Can UV cure inside opaque or more massive printed object?

Question

From what I understand, UV curing of resin prints works by starting a chemical reaction that hardens the resin permanently.

Also, a curing step after print is needed to speed up the print and also to reduce the curing during print, which would cure resin beyond the current layer.

However, what is the transmissivity of UV light in partially cured prints? If I print a more massive object, or if I use an opaque resin, how deep will the object harden properly? Absorption is always exponential, meaning that it decreases quickly with depth.

Depending on the resin, how thick prints can be effectively cured? This information is not provided with the resin, which I actually would expect from reputable manufacturers.

score 1 · Answer 1 · answered May 14 '20 at 12:51

From what I know about the various resins, once they are printed they are cured enough. Some of the transparent ones require additional curing, however the manufactures of the machines only test based on the max size that their machines can print.

The resin never stops curing apparently, going into diminishing returns; curing slower and slower as time goes on. Only way you will know for sure is by experimentation. You should get a poly-carbonate tube, place black tape along one side then spray paint the rest black. This will leave a window that you can use for future observation. Seal the bottom, and pour in uncured resin. Leave it under a UV source for 2 weeks then peal the tape off to see how much of it has cured. Then come back here and ask Trish to do the math on what the co-efficients of absorption are for the material :)

score 1 · Accepted Answer · answered May 14 '20 at 14:29

My background

I am not a polymer chemist. I know some, and I've talked about UV curing resins with them. These are my conclusions based on that informal education. Your expertise may well be better.

Minimizing curing on the printer is good

For the maximum print rate and best layer adhesion, the resin should cure during the printing exposure the minimum amount that is consistent with the object supporting itself during the print, being removable from the bed, being rinsed in isopropyl alcohol, and stand on its own during curing.

If you are curing too completely during exposure, you won't get optimal mixing of polymer chains between layers. The layers will be attached by the glue-like action of the new layer on the old, rather than having their polymer chains cross-linked and extending between layers.

If you are curing too little, the object won't be strong enough to support itself and survive post-processing. Soft, gummy bits may dissolve, or be swept away by currents in the cleaning solution. Small details may not be robust enough. Supporting structures (as added supports or parts of the model) may not be strong enough to resist gravity and handling.

If you are curing too completely, your print will take longer than it could take.

Curing is not binary

Curing will take place spontaneously over a long time. If it didn't, a bottle of resin might last a very long time. But, curing is not a self-catalyzing process that runs quickly to completion. If it were, then the first stray UV that came through an open bottle top would turn the contents solid. If exothermic (which it seems like it must be), the bottle would get hot.

UV curing may fail

UV curing the inside of a large, opaque object probably doesn't happen. Before I get all excited, though, I would need to place some bounds on "large" and "opaque".

Absorption of UV light depends on the pigment or dye used in the resin. This absorption is never absolute. It is not 100% gone after the first, thinnest penetration. The light is attenuated by an amount per distance it penetrates.

Transmission is the complement of absorption, and the numbers are easier to work with, so lets work with transmission factors of T rather than absorbtion factors of A. $T=1-A$

If a factor of T is passed for millimeter, then one centimeter into the object the light intensity is $T^{10}$ of what it is on the surface, which is a small, but non-zero, number.

Keep in mind that opaqueness depends on wavelength. For example, Window glass is transparent (very little absorption) to visible light, but highly attenuating to UV light. Were I to design a black resin, I would look for a black pigment that was relatively transmissive to UV.

Will it cure?

A low UV dose delivered to a 0.2mm layer of resin will partially cure the resin. 1 mm into an object, the dose is lower, but it still exists. 1 meter into the object, the absorption is probably too high pass a useful level of UV.

If the transmission factor is 0.8 for a 0.2 mm layer, it is $0.8^5$ for a 1 mm layer (0.33). It would take only three times as much UV exposure to cure a 1 mm thickness as a 0.2 mm thickness. If the object were 1 meter thick, the transmission to the inner bit would be $0.8^{5000}$, which is a very tiny number, roughly $2.82×10^{-485}$.

Finally, consider if the object is or must be truly be solid. UV curable resin is relatively expensive. Many UV printable objects include drainage paths for uncured resin to flow out of the object during printing. Perhaps your object could similarly be hollowed out.