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Plastic materials can be shaped in many ways. In extrusion, the hot thermoplastic material is forced by a screw through a die of the required shape. Sheets of plastic can be produced in a wide range of thicknesses by calendaring, pressing the material between hot rollers. Thermoplastic materials can be moulded by injecting the hot material under pressure into a cool mould. Thermosetting materials can be moulded by placing powdered plastic between two halves of a die and applying heat and pressure. Thermoplastics, such as polythene and PVC are polymers which can be melted and reshaped several times, whereas thermosetting plastics such as Bakelite and other phenol formaldeyde resins can only be heated and shaped once.