Molds that cannot be utilized without standard frame. Unit inserts slide into a standard frame. The frame typically stays mounted in the molding machine. This not only saves mold change over time but also decreases the cost of the mold. There are a few standards for unit inserts, Master Unit Die, Round Mate, are two common ones.
Make sure when purchasing a unit inert that you are getting a standard one and not a custom or proprietary insert. Also realize that if you want to move your mold to another molder, the new molder will have to have that frame size or you will need to purchase a frame.Free Stand Molds:
Free stand molds are molds with all features required to run on a standard molding press unlike unit inserts that have a separate mold frame.