Shrinkage should be simple but in practice it's complicated. Please specify which of the 5 or so common issues you care about:
1) part is wrong size dimensions
2) corners lift or part won't stay on bed
3) overhangs lift and get hit by nozzle
4) bridging strings rip/tear/break
5) topfill rips/tears/breaks
6) other?
I looked up the density versus temperature curve for ABS and PLA and they aren't perfectly linear but close enough. I was surprised. I expected a different result below the glass temp than above it. Also they both had almost IDENTICAL graphs! Yet ABS seems so much worse! Well - it isn't. It just has a different glass temperature.
What? you ask?
2) corners lifting, or part coming off glass issue:
This is the most important issue because if your part comes off the bed it's always a complete failure. Some people can tolerate the other issues.
PLA has a very low glass temp - you print around 220C, glass temp around 60C, room temp around 20C. So it is basically a liquid above 80C or at least very soft (like clay). The upper printed layers cool and pull hard on the lower layers lifting the corners. But this effect doesn't start until the upper layers are below 60C or so. Above 60C the material simply thins out a bit and since it is a liquid it just flows around and doesn't pull very hard at all on the lower solid layers. So the cooling/shringing/lifting-corners effect only happens from 60C to 20C. That's only a delta of 40C.
ABS glass temp is closer to 100C. So this happens from 100C to 20C or 80C! Much more of a problem for issue #2 above! Very roughly twice is bad. One fix is a heated chamber (if chamber is at 60C now you have 100C to 40C or only 50C delta - about the same as PLA). But people have learned to instead use hairspray on glass with heated bed which STICKS LIKE HELL and brim and this keeps the corners from lifting by just having it stick so well. So it doesn't adress shrinkage - it addresses getting it to stick so hard you can pick up that little UM robot and swing the entire printer around the room without it coming off.
THIS IS JUST ONE ISSUE PARTLY EXPLAINED. There are many other issues. Also I've printed nylons but I'm not certain what their characteristics are. They definitely have shrinking issues also. They have higher glass temps but seem to shrink less than ABS. Not sure though.
Here's another issue: PLA I would expect to be worse than ABS when it comes to other issues: when printing a vertical circular hole, the PLA cools so fast (faster than ABS because ABS is usually printed with no fan and because the layer below is warmer maybe) that it's like putting down a tight rubber band that pulls that circle inward and makes the hole too small. ABS turns to solid so fast that I suspect it doesn't have this problem.
When printing PLA you want 100% fan (actually you want 500% - yes 5 fans print much better). With ABS you mostly want 0% fan with the exception of certain overhangs and bridging. The old slicers tuned to ABS turn the fan on and off constantly - sometimes 5 times on one layer. Too much fan with ABS and you get bad bonding between layers.
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kris 29
Difficult to do anything against shrinkage since it is a material property that more or less behaves linear with temperature.
One thing you can do however, is to make sure that whatever you print cools uniformly. Hence heated build platform & heated build volume.
For that I use a greenhouse to make sure the part I print stays uniformly at a higher temperature until it is completed.
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