Thursday, September 6, 2012

05 Molding Design Nº 7

One of the things that can be done with a moulage mold, that is not practical to do with a plaster mold is to make a one-piece block mold over the model that can be cut apart with a mold knife. The moulage is soft enough to be easily cut after it sets. The mold knife, as well as cutting the moulage mold, also makes the registration keys. The opportunity to make a block mold eliminates a lot of work making a clay build-up that is needed to make a regular two-piece mold. Also, because a moulage mold is semi-flexible, an exact parting line is not needed, since the mold can be flexed over slight undercuts. Click on any image to enlarge it.

In this example of making a block mold, I am using some photos from older posts to illustrate the concept. Here, the sculpture wax version two of the upper torso is being prepared for a moulage block by building up a spare with Roma Plastilina, an oil-clay containing sulfur, which means it is not usable for making a spare for a silicone rubber mold. However, for a moulage mold, it is okay to use it. Note that the spare is going to rest on the molding board.

The cottle for the mold is made from cereal box cardboard, taped with masking tape. The molding board is a painted piece of sheet metal. A wooden molding board, sealed with several coats of shellac would work just as well. The point I am trying to make here is that a heavy-duty cottle is not needed. This type of cottle can also be made from a roll of linoleum, as mentioned in Plaster Mold And Model Making by Chaney & Skee. The model is placed on the molding board, and fastened to the molding board with a coil of clay all around the spare.

Once the moulage has been melted, poured, and has cooled, the mold is removed from the cottle and the mold knife is used to cut the mold all the way from the top to the bottom on two sides. Then the mold is opened, the original is removed, and the mold is put back together, and put back in the cottle. This type of cottle does not require banding because it is already a band. Make sure it is taped together well. One other thing I would like to mention that makes it easy to cut the mold near the parting line is to make some marks on the outside of the cottle that indicate where to start the cuts, after the moulage has been poured. In this example, I cut the mold down the middle of the front and back of the torso so that the arm sockets could be released easier. The arm sockets were the deepest undercuts in that model.

The carving wax is melted and poured, then the mold is removed from the cottle and opened to take out the hollow carving wax casting. Here, the mold is being opened to remove the carving wax casting. It is easy to see the registration keys made by the mold knife.

Making a Mold Knife

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