I'll be covering two distinct mold types and how to create them for your model. There are two part molds that can be easily assemble/disassembled. The other kind we'll cover are known as flexible molds. These molds can be reused for casting purposes.
Two part molds can make your model in two halves separately, or be combined to create the model all at once. I've created a two part block that can be scaled for your project and used for both methods. You can download the two-part mold from Thingiverse.
Using a 3D modeling program of your choice, overlap the custom model with the two part mold block and cut a hole with it. I'll be using Blender to accomplish this. If you'd like to follow along with me, use a boolean difference modifier to cut the object from the mold. You can learn more about cutting objects with the boolean modifier here.
If you’d like the model to set as one piece, you will need to cut a hole in the block for silicone to pour in through. The two halves can be held together with rubber bands or tape. I created spherical notches to keep the mold perfectly aligned.
Flexible molds are really neat because of their versatility. We're essentially creating a mold for another mold. The final product will be a flexible piece of silicone rubber than be used for future casting. This is ideal for things like jewelry, cosplay, and other crafting needs.
To create a flexible mold, start with a block and position a smaller block to overlap with it. This block will form the edges of our flexible mold. Using a boolean difference modifier, cut a cavity from the starting block. Place the desired design inside the cavity. Consider the angle you use when placing the design, as this will affect the final mold.