While I have had success with certain Alumalite products, IMO, there are excellent and (somewhat) affordable commercial grade rubbers and resins out there.

There is a definite shelf life, don't buy it for backstock, use it as rapidly as possible.

I made the mistake of setting up a mold, then 'using up' some rubber I'd got at a chain store clearance, which then sat at our WHQ for a year or so.

Rescued the master, but learned a lesson.