I’ve wondered why we need post cure. A part seems soft when it has not done a post cure yet. Is this only the surface that gets cured? How does the UV light finish the cure inside the part if the resin is blocking the UV?
I ask this because I use my printer for new parts or replacement parts. Would it be better to use clear resin so the inside gets the UV curing?
Maybe there should be an option to have the printer do a more thorough print with longer exposure time.
you can use a uv pen to get to the areas that you are concerned about but the parts are also heat cured as well
How does the UV light finish the cure inside the part if the resin is blocking the UV?
It doesn’t—at least not very well. Beyond a certain depth, UV light is scattered or absorbed by the resin, especially in pigmented or filled formulations, so it loses the ability to initiate further polymerization directly.
However, as @MattRForerunner noted, curing isn’t entirely dependent on UV at this stage. Many resins are formulated to generate free radicals during the initial print that remain reactive for some time, enabling continued polymerization post-print. Heat also plays a role—raising the temperature increases molecular motion and helps those reactive species complete the crosslinking process.
Given enough time (and heat), the part will eventually reach near-full cure even without UV. But ironically, the same chemistry that helps it finish curing will eventually lead to degradation—a slow unraveling of the polymer network over months or years, depending on conditions (applying thin surface finishes like clear coatings or sealants can help extend the life of the resin by hermetically sealing it against moisture, oxygen, and UV if that’s a concern).
Are you experiencing or anticipating a specific issue?
Thanks for that good explanation. I was just curious why post cure was needed. And why some parts could warp in post cure.
Warping essentially boils down to atomic and molecular geometric rearrangement in to more stable configurations… sometimes that means atoms get closer together (or futher apart), sometimes that means molecular bonds are formed (or broken due to tensile stresses, for instance)… The more this happens at the time of print, the less it happens later. But all things sort of do this in one form or another. Ceramics fired in a kiln are very dimensionally stable after cooling because they’ve been given a ton of noisy energy (heat) to allow them to wiggle around in to the most stable configurations… but polymers don’t really get that opportunity, so some part of that happens over a longer period of time.
Pretty cool stuff, imo. But definitely inconvienient.
If you’re doing the same prints over and over again, it’s theoretically possible to control the process in a way that the warping itself becomes part of the process… Like letting steak sit off of the grill for a few minutes before you cut in to it, so the heat that’s inside has a chance to finish doing it’s work before you let it all out.
Weird analogy, but additive manufacturing is part mechanical and part chemical. We don’t get the luxury of “well I milled off this chunk to nanometer precision, and now it’s that way forever if I just let it set in a shelf.” But it’s WAY MORE CONVIENIENT when it is the right tool for the job. 
Hi @bitsyncmaster,
Great question, post-curing is a critical step in the SLA
printing process. You might find this article interesting for a deeper dive:
Introduction to Post-Curing SLA 3D Prints
Also @cwharris’ comments are very helpful regarding this topic.