How does sunlight affect the mechanical properties of prints?


I am having an issue with my prints. Some prints are significantly stronger than others, Eventhough it is the same part. It would seem that prints that has turned yellowish are not as strong as prints which are still clear in colour. It should be noted that i am curing my prints in sunlight, and the curing time varies alot.

Have some of you experienced the same and have you found a solution for this?


after the curing is sufficient, paint with a UV protective coating.

You shouldn’t leave the print out in the sun too long. 30 minutes is probably fine

Define “stronger”?

Parts that show a higher tensile strength but exhibit a glass-like fracture when they break could be defined as “weaker” than parts with a lower tensile strength that yield to a much higher level of stress before breaking.

Parts should become harder and more brittle the longer they’re exposed to UV. How much UV light they get if you just put them in the sun will vary depending on the weather. Like trying to take a picture by guessing at what the exposure settings should be, some will be over exposed, some will be under exposed. You should buy or make a UV curing box so you can control the level of exposure of your parts, and in so doing get more consistent results…

The parts continue to cure and get more brittle as long as they are exposed to UV light.

I think the issues are as you describe. I have not had control of the curing process leaving them in the sun too long and also the issue that ScottBarbour describes seems to be the case.

I will try to control the curing process, and reduce the exposure to UV after curing.

I think that is such a good point. Which value or characteristic is most important to you (in my case, surface). Many ‘Tough’ resins are actually formulated to experience less cross-linking, allowing them to yield more to bending forces before breaking. On the other hand, a heavily cross-linked / polymerized piece may show more hardness and resistance to bending, but will reach the catastrophic break point sooner (brittleness). Well-polymerized will usually hold a surface finish better. To an extent, intense post-curing will limit the ultimate depth the post cure radiation will be effective in thick stock; and may cause uneven stresses in the polymer matrix. I think that the FormLabs folks have confirmed the wisdom of heating the stock and applying less-intense light over longer time. For post-curing, I have the nail light, a hand-built 100W 395-405-430-465nm universal curing wand, a flash-lamp chamber (pulsed landing lamps from airplanes), an Elipar commercial vacuum chamber surrounded by a few fluorescent lamps post-cure, a hand-built 300W metal halide (white) lamp system, and the sun. Hands-down, my best surface results so far have been with the vacuum curing system (ELIPAR) for 14 min. (yet probably near the lowest intensity of the choices). I don’t have Instron lab equipment to test tensile, torsional, and shearing.

FL has published a white paper on curing. It’s very interesting. The strongest parts are achieved with 405nm light and higher curing temperatures. They cure faster at higher temperatures. Parts are substantially stronger when cured optimally. And strength decreases some with too much curing. So just putting your print in the windowsill is probably not providing anything close to an optimal cure.

FL clearly wants people to download it, so I won’t post it here. If you got the email about PreForm 2.5.0 and you follow the link to the webpage that describes this revisions features/changes, there’s a link to the white paper there.

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