Extrusion Equipment Design And Selection: Part 2 - Single-Screw Extruders

Single-Screw Extruders

From an engineering point of view, an extruder is simply a pump that provides the pressure necessary to force the process mash through a restrictive die. During the transport through the barrel, it is common that massive amounts of heat are added to the mash through friction generated between the mash and stationary and rotating components of an extruder. Therefore, an extruder is often considered a "heat exchanger" . The pressure and temperature profiles experienced by the process mash can, within limits, be chosen and controlled by variations in screw design and operational conditions. Due to the pressure applied to the barrel, the shape of the final product can be easily controlled through die selection and design.

The major features of the single-screw extruder are shown in Figure 1. In most cases, a preconditioner is used in conjunction with the extruder to increase moisture and heat absorption into the process mash, reduce mechanical power requirements and increase capacity. The conditioner normally operates at atmospheric pressure and provides a means in which either water or steam or both are uniformly incorporated into the process mash. In addition, additives such as vitamins, flavors, colors and even meat slurries may be incorporated. The conditioner provides retention time necessary for the mash to absorb the heat and moisture needed before entering the extrusion 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. In the feed zone, the conditioned mash 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. The "transition" zone of the barrel is identified by the fact that the mash changes, rheologically, from a powder to an elastic dough. In the "metering" zone, sometimes referred to as the "cooking zone" , extreme pressure is applied to the mash and high levels of heat are induced by friction causing the temperature of the dough to increase to well above 100 c. From a thermodynamic point of view, 75% or more of the work done in the extruder barrel is done in the metering 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 component.

Depending upon the specific design if the extruder, various manufacturers use different screw configurations to create elevated compression in the transition and metering zones. In many cases, either decreasing flight height or decreasing screw diameters are used to create compression rations in the range of 1:2 to 1:5. As compression on the extrudate is increased, the mechanical energy created by the screw turning is dissipated as heat into the extrudate.

In many designs, the surface of the barrel is grooved, either in a spiral or straight, so that the barrel "grips" the extrudate so that the rotating screw can force the material forward toward the die. Mixing of ingredients within the extruder barrel is limited by laminar flow within the flight channel. To increase mixing potential, it is sometimes advisable to modify the screw profile to include cut-flight sections that allow backward flow of extrudate.

Single screw operations depend on the pressure requirements of the die, the slip at the barrelextrudate interface and the degree to which the void volume in the barrel id filled. Feed rate, Screw speed and design and the characteristics of the extrudate dictate screw fill. The interaction of all these variables creates the limits in the operating range and flexibility of a single screw extruder.