Principles of extrusion cooking technology:

by Emily Wilson
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For either single- or twin-screw extruders, the screw design can be machined out of a solid metal shaft, or modular screw elements can be configured on a keyed or splined metal shaft. The modular design allows flexibility both for reconfiguration at any time during product development and for the manufacture of different products.

Grooved barrels (spiral and longitudinal) may be more prevalent for single-screw extruders, and may be necessary for successful operation. The ability for a single-screw extruder to pump material requires drag and no slip at the barrel wall. The presence of grooves in the barrel adjacent to the rotating screw facilitates drag and thus flow.

Screw rotation can be clockwise or counterclockwise, depending on the handedness of the screw design. For twin-screw extruders, rotation of each screw relative to the other needs to be considered. Screws that turn in the same direction are known as co-rotating, whereas screws that rotate in directions opposite to each other are known as counter-rotating.

In terms of twin-screw extruders, the co-rotating design is common to the feed and food industries. Because of high localized pressures, large and undesirable separation forces can be created in the counter-rotation design. Separation can force the shaft and screws toward the barrel wall, invoking wear. Thus, counter-rotation systems are limited to low operation screw speeds.

Popular twin-screw designs employ self-wiping features by having a high degree of intermesh between the adjacent screws. Other possible twin-screw designs include non-intermeshing and tangential, but these two designs would not have adjacent flights available to provide a scraping action to clean materials from the flight and prevent accumulation.

Both single and twin screws can have a number of parallel flights along their lengths. Common screw flightings are single, double, triple and even quadruple. For example, a double-flighted screw has two parallel flights. However, as the number of parallel flights increases for a given length of screw, the free volume of the screw often is diminished. One rotation of the screw should deliver material a distance in the axial direction equivalent to the pitch.

Screws of the same pitch can intermesh regardless of the number of parallel flights. The number of independent channels created by two fully intermeshing screws is equivalent to one less than twice the number of parallel flights (p) on one screw (i.e. 2p — 1).

The continuous channels created between the flights of these screws can be interrupted with shearlocks or kneading paddles. These may be arranged to resemble the flighting and pitch of screws and typically are self-wiping in the twin-screw extruder. However, this path is discontinuous and allows material to travel in either direction through the radial gap and chambers created by closely intermeshing paddle sections.

The die offers one of the last resistances to material flow from an extruder. The geometry of the die, including the geometry and length of the lands, influences the overall resistance that a given die can offer to a material. The die land is a surface against which the melt must flow. Often, lands are short in length (<1 inch) and are parallel or tapered to direct the flow of material.

A simplified operating concept indicates that the volumetric flow rate from the extruder must be equivalent to the volumetric flow rate into the die. This assumes an incompressible fluid and, hence, a constant density, thereby representing application of the law of conservation of mass between the extruder and the die assembly.

The volumetric throughput for the extruder is not the theoretical throughput but rather the actual throughput. If leakage flows exist, the theoretical throughput is reduced by the magnitude of the leakages.

For simple single-screw extruders, the volumetric throughput of the extruder can be described by representative components for drag, pressure and leakage flows. Leakage flow over the flight tip is considered very small when the gap between the flight tip and barrel wall is less than 0.001 of the screw diameter (flight tip to flight tip).

There are two additional components to the drag flow — the down channel flow and the cross channel flow. In general, as the drag flow conveys material toward the die or another restrictive device, pressure is created. This pressure can be quite high and is typically on the order of 300 to 2,000 psig for food or feed proceeses. The direction of pressure flow is always from the highest pressure to the lowest pressure.

Volumetric flow rate of the die is a function of the pressure gradient across the die in the direction of flow. Any change is die geometry will change the die resistance. For example, as the land length of a straight circular die increases, the resistance to flow will increase.

For an ideal Newtonian fluid, this could be modeled from Poiseuille's equation for a circular pipe. However, bio-polymeric materials do not behave as ideal Newtonian fluids and may be able to be characterized by non-Newtonian behavior.

Assume that a Newtonian fluid was extruded with a given single-screw configuration with shallow, rectangular channels using a defined set of operating parameters. The volumetric flow rates (Q) can be estimated using the following equations for the extruder and the die, respectively, where a and b are constants characteristic of the screw geometry, N is the screw speed, m is the viscosity, k is the die conductance (or alternatively, l/k is the die resistance), and P is the pressure generated for a given length of screw, L.

Q extruder = aN — bP/(Lm)

Q die = kP/m

The intersection of the extruder and die operating lines is an approximation of the theoretical operating point for this combination of extruder and die (see chart at left). Therefore, the extrusion process is defined as the relationship that exists between the extruder and the die. If either the extruder or the die is changed, the process will be different.

Interdependence of the extruder and die at various extruder operating conditions has been shown. A reasonable operating window can be bound partially on the lower side by economic constraints. If production or volumetric throughput is low, then profits are likely to be reduced. An upper bound might be influenced by extruder stability or limitations of the equipment.

Other boundaries of a possible operating window may be related to minimum and maximum extrudate temperatures (see chart below). Through a combination of trial and error or application of the operating equations, a window of acceptable operating conditions can be determined.

As in the single-screw case, the selection of the operating conditions for a twin-screw extruder should be made with an understanding of how material is transported along the barrel. This transport mechanism can include displacement in addition to drag and pressure flows.

Positive displacement is dependent on how the extruder screws intermesh. The extent of intermeshing and the closeness across the entire region of intermesh will dictate whether distinct and discrete C-shaped chambers are formed in the channels of two adjacent screws.

Screws that are closely intermeshing create C-shaped chambers that are essentially closed off at the screw-screw interface. As a result, the direct transmission of pressure from the die cannot create the backflow that occurs in an extruder with continuous and open flighting. The latter applies to single screws and, partially, to self-wiping twin-screw extruders.

The flexibility that twin-screw extruders offer is due partly to the complexity that can be achieved in modular screw designs by incorporating screw elements that have various conveyance and restrictive flow characteristics.

This is the final segment of a two-part series on extrusion technology by John L. Brent Jr., an assistant professor in the Department of Grain Science and Industry at Kansas State University, Manhattan, Kansas, U.S. This article was presented earlier this year at the International Symposium on Animal and Aquaculture Feedstuffs by Extrusion Technology in Aguas de Lindóia, Brazil. It also has been published as a chapter in the book "Advances in Extrusion Technology: Aquaculture/Animal Feeds and Foods," edited by Y.K. Chang and S.S. Wang, and published by Technomic Publishing Co., Inc., Lancaster, Pennsylvania, U.S.

 

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