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According to Eyler, "Building forms is like normal carpentry, but a bit more precise. On an inverted cast everything is formed as a negative." Materials he prefers include melamine, plastic and rubber.
Not only do materials commonly used for making molds need to be able to handle the weight of the concrete and create a smooth face, they also need to withstand moisture.
"You need water-resistant or waterproof materials," reports Gobillot. His company uses plastics and laminated wood. For a finish that looks like hand-troweled, they've used MDO (medium density overlay plywood) to build molds. "There's 10 ways to do
what we do and many materials to get to the same end." But he cautions, "Not every lumberyard carries good materials for working with concrete."
Most contractors who make precast concrete countertops use caulk or some other material to slightly round the edges and prevent the concrete from leaking out of the mold. Silicone caulk is one of the preferred methods. Once you have your basic mold constructed, then the real fun - and creativity begins, says Fu-Tung Cheng, principal and chief executive officer of Cheng Design in Berkeley, Calif. "The form can be the ultimate simplicity. What you do from there is what it's all about. It's your imagination - it's unlimited."
Rigid foam, rubber molds and other materials are used to create the negative forms for sinks, faucet knockouts, trivets and stand offs, integral drain boards, and the like. Embedded objects are affixed to the bottom of the mold - upside down, of course, so they are right side up when the countertop is de-molded and inverted. And then there is the concrete mix and what you do with it.
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