When creating a large mold using alginate (dental impression material)- think a face casting or torso casting as our example, a mother mold or shell mold is required to support the alginate mold once it is removed from the model. That is because alginate is rubber-like and not rigid and will collapse on itself. With alginate mold making, a mother mold is made either by using overlapping plaster bandage strips, or in a second technique, cheese cloth impregnated with rapid setting plaster. Either method is satisfactory.
However, in either method the shell mold will not remain attached to the alginate. This means that when demolding, the shell mold most likely will come off without the alginate mold being attached. Then, no matter how careful one is in demolding, putting the two back together again for the casting step can be a nightmare in registering. The two pieces will not fit perfectly together as they were first constructed. Mis-registering leads to unsightly mold flaws such a ripples, wrinkles and tears.
However a number of experience mold makers have solved this problem by an embedding process using a special soft cotton fiber sandwiched between the alginate mold and its plaster shell. Since alginate does not stick to anything, embedding fiber into the alginate surface before it sets will give the shell mold of damp plaster, something to bind to when the plaster has set.
The process begins by is to spraying the surface of the alginate with an alginate retarder to keep the alginate from setting and maintain its tackiness long enough so it binds to the cotton when the cotton is touched to its surface. Thus, when the alginate has cured cotton fibers are actually held fast by the alginate itself. Then as the plaster bandages set, both materials will capture the cotton fibers between them and lock the molds together so both the alginate and plaster shell can be removed as once piece.
But not any cotton fiber will do. Most cotton is rolled too tightly to be fluffy enough to give up its cotton fibers to the alginate surface. We have found that veterinary cotton is by far the best material to use for this process and we use it exclusively for its excellent properties. Its fibers are extremely supple and will easily embed into uncured alginate surfaces.
The following are instructions in how to prepare the cotton fiber for use with the best practice alginate mold making method The first step is to unroll about six inches of the cotton roll and cut it into a six-inch square. Then separate the square into two halves by peeling them in half and exposing its centers. The inside surface of the square has the fluffiest texture fiber. Spray the surface of the alginate liberally with the alginate retarder and gently pat the fiber square to the surface of the alginate. Don’t apply any pressure as you will disturb the alginate surface. Just a gentle touch is enough to pull the fibers from the square to the mold surface. When the alginate surface is covered, the technique will leave behind enough fiber to secure the shell mold. One half of a 6-inch a pad of cotton is all that is required for a torso mold. If the alginate has set, this method will not work. However, by spraying the cured surface of the alginate mold with an alginate retarder, the surface should soften enough to stick to the fiber, as the retarder will actually temporarily dissolve the surface enough to embed the cotton fiber.