Sunday, December 2, 2012
08 Joint Design Nº 25
I put all the coddles around the model and build-up, but I forgot to apply a parting agent. So I took the clamps off one wall of the coddle and made access for painting the model and build-up with a 50/50 soap/water parting agent. It is a good idea to never get in a hurry when making a mold. Click on any image to enlarge it.
This is a small mold, so I estimated the amount of plaster I would need. I used a water to plaster ratio of 2:3 for making this mold. The unit of measure I used was 1/2 pound. So first I weighed one pound of water, then added one and one half pounds of plaster to the water. I let the plaster slack for a minute or two, until all the plaster was wet. Then I stirred the plaster for a few minutes, until it became very creamy. I poured the plaster over the top of the lightbulb until it reached the mold mark I had made. Then I set the mixing bowl aside and rapped the sides of the coddles a little bit with my stirring tool, in order to settle the plaster. Then I walked away from the mold for and hour and a half and let it set up.
I am using a plastic mixing bowl, so I just let the left over plaster set up in the bowl. I can flex it out later. Never put plaster (dry, wet, or set) in the drain pipes. It is a good idea to always mix up a little bit more plaster than you think you will need. It is better to have a little bit more, than not enough.
After the plaster set up, I removed the coddles. This is a shot of the mold after the first wall was taken off. The light bulb was supported by a plastic cup, with the bottom cut off. The build-up was a piece of matte board, sealed with shellac.
The mold has been released from the coddles. The matte board build-up has been removed. The light bulb model is still embedded in the plaster.
Here is the ball mold half with the light bulb model removed. The light bulb and the build-up are also shown. I used the old kitchen knife to stir my plaster when I mixed it. I also used it to scrape the clay that sealed the coddles, off the molding table. I now have a 66mm diameter ball mold half.
I have several options at this point. I can use the mold half as a press mold, and press clay into it to make a hemisphere. If I make two hemispheres, I can join them together to make a ball which can be used to make the other half of the mold. Or, I can do the same thing with carving wax. Another alternative is to line this mold half with clay, then make the other half of the mold, with a spare. With a two-piece mold, I could cast several hemispheres in carving wax, and weld them together with my wax pen. I will decide on something.
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