Block Mold - Multi Part | Page 3
It is time to mix the silicone material. In this example we are using MoldRite 25, a two part, addition cure silicone with a 10-to-1 mix ratio. We begin by placing an empty container on our gram scale and pouring out the base in it.
Once the base is poured into the mixing container, we then add the catalyst to it. This is added at the rate of 1/10th of the gram weight of the poured base. When the two parts are combined they must be mixed well, so that there is no marbling of the colors and there is one uniform color throughout.
The silicone mix is now put into a vacuum chamber for the de-airing process to remove the air bubbles. Air bubbles will cause mold deformations and unwanted cosmetic issues
Use a container that holds at least four times the amount of material you are vacuuming to allow for the expansion of the material. Once your vacuum chamber reaches 29-inches of mercury (at sea level), the material will begin to rise (resembling foam) for a minute or two. When the material falls it will plateau and not rise any more. You will want to continue operating the vacuum chamber for another 2-3-minutes to make certain all of the air has been removed from your material. Open the petcock of your vacuum chamber to allow air inside, remove your material from the vacuum chamber.
Pour the material high, slowly and in a narrow stream, starting from the corner of the mold box, or mold, letting the material flow freely into the box or mold cavity. This method will usually not introduce any new bubbles into the vacuumed material.
Now that that the model has been de-molded, flip it over so that the silicone mold is facing downward and the model is facing toward the ceiling.