Over curing a part

This seems like something that should have been asked already, but a search turned up nothing.

Can I over cure a part? Is there a point where the resin is fully cured and more UV just doesn’t do anything? Lets say I left some parts in my UV chamber for a week, could that cause deformation?

The reason I am asking is I will be printing a part with a hollow area (of course it will have the appropriate drain holes), and I need to be sure its fully stable before I send it off to the customer. So my plan was to just cure the crap out of it, and hope the UV penetrates a few mm of Black resin.

(I don’t actually intend to cure it for a week… just a hypothetical question)

I use a UV nail curing box, and when leaving parts in for several hours, I have seen several effects:

  • Warping can occur if the part is very thin and doesn’t have support. I have left the support structure attached to the part while to cures to prevent warping, but this makes removing the support structure more difficult.
  • The part will become increasingly brittle the longer it is exposed
  • The clear resin can start to yellow in color
  • I make sure to dry the part well before putting the part in because I have had part blister if it had any liquid on its surface while curing

Charles had a great summary of the effects. I can give a bit more detail on the material property changes that will happen. Post curing continues polymerization process that happens in the machine. Increased polymerization will generally lead to:

  1. A bit more shrinkage. This can lead to warping, especially if the post-curing is uneven.
  2. increase in modulus (AKA stiffness)
  3. increase in strength
  4. decrease in elongation at break (increase in “brittleness”)

Is there a good way to know when is enough?

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In my experience i never post cure sla parts longer than 15mins.

I would just carefully syringe IPA through the part using your drain holes until all resin is out then blow compressed air in hole at a low psi to dry.

If you look at my post yesterday of the small radio i built,it was hollow with 2mm wall and using the method above it was touch dry before curing.

I usually cure it until the surface is no longer tacky to the touch. This also means that if I place two printed parts in contact, they won’t glue themselves together because the outside has been fully cured.

In the reverse of that, is it possible to use resin to “glue” parts together when they are still tacky, and bond them together by post-curing?

I’ve noticed that our parts in clear resin get a yellow tinge to them, regardless of whether or not they have additional curing. Is there any way to prevent that?

Certainly! I have used a little extra resin and a laser pointer to spot glue parts together.

I have not had clear parts yellow unless I over cured them. Even when they did yellow, I could often lightly sand off this outer yellowed layer.

Thanks, I’ll try that!

You can use a UV-resistant clearcoat on the parts - see this post.

And repair. I sometimes break off either small features, or chunks (chunks come off with the support removal). The fix is to dob on small drops of resin and then use the finger nail blue box to cure. Sand down to the original shape and you’re back in business without a total re-print.

If you don’t have a curing box, then I can recommend “Bondic”. Search the web (Amazon) to locate it. It’s a small tube of resin that works perfect with the Formlabs resin and the kit comes with a small LED blue light for curing. It cures in seconds and again, ready to sand to finish.

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