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Heatbed distortion due to thermal expansion  

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jkoether
(@jkoether)
Active Member
Heatbed distortion due to thermal expansion

I've had my mk3 a few years now and have octoprint with the mesh bed visualizer installed.  I've always noticed that the bed seems a little out of flat, but never liked the nylock or spring mod since it would reduce the rigidity of bed during movement.  I did get some .002" washers to make some gross corrections when I had to replace X-axis rods and bearings, but honestly I wasn't too worried about it since it really doesn't seem to affect the print quality.

When heated, my bed was about .3 mm low around the outside edges as compared to the center.  As an engineer who works with structures that undergo thermal transients, this makes a lot of sense to me.  If you you have two plates fixed together and you only heat one, the structure will bend due to the one plate expanding more than the other.  So as an experiment I took an updated mesh bed measurement showing that the perimeter was about about .25 - .35 mm lower than the middle.  I then loosened all screws except the middle with the bed heated the relieve the the thermal stress, and then re-tightened them.  And just like that, the bed is now level to within .15 mm!!

To confirm this, I ran through a hand calc and confirmed this is exactly what you would expect.  I used a CTE of 16 for the PC board and 23 for Aluminum.  With these inputs I calculated that heating the bed to 60C results in the bed curving so that the edges are .25mm lower than the center, assuming that the aluminum only reaches 30C (wild-ass guess). This would of course be worse at PETG temperatures.

Has anyone else tried tightening down their heatbed while hot to reduce the thermal expansion related distortion?  I was surprised how easy and effective it was.

Posted : 12/04/2021 1:39 am
Baklin
(@baklin)
Reputable Member
RE: Heatbed distortion due to thermal expansion

I use the nylock mod but with silicone tubes and do the first calibration with the bed at 85 degrees. 

 

The difference between hot and cold is big. So doing it hot gives a better and repeatable result.

Posted : 12/04/2021 10:47 am
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