Plastics processing techniques

The processing techniques of plastics depend on the nature of the polymers and the destination of the finished products. Some methods are regularly used for industrial manufacturing: injection, blow injection, extrusion, blow extrusion, thermoforming.

The injection

The plastic, in the form of granules, is poured into a hopper to feed a worm housed in a heated tube. It is compressed, kneaded and heated. This mechanical and thermal treatment provides a homogeneous slurry that is pushed by the rotating screw to an outlet nozzle. The material expelled under pressure by the injection screw through the nozzle fills a closed and cooled mould. Upon contact with the cold mould, it takes shape and solidifies.

This process allows a discontinuous transformation of thermoplastics. The finished or semi-finished products of complex shapes are obtained after demoulding in a single operation.

•  Blow injection This process is used to make most bottles and vials. We use semi-finished parts obtained by injection called preforms. These preforms are then warmed and then a stem stretches it to the bottom of a mould. Finally a powerful air jet slams the material against the walls of the mould. The preform then takes the shape of the mould and cools. As for the injection to change shape just change mould.
  
•  Extrusion Extrusion is a process of continuous transformation. As with injection, the pellet enters a heated tube with an endless screw. The homogenized soft material is pushed, compressed, then passes through a die to be put to the desired shape. It is a kind of Churros machine that allows to obtain products of various forms. With this technique, we manufacture long products such as tubes or profiles for the windows of plates and sheets of plastic… The tube or profile comes out continuously, it is cooled and then cut to the desired length.
•  Blowing extrusion This process makes it possible to manufacture hollow bodies. It starts by extruding a solid tube called the parison. This hot plastic tube coming out of the die is cut and a cold mould in two parts closes around it. Then, air is injected into the material by a blow rod and the polymer is pressed into the inner walls of the mould which is quickly cooled. The process ends with a demoulding
•  Inflated extrusion It is a variant of extrusion that makes it possible to manufacture plastic films. This process consists in exiting the extruder to expand with compressed air a polymer sheath previously formed. The outlet of the extruder is vertical, compressed air is injected into the melted material which inflates and rises vertically into a long film bubble. After cooling the rolls flatten the film into a flat sheath that wraps on coils. We produce films used in the manufacture of packaging, garbage bags, etc.
• Forming or thermoforming This is a secondary process in which the material arrives in the form of plates, tubes or profiles. The material is softened by heating before being deformed and shaped by a metal mould. It is possible to make parts with thin walls and large parts. This technique is used to manufacture for example yogurt pots, shower booths, bathtubs, body parts.