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First 2 layers wildly different

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Posted · First 2 layers wildly different

Hi all,


First time posting here. Many thanks in advance to all who see this post.


I'm having issues with the first and second layer of my prints. We are using a one-off custom pellet printer and while I've been able to import most of the settings I previously had from S3D (yes I know, it sucked), I haven't quite been able to get the first layer(s) properly set. As we are using a (roughly) 3mm nozzle, I've had to bump up the flow rate to 125% which is what we previously used as well on the other (they-who-must-not-be-named) slicer software.


It looks like I've got too much squish on the first layer but then not enough on the second as I see gaps between the lines. However, the wavy surface would also indicate the nozzle is too close...which is a contradiction (or so I think).


Is there anything that jumps out at anyone or am I just overdoing the flow rate and need to tweak the z height. Other layers when printing in vase mode don't seem to have too much of a problem but I also have an issue with perimeters/shells to quite touching each other. I think I need to figure out the first layer issues before moving on with that as I can't tweak too many settings at once.


Have included the .3mf file too, it's just a simple round cylinder that I drew in F360.


Just in case the images don't show up properly here:








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    Posted · First 2 layers wildly different

    I am just guessing here, so...


    The wavy aspect looks like the previous lines shining through. Thus: smooth previous lines will show through less than lines with big gaps.


    Have you checked if layer-height, speed, temp and cooling fans are the same everywhere, on all layers? Often they differ for the first layer(s). If they are all the same, I would guess that the bed is too close. At least if it would be a standard 3D-printer; hard to say about a custom one. I don't know if effects of pellets not molten enough might play?


    Long ago we had a plastic company in our neighbourhood: they extruded pellets into lollypop straws. Each time they started up the machine, after warming up, they had to extrude and waste some material, before the flow was steady enough for production. I could imagine that you have similar startup effects at the beginning of each print: different temperature and viscosity of the melt? If you can, try manually purging some material, and then immediately start the print? It could help diagnose things?


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    Posted · First 2 layers wildly different

    As Geert mentions, your extrusions aren't welding to the adjacent stripes allowing a view of the layer below and unless it's a "look" you were going for, you are still under-extruding but what looks like a lot.

    If the higher layers are coming out good I'd kick the flow up further on the early layers.  Go to 150% and see what it looks like.  You could manually step the flow back down using M221 S??? at the start of the first three or four layers until you got back down to 100%.  Custom machines often need custom touches.

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    Posted · First 2 layers wildly different

    Dear @geert_2 and @GregValiant,


    Many thanks for the replies.


    We do purge material before every print and I make sure that we run a 30cm prime line in addition to a skirt right before printing. I ran through a few tests on the first layer @125% and 150% with differing z heights, seems like 0.1 or 0 is the best for a 125% flow rate. 


    Although I'm able to get the first layer set where I want it (without ridges caused by the nozzle being too close), am I assuming that the second layer (with gaps) is still considered the initial (bottom) layers or is that setting overridden by  layer height?


    I ask because I see a barely imperceptible difference in the second bottom layer compared to the first one.














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    Posted · First 2 layers wildly different

    On a standard UM2, the first layer is printed at 20mm/s, while the standard speed for the rest is 50mm/s. Speed gradually increases (at least it seems so) from this 20mm/s to 50mm/s during the first couple of layers. But at 20mm/s the material is sitting 2.5x longer in the nozzle than at 50mm/s, so it is much more liquid. And the pressure on the feeder wheel is less, making that feeder slip less onto the filament. The square pattern on the feeder wheel translates into a *diamond* pattern in the filament due to this "partial slipping". The square holes are being stretched out, which gives a lower flow than set if there would be no partial slipping. So, at lower printing speeds = more time to melt = less pressure = less slipping = more material fed = higher flowrate = better filled layer. That, combined with a nozzle that is often set a little bit too close to the bed (for a better adhesion and smoother bottom), gives more dense first layers. And less dense filled upper layers.


    If the first layer is overextruded, due to this lower speed, or being too close to the bed, it contains too much material. That overfilled-effect shows in the next layers, they get a bit overfilled too.


    These things could cause the first layers to be different from the body of the model. At least as far as I have seen.


    I don't know how all this translates to pellet printers, which I guess work with a sort of conical worm screw? But I have no idea how to calibrate such a thing. I think you should first get the *higher* layers good, thus the body of the model, until these higher layers get a smooth and closed surface, without underextrusion. And then go to adjust the settings for the first couple of layers. That is how I would approach it, probably.


    Anyway, consider all this as "educated guessing", at best, not as truths. It's hard to give good advice on something you don't know...  :-)


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