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how much does pla generally shrink?


SG91
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Posted · how much does pla generally shrink?

Hi,

It depends of the material, probably the brand and maybe the color also. I'm not an expert in this but generally for PLA we talk about 2 to 5% shrinkage.

I think it can also be dependent of the model itself and the constraint that plastic is generating on the overall object..

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    Posted (edited) · how much does pla generally shrink?

    Okey cos I did A tube 15mm outer diameter with 2mm thick wall.

    And 10.75mm inside diameter.

    it just fitted tight on A 10mm axle . so I guess for that dimension it shrinks 0.75 ?

    Its about 7% ?

     

    Hi,

    It depends of the material, probably the brand and maybe the color also. I'm not an expert in this but generally for PLA we talk about 2 to 5% shrinkage.

    I think it can also be dependent of the model itself and the constraint that plastic is generating on the overall object..

     

    Edited by Guest
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    Posted · how much does pla generally shrink?

    In addition to shrinking, you have another effect when printing holes: the hot PLA has a somewhat rubbery consistency. On the inside of a hole, it will slightly contract while still molten. on the outside this isn't possible, because usually there is already an inner perimeter in place.

    Is it tight over the entire length of the tube? The first layers usually get squished onto the glas and therefore the PLA might build what is called an elephants foot.

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    Posted · how much does pla generally shrink?

    Vertical holes are MUCH smaller than intended due to what avogra says - not because of what you might have thought.

    PLA only shrinks about .3% from glass temp to room temp. ABS .6%. That .6% makes it twice as hard to keep ABS from lifting off the bed as the part cools but has little to do with dimensional accuracies.

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    Posted · how much does pla generally shrink?

    I often have to print 3mm holes to take a M3 bolt. I always specify a 1.6mm radius and the bolt is a perfect fit.

    It isn't shrinkage, just the PLA squeezing into the space.

    The elephants foot on the bottom layer adds a bit more but that is only 1 layer thick.

    The overall size the objects I print are certainly within +/- 0.2mm of the specification.

     

    Okey cos I did A tube 15mm outer diameter with 2mm thick wall.

    And 10.75mm inside diameter.

    it just fitted tight on A 10mm axle . so I guess for that dimension it shrinks 0.75 ?

    Its about 7% ?

     

    Hi,

    It depends of the material, probably the brand and maybe the color also. I'm not an expert in this but generally for PLA we talk about 2 to 5% shrinkage.

    I think it can also be dependent of the model itself and the constraint that plastic is generating on the overall object..

     

     

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    Posted (edited) · how much does pla generally shrink?

    Okey thanks guys for replies!

    About the tube, It turned out to be alittle rough inside

    And it did go away by some friction. So now it moves nice and easy back and forth.

    Still kinda tight but could have done it even smaller then.

    What known Pla And Abs does shrink less ?

     

    I often have to print 3mm holes to take a M3 bolt. I always specify a 1.6mm radius and the bolt is a perfect fit.

    It isn't shrinkage, just the PLA squeezing into the space.

    The elephants foot on the bottom layer adds a bit more but that is only 1 layer thick.

    The overall size the objects I print are certainly within +/- 0.2mm of the specification.

     

    Okey cos I did A tube 15mm outer diameter with 2mm thick wall.

    And 10.75mm inside diameter.

    it just fitted tight on A 10mm axle . so I guess for that dimension it shrinks 0.75 ?

    Its about 7% ?

     

    Hi,

    It depends of the material, probably the brand and maybe the color also. I'm not an expert in this but generally for PLA we talk about 2 to 5% shrinkage.

    I think it can also be dependent of the model itself and the constraint that plastic is generating on the overall object..

     

     

     

    Edited by Guest
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    Posted · how much does pla generally shrink?

    For vertical round holes they will ALWAYS be significantly smaller. Think of molten plastic as like stretchy snot. In the first few milliseconds it cools enough such that it is similar to a rubber band as it is being placed. It's this stretchiness caused by the initial cooling (while still liquid) that is the problem. As it rounds the circle forces are pulling inward. Not a problem for walls or other straight lines. Dimensional accuracy from wall to wall is excellent.

    For horizontal round holes you don't get this issue. This is a problem for ALL plastics. Even if you go the most common manufacturing technique - injection molding - there are all kinds of things you have to adjust in your cad model to get it to come out right - even corner angles in a box have to be tweaked in cad so they come out at 90 degrees when they come out of the mold.

    The best solution by far is to always make your vertical holes about .6mm larger (in diameter) than needed and to experiment a bit (print just 4 layers of the part and compare to the part that has to fit in the hole, then adjust the cad model).

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    Posted · how much does pla generally shrink?

    My $0.02:

    Printed holes are smaller mostly because they are NOT circles. The holes are modeled as polygons that have short chords along the perimeter of the desired bore size. It is these chords that make the hole "smaller" more than any shrinkage or elephant foot. higher resolution STLs will help mitigate this somewhat but as was mentioned elsewhere it is better to model the hole slightly oversize to make the opening the right size for a bolt.

    Remember that a pin will make contact on only three points inside a bore. with very small holes it is sometimes better just to make it triangle-shaped if you need it accurately printed.

    Shrinkage DOES have a dimensional effect on linear dimensions that becomes stronger as the footprint gets larger on the bed. For prints under 200mm square I use .6% (as in .006) for ABS and .3% for PETG and PLA. under 100mm square I use .5% and .2% respectively. I make parts that are used in machines and fit together precisely, I normally can achieve dimensional accuracy within .010mm-.020mm without too much problem but that usually requires a test print to evaluate first, especially over 200mm. This is true for both my MassPortal Grand Pharaoh XD and FlashForge Creator Pro

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