The Machinery Behind Plastic Extrusion

One of the most common methods in plastic manufacture today is the extrusion process

. This produces plastics with high strength and is commonly employed for creating cylindrical products such as tubes, piping and hose pipes. It can also be used to create flat sheets or films by utilising different machinery.

The plastic extrusion process begins by feeding small dried granules of plastic, also known as pellets, into an area of the machinery known as the hopper. From the hopper, the pellets are gravity fed into the screw, an area that rotates at 120rpm slowly heating the resin at melting point until it reaches its molten state. It is at this stage that any additives, colorants and / or setting agents are introduced and are mixed thoroughly by the rotating screw. This will give rise to the properties and characteristics of the final product. Once the resin mixture reaches the end of the screw, it is then projected into an area known as the die. This area is where the final form of the plastic is determined as the die moulds the mixture into the desired form, often tube like structures. The plastic is then cooled and left to solidify. When the plastic has partially solidified it is pulled through a cooling water bath to speed up the setting process. Here the plastic must still cool relatively slowly to ensure it regains its high strength. As mentioned above, the die will form a tube like structure but if flat films or sheets are required another process is involved. The plastic is then pulled through a series of heated rollers that flatten the plastic into uniform dimensions and left to cool on cooling racks.

A lot of skill is required to conduct the polymer extrusion process properly and to ensure the resulting plastic retains its strength and no internal or external cracks are produced. To ensure that no surface deformities exist or weak bonds are produced, the machinery involved in the extrusion process must be carefully monitored and the exact pressure and temperature are continuously maintained. Cracks will appear in the surface of the plastic if there is too much friction between the plastic and the machinery; therefore the machinery should always be sufficiently lubricated. If the temperature is not maintained properly and dips below the optimal temperature, the plastic will temporarily stick to the die. Problems also arise when the plastic does not cool evenly once moulded. If the plastic cools too quickly on the outside impurities begin to move to the middle of the plastic as the outside of the plastic oxidises along its surface.

If the machinery is of low quality and not maintained well, remnants of plastics from previous extrusions can become stuck along the surface of the machinery causing lines and cracks in the plastic. These plastics will not be able to be used and therefore a waste of resin. This is why it is vitally important that the whole extrusion process is regulated and monitored carefully at every step.

by: Ensinger

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