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Dimensional accuracy problem with holes

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I'm having problems to print separate parts which should fit together. So I created a test print.

I have a cube with the dimensions of 20x20x10mm

I subtract another cube with 10x10x10mm from it so I get a hole of 10x10mm in the middle of the first cube.


When I print this with my Ultimaker 3 all dimensions are perfect except the hole in the middle. It is 0.5mm to small in both dimensions (x/y).


I found the "Horizontal Expansion" parameter in Cura which allows me to correct this but this affects my outer dimensions as well.

I tried various materials (PLA, ABS, InnoFlex Pro1) and got the same results.


Does anyone have an explanation for this behavior? 

Any suggestions how can I avoid this?

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Are you taking your measurement from the top of the cube (as printed) or the bottom of the cube? It would be helpful if you could post some of your settings; layer height, print speed(s), temps, wall line width and no. of walls., outer before inner walls, alternate extra wall; compensate wall overlaps, infill before walls.


I am surprised you have such a large error.

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This is very common.  vertical holes too small.  It's much worse with round holes than square holes - with square holes typically it's only the corners that are a problem and typically the sides are fine.  0.5mm shrunk diameter is typical although I usually see closer to 0.3 shrinkage and I typically add 0.4mm to my vertical holes in CAD as a first estimate.


To answer your question - this is because PLA cools quite quickly (in milliseconds) as you are printing and so it shrinks a bit so it's under tension - PLA sticks to itself when liquid - like snot - like mucus.  This is a very very useful property (try printing with PVA - horrible in comparison) but when you are printing it's like a liquid rubber band and when printing circles it pulls inward or if squares it pulls inward mostly on the corners.


There are other reasons why holes are too small - CAD software doesn't output circles but instead polygons with each point on the circle and that makes the polygon *smaller*.  This is typically only an additional 0.1mm so not as significant (depends on how many sides to your polygons).  Also PLA shrinks by about 0.3% - also not as significant as in your example .3% of 1cm is .03mm.

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Oh by the way - so the problem is worse near the heated bed but the heated bed is so amazingly helpful for other reasons it's worth it.  But if ALL your parts are smaller than 1cm you could lower the heated bed to 45C.  Or even go with blue tape instead (but if you use blue tape you have to wash it first with isopropyl alcohol).  Also if you print cooler the problem isn't as bad.  I've printed PLA as low as 180C but you have to print much slower as it's more like peanut butter than honey at that temperature.  I usually print at 200C on UM3.

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