Sunday, October 17, 2010
Plaster Mold Making
In order to make a porcelain BJD, it is necessary to make plaster molds to pour porcelain slip into. More care is taken to make these molds than was taken to make the rough shell molds. These are the molds that the final ball-jointed doll will be cast into. At this point in the process, finished carving wax doll parts have been made, ball-joints added; they've been test strung and sanded smooth.
All of these parts will be put in a clay build-up which divides the parts along the parting line. Coddles will be placed around the clay build-up and sealed with clay. A clay spare may be added. The model will be brushed with a parting agent. The volume of plaster needed will be calculated. Water is put in a mixing bowl. Plaster is sifted into the water. The plaster is allowed to slack for one to three minutes. Mixing is then done by hand, or with tools. The mix is poured. Excess plaster is discarded into the trash bin. The mixing bowl is rinsed in a bucket of water (Never put plaster, dry, wet, or set into the drain!). The mix sets up into a hard solid. That makes the first piece of a multiple-part mold.
The coddles are removed and the mold is turned over. The clay build-up is carefully removed, without disturbing the doll part. Registration keys are carved in at least three places. Coddles will be placed around the plaster mold and sealed with clay. The other half of the clay spare may be added. The model will be brushed with a parting agent. The volume of plaster needed will be calculated. Water is put in a mixing bowl. Plaster is sifted into the water. The plaster is allowed to slack for one to three minutes. Mixing is then done by hand, or with tools. The mix is poured. Excess plaster is discarded into the trash bin. The mixing bowl is rinsed in a bucket of water (Never put plaster, dry, wet, or set into the drain!). The mix sets up into a hard solid. That makes the second piece of a multiple-part mold.
If a third mold piece is necessary, the same procedure that was used to make the second piece of the mold is followed. The clay build-up for the third piece of the mold is carefully removed, without disturbing the doll part. Registration keys are carved. A parting agent is applied. The volume of plaster needed is calculated. The plaster is mixed, poured, and allowed to set up.
Once all the pieces of the mold have been made, the mold is taken apart and the edges are beveled. The mold is cleaned, then put back together and held with thick rubber bands, so the mold will not warp. The mold must be put in a dry ventilated place so it can dry. A dry plaster mold will absorb water, and this is the property of plaster that makes it so useful for slip casting porcelain.
In the above diagram, the mold making process is shown.
1. The torso is being molded. Here the torso is shown in a clay build-up. The clay is green. It is important to build the clay up to the parting line of the torso. There must be no undercuts.
2. The torso is shown with a clay spare added, as well as clay build-ups for the hip sockets. The coddles have been put in place and clamped securely. The edges have been sealed with clay. Parting agent has been brushed on the torso. The first half of the mold is ready to be poured.
3. After pouring plaster into the first half of the mold, the mold is turned over, and the clay build-up is carefully removed. The other half of the clay build-up for the spare, and the other halves of the clay build-up for the hip sockets are made. Registration keys are carved into the first half of the plaster mold. Parting agent is applied to the torso and to all the plaster area that is showing from the first half of the mold. The second half of the mold is ready to be poured. The coddles should be clamped and all the edges sealed with clay.
4. In (4a) we see that both halves of the plaster mold have been poured, leaving the clay build-ups for the hip sockets. These clay build-ups are removed, cleaned, registration keys carved, and parting agent applied. In (4b) plaster is mixed and poured into the hip socket areas. In (4c) we can see that the mold has been separated, showing the clay build-up for the spare, the torso embedded in one half of the plaster, and the two plaster pieces for the hip sockets in place. The torso is removed, the clay build-up for the spare is removed, the mold edges are beveled with a knife, the mold is cleaned up, banded together, and put aside to dry.
My coddles are made from dimensioned lumber from the hardware store. 1x4 lumber is actually 3/4ths X 3-1/2 inches X length. 2x2 lumber is actually 1-1/2 X 1-1/2 inches X length. I cut all the pieces, then glue and screw the 2x2 to the 1x4. After the glue is dry, I coat all the surfaces of the coddles with several coats of shellac, letting each coat dry inbetween coats. My coddles have been in mold making service for many years now.
How long should you make your coddles? What are you going to be molding? For dolls, a torso is about the largest part that needs to be molded. The length of the torso, plus a spare, plus hip socket parts, plus a few inches extra length should be sufficient. For example, a torso that is 6 inches long, with a 2 inch spare, and 3 inch hip socket pieces, plus 2 or 3 inches extra, adds up to about 14 inches long. Of course, the doll I am making is a 60cm doll.
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