The maximum temperature that cured ArtResin can tolerate is 120F or 50C. At temperatures as high as that, the cured pieces may become a little flexible but once they cool off, they will harden up once again. If your cured resin is exposed to temperatures beyond 120F or 50C, however - for example, you leave a cured piece in a hot car for a long time - it could cause irreparable damage.
A word to the wise? Keep it cool!
ArtResin: Made For Artists, By Artists.