Vertical "spikes" on outer corners of prints

This one has me more than a little bit baffled.

Basically, if I print something on my Mini that has any “sharp” outer corners in the XY plane, with near-vertical walls, these corners “spike” upwards, higher than the nozzle.. When I first encountered it, I thought that I was seeing simple curling of the print… but the bottom layer of the print is 100% adhered to the bed, no detachment whatsoever. The print simply, somehow, becomes several layers thicker at these corners.

The test case that shows this best is when I try to print a simple, flat, spur gear. The tips of the gear teeth will all show this extra thickness, but the inner corners (between the teeth) don’t. The effect seems to accumulate over Z height – the further the print gets, the worse the corners spike, until the nozzle is literally smashing its way through these corners. However, curves, flats, and infill all look fine – it’s only sharp outer corners that have this problem. While gears show it the worst, I also get it on simple calibration cubes.

I’ve tried testing this with multiple models, and by printing the same model under different slicers (Cura and S3D), and get the same result (which is why I’m posting this in the hardware forum). I’ve also tried changing speeds, temperatures, and flow rate %s, without seeing any apparent affects on these spikes, even if I alter the settings so much that other parts of the print begin to fail. It happens with both PLA and HIPS – changing material doesn’t seem to have any real affect on the problem.

OTOH, if I print something circular or domed, or something square-ish with a more gradual slope (pyramid, say), I don’t see anything that resembles this issue, and print quality looks good.

I’ve gone searching the forums, and various online resources (like the S3D print-quality guide), but I haven’t found anything that looks like what I’m seeing. Analysis of the G-Code doesn’t show any up-and-down Z motions in the corners that might explain this.

I’m honestly confused as to how certain parts of the print could get higher than the current nozzle height, if the nozzle isn’t “climbing” at those corners. The only idea I’ve had so far is that I’m encountering some sort of over-extrusion just at the corners, possibly caused by the Extruder speed not scaling down to match the XY speed when the head has to decel to make those sharp corners. But in that case, I would rather expect the over-extrusion to push out sideways, rather than piling up vertically behind the nozzle.

I could swear I’ve seen a description of this problem along with a solution, but I can’t find it now, so this is what I remember.

With PLA, more cooling seems to help. A small desk fan blowing across the part after the first layer or two. If the part is small, print two of them at the same time to give the previous layer more time to cool.

Changing the print speed may help (I seem to remember that there was some disagreement about whether to go faster or slower).

I was just recently introduced to KISSlicer ( and it does a remarkable job out-of-the-box. It’s algorithms seem better than either CuraLE or Simplify3D resulting in less stringing and blobbing maybe this corner curl as well.