Core Ones, the back of the print head (the fan shroud if I recall correctly) can just touch the cover for the heatbed cable when probing the bed height in the back left area. That may result in probing failing entirely (if the nozzle never touches the bed), or giving too low heights (if the bed gets pushed down a bit by the collision with the cable cover).
If a collision between print head and cable cover occurs in your printer: The plastic cable cover itself should just fit; it's the hardware underneath which may be pushing it upwards. Unscrew the cover; ensure that the cables underneath are not bent upwards; maybe also replace the screws below the cover with types with a lower-profile head.
RE: PETG First Layer issues (burn marks)
In my opinion, you have very irregular filament flow in this area, as if the filament isn't holding on to the bed and is being dragged down.
If you have a different bed plate, I would try this one.I print a lot with PETG on a satin bed without any problems, but I regularly clean it with dish soap.
I also recommend this instead of, for example, isopropyl alcohol.
RE: PETG First Layer issues (burn marks)
Posted by: @avarice
This doesn't seem like a burn mark to me. Could you zip the .3mf with the model and upload it here?
It´s a gridfinity baseplate one of the only things I ever printed in PETG that took this much space. There is a black spot on the bed and no extrusion. I stopped the print before it was complete.
Could it be just an area where the first layer becomes very thin for some reason? I.e. the print head gets too close to the bed?
I think so thats why I tried raising z offset, but 0.1mm was not enough and that raises z everywhere.
Any chance there is a bit of debris under the print sheet, or a dent in the sheet?
So, it´s definitely not the print sheet being bent or something as I have pretty much equivalent marks on the other side of the sheet when I printed with it flipped over.
I already tried to print the same model once and it did not work back then too with very similar results. I then did try to tighten the nozzle (as I had changed it). Move the bed all the way to the bottom turn off the motors and turn the leadscrews to make sure it´s really at the bottom. Do all the calibration steps again. Tighten screws on the bed and the once that connect the bed to the rest. I also moved the model a bit to the right and then it printed fine. I then tried to print that sheet of PLA and that worked just fine. So back then I though something I did must have fixed it. But now I saw the same issue again printing the same part.
It is known that in some Core Ones, the back of the print head (the fan shroud if I recall correctly) can just touch the cover for the heatbed cable when probing the bed height in the back left area.
I´ve read that too that they recommend a different head screw now. I looked out for that in the probing sequence but did not see it hitting the cover. I can try to replace the screws. I don´t have a lot of fate in that fixing it though. I also think if that where the issue I should see similar behavior with the full sheet of PLA. Or at least some change in the top left. I do use different print sheets for PLA and PETG though. I have used the smooth pei for PLA and satin for PETG.
RE:
It is known that in some Core Ones, the back of the print head (the fan shroud if I recall correctly) can just touch the cover for the heatbed cable when probing the bed height in the back left area.
I swapped out the screw for one of the black round head once. Did not really do anything. I don´t think it was a problem before. If its hitting Id expect it to fail probing or print higher up. But as far as I can tell what I'm seeing is it printing too low in that area.
I now created a little 0.2mm height square in prusa slicer to test with printing that square in the top left corner I get the following result.
This is printed with a 0.5mm z offset. For most of it the z offset appears far too large, but there are spots where it´s ok and there it normally would not print anything at (assuming that i always cancelled the prints when i saw it fail as i don´t want it to destroy the build plate). There is still that one line at the edge where it did not print anything. But maybe that part of the build plate is already too damaged by now from testing this. Either that or 0.5mm is still not enough for that spot.
RE: PETG First Layer issues (burn marks)
I also think if that where the issue I should see similar behavior with the full sheet of PLA. Or at least some change in the top left. I do use different print sheets for PLA and PETG though. I have used the smooth pei for PLA and satin for PETG.
But wouldn't that be perfectly consistent with the "lightly hits the cable cover during probing" hypothesis? Those two sheets have different thickness. I can't get to mine to measure them right now, but community-provided measurements I found are 1.2 mm for the smooth PEI sheet vs. 0.65 mm for the satin sheet. So the print head needs to get half a mm closer to the bed when you print PETG, which might just be the critical bit of difference that lets it touch the cable cover.
RE: PETG First Layer issues (burn marks)
I also think if that where the issue I should see similar behavior with the full sheet of PLA. Or at least some change in the top left. I do use different print sheets for PLA and PETG though. I have used the smooth pei for PLA and satin for PETG.
So the print head needs to get half a mm closer to the bed when you print PETG, which might just be the critical bit of difference that lets it touch the cable cover.
That might be the case. I remember having a similar issue in the back left corner (0,250) that was cause by the print head colliding slightly with the cable cover. That was fixed by replacing the button head screw with a flat head screw.
RE:
I also think if that where the issue I should see similar behavior with the full sheet of PLA. Or at least some change in the top left. I do use different print sheets for PLA and PETG though. I have used the smooth pei for PLA and satin for PETG.
But wouldn't that be perfectly consistent with the "lightly hits the cable cover during probing" hypothesis? Those two sheets have different thickness. I can't get to mine to measure them right now, but community-provided measurements I found are 1.2 mm for the smooth PEI sheet vs. 0.65 mm for the satin sheet. So the print head needs to get half a mm closer to the bed when you print PETG, which might just be the critical bit of difference that lets it touch the cable cover.
I did replace the screw on the cable cover. I also set a 0.5mm z offset in software and you still see some spots where its too close. So removing that Ill be back to square one with it scratching / burning the bed. I just wanted to be able to test without destroying anything. The goal is for it to look too far everywhere then its gonna be fine when I remove the offset again. So it is not actually too far to start with, but too close. I think the bed thickness should be accounted for on the core one as it measures where the bed is when the print starts. Earlier prusas had a per print sheet offset. That setting appears to be gone.
RE: PETG First Layer issues (burn marks)
This is actually caused by the contact of the cable cover on the pad with the extruder air duct when probing the pad. The evidence is the shiny trace on the left next to the layer on the pad in the second picture, which is caused by the nozzle moving over the next measuring point of the pad during measurement because the pushed-off pad rises and touches the nozzle. All you need to do is replace the screw that holds the terminal cover on the pad (the middle one) with a screw with a smaller head. I also encountered this, but I solved it by redesigning the terminal cover on the board and the extruder air duct to get more clearance between the parts, just to be safe.
RE: PETG First Layer issues (burn marks)
Replacing the middle screw really fixed it! I did not think that this would help but I also assumed that in probing as long as it does not get stopped by the cover outright it would be fine and I did not really see that happening. It must have "powered threw" the resistance when probing and then it thought it needs to go lower because of it.
I also tried with both the smooth PEI sheet and the satin sheet on top before. That works out fine too (more of a science experiment to check if print sheets can be too thick). The smooth PEI sheet is thicker than the satin sheet.
I solved it by redesigning the terminal cover on the board and the extruder air duct to get more clearance between the parts, just to be safe.
Do you mind sharing that design?
RE: PETG First Layer issues (burn marks)
I did replace the screw on the cable cover. I also set a 0.5mm z offset in software and you still see some spots where its too close. So removing that Ill be back to square one with it scratching / burning the bed.
Posted by: @maximal
Replacing the middle screw really fixed it!
I am glad your Core One's issue is fixed, but am a bit confused now. Why do you argue, in response to my suggestion, that it didn't help and you are back to square one -- and then confirm for Miroslav, who suggested the same thing a day later, that this fixed the problem and he gave the "best answer"?
RE: PETG First Layer issues (burn marks)
I am glad your Core One's issue is fixed, but am a bit confused now. Why do you argue, in response to my suggestion, that it didn't help and you are back to square one -- and then confirm for Miroslav, who suggested the same thing a day later, that this fixed the problem and he gave the "best answer"?
I appreciate your help too! Maybe I just did not get it then. I did replace two screws then and that did not fix it (the other two screws not the center screw).
But like I answered to Miroslav I really did not believe that this would do anything to fix this at the time because I did not think that it hit the cover as it finished probing just fine and did touch the bed.