Q: I make a lot of rubber molds, and I’d like to mark them for identification, but there doesn’t seem to be anything I can write on the outside of them with that is permanent. Any ideas?
A: I’ve got several for you. If you’re using the one of the transparent silicone rubbers, you can embed a small printed paper label within the mold when you pour it. Or, cut a slit into the rubber and insert such a label after making the mold. Either way, you can read it through the rubber.
If you’re using opaque rubbers, then it gets a bit more complex. What I did, and ended up doing with ordinary vulcanized molds too, is to take some small pieces of aluminum sheet, and using reversed metal letter-number stamps (right-reading, so they stamp a reversed letter), I made a series of little plates comprising a small font of the letters and numbers I needed. These can be stuck to the inside of the mold frame, so they will leave an impression in the rubber. The plate sinks into the rubber and the little plates are pried out and saved for reuse. The letter or number then reads correctly and is raised from that recessed surface. This works for generic labeling, giving the mold a stock number, date, etc. It’s not as useful if you wish to put detailed info onto the mold, but you get the idea. A bit of work is required to make the initial set of stamped pieces, but after that, it’s quick and easy to use.