Concrete for mold making
Don't use quick-crete mix your own, for one thing you don't want gravel in detailed or smooth are casting, just sand, the finer the better. I have had a lot of success using a mix of sand and marble dust but I was casting giant pacific clam shell replicas so I wanted that very white sparkly surface.
Getting it really smooth requires a sealant, concrete by itself will not be perfectly smooth just because of the sand. Using a vibrating table will help a lot with getting a smooth void free surface.
I use a pool lining mix ratio since it is a gravel free minimum void mix, something between 1:2 or 1:3(portland cement:sand) but a little experimentation is best. The water ratio is more important, you want as little water as possible, 1:3 or less, but it is effected by climate so again experimentation is best. the DIY stack has help for counter-top cement which is pretty much what you want. https://diy.stackexchange.com/questions/90389/whats-the-best-ratio-for-sand-only-concrete
I have heard good things about plasticizers but have not used them myself.