There are occasions in robot building, as well as general hobbies, where you require a custom plastic or rubber component. While there are a variety of methods to create custom pieces from milling and lathing to 3D printers; for the majority of custom parts none are simpler or less expensive than creating custom molds or castings. Some designed pieces might be too intricate or complex to be moldable, but for the rest, mold making can be an invaluable tool.
At the hobby level, there are two basic styles of mold making that I will cover here: layered and poured. In either method you must first start with a design template, or blank, which the mold will be crafted after. If you are duplicating an existing part, that is your blank. For more customized pieces you will have to create the blank first.
I have found no easier way of creating a blank than using cheap, oven setting children's "clay". Most of the time these compounds are not clay at all, but rather a soft plastic polymer that hardens when heated, and are available at craft stores. The current clay of choice for my projects is Sculpey, as it is inexpensive and comes in a variety of colors (the bright colors help make it easier to see and thus clean clay residue from completed molds). The clay can also be used for single use one piece molds by pressing the shape of an object into the clay.
To create your mold template piece you just squish, carve, cut and form the polymer clay into the shape needed and then bake in the oven at the required temperature and duration. You can often find needed sub shapes for your blank in other objects, such as a round shape from pushing the clay into a water bottle cap or the middle of a spool of wire. Just don't create it on the kitchen table; a nonporous surface is best to avoid any potential staining of your work area.
Another nice feature of using polymer clay is that you can fix any problems you might find with your blank easily just by adding a little more clay. Below are two images of a mold blank for a ball bearing holder. The first image is of the initially baked clay, where you can see a chip is missing. In the second image I merely filled in the chip with unbaked clay; it is not permanent, but it does not have to be.
Once you have a suitable template, you are ready to get started on creating the mold, but first you must figure out what style of mold making best suits your purposes.