Printing to fairly precise dimensions -- first order shrinkage estimation


#1

Hi all,

I imagine some people here have tried printing objects to fairly tight tolerances. Have people compiled a reference on how much the various materials tend to shrink, post-wash and cure? At some point I thought I found some technical documentation from Formlabs with these estimates, but I am unable to find them now.

I certainly understand this is a very non-linear phenomenon so the answer will depend on the shape of the object printed, and the resin used. I’d like to try, for example, printing off a lego-style brick for my son. If I had some first-order estimate of shrinkage it would give me a starting point.

Failing that, I will print off a 1cm^3 block and measure shrinkage.


#2

It’s definitely going to vary a bit depending on the geometry of your print. When tolerances are crucial, I usually just do a test print then adjust and iterate until it’s close enough. To achieve truly perfect and consistent results you’d probably need to do some subtractive machining afterward (e.g. undersize your holes then drill them out).

In any case, I would suggest measuring your block and inputting an X/Y and Z fine tuning as appropriate. Here are some blocks I’ve used for calibration in the past if you want them:

I don’t think you need super-tolerance for a Lego brick. I recall people have done them before.

If you do figure out a scaling factor etc. that works, let us know!


#3

I will try to do some more systematic tests in the future. My initial test was perhaps not controlled-enough.

I printed a 1cm cube in default grey resin at 50 micrometers. My mistake was to make the cube oriented to be parallel to the printing plane. On five sides of the cube that resulted in a good print, but the side pointing towards the build platform was not smooth. My guess would be resin was trapped on the top of the cube, and the scattered UV light from the other print layers were enough to cure the trapped resin a little bit.

In the end, the 1cm cube, after wash and cure had dimensions 9.96mm x 9.92mm x 10.36mm.

I will try this print a few more times, with a few other orientations and materials.

With some possibly reasonable assumptions it looks like shrinkage is in the 0.4%-0.8% range in the two build-plane directions. Hard to say from this example what the shrinkage is in the orthogonal direction (i.e. direction the build plane moves in). Some more examples should help.

I will build a little spreadsheet. And put some text on my cube, so that it’s easier to keep track of the build orientation, after the build.


#4

I printed out a few 1cm cubes in clear today. Two runs with slightly different orientations each time. Each time they were alighed slightly off from the build tray coordinates.

Front-Back: 9.85, 9.87mm.
Left-Right: 9.94, 9.91mm.
Top-Bottom: 10.48, 10.48 – these are too large due to excess cured resin on the build platform side.

So in clear the shrinkage appears to be a bit larger than grey, in the 0.9% to 1.5% range.

I’ll see if I can find better shapes to print in that allow better estimation of vertical shrinkage.