Is it possible to "over cure" standard resins or not?


I’ve seen competing (and conflicting) advice from Formlabs in these forums and elsewhere, so I was hoping someone could provide a statement as to whether or not it’s possible to “over cure” parts printed in the standard resins?

Here’s what I’ve read from official Formlabs sources at various times:

  1. Parts printed in standard resin will not become more brittle over time (as they sit out in the open, exposed to sunlight and other random UV)
  2. The whitepaper on UV curing from FL seems to indicate that using a cure box with a UV intensity over a specific value will cause the parts to cure less well than parts cured at an “ideal” intensity level
  3. Parts that are left in a UV cure box for “too long” can become brittle
  4. Over-curing is better than under-curing, so don’t worry about post-curing parts for extra time

So: do parts printed in standard resin become more brittle if they are over-cured, and if so, is “over curing” in a UV box equivalent to continued exposure to sunlight post-cure, and if so, does that mean parts become increasingly brittle over time?

(Also, out of curiosity, as far as I can tell, while the page here shows a curing temperature of 45 or 60 degrees C as ideal, none of DIY cure boxes linked include any kind of heater. How important is heat to a proper cure?)


Hopefully there hasn’t been too much conflicting advice :wink:

There are two things going on with post-curing and UV exposure generally. The first is a completion of the photo-polymerization reactions. Parts are not fully reacted and cured out of the printer so post-curing completes any unfinished reactions. This is much of the reason for the changes in aesthetic and mechanical properties.

The second is material degradation due to UV irradiation. Most everything organic is damaged by UV exposure. With extreme levels of UV exposure, the part will degrade and this is why ‘over-curing’ will cause brittleness. Placing a part outside is generally not nearly enough to cause any notable amount of degradation. Our materials are especially resilient to UV exposure as compared to many common engineering plastics and won’t significantly degrade outside (we’ve accidentally tested this). Curing a part over night in a high intensity UV chamber on the other hand might be enough to damage the material. This is not unique to resin and would occur in most organic materials.


Thanks for the quick reply.

One follow up: how critical is maintaining the correct heat during the UV cure? If I cure for longer at a lower heat, do I get the same results?

It seemed from the whitepaper that the intensity of the UV light had an impact on the cure independent of the cure time (that is, too little UV and it didn’t matter how long you left it in the box; too much UV and the cure would be compromised even if done for less time).

My overall impression is that, although the UV cure is critical to part performance, there isn’t much guidance on a proper UV cure box. No doubt like most FL customers, I have a box with some UV lights (that incidentally generate some heat), and printed parts are definitely stronger after an hour cure than before. But I have no idea how close I am to the ideal.


This is something we’re continuing to investigate as well.

Temperature does pretty significantly impact final mechanical properties. We have a whitepaper on the topic that you can find here. You’ll notice that higher temperatures do allow overall greater mechanical properties and this cannot be compensated for with increased exposure.

My understanding is that temperature is excitatory to un-initiated photo-polymers. This lowers the required UV energy to initiate the photo-polymerization reaction. This is especially important for dense parts where UV energy has a more challenging time penetrating through. Increasing temperatures and lowering the barrier to photo-polymerization allows more dense geometries to be fully cured.


I have to chime in - we have been curing primitively for a while (405nm leds, other arc lamps etc - are in process of getting a proper, warmed cure box). The parts that were “primitively” cured get brittle over the course of 2-6 weeks lying in a typical lab environment. I have seen this for Tough, Black, and especially Grey. Thin-walled boxes (3" x 3" x 3" with 1-2mm wall thickness) will have some give when pushed on for a week or two, and at around the month mark, they shatter.I hope that going to heated cure will help this a bit, but ultimately, I think I’ll have to paint important parts.


Yes, I agree. Formlabs seems to have experiments that show no change in brittleness, but my personal experience is that parts become increasingly more brittle over time, after curing, to the point where they shatter easily.


Agree, cure your parts for a full day in UV. Then let it sit in the window sill for three weeks. Small features just shatter when touching.


Isn’t there 2 things that happen, post curing finishes off the polymerization of the process then after that point because the part is not uv resistant it starts to degrade with continued exposure?


I have definitely experienced the negative side to this topic. After reading that it’s better to over-cure rather than under-cure I felt much better about receiving the prints fresh from my friend’s printer and not worrying about curing them the rest of the way. However, once these prints had been prepped, painted and sold, their once-observed minute flexing capability and overall sturdiness was lost. Reports of cracking, shattering from a small impact and overall brittleness seemed to be reoccurring. I can’t say yet what the turning point was from optimal UV to too much because it seemed to vary from print to print. This may have something to do with how much they were partially cured by the person printing them before coming to me.


Thank you for this response… would you consider any of the more resilient resins more… PRODUCT ready? Something that could be put to use and work for a considerable amount of time as an actual part based on its mechanical properties?


That’s going to depend significantly on what sorts of products you’re making and how they’re being used. We’re excited by the potential of the Fuse 1 to create consumer ready goods, and that might be something to look into.


Could you describe your application bit more? Different resins are good for different types of applications.


  1. Grey resin is amazing for models/figures.
  2. Rigid resin is usually my go to resin for mechanical applications.
  3. Tough resin, for any applications where part might experience an impact.

Unfortunately you can not expect mechanical properties comparable to thermoplastics (such as nylon) from photo-curable resins; it will probably take good couple years for material science to catch up to thermoplastics. If you want really strong parts you should look into SLS or fiber reinforced FDM. None of those techniques will give you “product ready” finish, but parts would be stronger then SLA parts. You can also explore making moulds on a Form 2, and casting them using two part material. This process gives the best surface finish and good mechanical properties. BJB and Tap Plastics have extensive tutorials on YouTube on how to use Form 2 to make very nice end finished parts.


How much does painting/coating extend the life of parts?


Have you tried painting?


Polymerization is a chemical reaction, and like 99.99999% of chemical reactions, higher temperature accelerates the rate (see: Arrhenius) of reaction. So at a guess, the UV cure time should be limited to prevent overcuring, but the exposure time needs to be long enough to complete any curing that hasn’t completed yet. So raise the temperature to accelerate that process, means you need less UV energy and so, reduce the chances of overcuring.