First up, you have to understand that plaster is a 'heavy' mold, meaning unless your cardboard is somehow reinforced, using plaster is just gonna collapse it. Take me for example, I'll sculpt in clay and then plaster mold the finished clay item.
It would help if you were more specific on what materials you're gonna be primarily using in your build, what's readily available to use and besides cardboard crafting, what sort of 'chemicals/materials' [resin, fiberglass, glaze, bondo etc], you've had experience with.
There are some good books you can read up on, but for the life of me, I can't seem to remember the titles...apologies, am getting a little old.
love your sig tim. "recoil" srry but i myself have no experiance in molding. but like tim said plaster is heavy and because of that we need to know what your molding it off of. is it a pep helm or did you sculpt?
This is a tremendous book for learning how to make molds
Many guy's form the 501st have used and reference this book. I own it myself.
Look it up at Amazon.com. Make sure you look in their "New and Used" link
when you find it. Many times you can get brand new books from their affiliate
sites up to 75% off. I always check book prices this way.