Polyurethane foam refers to a number of different types of foam consisting of polymers made of molecular chains bound together by urethane links. It can be flexible or rigid, but has a low density. Flexible polyurethane foam is most often used in bedding and upholstery, while the more rigid variety is used for thermal insulation and in automobile dashboards.
Flexible polyurethane is made in blocks and then cut to its desired shape. This is the way in which furniture cushions are made, for instance. Some materials can substitute for this application of polyurethane foam, such as polyester fiber or steel springs. However, polyester fiber does not recover nearly as well as foam does after compression. Steel springs recover well, but can become noisy as they get worn out. They also require some form of cushioning between them and the user.
In addition to the flexible and rigid types, semi-flexible polyurethane foam also exists, and is used extensively in automobile interiors. Seats, armrests, and headrests are frequently made from the semi-flexible variety of this material. In the case of seats, the foam is formed in a mold, and then removed and upholstered.