How plastics are made

Which plastic production technique is used depends on polymer type and finished product application. Several methods are regularly used in industrial manufacture: injection moulding, injection blow moulding, extrusion, extrusion blow moulding and thermoforming.

•  Injection moulding

The plastic material in the form of granules is poured into a hopper feeding a worm screw housed in a heated tube where it is compressed, mixed and heated. This mechanical and thermal treatment creates a uniform flowing paste that is moved along by the rotating screw towards the outlet nozzle. The material is expelled through the nozzle under the pressure created by the injection screw and fills a cooled, enclosed mould where it takes its shape and solidifies.

This process is discontinuous and produces finished or semi-finished thermoplastic products with complex shapes in a single operation, once removed from the mould.

•        Injection blow mouldingThis procedure is used to manufacture most bottles and vials. Semi-finished objects obtained by injection called preforms are heated before a rod draws them onto the bottom of a mould. A powerful jet of air then presses the material against the mould and cools it down. As is the case with injection moulding, all that is needed to change the shape is a different mould.
•        ExtrusionExtrusion is a continuous production process. Like injection moulding, the granules are drawn into a heated tube by a worm screw and the uniform soft material is compressed and pushed through a die to give it its desired shape. Extruders resemble machines for making Churros and can produce products with different shapes. This production technique allows very long products such as pipes, window profiles, plastic sheets and panels, etc. to be produced, whereby the pipes and profiles emerge continuously, are allowed to cool and are then cut to the desired length.

•        Extrusion blow mouldingThis process is used to manufacture hollow items. A solid tube called the parison is first extruded, then cut when it emerges from the die while still hot, before a two-part cooled mould closes around it. Air is then injected into the material through a blowing tube and the polymer is forced against the walls of the mould where it rapidly solidifies. The process ends when the product is removed from the mould.
•        Extrusion blowingThis is a variant of extrusion used to manufacture plastic film and consisting of expanding a preformed polymer parison with compressed air as it emerges from the extruder.The output stage of the extruder is vertical and compressed air is blown into the melted material which inflates and rises vertically in a long bubble of film. After cooling, rollers flatten the film into a flat sheath for winding onto spools. Extrusion blowing is also used to manufacture packaging, refuse bags, etc.
  

•        Forming or thermoformingThis is a secondary production process in which the material arrives in the form of panels, tubes or profiles and is softened by heating before being moulded into shape by a metal mould. This technique is used to make very large items and items with thin walls such as yoghurt pots, shower enclosures, baths and vehicle bodywork.