in Liquid Rubber & Plastic for Moldmaking & Casting

EFX, Displays & Theatre

     A solution of baking soda, food coloring and water is brushed onto set alginate to cause a second coat of alginate to bond to the first. Adsorbent cotton is patted into the wet alginate to cause the plaster shell to bond to the alginate. The plaster can be strengthened with cheesecloth if necessary.
     As soon as the Hydrogel^® mold is removed from the subject, plaster or wax should immediately be cast into it, because the alginate compound shrinks as the water dries from it. Only one casting is usually made into Hydrogel^® molds. The complete description of the process is shown in the Special Effects Workshop Video #2, Body & Face Casting with Hydrogel^® Alginate, available individually for a sale price of $49.00 or as part of the complete set of four videos described in the ad below. A Trial Unit of Hydrogel^® can be ordered.
     Poly Skin Wax is a liquid at temperatures as low as 125°F and is somewhat elastic after solidifying, allowing removal without breaking apart. It can be poured or brushed onto surfaces to be reproduced, or molds can be made by dipping objects such as hands or feet into the wax. Careful temperature control between 125°F and 135°F is necessary to avoid burns. The complete process is described in the Manual & Catalog and a Trial Unit can be ordered.

RTV Silicones:
Which One to Try First?

     Polytek^® offers over 14 different RTV (room temperature vulcanizing) silicone mold rubbers, in addition to many polyurethane, polysulfide, latex, and products based on other chemistry. Which moldmaking silicone is the best candidate for your new project? How does one choose?
     Since first-time uses are often learning experiences which educate the user and lead to better-suited methods and materials, we suggest starting with a softer, easy-to-use, "middle of the road" rubber. Even though a harder silicone may be more durable or more dimensionally stable, a hard rubber may be difficult to demold or may be too high in viscosity to mix and pour without vacuum to help remove mixing bubbles.
     TinSil^® 70-20 may be the first choice because the tin-catalyzed TinSil^® series rubbers are less expensive and less subject to surface inhibition than the platinum-catalyzed, addition-curing PlatSil^® line. It has low viscosity for good flow and air release, is soft enough for easy demolding, and is low in cost. Remember that tin-catalyzed silicones give off a small amount of alcohol while curing, which may cause very slight shrinkage and poor surface cures of certain polyurethane plastics in a fresh mold. Tin silicones may become brittle after some time in storage.

PlatSil^® 71-30 May Be the Best Choice
     PlatSil^® 71-30 may then be the best choice. Although slightly more expensive, there is no shrinkage during cure and no volatiles are  produced. PlatSil^® 71-30 has a relatively low viscosity and thus can be mixed and poured without vacuum. When cured, it is soft enough to be bent by hand for easy demolding. It will last for many years without degrading, if properly stored.
     PlatSil^® 71-10 is a fun product which we often use for first-time silicone molds because it cures super fast and is ready for use in only 30 minutes. It has an extra low viscosity which allows very easy mixing and pouring and has an adequate 5-minute pour time. While it is not super strong, its ease of use and fast demold often make this unique product an excellent choice for a first-time mold rubber.
     Remember, with any two-part rubber or plastic, read as much as possible about it before getting started and prepare your model or mold carefully. Weigh or measure exactly, according to directions.

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