How to Make Molds Out of Plaster
Learning to make molds out of plaster is a great introduction to the art of casting, a common method for making sculptures and a variety of commercial products. This plaster mold can be used for casting or molding wax, paper, or latex. Depending on your process, the mold may be broken after casting, or saved to use again.
Things You'll Need
- plasticine (kids' craft clay) or real clay
- petroleum jelly
- carving tools
- shoe box
- large old paint brushes
- plaster powder
- bucket
- stick
Instructions
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Carve an original object that you want to make a mold of, out of plasticine or clay. The model should have a limited number of small parts and sharp angles--the closer to a solid block or sphere it is, the easier it will be to make a mold out of plaster.
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Figure out where the mold will need to be divided in order to come off the cast object. For example, the mold of a sphere should be divided in half so that one piece can pull off of each side. Objects with parts pointing in various directions need to be divided into more sections.
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Make a thin sheet of clay and attach it around the model wherever the mold must be separated. For example, an approximate sphere should have a sheet attached all the way around its middle, to divide the mold you will make into two halves.
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Cover the model with petroleum jelly if you used actual clay. Kids' modeling clay shouldn't require extra lubrication.
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Prepare your work surface, and be ready to work quickly.
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Mix the plaster with water in the bucket according to the package directions, mixing with the stick. Fill the shoe box partially with plaster (but make sure it's deeper than the part of your model that will be submerged), and let it set just slightly, enough that the model doesn't sink in the plaster. Push the model halfway into the soft plaster (up to the main clay sheet).
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Quickly use the paintbrushes to scoop and spread more plaster onto the other half of the model. Make it several inches thick if possible, but try to avoid covering the dividing sheets completely. Wait for the plaster to dry completely.
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Remove the shoe box if necessary, and separate the pieces of the mold at the clay sheets. If any of the dividing sheets are completely buried in the plaster, chip down toward them carefully from the outside of the mold, using carving tools.
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Remove any extra clay and hold or glue the sections back together to create a complete plaster mold. Keep in mind that you will need to leave an opening to add a casting material, and after casting you will need to either separate the pieces again or break the mold.
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Tips & Warnings
The problem with having lots of points on your model is that in most cases, each part pointing in a different direction requires its own mold section. So, a simple shape can be made using a two (or even one, if the flat base is placed at the opening of the mold) part mold, but a more complicated form may require a mold with many different pieces. This isn't necessarily a problem, it's just harder to do correctly, and may be worth saving until you have a bit more mold-making experience.
Resources
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Comments
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I just made a mold form out of plaster over a product called Smooth On, a silicon rubber from Reynolds Advanced Materials, which I put on over the clay original of two 20" famous men. Let me tell you.....READ ALL DIRECTIONS ON PRODUCTS BEFORE BUYING OR MAKING. I am on my second mold because I didn't read that this product CAN NOT go over plastilina. Well upon making the first mold using silicone you buy at Home Depot or Lowe's the cream color silicone didn't set up but the brown and black did....go figure...Well I put the plaster over that and the plaster turned out fine but the mold inside with the silicone didn't. So I contacted Reynolds Advanced Materials and they had the right stuff. Problem is that when a friend helping took the silicone off the master he dug a groove into the original clay and I had to rebuild it. I used plastilina which looked great. I sprayed it with clear acrylic spray to seal it before I used the Smooth On rubber. Well the rubber only dried over the natural clay but stayed sticky over the plastilina. Had I mentioned that to the gentleman I ordered the rubber from he would have sold me something different. Had I read the directions that clearly says Does Not Work Over Plastilina containing sulfur I would have had no problem. So just a word of advise. READ ALL DIRECTIONS BEFORE YOU START. I put the plaster over the rubber and I am taking it off today. I will let you know how it turns out. Keep your fingers crossed. Plaster is also very heavy. I can't do the mold myself. I am pouring cement into it and will never be able to lift it. I can't lift half of the mold from the other discarded one I made. Make sure you are strong enough to do the project. I usually do direct cement meaning I build up the cement over an armature.
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excelent
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psigur
Jun 15, 2010
Are there any videos on how to make a plaster mold? -
dogboy101
Dec 19, 2009
if you want to be able to resuse your mold, you can press play doh into it to take the shape, and then make another mold from that after it dries
