The density of plastic is typically 0.9-1.4 g/cm3, while a lightweight metal such as aluminium has a density of 2.7 g/cm3. Steel has a density of 6.6 g/cm3.
Because of their light weight, plastics reduce production and supply-chain costs in several ways.
Less raw material needed
Less energy in production
Easier handling/carrying
Less fuel in transport
Less air pollution in transport
Less waste
Think of the weight difference between a glass bottle and a plastic one. In most cases, a solution involving plastic will be environmentally superior to one using any other material.
