Saturday, January 15, 2011
Rough Shell Mold for Lower Leg
Yesterday I started on a rough shell mold of a brown wax lower leg doll part. I used some cereal box cardboard to make a build-up. I laid the brown wax part on the cardboard and traced around it with a pencil. Then I cut out the cardboard so it fit around the part. I put some coils of oil-clay around the seam, on the bottom of the cardboard. The printed side was facing up. Then I mixed up some plaster and spooned it over the wax leg part. I built the plaster up around the edges, using the spoon the scrape off the plaster that went over the edge. I used the edge of the cardboard as a guide. I let it set up for an hour, then flipped it over and trimmed it up with a knife and a Surform tool. Then I carved some registration keys with a spoon.
The cardboard was floppy, so I got a curved edge on this rough shell mold. It doesn't hurt anything for this type of intermediate mold.
This is a shot of the wax leg part embedded in the first part of the rough shell mold. I have built-up the spare with oil-clay. I will carve the spare out later, when all the parts of the mold are finished.
This is a shot of the cereal box cardboard build-up that I used to make the first part of the rough shell mold. This is much faster than making a clay build-up.
This is the knife, Surform tool and spoon, along with the plaster trimmings.
I decided that I needed a three part mold for this leg part because it had some undercuts that needed the extra mold part. Here you can see the flattened piece of oil-clay that I used for a divider between the mold parts. There is also the soap parting agent (1 part liquid hand soap to 1 part water), and the brush I use to apply it.
This is the back of the oil-clay divider, showing some oil-clay supports.
I always set my plaster mixing bowl to zero before I start measuring water and plaster. This is also called the tare weight.
I made a cardboard coddle using a pizza box and masking tape. Each side of the coddle was cut to the length of the plaster. Then I banded the coddle with thick rubber bands and some scrap pieces of wood. These pieces of wood just pushed the cardboard up against the plaster, so it would not leak too much. I carved registration keys, and applied parting agent.
Then I mixed and poured the plaster for the second part of the mold. When it setup, I removed the coddles, and the oil-clay divider, then trimmed the plaster, cut registration keys, and applied parting agent.
I did the same thing for the third part of the mold after the second mold part setup (about 1 hour for the second mold part to setup).
After the third part setup, I removed the pizza box cardboard coddles and used my knife blade and a small hammer to gently open the mold parts along the seams. I used the Surform tool to bevel the edges of the mold. I carved out the spare with my knife. These are the three pieces of the rough shell mold for the lower leg part.
This is another view with the second mold part sitting on the first mold part.
This is the finished mold, ready to be saturated with water so that carving wax can be poured into it.
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