Fine Tuning and Post-Curing Shrinkage

Hi,

When I print a 10mm cube directly on the build platform, does the printer scale the size up slightly to account for post-curing shrinkage? Or should I expect the “green” part to always come out 10mm x 10mm on the platform regardless of which resin is used?

e.g. After some rough measurements I determined my cubes printed in Black v4 resin shrink an average of 0.3% in X and Y after 30 minutes of curing at 60°C.

I’m doing X/Y Fine Tuning and need to know if I should be targeting the green or post-cured dimension (i.e. whether the boxes should be 10mm or 10.03mm when still attached to the build platform).

Thanks!

Formlabs targets dimensional accuracy for resins following the recommended post-cure schedule for each resin.

In other words, when your printer is in tune, and you specify a 10mm cube in CAD, and print that in Black v4 resin, it should measure very very close to 10mm after it is post-cured.

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Thanks!

By “recommended”, I assume you mean the recommended temperature and cure times listed here (and not the full cure times):

https://support.formlabs.com/s/article/Form-Cure-Time-and-Temperature-Settings?language=en_US

So for Black V4, that means 30 minutes, right?

I don’t suppose you can share the X/Y correction factors for the various resins? If I knew exactly what width (X) and length (Y) the laser is supposed to be tracing across the layer, I could dial into the target without any error introduced from post-curing variance.

Even just knowing I’m close at 0.3% for Black would be helpful. Thanks!

Yes, the recommended times, not the full-cure times. But if you prefer to do the full cure on your Black v4 parts and that’s all you print, you could fine-tune it or calibrate it to that instead, or whatever resin/cure schedule you want.

I don’t think I can give you anything Formlabs doesn’t publish on the spec sheet. What I can say is that Formlabs uses the same kinds of methods to calibrate resins - print an object, measure it, do some math, re-size the object, print it again, and so forth. So you should be able to arrive at good correction numbers for your Form 2 by doing exactly what you’ve been doing - printing cubes with a known size, and then measuring them after post-curing to see how close they are to that size.

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In my experience post-cure parts are roughly 3% smaller than expected using standard resins.

That said, it may have changed since PreForm has been updated, and with the new firmware updates. I have not run my tests in about a year.

Moreover, shrinkage will not be uniform. It will be different in the directions of the layers, and the transverse layer direction. And that’s just local. Globally you can get significantly more twisting as one local shrinking can compound the deformation globally.