"Hourglass" warping on ABS prints

[Andrew Warkentin]

I have got my printer working reasonably well (it is an MG Prusa, and I
am using Slic3r), but ABS prints are still warping a bit. The bottom
layer sticks to the print bed reasonably well (there is almost no
warping off the bed at the corners), but the layers after the first few
tend to shrink, giving pieces like the 20x20x10 calibration block a bit
of an hourglass shape. I am not quite sure what the problem is. The
heated bed temperature tops out between 100-110, so I don't think that
it is too hot. Suggestions on how to reduce this warping would be
greatly appreciated.

I have attached my Slic3r configuration.

[Triffid Hunter]

On Sat, Aug 11, 2012 at 11:47 AM, Andrew Warkentin <andre...@gmail.com> wrote:
> the layers after the first few tend to shrink,
> giving pieces like the 20x20x10 calibration block a bit of an hourglass
> shape.

this is a symptom of bed being too hot. You must drop it below Tg
(105c for ABS) after the first layer is complete to avoid this.

When the bed remains above Tg, then the first layers never harden
properly. The shrinking forces from the thermal contraction are thus
able to pull the still soft perimeters inwards.

If the plastic is allowed to drop below Tg, then it can harden fully
and thermal contraction forces remain locked in the lattice, unable to
cause physical deformation.


You will get exactly the same deformation with PLA if you leave your
bed above 60c.

[Andrew Warkentin]

Dropping the bed temperature to 80c after the first layer fixed that
problem.

Another problem that I have been having is that vertical holes come out
too small. Horizontal holes don't seem to be affected. Rectangular holes
aren't affected that much, but anything with more sides comes out
significantly smaller (e.g. on
https://github.com/prusajr/PrusaMendel/blob/master/metric-prusa-lm8uu/bar-clamp.stl,
the horizontal holes are around 8.5mm diameter, but the vertical hole is
7.5mm). This happens with both ABS and PLA.

[Nolan Poe]

This could be a lot of things, but I suggest looking at nophead's post here:  http://hydraraptor.blogspot.com/2011/02/polyholes.html 

This could be the problem, though I really don't know.