Single or twin-screw extruder: what are the options?


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Selecting an extruder can be a confusing process for buyers. Many options are available in the marketplace when selecting extrusion systems for product. For example: is a single- or twin-screw extruder required? Should it be a "wet" or "dry" extruder? Should it have internal steam locks or a single face die plate? Should it have continuous or interrupted flights, and so on. Appropriate selection depends on several factors.

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Single or twin-screw extruder: what are the options?

  1. 1. Digital Re-print November | December 2013 Single or twin-screw extruder: what are the options? Grain & Feed Milling Technology is published six times a year by Perendale Publishers Ltd of the United Kingdom. All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2013 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1466-3872
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  3. 3. FEATURE Single or twin-screw extruder: what are the options? by Dr Mian N Riaz, director, Food Protein R&D Center, Nutrition and Food Science Department, Texas A&M University, USA S electing an extruder can be a confusing process for buyers. Many options are available in the marketplace when selecting extrusion systems for product. For example: is a single- or twin-screw extruder required? Should it be a "wet" or "dry" extruder? Should it have internal steam locks or a single face die plate? Should it have continuous or interrupted flights, and so on. Appropriate selection depends on several factors: • Physical and sensory properties of the end product • Formula ingredients: their physical nature (i.e. whether the product will utilise high levels of fresh meat), moisture content, whether they are constantly available or only during certain seasons, and the potential for occasionally using substitute ingredients. • What kind of product do you want to extrude? Is it food grade, or feed or pet food? Should each piece be multicoloured or centre-filled? Is the shape general, exotic, or detailed? What is the target bulk density? In the case of feed, how much fat needs to be added to the formula? How much can be applied to the surface? • What is the production rate? The size of an extruder depends on market size, since extruders function best when operating at full throughput per hour. • What is the energy source? For product heating, will steam or electricity be more economical where the extrusion plant is built? If it is a small operation in a developing country, would a tractor power take-off drive be more suitable? • What about capital availability and the recovery date target? Would a used extruder fill a start-up operation’s needs better? 16 | november - december 2013 Segmented screw/barrel singlescrew “wet” extruders Segmented screw/barrel single-screw extruders are the most widely applied cooking extrusion design in the food, pet food and feed processing industries. “Wet” means that steam and water can be injected into the barrel during processing. Typically, the barrels of these machines are also equipped with heating and cooling jackets. They can process more tonnage of extruded products than any other extruder design, and can produce a range of products, from fully cooked, light-density corn snacks, to dense, partially cooked and formed pastas. A typical single-screw extruder consists of a live bin, feeding screw, preconditioning cylinder, extruder barrel, die and knife. The live bin provides a buffer of raw material so the extruder can operate without interruption. Typically, the height of raw material in the bin is kept within defined limits by high- and low-mounted sensors which activate a conveyor supplying the bin. The bin is designed to prevent the bridging of its contents, and the blocking of the feed screw leading to the preconditioner. The speed of the feed screw to the conditioner or extruder must be variable, in order to ensure a continuous uniform supply of raw material. This is crucial to ensure the consistent and uniform operation of the extruder. Because single-screw extruders have a relatively poor mixing ability, they are usually supplied with premixed material, which often has also been preconditioned with added steam and water. Generally, preconditioning prior to extrusion enhances extrusion processes which benefit from higher moisture content and longer equilibration time. Preconditioning of the raw material typically improves the life of wear parts in the extruder several times over. Although the weight of ingredients in the extrusion system is increased, preconditioners are relatively inexpensive to build for the volume they hold and the time they add to the process. Perhaps most importantly, product quality can be greatly improved by preconditioning the raw ingredients. Application The first major commercial application of the single-screw extruder in the food processing industry was the conversion of semolina flour into pasta using solid screws. This low-shear, low-temperature forming process first found commercial production in the 1920s and 1930s, and remains a standard process even though equipment has improved. Several recent developments in the single-screw extruder have further increased its efficiency and versatility. A brief list of the products made by single-screw extruders can be found in Table 1. Twin-screw extruders Recent years have seen increasing requirements for new products with intricate shapes and small sizes, which are beyond the capabilities of single-screw systems. Twinscrew extruders can fill some of these needs. The term “twin-screw” applies to extruders with two screws of equal length, both placed inside the same barrel. Twin-screw extruders are much more complicated than single-screw extruders, but at the same time provide much more flexibility and better control. Twin-screw extruders are generally categorised according to the direction of screw rotation and to the degree to which the screw intermeshes. In the counter-rotating position the extruder rotates in the opposite direction, whereas in the co-rotating position the screw rotates in the same direction. These two categories can be further subdivided &feed millinG technoloGy Grain
  4. 4. Image courtesy of Wenger Manufacturing, Kansas, USA FEATURE according to the relative positions of the two screws (intermeshing and non-intermeshing). The non-intermeshing twin-screw extruder is like two single-screw extruders sitting side by side, with only a small portion of the barrel in common. These types of extruders depend on friction for extrusion, just like single-screw extruders. In non- intermeshed extruders, neither pumping nor mixing is positive. Their design does not provide a positive displacement action for pumping the product forward. In intermeshing twin-screw extruders, the screws partially overlap each other in a figureeight barrel track, resulting in positive pumping, efficient mixing and self-wiping action (although only in co-rotating machines: mixing is limited in counter-rotating machines). This is what differentiates these types of extruders from non-intermeshing and singlescrew machines. These extruders are like a positive displacement pump, forcing material in the barrel between the screws to move toward the die by the rotation of the screw. Co-rotating self-wiping types of extruders are most commonly used in the food industry. When they were developed, these extruders significantly increased the variety of products that could be made using extrusion technology. The twin-screw extruder consists of several sub-components very similar to singlescrew extruders (live bin, feeding screw, preconditioning cylinder, extruder barrel, jacketed heads and rotating screw). The bearing assembly in twin-screw extruders is much more complicated because more components (such as drive and torque dividing gears) are required. Twin-screw extruders also have three processing zones: feeding, kneading and a final processing zone very similar to single-screw extruders. These zones were described in the single-screw extruder section above. Applications Twin-screw extruders became popular in the food industry in the mid-1980s to the mid-1990s. Originally developed for processing plastics, food companies began using twin-screw extruders for products like sticky caramels and candies that could not be made with single-screw machines. Very soon, twin-screw extruders became popular with food manufacturers for many specialised food items. However, new developments for singlescrew extruders are beginning to change the relative advantages of the two technologies. Variable speed drivers now give single-screw extruders a flexibility approaching that of twin-screw machines. Improved gravimetric feed systems and mass flow meters allow for the more precise measurement of recipe components. These recent improvements, along with systems for computer control, have made it possible to process several foods formerly made with twin-screw, possibly limiting the market for twin-screw extruders. Single-screw vs twin-screw Single-screw extrusion has been successfully employed in food and feed production Offices and Factory: Ctra. de Arenas km. 2,300 13210 Villarta de San Juan • Ciudad Real- Spain T: +34 926 640 475 • F: +34 926 640 294 grain silos &feed millinG technoloGy Grain hoppered silos Madrid Office: C/ Azcona, 37 • 28028 Madrid - Spain T: +34 91 726 43 04 • F: +34 91 361 15 94 november - december 2013 | 17
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  6. 6. Image courtesy of Wenger Manufacturing, Kansas, USA FEATURE for the last 60 years. Because of consumer demands for innovative food products in the market, extruder manufacturers adopted and developed twin-screw extruders around 30 years ago. Twin-screw extruders have greater ability and flexibility for controlling both product and process parameters. They are a flexible design, permitting easy cleaning and rapid product changeover. Because of the ability to better match the desired shear, the twin-screw extruder has more control over product variability. Screw speed can also be used to compensate for some variations in the properties of the starting material. Because screw speed is such an influential variable, the twin-screw extruder is a better choice for plants producing a wide variety of high-value products at low volume. Single-screw extruders limit formulas to a 12-17 percent fat level. Fat content above that level reduces friction due to lubrication, and does not help the hardware transform mechanical energy into heat for cooking purposes. On the other hand, the fat level for recipes designed for twin-screw extuders can be as high as 18-22 percent and still maintain the required mechanical energy. This is only possible due to the greater number of screw configuration options provided by twin-screw extruders, compared to singlescrew machines. In single-screw extruders, with the help of steam injection a fat level as high as 17 percent can be achieved. However, the addition of steam injection to twin-screw extruders allows the product to be processed more consistently, which as a result 18 | november - december 2013 allows better binding of the fat and reduces its leakage from the products during handling and packaging. Moisture content is very critical during the extrusion process for starch gelatinisation and protein denaturation. The average moisture content of a typical formula ranges from 20 to 28 percent. Moisture, in the form of steam or water, is added to the preconditioner and extruder barrel to help soften raw ingredients and reduce their abrasiveness. Twin-screw extruders have the ability to run equally under narrow or wide ranges of moisture. Ultimately, processors should consider using twin-screw extruders in situations dealing with: • Frequent product changeovers • Products with a high internal fat content (more than 17 percent) • The addition of a high level of fresh meat in the product (up to 35 percent) • Uniform product size and shapes • Ultra-small product sizes (less than 1.5 mm) • Products made with low density powders • Special formulations (e.g. products with high levels of protein and fibre, or which require a high level of moisture, or require the use of sticky raw materials like soy isolates or wheat gluten) Table 1: Single-screw extruder products Direct expanded corn snacks Texturised vegetable protein Ready-to-eat breakfast cereals Full-fat soy Pet foods Floating and sinking aquatic feed Baby foods Rice bran stabilisation Precooked or thermally modified starches, flours and grain Breading Table 2: Twin-screw extruder products Co-extruded snacks and other food items Precooked pasta Noodles, spaghetti and macaroni Third-generation snacks Texturised vegetable protein (soy and wheat) Semi-moist food Pet treats Meat analogue Rice bran stabilisation Multicolour food and snacks Cereals and corn flakes Corn chips and tortilla Loose fill (packaging material) from starches More inforMtion: Ultra-fine aquatic feed High fat aquatic feed (salmon) Premium pet food (with fresh meat) &feed millinG technoloGy Grain
  7. 7. FEATURE Maximize your process with lowest operational costs Hydronix Moisture Sensors Save You Money Hydronix digital, microwave moisture sensors provide accurate and cost effective moisture measurement n feed meals and pellets, grain, cereal and pulses. Hydro-Probe XT HAMMER MILL • Low energy consumption • Low noise level • Largest grinding surface in the industry • Minimum service down time Our sensors are successfully used in many applications to ensure product quality, maximise yield and save energy. Typical uses include: • • • Controlling the moisture in the grain drying process to save energy and ensure quality Optimising the efficiency of expensive additives such as mould inhibitors Controlling moisture content during the pelleting process Hydro-Mix VII Hydronix sensors are: • • • w w w. a a r s e n . co m &feed millinG technoloGy Grain Suitable for chutes, silos, mixers or conveyors Not affected by dust or colour Temperature stable november - december 2013 | 19 GFMT half page vertical 90 x 270 plus 3mm bleed not left.indd 1 30/11/2012 13:44:07
  8. 8. LINKS November - December 2013 This digital Re-print is part of the November | December 2013 edition of Grain & Feed Milling Technology magazine. Content from the magazine is available to view free-of-charge, both as a full online magazine on our website, and as an archive of individual features on the docstoc website. Please click here to view our other publications on first published in 1891 • • Fit-for-purpose grain and feed packaging: an environmental perspective • • Organic feeds: the future for sustainable poultry farming? • See the full issue Visit the GFMT website • Contact the GFMT Team • Subscribe to GFMT In this issue: • Single or twinscrew extruder: what are the options? • Animal feeding in the future: reaching genetic potential through smarter nutrition? PORTS: VIGAN industry report • Market-aware farming: commodities training at Writtle College INCORPORATING PORTS, DISTRIBUTION AND FORMULATION A subscription magazine for the global flour & feed milling industries - first published in 1891 To purchase a paper copy of the magazine, or to subscribe to the paper edition please contact our Circulation and Subscriptions Manager on the link adove. INFORMATION FOR ADVERTISERS - CLICK HERE Article reprints All Grain & Feed Milling Tecchnology feature articles can be re-printed as a 4 or 8 page booklets (these have been used as point of sale materials, promotional materials for shows and exhibitions etc). If you are interested in getting this article re-printed please contact the GFMT team for more information on - Tel: +44 1242 267707 - Email: or visit