Layer contraction with height

Set up: A1 & AMS-Lite
Filament: Bambu Labs PLA Matte (dark blue, white)
Hot end: 0.2mm stainless steel
Settings: Default, except:

  • Sparse infill density 12%
  • Sparse infill pattern Cross Hatch

I have a finely detailed project which prints a base disc (dark blue) and then builds a design (white) on top of the disc (a logo). The white logo has a ring around it which matches the diameter of the blue disc below. I’ve printed 3 so far and each time the blue disc diameter shrinks slightly at each layer. Then the white ring is built with perfectly straight edges. This results in a slight bump at the color change instead of a perfectly smooth transition.

Here you can see the diameter change in the blue disc, it’s uniform around the entire disc:

Here you can see the sliced model showing they should be perfect aligned:

Since this print takes 5 hours and I have very little experience, I’m hoping for some confirmation / thoughts before I go messing with settings and starting the next one.

Since I’m printing with the 0.2mm nozzle, the 4 wall loops on the edge of the blue disc isn’t much material. In the white, it’s a ring so there are 4 wall loops, then immediately 4 more wall loops for the inside and outside edges of the ring.

I’m thinking the issue is that the disc walls are contracting due to the sparse infill and only 4 wall loops not providing enough “resistance” to that force. Does that seem reasonable? Is it more likely to get a better result by increasing wall loops on the blue disc? Increasing infill density? Changing infill pattern? Something else?

My current thought would be to reset the infill to the default: 15% @ Grid and, in the blue disc, increase the wall loops to 8. Thoughts?

Since you say that the blue disk doesn’t have much material due to the 0.2 nozzle, I would try activating the ‘‘Detect Thin Wall’’ option and changing the infill to Gyroid. Or try changing the outer wall line width (putting it slightly lower than the standard 0.42).

Thanks, I’ll give that a try. I did a print with 8 wall loops and the 15% @ Grid sparse-infill, but the result was exactly the same.

Edit: I’m wondering if the contraction isn’t due to cooling of the material per se, but due to vacuum pull as the enclosed sparse infill cools. Not sure what I can do about that besides increase the infill density though.

Upon further examination and comparison, the 8-wall-loop version was not exactly the same. It was somewhat improved, but not fixed.

I just finished one using 25% @ Gyroid sparse infill with 4 wall loops which was significantly improved, but not perfect. I’m going to try keeping the infill change and increase the wall loops back to 8 and see if their powers combined can do the trick.

Running the slicer with “Detect Thing Wall” didn’t change anything about the slicing, so I left it off.

Well, combining the 25% @ Gyroid sparse-infill with 8 wall loops didn’t seem to make any difference.

Your problem is thermal. When you’re printing the blue, each layer is laid on the last fairly quickly. The plastic is still hot. But when you switch to white, there is a lengthy delay before the next layer is printed. The plastic cools down a lot more than a non-color-switch layer. The plastic contracts as it cools. So when white is finally loaded and the print head goes back to printing, what it’s printing on top of has “shrunk” quite a bit and you get a perceptible step.

This problem also occurs when you print something with an interior “floor”, like a box that’s open at the top. The layer time changes significantly when that interior surface is printed (because it’s a slow “top surface”), and what’s already been printed contracts a bit as it cools. So when the next exterior wall is printed, it’s offset a little relative to the layer below. You get the same kind of “step”.

I call it a “water line”.

You need to reduce how much the plastic can cool while the filament switch and purge is underway. You don’t have an enclosed printer, so doing this will be more of a challenge. But I’d start by increasing the build plate temp as much as possible, while lowering the filament extrusion temp as much as possible, and if that didn’t do it I’d put a box over the printer while it’s printing.

And if none of that worked, I’d break it in to two separate parts, print each in its respective color, and glue 'em together after.

Note: more plastic (like higher infill or more walls) might actually exacerbate the problem. The more plastic there is, the more force is produced as it contracts. Going the other way might work better, no infill, 1 wall, though that might not work for what the print has to be able to do after it’s printed…

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Thanks for the info, I’m kicking off the next one right now using 100C bed temp and 200C nozzle temp. I’ll let you know how it goes.

Edit: I’m also going to reduce the flush volume on the color transition so it goes faster. From the waste tray it appears to be completely transitioned before the halfway point in the flush.

Hmm… I had first-layer adhesion issues so I returned the nozzle temp to 220. Then the print detached from the bed halfway through. So I’m going to return to the default settings and just try the reduced flush volume. I have a deadline on getting these printed and I’m starting to run out of slack in my production schedule.

An unintended experiment. The white filament failed to load and I couldn’t find a way to retract it and cut it without canceling the job. So here we can see that the top layers of blue actually hold the perimeter correctly, so it’s not (entirely) the color transition causing the issue.

I’m trying my next one with a modifier halfway up the blue disc to add a few layers of solid infill to see if that helps straighten out the edge.

Adding a few layers of 100% infill produced a little bump all around, but I’m not convinced it made any meaningful difference in the overall shape beyond that. I abandoned trying to improve this contraction since the prints were succeeding and I needed to get them all done. Perhaps later I’ll experiment some more.