|Plastic extrusion is defined as a manufacturing process (high-volume), in which the raw plastic / raw plastic material is melted and then formed in a continuous profile. With this process various products can be manufactured such as pipe/tubing, window frames, weather stripping, wire insulation, and adhesive tape. In general, the plastic materials that are used in the process of extrusion are Polyethylene, Polypropylene, Nylon (Polyamides), Polystyrene, Polycarbonate, Acetal, Acrylic, and Acrylonitrile Butadiene Styrene (ABS). These are just the primary plastic materials that are extruded. Apart from these, there are many other plastic materials that can be / are extruded all around.
Plastic Extrusion Process
Through the feed throat, the material enters and comes in contact with screw. The feed throat is an opening near the rear side of barrel.
This permits the plastic beads to slowly melt as these are forced through the barrel, lowering the risk of overheating that may result in polymer’s degradation. Additional heat is contributed from the intense friction and pressure that takes place in the barrel. Actually, if a specific extrusion line is operating on a fast-enough material, the heaters can be switched off & the melt temperature can be maintained by the effect of friction and pressure alone in the barrel. In a large number of extruders, there are cooling fans for keeping the temperature less than a set value, in case there is too much heat generation. In case the forced air cooling seems to be incommensurate, then cast-in heater jackets are used. These commonly use a closed loop of distilled water during the exchange of heat with the city water or tower. Further steps of plastic extrusions are as follows:
The assembly of screen pack / breaker plate also serves the purpose of creating a back pressure inside the barrel, which is needed for proper mixing and uniform melting of the polymer. The amount of generated pressure can be easily ‘tweaked’ by changing the screen pack composition i.e., number of screens, wire weave size of the screens, & various other parameters. The combination of breaker plate & screen pack also serves the purpose of converting the “rotational memory” of molten plastic into the “longitudinal memory”.
After the molten plastic passes through the breaker plate, it enters the die. The die provides a specific profile to the final product and it must be designed in such a way that the flow of molten plastic is uniform from a cylindrical profile to the product’s profile shape. Until, it is a continuous profile any imaginable shape can be developed.
At this stage, the product is now cooled by pulling the extrudate by way of water bath. It is well known fact that plastics are extremely good thermal insulators & hence are difficult to cool swiftly. In a pipe or tube extrusion line, a carefully controlled vacuum is used to act on the sealed water bath. This is done for avoiding the collapse of newly formed and still molten pipe or tube. For products like plastic sheeting, the required cooling is acquired by pulling through a definite set of cooling rolls.