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Basic Information for Flexible Molds 
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Unread post Basic Information for Flexible Molds
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There are three materials that are most often used for making flexible molds. These include latex, urethane rubber, and silicone rubber. Which material you use can be determined by the project you are working on, your experience with mold making, and your budget.


Latex - This material is inexpensive, and it is easy to use because you don't have to mix anything. You just paint it on, and then wait for the ammonia to evaporate out of the latex. As it evaporates, the latex acidity changes, and it cures. To make a latex mold, you simply paint on layers of latex until it reaches the desired thickness. Latex also has the advantage of being able to stretch and roll up. This allows you to make a mold with no seam lines. You simply fill the latex mold with the casting material, let it set, and then peel the latex off like a rubber glove. The downsides to latex are the ammonia smell that will permeate your work area, the time-intensive need to paint on many layers and let them dry, and the fact that the latex will shrink, often up to 10%. I also have noticed that my old latex molds have begun to shrink even more, and they have grown stiff and brittle.


Polyurethane Rubber- I don't like to use this material. It is primarily used for industrial projects rather than small-scale hobby work, and it takes a lot of extra work to prepare models for molding. Polyurethane rubber will permantly "glue" itself to your sculpture if you haven't fully coated it with the proper release... and I really don't like the toxic smell!


Silicone Rubber - In my opinion, silicone rubber is the ideal product for making molds of costume pieces. It is strong and flexible, so it can be used on rounded or undercut designs. Silicone is easy to mix, it has no bad odor, it will not stick to your sculpture as long as the surface has no pores, and it will capture every detail of your design. Some silicone products will shrink, but only by a very small percentage. The main problem with silicone molds is the cost. The rubber is quite expensive, with many projects costing from $20 to $50 for the mold materials. The finished product is well worth the cost, though. You will have a durable mold that will be able to make many castings, it will last for years, and it will not stick to the materials that you cast in it. Silicone is also able to be used with a wide variety of materials, such as fiberglass and casting resins, and high temperature castings that would damage other kinds of molds.

There are a variety of silicone choices when it comes time to make your mold. I prefer to get my materials from Smooth-On. They have good customer service and good products. For ease of mixing, Smooth-On's OOMOO silicones are great. They don't have bubble problems, they're easy to pour, and you don't need a scale when you mix them. They don't last long, however. After about a year, you will see your molds begin to deteriorate. So if you want to have a long-lasting mold, it's better to get a higher quality silicone. For long-lasting molds, I prefer Smooth-On's Mold Max, Smooth Sil, or Dragon Skin silicones. Those will last for many, many years! They're a bit more difficult to work with, though. They are thicker, they tend to get a lot of air bubbles (and should be degassed in a vacuume chamber), and some of them have to be measured on a gram scale in order to get the amounts of each component exactly right.


Pam :-)

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Wed Jul 15, 2015 5:47 am
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