Common problems associated with the selection and use of food product extruders include lower than desirable throughput, high scrap rates, long post-extrusion cooking or drying times, excessive energy consumption, and high equipment costs. Frequently, these stem from choosing an extruder design that is ill suited for the application. A basic review and understanding of extruder design and operation can help prevent these frustrations.
Extruder Design Considerations
Die design is as important to the quality of the extruded material as it is to extruder throughput. Open area, land area, and material of construction directly relate to back pressure and, in turn, product density and screw efficiency. Open area refers to the actual cross section of the die and is typically expressed in ratio to the cross section of the screw. The land area of the die is the portion which forms the shape of the product.
Material of construction will affect pressure through friction. For example, steel has a much higher frictional coefficient than acetyl plastic; therefore product will “slip” through an acetyl die with greater ease, developing less pressure than through a steel die of identical design. This can be advantageous in applications where the extruded product is resistant to flow. However, a well lubricated product will require more drag to form a solid profile. Die restriction can be thought of as a valve in water line; the more it is closed, the greater the pressure becomes.
Flow characteristics of the extruded material also impact screw and die design. A product with the consistency of fruit paste will flow quite easily through any die orifice, but is not easily conveyed by the screw. On the other hand, a material such as cornmeal does not flow well through the die, but is conveyed quite efficiently by the screw. There are many formulation variables to consider and extrusion aides are available to increase the lubricity of a product.
The overall design of the extruder with respect to how the material passes through is vital. For example, the buildup of residual material throughout the extruder must be avoided. An important factor is the clearance between screw and barrel because, as clearance increases, product is allowed to stagnate and build up in some areas. Excessive clearances and “dead spots” must be eliminated by designing in accordance with current Good Manufacturing Processes. This will help prevent difficult cleanup and maintenance problems.
Excessive clearance between screw and barrel also contributes to a decrease in throughput and die pressure. In fact, a drop in die pressure can be an indication of wear on the screw and barrel. It is important to monitor wear to ensure maximum efficiency; to do so, the use of a pressure transducer is recommended at the output flange of the extruder. Because tighter clearances mean higher efficiency, it’s important to have a screw and barrel with these characteristics, along with surface treatments that resist wear. An extruder that maintains a tight screw and barrel clearance is crucial to maintaining high throughput. Wear resistant surface treatments prolong the life of the barrel and screw over extended periods of use.
Of course, temperature is important to extrusion for a number of reasons relative to each product or process. When designing cooling channels, it is therefore imperative to utilize as much surface area as possible and create flow patterns that transfer heat more efficiently. This is achieved through improved barrel, barrel jacket and screw design. Giving more attention to ΔT, dead spots, and turbulence can greatly increase cooling efficiency.
Jeff Schweizer is President of Diamond America Corp., a position he has held for the past 5 years. He has nearly 30 years of experience in the design and manufacturing of extruders used in food processing and other industries. More information on food extrusion is available at http://www.diamondamericacorp.com/food_products.asp.
Be on the lookout next week, when Jeff will tackle The Right Extruder: Process, Sizing And Options.