Odd bulging problem

I was trying to print a cabinet for 4 drawers, a 10x10x10 cm cube with 1.8 mm outer walls and 0.8 mm inner divider walls. With 0.25 layer height and 1.8 w/t these printed nicely with 4 and 2 tracks. I was printing with PP3DP's ABS at 260C, print bed was at around 105C. Print finished late at night so I just left it there until morning.

However, the next day I found that the inner walls had bulged into random directions like crazy. The outer walls are dimensionally excellent in every way, as are the inner walls at the top and bottom, but the middle parts of the inner walls are totally out there.

So... what happened?




(P.s. I "solved" this particular problem by printing a version with the middle 80% of the dividers removed)

What do you mean by the middle 80% ?

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[www.hydraraptor.blogspot.com]

I mean I printed it like this instead:



That worked nicely and saved some plastic as well.

My guess is that the inner walls cooled quickly, but the outer walls cooled slower as they are thicker. As the outer walls cooled, they shrank a little, both the side and end walls pulling in, more in the centre than on the edges. Probably only 0.5mm, but enough to cause inner walls to bend.