What is Plastic Extrusion Part 2: The Extruder and Screw

In the first post in our series ‘What is plastic extrusion?’, we discussed the basics of the plastic extrusion process. If you haven’t already read this post, you can check it out here. In the second post of the series, we are focusing on the extruder and the screw and how they influence the extrusion process.

The extruder is a device consisting of mechanical and electrical components connected together. Residing within this combination of mechanical and electrical components is the screw or the heart of the extruder. These connected components support the heart. The extruder control is like our brain, it tells the rest of the extruder what to do using feedback from its various components. The extruder motor and gearbox can be compared to our legs and arms, they help get things done. However, without a heart, none of these bodily functions would work. If the extruder did not have the screw it couldn’t achieve its objective of melting a solid plastic into a flowable mass. This is not to discount the importance of the other components, but a poor performing screw or heart makes it difficult for the extruder to perform as expected. So what is the screw’s objective?

The screw is connected to the gearbox which is connected to an electric motor. The screw resides inside a barrel or cylinder which is encapsulated with heaters. As the screw rotates inside the barrel it takes energy from the electric motor and imparts into the plastic pellets causing them to melt. The screw has various geometric features which causes the plastic to transform. The plastic enters the extruder at a given temperature and it must be raised to a certain temperature to flow. This is achieved through shear and exposing the plastic to the heated barrel surface; the screw does all the work to the plastic.

All plastics have certain characteristics, a melting or softening point, specific heat, specific gravity to name a few. Each of these characteristics plays a part in the amount of energy required to process the plastic; but also the shape or geometry of the screw and extruder design. Let’s focus on the screw geometry or what we call the screw design.

Assuming the extruder is properly designed a significant amount of due diligence is required to properly specify and design a screw. An improperly specified or designed screw can make a really good machine design perform poorly. As we continue our discussion on “What is plastic extrusion”, we will present what is important to discuss with your machinery or screw supplier in the future.

Milacron both designs and builds full extrusion systems in house, maintaining complete control of your precise equipment needs. From extruders, to new and rebuilt extrusion barrels and screws, to pipe heads, dies and downstream equipment. To learn more visit us at milacron.com or contact us.

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