Working with Plaster of Paris in mold-making?

I am planning a project that will require me to partially cover a form with plaster, top down, enough to allow it to be overturned and set down on that section when it has set sufficiently. Then I'll continue to cover the form with plaster, working up from the now upper edge, or down to it from the "top". It will be a one-piece mold, the form being removed later.

I'm concerned about the bond being strong between the two sections of plastering. If I work quickly, will it hold up or will it tend to crack and come apart under stress when I fill it with concrete later?

New to this, so any advice will be appreciated.

TIA.

R.