Kink in spiral vase mode

I'm using spiral vase mode pretty much all the time and generally I'm very happy with it. However, I commonly see a shadowy seam that spoils the finished print. It appears to be caused by a horizontal kink in the path which varies a little from layer to layer. The kinks line-up with the point where the print transitions to spiral vase mode after printing the base. You can see the effect in the screen shots from Repetier. The first print looks very similar to what I see in the real-world print. The other shot shows the first spiral vase layer coming in from the left and kinking outwards above the point where the base joins the periphery.

Shots are from Slic3r version 1.1.7. I tried 1.2.1 and it's exactly the same. I've checked the .stl mesh and it's perfect.

Any ideas what is causing this and if it can be fixed?

jack



Edited 2 time(s). Last edit at 12/23/2014 07:01AM by -jack.

OK, I answered my own question. Slic3r only spirals the z axis. So instead of a true spiral lying on the theoretical surface of the model, we have a succession of tilted slices. If the wall is not vertical then as the path moves around one of these slices it gradually departs from the theoretical surface and does a horizontal kink at the end of the lap to get back where it should be. You can see the effect quite easily by slicing a sphere and zooming in on the top. As the slope of the wall increases, so the kinks get bigger.

I guess Slic3r does this to save computing time and I guess I'll just have to live with it.



Edited 2 time(s). Last edit at 12/24/2014 10:07AM by -jack.

Decreasing the layer height should make the kinks smaller and therefore less noticeable.

Dave

Thanks for the reply Dave – and on Christmas day too.

Good point about layer height. We're printing with clay so we use more layer height than for plastic. I can try to squeeze the layer height down a bit but it's tricky with clay.

I found that the position of the shadow seam is set by the fill angle so at least I now have some control over where it appears in the finished pot.

jack