Noticed something weird when I'm printing rounded objects (like a sphere).

Noticed something weird when I’m printing rounded objects (like a sphere). It seems like the outside (the “shell”) gets ahead of the z-axis, eventually it gets higher than the nozzle and the nozzle pushes the object off the build platform.

I’ve only seen this in a model my daughter designed in Tinkercad so far but then again most of the other stuff I’ve printed has flat walls (nothing curving in and out horizontally).

Should I be slicing things like this differently (no shell, etc.) or is this more likely an alignment, step size, etc. problem?

Could it be the hot plastic shrinking, causing the edge to curve up slightly on each layer, sucsessive layers adding to the destortion?

This is normal thermal warping.
Try to lower the temperature and cool the freshly extruded plastic to lower the temperature difference and thus the expansion.
Vertical slots are one possible method of making difficult models less prone to fail due to edges warping up.
A heated chamber is the golden solution but difficult to do in hobby printers.

I’m assuming you’re printing ABS? ABS (and PLA too, but to a much lesser extent) has two effects that result from the plastic cooling and thus contracting:
A) “Warp”: That’s why prints come unstuck from the platform and end up looking like a banana.
B) “Curl”: Probably what you’re seeing. It’s usually most apparent on convex parts of an overhang, where the outmost edge would try to contract, going in- and upwards. This is usually caused by the layer below it not giving enough resistance to the contraction. There might be one of the following things wrong with your setup:

  • Nozzle temperature too high
  • Bed temperature too high
  • “Cool” feature needs to be set more aggressively (especially “minimum layer time”)
  • If you’re printing PLA, your fan might not be doing its job well enough. If you’re on PLA and don’t have a fan yet, get one.

this may interest you good sir

Ah sorry I should have shared more details:

Prusa Mendel using 1.75mm PLA on non-heated blue tape build platform.

What @Thomas_Sanladerer describes as “Curl” sounds like what I’m seeing. I’ll try and grab a pic but essentially as the outward (overhang) curve is constructed it grows at the outside of the circumference and starts to interfere with the nozzle. I do have a fan and I’m using the basic "cooling’ settings in Slic3r but I’ll try dropping the nozzle temp (at least after the first layer or so) and then see if more aggressive cooling settings make a difference.


Are you having problems early on in the print, or later on?

For stuff later on, using Z-Lift in slicer will allow you to go over the curl and reduce the likelyhood of knocking the part off the bed.

I have a cheap pair of tweezers I use for cleaning up the nozzle. I use the long, flat side of it as a heat sink and flattener. A few seconds of holding the curl down will draw enough heat out of the plastic that it will set. Gotta futz with it every couple of layers. I let the printer still run while doing this. Nimble fingers.

@Mike_Creuzer when you say Z-Lift is that the same thing as “Lift Z” in Slic3r, or is that something else (there seems to be a few simular things)?

Same thing I believe. Yeah. Printer Settings, Extruder 1.

For some reason, I am currently having troubles with the first layer and Zlift. The Gcode looks right, my printer just wants to go up and up and up. shrugs I slice it twice and splice the files together after the first layer.

Hopefully you won’t see the same issue.

Z lift will probably help with prints coming off your platform, but should only be a temporary solution until you fix the underlying problem. half way down, discusses ‘lift’.

What does the Z lift setting do?
Lift the extruder while not extruding?
That would not help.

Z lift picks up the extruder nozzle for non-print moves.

It can pick the tip of the nozzle up above the top of the curl so it won’t knock the print loose. As @Thomas_Sanladerer pointed out, it’s a band-aid until the curl is fixed.

I only have curl problems on shallow, unsupported overhangs at this point. I still run Z-lift as default as I forget to turn it on for skinny towers, which is the other thing I have problems knocking over.

Problem is, the nozzles moves right through the warping while extruding more often then it does non extruding moves.

I did some fiddling with the temperature when I switched filaments (from the stuff that came from Makergear to the Faberdashery samples) and it looks like, at least with the Fab stuff I could run 10-15 degrees cooler than I was before without problems so I’m going to give this another go with the lower temp and see what happens.