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Information for Preparation
Silicone molds have advantages and disadvantages compared to latex and flexible urethanes. They offer better detail than the alternatives, and require little release agent cutting casting costs, while offering a mold life comparable to other methods. A drawback is that silicone block molds can be expensive, and they can take up a lot of space and weigh a ton.
To make a silicone mold, the fossil or cast to be copied is first put on a clay mat. Careful attention must be paid to minimize trapped air bubbles and to place the seam lines in a less visible area for the completed cast. One should decide whether a one, two or more piece mold is required, and plan accordingly. The clay mat will act as the second side of the mold as the first side is being poured, so you want to make sure all the air vents, pour spouts for casting material, and keys/locks for mold alignment are included in this. Also helpful is to inscribe what is being molded into the clay, as it is difficult to mark the mold later.
Bones laid out in clay. Called a gang mold, several bones are embedded in rubber to save materials.
Bones laid out in clay. Called a gang mold, several bones are embedded in rubber to save materials.
Ready to pour second layer of silicone. Note the cavity left by the clay ropes from the first side. This will allow for an interlocking registration between the sides, ensuring an accurate cast.
Next, you need to make the side silicone-tight. This includes ensuring that the clay mat is sealed completely and neatly to the bone or cast, as well as to the clay, wood or Lego retaining wall. Eliminating leaks saves material, time, and money. Once this is accomplished, the item being molded should be given a light spray of urethane mold release agent (like Parfilm or Stoner brand) to ensure the item can be demolded without damage. This side is now ready to pour.
Different silicones have different physical properties, and you should choose the most suitable one for the project. Things to take into consideration are how flexible the mold is needed to be as well as curing speed and costs. Silicones should be mixed according to manufacturers recommendations, and poured so that the molded item is submerged about 5mm. "Kroutons", scrap pieces of silicone that have been cleaned of release agents and dirt may be used in the bulk of the mold to take up volume. The silicone in these pictures are made by GT Products.
After the first side has cured (usually the next day) it is time to flip the mold over. Trim the clay wall or remove legos so that the fresh silicone will lay flat. Remove the clay pad and keys, but retain and possibly adjust the vents and pour spouts. Next, ensure the walls for the second side are high enough to retain silicone over the molded item. A thick release agent is then applied to all exposed silicone. This is a very important step, and overlooking it may result in a fossil completely encased in a solid block of silicone. I prefer using "Rocket sauce", a mixture of petroleum jelly and mineral spirits. Paste wax may also be used, but it is more difficult to get a consistent cover over the silicone. Lastly the molded item and the silicone are sprayed with the same urethane mold release that was used on the first side.
