Understanding the Extruder Barrel, Where cooking and shaping happen

    Continues with a discussion on extruder barrels, where the final action happens on extruded products with regards to cooking and shaping. Typically, extruded products are cooked to where starch gelatinisation is in the 90–100 percent range. There are many types of extruder barrels, but in all of them rotating screw elements turn inside a tube where friction and temperature elevate, creating a viscous mass that flows out of a die orifice under pressure. Some tubes are smooth; others have straight or spiral ribs. Straight ribs promote retention time, while spiral ribs move product through faster.

    Screws come in many forms, from single flights through multiple flights stacked from inlet to discharge, so that the advancement of materials is basically controlled in three zones: the feeding zone, kneading zone and final cooking zone.

    The feeding zone uses deep single-flighted screws to move product out of the inlet so more material can enter. Different capacity designs are generally based on the actual diameter of the screws or tube. At 20 tonnes per hour, you need to move product quickly in this area so more feed can enter from the preconditioner. As screws get larger in diameter, volumetric capacity grows exponentially.

    The barrel length can be broken into thirds. The first zone is the feeding zone, where water can also be added. This is followed by the kneading zone, where additional flights on the screws assist in temperature elevation and pressure begins to increase. Between most extruder screws, a round diameter short shear lock or steam lock is positioned, and their diameters increase as you move forward in the barrel. Their function is to disrupt the flow and cause the material to exit the screw behind and pass over the lock, creating even mixing. Generally, these locks are completely surrounded by product, so if steam is injected, it does not move backwards or forwards and is incorporated into the mix. Selected areas in the middle of the barrel – where the barrel is now full – promote steam inclusion. This centre zone is where materials start to melt into a fluid-like mass.

    The final cooking zone is where additional mixing elements – double, triple and even quadruple flight screws – promote final cooking. Final melting of the mass occurs here before exiting the die. Historically, different final cooking screws had decreasing volumes, either from being tapered or by design, so that the highest pressure occurred behind the die. Variable speed screw drives assisted in speed change to aid final cooking when needed: more speed means more cook; less speed means more retention time.

Devices available from different manufacturers had the ability to open or close an orifice, which could increase barrel fill. This, coupled with speed control, allowed elevation of higher meat inclusions as the industry desired. This development allowed cook to be controlled at this location instead of right at the final die. It also allowed for less downtime, avoiding the need to change screws based on formulation changes. Need more cook? Reduce the orifice opening. Need less? Open it up.

    In summary, the extruder barrel finalises the cook and level of cook by element design, water added, steam added and possibly other liquids as well. The same applies to twin screw extruders, where the screws intermesh and the result is a more positive pump, sometimes needing extra restriction for friction development. In single screw designs, there is no limitation in screw pitch (distance from one flight to make 360 degrees around the screw), number of flights, or increasing or decreasing volumes. Both single and twin screw designs have their advantages, and the choice depends on what you wish to make.