What's That Process? Part 3
By special request, the “What's That Process?” file is being extended to include other process heating topics that do not specifically relate to drying. This month, I'll begin a look at extrusion.
ExtrudingExtrusion is the process of forming a material into a predefined shape under pressure and commonly at elevated temperatures. While this is a simple definition of the process, extrusion and its many forms are quite complex. Not only that, it is used in such a variety of industries, forming so many differing products under such diverse conditions, that one definition hardly fits. Let me elucidate.
When you press your tube of toothpaste to apply the round bead to the brush, you have extruded the toothpaste out of the tube. When a gigantic press forces aluminum at high temperature through a steel disk, it is extruding a section. When a machine forces a mix of corn through a happy face shape to make a cereal, it is extruding the cereal. When a plastic producer forces molten plastic through a plate to form a shape, they are extruding the plastic. When a machine forces a filter cake through a perforated plate to make noodles, it is preforming or extruding the noodles. I could go on and on. What is common here?
Well, for one, all of the processes require pressure to force the material through a hole or holes. The plate with the hole(s) gives the material its final shape, and this plate is termed the die. The shape of the cross section of the so-formed material is termed the profile.
Second, the material can be forced through the die. This is important and principally the reason why this process is included in the process heating arena. (More on this in a bit.)
What is not common in all of these processes is the application of heat -- not all extrusions occur at elevated temperatures. If the product can flow under pressure without heat, extrusion can take place at ambient temperatures. However, for many products, heat is required. Extrusion is considered a heating process because the extrudate is either extruded at high temperature and requires cooling after extrusion or, very commonly, the extrudate is then processed in a dryer or the like.
The method upon which the product is heated, if it requires heating before extrusion, is obviously dependent on the material. If it is metallic, it will likely be heated in a furnace of sorts. Other products may be heated in process vessels or mixers. The material then is fed into the extruder, which shapes the product.
Products such as pasta, dog food and some cereals are then dried and packaged as the final product. Metallics are formed and cooled. (I do not consider rolling of steel, either hot or cold, as part of the extrusion process family.)
Still within the extrusion family but requiring a separate discussion is the extrusion of plastics. Plastic extrusion has evolved to the point where all of the required processing to heat and form the product occurs in one machine termed an extruder. There are many different types of plastic extruders ranging from high intensity mixers through single screws to twin-screw co-rotating compounders. All plastic extruders invest mechanical energy into the plastic, which uses the property of shear to melt the plastic and form what is termed a “melt.” Both single-screw and twin-screw extruders are composed of barrels with rotating screws internal to the barrel. The design of the screws allows the plastic to be worked to a molten state and then provides sufficient pressure to force the melt through the die. For extruders where the design does not provide sufficient pumping efficiency, a melt pump is used to provide the pressure to force the melt through the die.
Plastic extrusions are vast. They include items such as pipes, fascias, moldings and other types of continuous strips. Commonly, the extruded plastic is pelletized either at the petrochemical refinery where the plastics are created, or at reprocessing or compounding facilities. These pellets then are used for other process heating applications such as injection molding, blow molding, thermoforming and rotomolding. These process heating topics will be covered next time.