 Introduction to materials | This Fall I had intended to take another bronzecasting class at Maine College of Art, but when that class was canceled, I opted to take a class in Silicone Moldmaking. This class included the use of a technique that was new to me ... the utilization of a "pressure pot" to cure and "de-bubble" both the silicone mold itself, and the polyurethane resin that would be used to make the casting. |  The pressure pot |
After presenting a general overview of materials, our instructor, Ben Asselin, explained and demonstrated the mold process that we would be using in our first project - creating a one part silicone mold....1) attach object to be cast to a cardboard base (hot glue) & add wax gates to vent any areas that might trap be prone to trapping air pockets; 2) use a piece of clear acetate sheeting to make a cylindrical mold box (leaving 1/2" clearance around the sides and top of the object), scotch tape the seams inside and out, attach box to the cardboard base with hot glue; 3) make lines with marker on cardboard inside cylinder to indicate where mold will be cut in order to remove object after curing; 4) determine the approximate volume of the container by filling it with rice and then pouring the rice out into a measuring cup; 5) mix equal parts of the 2 part silicone material and pour into mold box (we used *Moldstar 16 Fast - it has be mixed, poured into mold box, and put into pressure pot within 6 minutes of combining the parts!) ; 6) put filled mold into pressure pot, set at 55-60 PSI, for 30 minutes; 7) demold by cutting marked sides down just far enough to allow the original object to be extracted.
 Demo - Mold setup & mixing silicone |  Filling mold box |  Demolding after curing |
I chose to cast a dried pumpkin stem that had an interesting twist. Before setting the stem up for molding, I had to seal its surface with polyurethane to insure that the Moldstar wouldn't adhere to the porous textured surface. I needed to set the stem at an angle in order to get the most efficient flow of material going through the mold. Hot glue wouldn't hold it at the needed angle, so instead I inserted a 1" wood screw through the cardboard and into the bottom corner of the stem. I also created a small wax pour cup at the base of the screw and coated the screw threads to provide a smooth main gate. After adding 2 thin wax vent gates to the areas that might trap air, I mixed my silicone, poured the mold and placed it in the pressure pot to cure. The next step was to cut 1/4" deep wavy lines part way down the marked sides of the cured mold (this would serve to align the two sides when casting), and then cut straight in until the pumpkin stem and gating were revealed and could be removed.
 My mold after curing - pumpkin stem removed |  My mold secured - ready to receive the casting resin |  Original pumpkin stem & casting with gating still attached |
After I secured the mold with several rubber bands, I was ready to mix and pour the casting resin (*Smooth-Cast 305). This is a 2 part mixture that is mixed in equal parts by volume. It has a 7 minute pot life, so you have to make efficient use of time to mix, pour and get the mold into the pressure pot. I used a syringe to load the 305 into the mold. This method forces air out of the mold more efficiently and allows the material to fill in all the details of the internal mold space. Once filled, the mold was placed in the pressure pot for 30 minutes. After 30 minutes, I removed the mold from the pot and extracted the casting of the pumpkin stem. The casting was accurate and free of surface imperfections - no unwanted air bubbles! The pressure pot had done its job - now I just have to come up with a project for the pumpkin stem!
*Note: Moldstar 16 Fast & Smooth-Cast 305 are both products of the Smooth-On Company.
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