Plastic
Most of the toys which you find in a toy store are made of plastic. Why? Simply because plastic is a versatile material which can take any shape by pouring molten plastic over a mold. Since industry level molding is not possible, hobbyists prefer using off the rack plastic sheets, rods and slabs; these sheets and rods are cut, drilled and glued together to give it a required shape. There are different types of plastics / polymers available, and you need to make a right choice. If you are stubborn on making your own parts using plastic and mold, you need to use a technique known as “Vacuuforming”. There is also a list of different varieties of plastic and polymers variants which is available as granules, rods, sheets, blocks etc. You can either cut and drill them, or use Vacuuforming technique and make your own parts.
Vacuum Forming Plastic
Vacuum Forming (also called Thermoforming or Vacuuforming) is a simple and versatile process for designing your robot parts. The idea is to heat a sheet of plastic and then suck it down tightly over a mold. The mold can be made out of wood, plastic, aluminum etc. Once the plastic cools down, it retains the shape and you have your quick robot body ready. Plastic sheet can be either Styrene (a.k.a polystyrene) or Acrylonitrile Butadiene Styrene (ABS) although I recommend Styrene as it is easier to heat up and form shape. Parts developed using this technique is widely used in model airplanes, toys, boats, etc.
Although the concept is simple, the catch is expensive and bulky Vacuum forming machine. With a bit of brain storming, you can use your kitchen Oven and create robot parts.
PVC
Polyvinyl chloride is a thermoplastic polymer which is amongst the most widely used plastic across the world as it is cheap, durable and easy to handle and is available in different shapes and colors. PVC is often expanded to create empty spaces inside the material which makes it thicker without additional weight. This material can be easily cut, drilled and sanded and is a solid replacement for wood.
Acrylic
Acrylic sheet, generally known as Plexiglas is a great material for building robot bodies which allows you to see-through your robots. An alternative for glass as they are shatter resistant (to a certain extent) and lighter, Plexiglas is available in different sizes and shapes. You can use superglue or hot glue to stick them together. Cutting, drilling and shaping Plexiglas is always a pain; you either crack it, or even worse as your cut the sheets, it melts and sticks back together. You can cut them slowly and carefully with a hacksaw. There are a few saw blades specially designed to cut acrylic glass; use them, or purchase an expensive laser cutter.
HDPE
High-density polyethylene is a polyethylene thermoplastic made from petroleum. There are many reasons why this can be your best choice for your robots. It is inexpensive, light weight, high impact resistance, high tensile strength, withstands high temperatures, non-toxic, non-staining, and an excellent cutting board material. They are available in different colors and sizes, white being the most common color. Cutting, drilling, machining, joining all happens with ease. Use a hand-driller to drill holes, hacksaw or a blade to cut HDPE sheets, fine sand paper to sand the edges and screws and nuts to join two HDPE sheets. If you are using it as your robot base, use a bit thicker HDPE sheet and this material is very soft and might bend and break. Making parts out of HDPE is again a nightmare; so, use this when you require a flat surface or a flat base for your robot. Another disadvantage being they do not stick well and distort a bit on cooling.
ABS
Acrylonitrile butadiene styrene is again a petroleum based thermoplastic which is stronger, more durable than HDPE; but again they are more expensive than HDPE. ABS does not shirk and has a good resistance in low temperatures. Analogous to HDPE, ABS can be processed (Cut, drilled, etc.) using standard mechanical tools like hack-saw and a hand-driller. They can be fastened using screws and nuts. If you make or have a 3D printer for manufacturing your own gears and other robot parts, then ABS (or PLA) should be your first pick. Be aware that ABS burns well and never attempt to use it in your battle robot if you opposition is throwing flames.
Carbon-fiber-reinforced polymer
Also known as CFRP or simply Carbon Fiber is an extremely strong, light weight, but expensive polymer which contains carbon fibers reinforced inside other materials. Mostly respected by the RC robot community, CFRP is used in light weight robots (like copters and air-planes) with extreme strength to weight ratio. Air plane wings, helicopter blades and light weight boats ask for this very material which is available in various shapes and sizes. Processing and handling a carbon fiber material is also easier. You can use a hacksaw or a blade to cut, a driller to drill holes and hot glue or superglue to fasten them. Care should be taken while cutting and drilling; since the material is made of fiber, the small and sharp fragments might break away. (Heard of splintering?)
If you wish to make your robot base or parts in plastic, then these variants would be sufficient for most of your robot material requirements. Other materials like Lexan, Urethane resin, Acetal resin, PLA are also used. If you think you robot would require anything more than the ones mentioned, a bit of research on these materials may help.
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