Extrusion blow moulding is the process used to manufacture plastic bottles and containers used in the packaging industry. The process involves a molten polymer being passed through a die to emerge as a cylindrical hollow pipe, called a parison. A hollow mould then closes around this, forming a tight seal, and air is blown into the open end of the parison. This causes the polymer to stretch and inflate until it fills up the mould and takes the shape of it. After a cooling period, the content is emptied.