Polyolefins are produced from oil or natural gas by a process of polymerisation, where short chains of chemicals (monomers) are joined in the presence of a catalyst to make long chains (polymers). Polymers are solid thermoplastics that can be processed in two ways – by film extrusion or moulding. During film extrusion the polymer is heated and forced, in a molten state, through a die to produce thick sheet, thin film or fibres. The thickness of the film can be varied to produce anything from lightweight food packaging wrap to much heavier film for agricultural use. The moulding process involves heating and compressing the polymer in an extruder, and then forcing it into a mould where it solidifies into the required shape.