Extrusion: what is it and how does it work ?

Note

It is highly recommended, to read the Introduction to extrusion Vol.1 – Materials, before you move on with this article.

What is extrusion?

Have you ever wondered how spaghetti is made? You probably guessed it, it’s extrusion. If you ever ate the famous snack “churros” at a fair you might have probably even seen an extruder. Figure 1 shows an example of a churro’s extruder at a fair, which pushes out churro dough into the frying pan.

The simple concept of extrusion is used here, which is just generating pressure (a pushing force) on the material, in order to push it through a die into the desired shape (with a fixed cross-sectional profile).

Figure 1, Churro extrusion at a fair in Strasbourg (Tal, 2019)

The same principle can also be applied to make long stronger objects, such as PVC pipes, aluminium beams and of course 3D-printing filament. But the problem here is that these materials cannot be pushed through a die just like with the churros machine, because they are just too solid and strong. This is why the material should melt first. If you have read this introductory article INTRO lv1 introduction to polymers, you probably know that 3D-printing filament always consists of a thermoplastic polymer. These polymers are usually manufactured in pellet form, also known as granulate (Figure 2). At room temperature this granulate is solid, so it must be melted into a liquid first before it can be formed into 3D-printing filament.

Figure 2, PLA granulate

A polymer extruder takes care of both the melting and forming of the polymer. This is done with the help of an extrusion screw. As you can see in Figure 3, the pellets/granulate are loaded into a hopper, which feeds the material into the extrusion screw made from hardened steel. This screw is placed within a cylinder, also known as a barrel. By rotation of the screw, the granulate is conveyed forward in the barrel.

Figure 3, Polymer extrusion (Kopeliovich, 2014)

As the granulate is being conveyed forward, it also gets heated up by the heating elements which are placed around the barrel. The granules which come in contact with this hot barrel will partially melt, and form a sticky substance. This sticky substance also known as melt film, increases the friction between the granules and the barrel. In the meantime the extrusion screw tapers up in the barrel (Figure 4), which increases the pressure.

Figure 4, Tapered extrusion screw (Dynisco, 2018)

As a result, enough friction is generated to completely melt the polymer down to a liquid. At the end of the screw the liquid polymer is pushed through the die (Figure 3), with the high pressure that is generated by the tapered screw. The shape of the opening of this die, sometimes also called a nozzle, determines the shape of the end product. For 3D-printing filament this is just a round opening.

So in short, extrusion works as following:

  • The polymer melts as a result of:
  1. Heat via conduction, thanks to the external heaters
  2. Heat via friction, thanks to the rotation of the screw
  • The polymer gets pushed through the die into the desired shape, thanks to the pressure generated by the rotation of the screw.

Usually after the polymer has been extruded it needs to be cooled down so it becomes solid again, and retains the desired shape. If the extrudate needs to be cooled down very rapidly, which is often the case with large-scale filament manufacturers, the filament is quenched in cool water. For smaller scale extruders air cooling is usually sufficient.