Maybe like putting it into an oven?
If so, I'd like to make some molds, if possible.
Thank you in advance.
No, except by freezing. Silly Putty (brand name) is made of long strand silicone molecules with the characteristic that they do not slide by each other quickly but do slowly - so a ball can bounce when thrown against a wall or floor but it will sag to a lump in your hands. If you want to make flexible molds, the nearest thing would be 100% silicone adhesive/caulk from hardware stores and home centers and you will have to be very careful to apply separator (spray fat, etc.) to the model to keep it from sticking. There are better choices, but you have to buy a bunch to get them, except for latex, which requires backing on the mold because it is so weak.
yeah, the oven sounds like the best idea.
bake some small samples as a test