Perhaps the most interesting of these processes is the manufacture of pipe where not only is the outside diameter controlled but also either a fixed or floating die is also used to set the internal diameter and hence the wall thickness.
Commonly extruded materials are copper (pipe for plumbing), aluminium (various extrusion profiles for tracks, frames, rails), steel (rod, track) and a multitude of plastics (pipes, rods, rails, seals).
It is common in the plastic extrusion process to use plastic chip, which is then melted and rather than drawing the material through the die to squeeze the plastic out of the die in a similar fashion to the extrusion of toothpaste from a tube.
Extrusion has found a great application in Food Processing. Various products like pastas, breakfast cereals, ready to eat snacks, fry-ums etc. are now manufactured by extrusion. Softer foods such as meringue have long been piped using pastry bags.
Food Extrusion was used as a shaping tool since time immemorial. In India, it has been used to shape products like chaklis and sev. In Italy, it was used for the manufacture of pastas. The first industrial extruders came into existence around 75 years ago (Mercier, Linko & Harper 1989). Initially used only for mixing and forming pasta and for the mincing of meat, they have morphed into high temperature short time bioreactors that transform raw ingredients into intermediate or final products.
The first industrial food extrusions involved the use of piston or ram type extruders to stuff casings in the manufacture of sausages and processed meats (Harper 1981). These were followed by meat choppers and mincers, which consisted of a screw forcing the meat out of a small die plate. These were the first twin screw extruders used in the food industry. The pasta industry became the second food industry to use extrusion with the development of hydraulically operated batch cylindrical ram macaroni presses around 1900. However, the application of the single screw extruder which revolutionized the industry was its use as a continuous pasta machine in the 1930s. The pasta press mixes semolina flour, water and other ingredients to form a uniform dough. The screw of the extruder works the dough and forces the mixture through specially designed dies to create the variety of shapes that pastas are available in now.
In the late 1930s General Mills used the extruder in the manufacture of ready to eat cereals. Extruded corn collets were developed around the same time. However, the concept was not commercially developed till 1946. The desire to precook animal feeds to improve digestibility and palatability led to the development of the cooking extruder late in the 1940s, which has greatly expanded the application of extruders in the food industry.
Cooking extruders come in a variety of sizes and shapes and provide the capability to vary the screw, barrel, and die configurations as required by the product. Temperature is controlled by direct steam injection or heating through external barrels. Preconditioning of the feed in an atmospheric or pressurized chamber allows ingredients to be partially cooked and uniformly moistened before extrusion.
Modern food extruders can be designed to combine a range of unit operations into one process which does not require much pre or post processing. They can carry out one or more of the following in one step: transport, grinding, hydration, shearing, homogenization, mixing, compression, degassing, cooking with partial melting and plasticization of the mix, starch gelatinization, protein denaturation, destruction of microorganisms and anti-nutritional factors, pumping, shaping, expansion, formation of porous and fibrous texture and partial dehydration. Depending on their design, they can be used to make a variety of products including pastas, breakfast cereals, puffed snacks (corn puffs/collets, kurkure, cheese balls etc.), meat substitutes like soya nuggets, fry ums, breading substitutes, modified starches, soft-moist and dry pet foods and confections.
The second revolution in food extrusion came with the use of variable pitch single screw extruders. These extruders further improved the mixing versatility of the extruder. The most recent advance for the food extrusion industry has been the use of twin screw extruders. The screws either rotate in the same direction (co-current) or in opposite direction (counter-current) to each other. These extruders, while more complex than single screw extrudes, offer better control over residence time distribution and internal control of shear for thermolabile materials. They are also more versatile in that they accept lower moisture feeds and are self cleaning due to the wiping effect of the screws.
Food extruders today are all screw extruders and the early ram and piston type extruders have disappeared from the industry. The various components of an extruder are a drive, feed assembly, extrusion screw, extruder barrel and an extruder discharge. The drive consists of a support / stand, a drive motor, a set of gears for variation of speed, a gear transmission (to reduce speed and increase torque) and a thrust bearing (to support and centre the screw and absorb its thrust).
The type of feeder section depends on the material to be fed. Different feeders are available for dry, wet and slurry like materials. For solids and dry materials hoppers / bins, vibratory feeders, variable speed screw conveyers and weigh belts are used. Water wheels, positive displacement pumps, variable orifices and variable head feeding devices are available for liquid or slurry like feeds. These feeders can be batch or continuous feeders as per requirements. Often the raw materials are fed with such feeders into a preconditioner from where they are fed into the screw section.
