UV Post Curing Temperatures


Over the past few weeks I’ve been in the process of building a UV Curing Chamber, which has been quite a journey.
Finally, I’ve been able to do an initial curing test of the chamber and I have some questions regarding the temperatures for curing.

In the “How Mechanical Properties of SLA 3D Prints Are Affected by UV Curing” whitepaper released by Formlabs, it notes that the optimal UV Post-Cure temperature for Clear V2 is 60 C using 405nm UV Light.

As I have the curing chamber set right now, the chamber sits right about at 60 C when I am curing my prints, tested in multiple areas using an infrared temperature gun. However, after completion of a 20 minute test with a part that I recently printed, the part itself was up to about 75 C.

As I understand, the process of curing UV resin results in an exothermic reaction (releases heat). So, what I am curious about is whether the temperatures of the chamber itself should be 60 C or if precautions should be taken to keep the part at 60 C?

For the FormLabs folks out there, were there additional tests performed at temperatures above 60 C when doing testing for the white sheet? I’d imagine this could be an issue / consideration to be taken with your new curing station that will be coming out soon.

The curing chamber we’ve built is a re-purposed convectional Toaster Oven, with two quartz heating elements on the top being run by a PID temperature controller and the convectional fan always on. The thermocouple is placed between the two elements at the top of the chamber, as they got much hotter than 60 C when I initially had it in the bottom or sides.

The curing process I followed in my first test was to get the chamber with the part in it up to 60 and once there, I would turn on the 405nm UV LEDs (which are conveniently wired through the oven’s timer so it goes for 60 minutes)

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So, after a little more research and more indepth reading of the white paper, I think I may have answered my own problem. The white paper mentions that the “Post-cure temperature should be limited by the HDTs
of the resins, which means 60 ºC for Formlabs Standard Resins.” where HDT is the temperature at which the parts will deform.

So, this means that my part being at 75 C is not okay. Whether that 15 C increase in temperature is due to the exothermic reaction of the resin, or improper temperature control, I am not sure.

My plan now is to retry the print and cure it at a lower temperature so I can aim for 60 C without modifying my chamber. If that doesn’t help, I’ll try to shield the area between the heating element and the part with foil, but making sure to not cover the elements (I don’t want to burn my office down) and provide adequate airflow for the convection.

Unfortunately, if the issue is the exothermic reaction of the resin, then this will be ongoing as the amount of heat energy generated by the reaction will change dependent on the amount of resin being cured, at best I would be able to create a table / chart per for volume increments.

I am curious how FormLabs intends to address this with their curing box? I suppose it would be, in theory, possible to do the chemistry math and determine the precise amount of energy released as heat from the chemical reaction, totalize that on a mass or volume basis for each resin then have the user input the volume of their print into the box, so it can adjust the set point temperature according to how much additional heat will be released by the part.

That said, unless they’re doing the same thing for UV exposure times, I don’t see them implementing a proportional heating process. It would be awesome if they were though, and would certainly make the $700 price much more realistic. Proportional heating and UV exposure based on the volume of the part.

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A bit of an update on my testing with the curing chamber. I’ve got a lot of projects going on at the moment, but I did manage to print and cure one part recently. I set the chamber to 50 C and it was sitting right around 60 after 60 minutes of curing. I did not notice any warping or changes to the part which was a relatively long, thin part (similar to arms on glasses).

I will say that it’s a bit of a pain to cut off the build supports after they’ve been cured though, and it frequently took some chunks out of my part. I’ll have to try smaller contact points for future prints when they’re this small.

Thanks for sharing your results. If you have issues with removing the supports you could take a soldering iron with a very thin and flat tip. This is like a hot knife it allows you to remove the supports without damaging the part.

I’ve tried smaller contact points but chances are high that the parts will be be distorted since the contact point will detach on higher forces.

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I tend to remove supports prior to post-curing as IPA swells and softens the touch points a bit making them easier to remove.

We’re working on new post-curing resources for the shipment of Form Wash and Form Cure. The photopolymerization reaction is fundamentally exothermic though it’s difficult to estimate the magnitude of the effect given your cure chamber.

Exceeding the Heat Deflection temperature of the resins by a bit often won’t result in adverse effects because HDTs are standardized to a given force (usually 0.45MPa). It’s unlikely that parts are going to experience this amount of force in a cure chamber unless you’re applying it intentionally. We’re keeping the exothermic nature of photopolymerization in mind with the settings and documentation associated with Form Cure.

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