The finished product you are attempting to create has a lot to do with what material you will want to use when making your mold, as well as the method you will wish to undertake. There are a few primary concerns that have the greatest effect on your finished mold, each is listed below.
- Object Shape - the number one determining factor in your mold creation. An object with a noncomplex shape with the largest side being flat is a good candidate for layered and single piece mold making. While I do not recommend plaster as a good complete mold, it does provide a suitable litmus test here by asking yourself, "If I shoved this in a gallon of plaster of paris and let it dry, could I remove it without breaking it and/or the plaster?" If the answer is "yes", then the object is a good candidate. If the answer is "no", then you will want to use a two piece mold. If the answer is "maybe", then you will need to consider the mold's flexibility.
Mold making materials come in a variety of flavors and colors (do not eat it though). The two most important aspects are set-time and flexibility. Flexibility is known as Shore Hardness, and can get a little confusing if you do not know what you are looking at. There are two Shore Hardness scales used by manufacturers (Durometer A, Durometer D) as well as Rockwell rating, each using similar numbers, but each on a totally different level. Generally, the lower the number, the softer the material, just pay attention to which scale is used as a 50 for Shore A is a lot softer than a 50 for Shore D.
You might ask why this matters, and I assure you I am getting to that. The outer wall strength of your mold is determined by the wall thickness and Shore Hardness. We will get back to that in a minute. The less flexible your mold is, i.e. the higher on the Shore scales, the less of an odd shape design you can put into it. If you mold is very flexible, you will be more likely to be able to wiggle a completed casting from the mold that has complex protrusions and shapes, than from a mold made from a more rigid material.
- Object Size - the size of an object greatly determines what type of material you will want to create your mold from. As I mentioned above, the Shore Hardness tells you the flexibility of the mold making compound, and as I said the wall strength comes into play with that. The larger an object is, the more resin you will have to pour into the mold to create the duplicate object, the more resin in the mold, the more stress and strain on the outer walls, and thus, the more likely the walls of the mold (inner and outer) are likely to deform. Instead of pouring resin into a mold to make a large cube, you suddenly have something that partially resembles an egg.
Wall thickness can only compensate to a limited degree, after that you need to create a more rigid mold. "But my object has all these twists and turns that I wouldn't be able to get out of a rigid mold!" Well, there is some hope; you have the option of reinforcing the walls of your mold in a variety of ways. For internal walls, you can use toothpicks or sewing needles or small pieces of plastic or anything else that is more rigid, encased in the mold itself. By inserting these objects into key areas prior to pouring your mold and then covering them with the molding compound, you can in effect get the best of both worlds. For outer walls, you can create a second mold around the first using a more rigid material (in this case plaster is a decent idea).
- Quantity - the number of objects you wish to create will effect what materials you use for the mold, as well as the method you want to use. Molds made from pressing an object into clay, or made from plaster of paris, are only good for one or two uses (a few more if you are really careful). Layered molds will generally withstand a dozen or so product creations before losing shape. I have one and two piece poured molds that are several years old and still kicking strong.
- Object Material - the last concern that I will cover is what the object will be made from. Materials that can be used to create a mold can be used to create an object, and vice versa. The catch is you want to make sure that the material the object will be made from is not the same type as what the mold is made from. As a general rule, if your finished object will be a plastic you will want to use a rubber mold, if the finished product is to be rubber, you will want to use a plastic mold. If the material will have both (don't ask me how, but I have done it), you will want to go with a rubber mold.
There are additional factors to consider when creating a mold such as cost, Hazmat concerns, availability, etc. Just keep all of the above in mind, don't break the bank, and read (and follow) all the warning labels and instructions; and you should be fine.