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Uneven contration in first layers

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I have been using UM3, with Cura as the slicing software, and printing with PLA and Nylon, and everything is going well, minus some small issues due to this being the first time using 3D printing.


However, now (a few months after studying this process and "mastering" PLA printing) I have encountered a new problem which I was not able to fix or find a solution yet.


I am printing some pieces which must have exact dimensions and have plane surfaces in some of their sides, and that's the reason which I'm using to choose the positions of those pieces. However, as you can see in the attached pictures, the print is suffering a kind of "warping" or bigger contraction in the first milimeters of the print. Even more, after that uneven contractions stop, the print starts to enlarge the dimensions in a regular pace, creating a "chanfer" effect, but only on the outside of the piece.


I don't know why this is happening, but could it be because of the "Draft Shield" (experimental setting) which is active in Cura? The difference in the layers changes after 7 mm of height, which is the value of the Draft Shield, but I don't know why the layers are becoming bigger layer by layer until they reach the desired dimenson.


It's also possible to see that the other pieces have a more compact look, which goes until the same layer where the dimensions start to become bigger.








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Bent layer heights on the first 1cm are related to the bed temperature.  Lower it from 60C to 55C and that should be enough to help quite a bit.  Normally you want the bed (actual surface glass temperature which is cooler than the 'bed temp") to be above 52C because PLA gets soft (like clay/plasticine) around 52C and this helps keep the PLA from warping off the bed.  


But in your case you want higher accuracy as a higher priority so you want to lower the bed temp.  You might even want to go down to 40C and put a layer of blue tape on there (wash the tape with isopropyl alcohol before printing on it to remove the waxy layer).


You do not want a draft shield as this will compound the problem.  PLA needs tons of cooling.


It would also of course help to drop the speed farther and print slower.   For extra accuracy I drop down to 25mm/sec (for all printing speeds!!!) and drop the temp by 10C.


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