I am mostly experienced with a molder who made his molds from aluminum plate, laminated with high strength bolts, wrenched by an impact wrench for part ejection, and shot mostly in nylon and acetal. He won awards for some of the parts, and always met our expectations wrt size and finish.
I had one thick Delrin part that he managed to mold with shrink marks, i.e., cavities, on the _inside_, and nice flat surfaces on the outside. Much to my regret, he has not been available for consultation since his death; I'm sure he's none too pleased about it either.
I tried insert molding, once, using steel inserts of ~2" dia in steel mold bases, with another molder. Our intent was to allow production of up to six different parts of similar size in one multicavity mold base, which was sort of the basis for using that molder, based on assurances that turned out to not be promises, much less guarantees. We found he could make good parts with one insert at a time, but he could never get even two good parts from one shot, and the inserts didn't really interchange from one position to another either.
If your mold base has clearance around the inserts, or thick walls, I wouldn't worry about it too much, >>>provided that I had personally made a site visit and witnessed that molder swapping aluminum inserts in steel bases as proposed and getting good parts<<<. If the large inserts are your idea, just buy the individual mold bases; they are cheap.
Mike Halloran
Pembroke Pines, FL, USA