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FREQUENTLY ASKED QUESTION
about Twin-Screw Extrusion Technology
Is Twin Screw food Extrusion Cooking in your future ?
Let Clextral, the extrusion experts help you evaluate your needs and answer your questions about extruded products.
Q .Is twin screw food extrusion really in the future ?
Q .If that's so, why don't we hear more about twin screw food extrusion ?
Q . What products are made on twin screw extruders ?
Q .Exactly how does a twin screw extruder work ?
Q .What's the difference between an expanded product and a formed product ?
Q .Exactly what is a pellet ?
Q .What are the advantages of twin screw food extruders over traditional cooking/processing methods ?
Q .How do twin screw extruders differ from single screw extruders ?
Q .If twin screw extruders have all those advantages, why would anyone buy a single screw extruder ?
Q .What features should I look for in a twin screw extruder ?
Q .What should I look for from a supplier ?
Q .Is twin screw food extrusion really in the future ?
A .Twin screw food extrusion cooking is NOW. Today, many companies are using extruders for producing both human, animals foods and industrial products. They are discovering the many benefits of extrusion cooking over traditional methods... increased productivity and reduced production costs.
Q .If that's so, why don't we hear more about twin screw extrusion ?
A .It is a relatively new technology, compared to traditional baking or mixing methods. Many new products have been developed on twin screw extruders, yet users are very reluctant to disclose their recipes and cooking methods. You may be surprised at the number of products made on twin screw extruders.
Q . What products are made on twin screw extruders ?
A .Today, all of the following food products are being produced by twin screw extruders :
Almond Paste
Bicard Crumbs
Baby foods, including rice, wheat and oat-based precooked foods
Bread
Confections, including licorice, gum candies, bar candies, soft centers and chocolates
Caseinates
Cereals, including corn flakes, bran flakes, wheat flakes, crisp rice, crisp bran, oat O's, whole grain, high fiber and ready-to-eat expanded cereals
Cookies, including co-extruded types with variety fillings
Fish feed, including feed for aquarium fish and commercial feed for salmon, trout or catfish
Flatbreads, in a variety of flavors, including fruit-filled, high fiber and whole grain
Flavors
High Fiber Foods
Instant Food and Drink Mixes based on cooked cereal flavors
Modified Starches used as functionnal ingredients
Petfood, including dry dog and cat food, plus semi-moist foods
Pet Treats, including speciality treats and snacks for cats and dogs
Processed Cheese
Snacks : expanded snacks, such as corn curl and corn ball style snacks, co-extruded snacks, snacks pellets
Shrimp Feed
Texturized Vegetable Protein (TVP)
The twin screw extrusion technology has been used for a number of years for processing plastic compounds, such as masterbatches, reactive extrusion and lately for cosmetics.
CLEXTRAL has developped an exclusive equipment called "bivis" very similar to an extruder to produce paper pulp using vegetal fibers, such as cotton, hemp, flax. The bivis machine can also be used as a reactor to process cellulose fibers in order to modify its mechanical and/or functional properties.
Q .Exactly how does a twin screw extruder work ?
A .It's simple. There are three main zones, the Feed Zone, the Cooking Zone and the Forming Zone.
In the Feed Zone, raw ingredients are continuously fed from storage into the extruder. The twin screws intermesh as they turn, mixing the ingredients thoroughly. Liquid, usually shortening, water or others, is injected through ports in the barrel to hydrate the grains before cooking. In this zone, the ingredients are worked into a continuous dough by the twin screw action.
In the Cooking Zone, the dough is subjected to heat from any or all of three sources, depending on the deisired product.
Mechanical Heat is generated within the barrel by varying the screw configurations.
Compression screws, reverse elements, shear locks and mixing discs can all generate mechanical heat.
Convective Heat is transferred directly from the barrel walls into the dough. This is a very effective method of heat transfer.
Steam Heat can be injected into the dough through ports in the barrel.
In the Forming Zone, the product takes its shape. Depending on the moisture level, temperature, pressure and die geometry, you can make an EXPANDED PRODUCT, or a FORMED PRODUCT.
Q .What's the difference between an expanded product and a formed product ?
A .EXPANDED PRODUCTS
To make expanded products, the pressure and temperature are increased, while the moisture level is accurately controlled.
When the product exits the extruder through the forming die, the change in atmospheric pressure causes the internal moisture to turn to steam. This puffs the fully-cooked dough into an expanded product.
FORMED PRODUCT
To create formed or pelletized products, you use a high moisture dough at a lower temperature. When the extrudate is forced through the forming die, the product takes the shape of the die, instead of expanding. The result is a dense intricately-shapped pellet.
Q .Exactly what is a pellet ?
A .The word pellet has a dual meaning :
In food production, pellets are high moisture, highly dense, often intricately shaped products, also known as half-products or third generation snacks. They are further processed by drying ; then frying, air puffing, flaking or microwaving.
In the feed industry, extruded fish and animal feeds are called pellets. These are traditionally produced on pellet mills.
Q .What are the advantages of twin screw food extruders over traditional cooking/processing methods ?
A .Compared to batch cooking, there are many advantages :
New Products
You can produce a wide variety of products on one twin screw extruder. You are not bound to one process by the equipment, as you can be with traditional baking equipment. TSE's give you superior flexibilty and process control which is not always possible with traditional methods.
High Productivity
The TSE acts as a complete processing plant. The ingredients are mixed, cooked, formed and sheared in one continuous process. Today, extruders can process over sixteen tons per hour of finished product.
High Product Quality
The twin screw extrusion process gives you almost unlimited control over the cooking process, and changes can be made quickly during cooking. This gives you complete control over the cooking and processing of your product. In addition, the HTST (High Temperature Short Time) cooking method on a TSE produces superior products. The high temperatures destroy harmful micro organisms, while minimizing the loss of nutrients or flavors in the food being produced.
Low Cost
Compare the purchase price of a twin screw extruder to a traditional cooking system, including mixer, oven, dryer and accessory equipment. In many cases, an extruder will cost less. TSE's are cost-effective to operate because they perform the functions of mixing, blending, cooking and forming.
Versatility
A wide variety of food can be produced on a twin screw extruder cooker, by varying the ingredients and processing conditions.
Energy Efficiency
In some processes, twin screw extrusion can actually reduce energy needs. Products can be cooked more quickly and efficiently because of more precise temperature and moisture control,
and shorter reaction times.
Q .How do twin screw extruders differ from single screw extruders ?
A .The twin screw was developed to overcome the limitations of the single screw extruder, and can be compared to the " next generation " of the single screw unit. With a twin screw extruder, you have the following advantages :
Better control of product flow
In a twin screw extruder, the dual screw action conveys the product through the barrel. In a single screw extruder, friction between the screw and the barrel is used to convey the rpoduct.
Single screw extruders offen experience slippage and surging. Slippage occurs when high pressure in the barrel causes the product to slip between the screws and the barrel wall. The result is that the product does not undergo proper cooking or processing. Surging occurs when the product is held back from proper flow. Presure builds up, and finally, the product bursts uncontrollably through the die. This can result in undercookked and misshapen products.
Better control of processing factors
All the processing parameters : temperature, presure, screw speed, moisture content and flow rate can be varied independently on a TSE. This gives you maximum process control. On single screw extruders, you can adjust the processing factors, but to a much lesser degree.
Better mixing
First, the " figure 8 " mixing style of a TSE offers better mixing than the single twist mixing on a single screw extruder. In addition, TSE's offer a wide range of mixing elements, including intricate mixing discs, reverse screws and compression screws that provide superior mixing. Finally, a TSE can give you more latitude on ingredients than a single screw unit can. A TSE can accept a wide range of particle sizes assuring proper mixing and processing of all.
Twin screw extruders are " self-cleaning "
The dual screws on a twin screw extruder are " self-wiping ", that is, complimentary screw flights from one shaft remove product from the other as they turn. Single screw machines do not have this feature, and are susceptible to dead spots where product not conveyed will adhere to the screws. This " burned on " product can break loose, block the die and plug the barrel.
Greater Flexibility
Most products produced on single screw machines can be made on twin screw machines. The opposite is not true. TSE's can process foods at higher moisture contents. They can accept dry ingredients at varying particle size, while single screw machines require uniform particle size for optimal processing. Twin screw extruders can process foods with higher fat content and higher sugar content than can be hanled on single screw extruders.
Scale-up is more predictable
Scale-up, the process of going from laboratory development of a product to full-scale production, is mainly a single screw problem. Scale-up on TSE's is not a problem as long as proper data is recorded during research and development of new products. Twin screw extrusion gives you very accurate control on production parameters, which is critical in scale-up.
Reduced start-up and shut-down costs
Start-up and shut-down can be handled more quickly on a twin screw extruder. This will save time and money, especially on high volume production lines.
Q .If twin screw extruders have all those advantages, why would anyone buy a single screw extruder ?
A .Single screw extruders are used in many areas of food and feed production today, simply because they were the only alternative to traditional methods. People learned to live with the disadvantages of single screw extruders, because the overall process was superior to past processing methods. Today, the twin screw extruder is more advanced, offering many advantages to the modern processor. As more companies see these advantages, the trend is toward twin screw extruders.
Q .What features should I look for in a twin screw extruder ?
A .First, examine the design of the extruder. Most TSE's use corotating, self-wiping screws driven by a variable speed drive.
Consider the following questions :
Is the machine construction heavy-duty ?
The screw drive is very important. Is it designed for long-term operation ?
Does it offer easy access to the screws ?
Very important in changeover and cleaning.
Are the barrel elements easy to replace ?
As with any processing equipment, wear is a fact of life. Quick change barrels can reduce downtime.
Are a wide variety of screws and mixing discs available ?
This is very important in product development.
How is the barrel heated and cooled during cooking ?
This varies from one manufacturer to another. Try to be certain that you're using the most cost-effective system for your needs.
What types of dies are available ? Can the supplier provide die designs to make your products ?
Look for dies with good flow characteristics, that insure proper product distribution for best product uniformity.
Is the die plate easy to remove for production changes or cleaning ?
Downtime for die changes or cleaning is always costly.
Are the controls easy to use ?
Look for an extruder that's easy to control, whether manual push-button or automate drive.
Q .What should I look for from a supplier ?
A .Experience
Even though you may buy a standard model extruder, from the standpoint of barrel length, screw diameter and production capacity, it will be a custom unit. Your extruder must be customized with the proper screw elements to produce your product successfully, from blending, through mixing and cooking. You should look for a supplier with experience in twin screw extrusion cooking.
Die design is very important. You need a supplier who understands the importance of the die for product success. A change in the die can do more than change your product's shape... it can change the size, and texture, even with all other factors being equal. The die is a fundamental part of the processing function.
Technical Knowledge
Ask potential suppliers about their successes in product development. Focus on their experience in your industry. Since twin screw extrusion is a science, it is not easy to transfer technology from one industry to another.
Product Development Assistance
Do your potential suppliers have pilot plant facilities and scientists and dedicated team to help you ? This can be very important in new product development.
Service
Your supplier will be your link to the exciting world of twin screw extrusion. You will work together every step of the way. Ask your potential suppliers about start-up assistance, on-line service and parts availability. You should feel comfortable with your supplier's assistance and service.
The Clextral Advantage
We invented screw extrusion for the food and feed industries. For more 20 years, we've been testing and perfecting twin screw extrusion to make this technology a useful alternative for food processors worldwide. We are experts on twin screw extrusion. We'll be glad to help.
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