General Molding Information
If you wish to create duplicates of an object or sculpture, or would like to shape a soft material such as latex, you will need to make a mold. A mold is a hollow, negative imprint of an object into which a casting material is poured and allowed to harden, creating a replica of the original object.
Molds can be made in one or two pieces (or more if you need to get really complicated!). They can be made out of a solid block of material, or they can be layered onto an object. They can be made of hard material or soft material, depending on what you wish to cast in them. Typically, to cast a hard material like resin, you would use a soft mold, and to cast a soft material like latex, you would use a hard mold.
I will briefly describe each type of mold below.
A one-piece mold is the simplest kind of mold to make. It can be used to create a mold of a relatively simple object, with a flat, non-detailed side. It will not work well (or at all) with objects which are round, have a lot of detailing on all sides, or overhanging parts.
One-piece molds are made by laying the flat side of an object against a flat, smooth surface (such as a sheet of plastic), and covering the object with your moldmaking material. When the mold has dried or set, simply remove the object and use the mold to cast duplicates of it.
Two-piece molds are trickier to make than one-piece molds, but they may be necessary if the object you wish to create a mold of is rounded, complicated, or heavily detailed in many places. Exceptionally complicated molds may have even more than two pieces, though those are rare.
To make a two-piece mold, you will need to decide on where you want the mold to be divided, and build a dividing wall there. I've seen dividing walls made out of oil-based clay, water-based clay, cardboard, metal shims — anything to keep your molding material on one side. It is a good idea to add nubs of clay (or, if your dividing wall is made of clay, press a few indentions into it) to form keys so your finished mold will line up properly. Pour the material on that side and allow it to dry or set. Flip the whole thing over and remove your dividing walls. This might require some clean up and/or repairs to your original object. Use some type of mold release on the back of the first half of the mold (so the second half doesn't stick to it), and pour it the same way. Once it has set, you can pull the two mold halves apart and remove the original object. You then bind the two parts of the mold together to cast it.
Materials I use for making hard molds are Plaster of Paris, and UltraCal 30. The two materials are fairly similar to work with, but there are a few key differences. Plaster of Paris sets very quickly, and remains relatively "chalky" once set (you can scratch it with a fingernail). UltraCal 30 sets quite a bit more slowly, and is much harder once set. UltraCal 30 also picks up detail more precisely than Plaster of Paris, but Plaster of Paris less expensive and easier to find at local hobby or craft stores.
Typically you will cast soft materials in a hard mold. For me, that usually means rubber latex, though it can also be used for foam latex and other materials. Some materials may require a mold release, so be sure to read all directions that come with whatever material you use. Rubber latex doesn't require a mold release in a plaster or UltraCal mold.
Tip! If you have any difficulty getting latex to release from a plaster or UltraCal mold, soaking the whole thing in water for a couple minutes will cause the latex to pull free easily.
Some common materials for making soft molds are latex, silicon RTV and urethane RTV rubbers. I will mainly cover latex and silicon, since these are materials I have used. The main difference that I'm aware of between silicon and urethane rubber is that urethane is meant to always require a mold release.
Latex and silicon are completely different in the way they work.
Latex comes as a liquid, in a single container. It requires no mixing. It has an ammonia odor. It sets into a solid form by drying, and so the mold made from latex must be brushed on in several thin layers, and allowed to dry between layers. This can take a while!
Silicon RTV come as two separate liquids, which must be measured and thoroughly mixed together in order to set. RTV stands for 'room temperature vulcanizing', which means, simply enough, that the silicon will set into it's solid rubber form at room temperature. Because it sets into solid form by a chemical process, rather than by drying, it can be poured as a block mold.
Use soft molds to cast hard materials. Among other things, plaster and some resins can be cast into a soft mold. Latex and silicon molds usually don't require a mold release, since they are flexible and can simply peel away from hard materials. Very little seems to stick to silicon, especially.
Block molds are made by forming a box or wall around the object you wish to make a mold of, and pouring your mold material around the object. Block molds usually require only one pour, so they are quite quick to make, but can use up quite a bit of your mold material. Silicon is fairly expensive, so you want to be as economical with it as possible. And block molds made from Plaster of Paris, or especially UltraCal 30, can end up being very heavy! Also, you cannot make a block mold out of rubber latex molding material. Latex turns solid by drying, so thickly poured latex can take weeks to set fully - if indeed it ever does!
As I mentioned, block molds require a mold box. There is an excellent video tutorial showing how to make various shaped mold boxes on TAP Plastics' Instructional Videos page.
Tip! For hard block molds, I have found it to be more economical to pour a thin base layer of UltraCal 30 over the object first, and then pour the rest of the mold in Plaster of Paris.
Glove or layered molds
Layered molds take quite a bit longer to make. They do not require a mold box. You simply take your mold material and brush or carefully pour it on in layers. UltraCal or Plaster of Paris should be allowed to thicken quite a bit between layers, though you don't have to wait until they are completely dry. With latex, you should wait until it is dry before applying the next layer. Silicon can be mixed with something called a thixotropic additive, which will thicken it to the point that it can be brushed onto an object rather than poured on. (I have not yet worked with silicon in this manner.)
The number of layers required depends on the material you are using, and the size of your mold. A large mold requires more layers for stability. Hard molds may also require layers of plaster or UltraCal-soaked burlap for added strength. A few layers of UltraCal or plaster will do. Latex may require as many as 20.