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Q.   We printed some Lego compatible blocks. When simply using the default Lego dimensions it does seem to work, but the block seems a little smaller and the fit is not optimal. The difference is small though, so were wondering whether this is due to shrink or other inaccuracies.

Most prints tend to end up slightly smaller than intended, but despite attempting to make sense of it with calibration objects designed for that purpose it seems hard to get a common denominator or simple factor.

have had warping shrink too,  but those are two somewhat different matters

shrinkage of PLA from glass temp to room temp. It's either 2.5 to 3% or .25 to .3%. So maybe if you scaled everything in Cura (XY only - don't scale Z) that was to be printed in PLA by .3% (and double that for ABS) then that might fix some of the dimension issues. I don't know.

It's more complicated as the lower layers are already cooled to room temp when you put the upper layers down and the upper layers are placed in the proper spot such that the lower layers hold them in place. Mostly. At least until they cool to glass temp. Once at glass temp both layers are "solid" but now the upper layer is still shrinking. No wonder you can see all the thin lines/layers in a print even at .1mm.

What most people do is print everything twice. After printing it the first time measure all the dimensions and if a side is off by say .3mm then change the dimension of that side by .3mm to compensate.
Also, at least for PLA, consider heating the bed to the "soft" temp (usually close to 65°C) for the first layer, and then dropping it to the "firmer" temp (maybe 55°C) after the first or second layer, so that the first layer stays stuck to the bed, but the second layer doesn't squeeze the first layer as much or shrink as much relative to it, and creates a more solid base to resist shrinkage for additional layers on top. It sort of spreads the strain of shrinkage across several layers.

we 've never seen any need for a brim to prevent corners lifting on 2" to 6" square objects with this approach.

It often depends on the shape of your object. Sharp corners tend to shrink and pull off the bed more (as in, it shrinks not just inwards slightly, but upwards) You can counter this by making the corners rounded, or adding a brim which helps it stick on the bed a bit better.

Large solid blocks of objects tend to shrink a bit more. Cutting shapes out of your object help the shrinkage, for larger objects I like to cut pyramids into the base of the object (so long as that surface is not seen) to even out the stresses.

But more often than not, PLA has hardly noticeable shrinkage unless you're printing big things.

we don't really need to consider shrinkage in the design unless the parts have to glue together.

PLA and ABS have very different issues because of glass temp and shrinkage.

First it's useful to know that both materials are printed at roughly the same temperature (say 200-260°C).

Then know that if you graph the density versus temperature both materials have mainly linear graphs. In other words they expand and shrink at about the same rate even passing through their phase transitions.

Also the slope of these curves is about the same for both materials. What you might think? But ABS shrinks more!

Not really - the big difference between the 2 materials is their glass temp. The glass temp for PLA is around 50-60 °C and ABS around 100°C. This means that when cooling from glass temp to room temp, ABS shrinks twice as much.

This means that the two materials have many differences caused by this single thing.

PLA Shrinkage rate is +/- 3%
ABS Shrinkage rate is +/- 8%
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