As the material leaves the preconditioner, it enters the extruder barrel. Conceptually, the barrel of a single screw extruder can be divided into three separate zones depending on what is happening to the process mash in that zone. Here the major transformation of the raw preconditioned material occurs which ultimately determines the final product characteristics.
In the feed zone, the conditioned extrudate is simply received from the conditioner and transported forward in the barrel to a point where the crosssection of the barrel is completely full and an elastic plug is formed. It is the initial section of the extruder barrel is designed to act as a feeding zone to simply convey the preconditioned material away from the inlet zone…show more content… It is the transition zone of the barrel that is identified by the fact that the extrudate changes from powder to an elastic dough. The compression ratio of the screw profile is increased in this stage to assist in blending water or steam with the raw material. The temperature of the moist dough is rapidly elevated in the final few seconds of dwell time within the extruder barrel.
In the final stage known as the cooking zone, extreme pressure is applied to the extrudate and high levels of heat are induced by friction to increase the temperature of the dough above 100 ˚C. From a thermodynamic point of view, 75% or more of the work done in the extruder barrel is done in the cooking zone. This is easily seen in maintenance records of any extruder that will show that the final screw and barrel sections require replacement much more often than any other…show more content… First, the die offers restriction to product flow thereby causing the extruder to develop the required pressure and shear. The final die also shapes the extrudate as the product exits the extruder. Die design and its effect on expansion, uniformity, and appearance of the final product are often overlooked.
The amount of expansion desired in the final product can be controlled by formula manipulation and open area in the die. Unexpanded but fully-cooked pet foods, aquatic and livestock feed generally require 550 to 600 square millimeters of open area per metric ton of throughput. Highly expanded foods require 200 to 250 square millimeters of open area per metric ton throughput.
Final dies may be as simple as single plates with a predetermined number of sized round openings, or they may consist of two or more plate elements. The first plate element of a two piece die serves to increase the resistance to flow and to aid in imparting shear to the extrudate. The second die plate in a two piece die is used to size and shape the extrudate by forcing it to flow through a number of orifices. Spacers may be added between the extruder barrel and the final die plate to even out the flow from the extruder screw to the final die plate and give additional retention time for cooking. A face cutter is used in conjunction with the die, which involves cutting knives revolving in a plane parallel to the face of the