What is the best way to store molds when not in use?
I always try to store them on a flat surface, away from direct sunlight. Room temperature (70 degrees) is probably optimal. If they are on an un-even surface, or there is something on top of them that exerts un-even pressure, they tend to deform around it and will stay that way if left too long. I have read some recommendations for treating them with silicone oil prior to storage, but have not personally tried it.
I cut luan floor underlayment or 1/4" plywood to the size. This is to give a flat solid support. I place them on the wood support in gift boxes. I have some that are over 10 yrs old and still as good as the day I made them. I have seen some that were mistreated by not lying flat. They never regain the flatness as they seem to stretch. Once they do that they never will lay flat to make good castings. One word of caution is not to store them in a damp place as they will mold over.
Read this…its the best I have come across - as most on Google are users advice and some of it leaves a lot to be desired…
https://www.polytek.com/tutorial/tek-tip-how-store-rubber-molds-extend-library-life
…and they even offer free advice by phone (see bottom of their website page).
I did not know storing dissimilar products touching each other was not recommended. Thanks for posting that link.