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PLA shrink factor

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PLA shrinks a little less than ABS, but as is visible when corners on a print lift, it does shrink. Is this something you can take into account and, if so, how do you do that? Is it a matter of simply scaling your model or is it something a little more complicated - and what factor should that be?

I have been searching around for technical data with some more or less exact numbers, but so far I have not been able to turn anything definite up. I understand different blends and brands have different numbers, but there should probably be at least a range.

I would love to hear your input :)

 

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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 pyrimids 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 nardly noticable shrinkage unless you're printing big things. I just printed a 6" cubed object and the corners pulled up maybe half a mm. But it still printed fine.

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

 

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The reason I am asking is that I 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 I was 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. I am sure I am not the first to meddle with this though.

I have had my share of warping shrink too, but I am slowly but surely getting a grip on that. In my mind those are two somewhat different matters :)

 

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The physics involved is amazingly complicated and beyond me even though I have thought on this subject hundreds of times. I only know solutions and causes of certain specific special cases in 3d printing.

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 200-260C).

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 50-60C and ABS around 100C. 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.

ABS I would expect should deal with overhangs better and diameters of verticle holes (cylindrical holes) better because there is less shrinkage from print to glass temp than with PLA. So the rubber band effect that PLA has I would expect to be lessened by ABS. This means I would expect overhangs also to print much better but I haven't really compared them.

But shrinkage *after* glass temp is achieved is much less (about half) with PLA so the entire part shrinks left and corner-lifting-forces aren't as strong.

 

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I forget the shrinkage of PLA from glass temp to room temp. It's either 2.5 to 3% or .25 to .3%. I guess it must be the latter (.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.

 

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I forget the shrinkage of PLA from glass temp to room temp. It's either 2.5 to 3% or .25 to .3%. I guess it must be the latter (.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.

 

Thanks for the info. I will try these numbers and report back.

 

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.

Wouldn't you also have to do that by percentage? If you add 3 mm to something that is obviously not calibrated correctly you still do not know what you end up with, although the error should reduce every time you do this. Up to a certain point where other inaccuracies take the overhand.

 

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Also, at least for PLA, consider heating the bed to the "soft" temp (usually close to 65C) for the first layer, and then dropping it to the "firmer" temp (maybe 55C) 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.

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

 

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ColorFabb's XT-CF20 doesn't shrink at all... might even expand a little if my latest print is anything to go by... thanks for the info here... everything makes a bit more sense now...

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to come back to lego: i heard that those bricks have very tight tolerances, in the range of 1/100 mm. so maybe the print isn't too bad at all but requirements for a good fit are even higher.

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