Polymers are made by joining together many monomers and create large molecules. Plastics are composed of chains of polymers; they may be shaped when soft, but once hardened, they are no longer malleable. Some of the different types of plastic include polyethylene terephthalate (PETE or PET), high-density polyethylene (HDPE), polyvinyl chloride (PVC), low-density polyethylene (LDPE), polypropylene (PP), polystyrene or styrofoam (PS), polycarbonate, polylactide, acrylic, acrylonitrile butadiene, styrene, fiberglass, and nylon. Polymers are resistant to chemicals, they are heat and electricity insulators, they are light, have varying degrees of strength, and can be processed in many different ways. Some of the ways that plastics are used in aerospace industry to increase efficiency are in the creation of aftermarket components, aircraft interiors, aircraft exteriors, and 3D printing.