Wednesday, April 2, 2014

09 Plaster Production Molds Nº 14

I have been thinking about registration keys. I basically make three kinds of registration keys. Two of them are almost the same, differing only in whether the key is positive or negative with regards to the mold piece being withdrawn. These are the flat rectangular or trapezoid shapes. I also make round hemispherical keys, carved into the plaster with a flattened metal spoon. The trick is to make the key big enough to provide good registration, but not so big that it sticks in the key hole. It is very important that the registration key hole gets coated with the parting agent, otherwise, the freshly poured registration key will stick in the key hole and lock the mold parts together.

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This is an idea of how I use the different registration keys for making a multiple-piece plaster production mold. Often, the type of key used is dependent on how the mold pieces are withdrawn. The diagram below depicts a four-piece mold, with one piece being the 4.5 X 7 inch bottom piece, with three pieces on top of it. At least two, preferably three keys are needed to register each mold piece.

These are two of the flattened spoons I use for carving round registration keys. I use the round end of the spoon. I twist it back and forth to create the key hole. I have also tried using a pottery loop tool, but for me, the flattened spoons work better.

This is a plaster mold for producing carving wax balls, to use for designing ball joints for dolls. I used the flattened spoons to carve the key hole in the first pour, and the registration keys were formed when the second pour filled-in the key holes.

Making the flat registration keys is simple. I roll out a clay slab that is 0.25 or 0.375 inches thick, then I cut out the shape of the registration key and stick it on the clay build-up. I make sure there is plenty of draft along the edges of the flat key so it will release easily.

This is another diagram about making the parting line on a carving wax model that has an overhang that gets in the way of drawing a mark under the overhang. The piece may be raised up on a block, and/or a smaller square can be used. In the case of the carving wax feet, I both raised each foot up on a wooden block, and I used a plastic card as a smaller square to get to the surface on which I needed to draw a parting line. The square had some carbon paper taped to one edge. After marking the parting line with carbon paper, I traced over that line with a Sharpie® permanent marker.

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