I filled up my resin vat yesterday afternoon with Elegoo standard gray resin. I used the dregs of an older bottle nearing the end of it recommended shelf life.
I printed a short print (approximately 1 1/2 hours), removed the print and then left the printer overnight. The resin vat was uncovered, but I had the red plastic top that comes with the printer squarely in place, and a third party black insulated printer cover covering the entire printer down to the feet. No gaps.
The printer should have been entirely blacked out except for a period of about 1 hour where it only had the red plastic cover on it when the printer was illuminated with artificial light (Warm white LED light panels).
When I came to use my printer again the following morning there was a thick skin of resin on the top of the tank.
This is a problem that I've never experienced before.
Could this be due to the resin's age, or is it likely that someone removed the cover of my printer for some reason (I don't know what reason, it's not really important right now), or is there another possible explanation, such as a contaminate (There are probably over 100 different chemicals or substances in the same area as the printer).
Drops of resin on my silicon mat that I hadn't bothered to clean up were still tacky, despite being exposed all afternoon and all night.
