Making Plaster Molds
How to make a simple plaster waste mold
 mold made of gypsum bandages (upper left) with molded leather mask lower right. Molding a form is an entirely different process than mold making/casting. |
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Mold making is fun, and an excellent supervised project for children from age 9 or so. The difficulty level depends largely upon the size of the original and complexity of the form. Keep it simple, for younger kids. Adults venturing into mold making should be prepared to experiment a bit, in order to get a good feel for the materials and methods. You may want to practice on a small rubber ball or an apple. I do not go into the diverse materials available for casting purposes. However, you will find the links provided on this page offer highly detailed information regarding not only plaster grades, but foundry casting methods and other mold making materials, as well.
I have included a few tips pertaining to life casting and many general mold making principles, but this page is predominately for the process of taking a plaster mold of a clay or organic original. |
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Precautions
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Getting Started The first thing to consider is the form you are taking a mold of, herein referred to as the original. The original object should be of a soft, smooth, material, such as wax or clay, for a plaster mold. It is also possible to cast organic materials, such as fish or fruit. I use styrofoam packing shapes, sometimes. Inspect the original object for undercuts. (see diagram) You will also want to establish a visual midline, if possible.
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visual or imaginary "midline" line divides negative mold into two perfect halves. This allows the mold to be easily removed from the original object. In this example, the globe is the original object to be cast. |
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undercuts are indicated by solid black prevents removal of mold section "A" from object (globe) Some undercuts may be filled with plasticine. |
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three part mold also removes undercuts. More is not always better, however. I usually reserve multi-part molds for complex shapes such as highly detailed figurines. |
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>> Molding Making Rule #1: if the original is soft, consider using a hard molding material. If the original is hard, use a soft molding material. There are exceptions to every rule, however. Examples of exceptions include a disposable original such as an egg shell, a balloon, or a rubber ball, which work very well for a plaster mold. Other exceptions would include delicate items, such as flowers and fabric items which would not support the weight of the plaster. Also, figurines and items with undercuts.<< |
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clay (soft) original --> plaster (hard) molding medium --> clay, ceramic slip, leather, (soft) casting medium(s). Casting can be achieved by pouring, pressing, or forming the "casting" medium into or onto a mold. |
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The General Process 1. Prepare the original for molding medium by painting with a light coating of petroleum jelly or mold release. Clay originals can generally be mid-lined with aluminum shims, for ease in separating the mold sections. The shims should be cut to a proportionate size with shears, and should protrude from the surface by two inches, ( if you plan to apply an inch of plaster) and are gently pushed into the clay original along the midline. Wear gloves! The aluminum shim stock will have sharp edges! Insert one at a time, to minimize surface alteration. Edges should overlap sufficiently to insure clean separation. After you have one complete row of shims along your midline, begin inserting the second row, flatly against the first. (The two halves will be separated by tapping a chisel between these two rows.) 2. Mix small amounts of plaster, and build the surface in layers to a thickness of approximately one inch, allowing each layer to set firm before adding another. Use reinforcing material, if needed. Molds larger than 24" in any dimension may require more than one inch of plaster. Molds smaller than 10-12" will not require a full inch. The discretionary objective should be to develop a sturdy vessel to receive the casting medium. 3. Plaster waste clean up must be done before plaster sets. Plastic bowls and utensils will clean up easily by flexing. Any dried plaster will crack and fall away. 4. Once the mold is set and dried, separate the sections by gently tapping a thin edged tool (chisel) between the midline shims. Remove the shims, but be careful not to corrupt the edges of the mold. Remove and discard clay, wash mold thoroughly. Casting the mold: Join mold sections and band or secure tightly prior to casting. Be aware that there may be some leakage. If leakage is excessive, use floral clay or plasticine to seal the edges. Clay slip cast in a plaster mold will shrink and pull away from the mold surface, making a release agent not only unnecessary but highly undesirable, as it creates a barrier between the plaster and the clay which prohibits the plaster's natural moisture drawing action. My advice... general mold making tips Working time is an important consideration in any mold making process. Know what the working time of the medium is, and plan accordingly. To the extent possible, arrange all work surfaces, tools, and materials, in advance. Make sure you have buckets and water available to wash hands and utensils before plaster sets. Reinforcement: There are number of ways you can reinforce a plaster mold. The best method is determined by the cause for reinforcement. Large area, thin walled, molds may need to be reinforced to avoid flexing during removal or casting. Consider using chicken wire for larger molds or gauze strips for smaller molds (generally works well for organic forms).
Many people use Pam cooking spray as a mold release. I do not, because it encourages black mold. Black mold is a scourge to your plaster projects. This can be avoided by thoroughly washing all surfaces. But, be warned, if you miss even a little... it can cost you a lot. In particular, if your mold might sit on a shelf for a few days or a few weeks before casting, I urge you to use another type of release agent. Plaster repair Plaster waste molds are generally used for one-offs. Each subsequent casting will cause the inner surfaces of the mold to lose integrity. After a few castings, small dimples may develop. You can stretch the life of your mold by using plasticine to fill dimples and correct irregularities. Outer surfaces may become chipped. I use a product called "Fix All" (commonly available at any hardware store) to repair plaster castings and outer surfaces of the molds. You must first seal the surface to be repaired with a coat of shellac or varnish thinned down to a milky consistency, in order to get good adhesion. (Otherwise, the old plaster will draw the moisture from the repair and cause it to become powdery. ) Only coat the area to be repaired. To repair plaster with plaster, soak the mold in cool water for 20 minutes. Repairs must be completed before the mold dries. You can add new plaster to set plaster only if both surfaces are wet and will have approximately the same drying time. |
Other Sources For Information:On the web...
Books and Periodicals...
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