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Extrusion Equipment Design And Selection: Part 4 - Single- verses Twin-Screw Comparison, Low-Cost Ex

Single- verses Twin-Screw Comparison

There is no doubt that twin-screw extruders allow the development and production of a greater array of products but at a significant cost. Twin-screw machines are 1.5 to 2.0 times the cost of a singlescrew machine of the same relative cost. The extra expense is due to the relative complexity of the screw design; the complicated drive components and the required heat transfer jackets. This may be somewhat offset by the ability of the twin-screw machine to process drier product thus requiring lower energy for drying. In addition, simply being able to produce a product that is impossible to produce on a single-screw design may justify the additional cost.

A very unique characteristic of a twin-screw machine is the ability to configure a single machine to perform two distinct tasks or functions at the same time. By configuring the first half or so of the machine as a high-shear, high-compression, cooking extruder, providing a vent into the barrel and, then, configuring the last half or the barrel as a low compression, low shear forming extruder, a twin screw can be used to produce high density, cooked products like pre-cooked pasta, sinking aquafeeds or pellets for flaked breakfast cereal. In doing this, a single machine is able to accomplish the functions of two separate single-screw machines producing the same product.

Low-Cost Extruders

As is often the case, entrepreneurs find a way reduce the cost of expensive technology and produce an item (in this case an extruder) that is much lower in cost, has much less flexibility in terms of application but that is capable of producing an acceptable final product. Such is the case with lowcost extruders.

These machines are very simple in design, are constructed of cast, rather than machined, components and perform a specific function reasonably well. In this instance, the application is in the production of full-fat soybean meal. Most of these machines use a simple, interrupted flight arrangement with breaker bolts to create the shear and pressure necessary to heat-denature the anti nutritional factors associated with the use of soybeans in non-ruminant feeds. A manual variable orifice die is used that can be set close to- or away from the end of the screw to increase or decrease operating pressure and process temperature.

While these machine designs usually produce acceptable FFS, they often fail when used to produce other extruded products, such as floating aquafeeds or pet foods, without significant mechanical and system modification. In most cases, a preconditioner is not needed when processing FFS, however, when attempting to produce pet- or aquafeeds, a preconditioner is necessary to precook and hydrate the starch and protein in the formula for successful production. Because of the relative poor fit (tolerance) between the screw sections and the barrel, much higher back flow tends to occur and production efficiency, product quality and nutrient survival tend to be low. However, there is no doubt that low cost extrusion equipment has a place in the feed industry and provides a feasible way to produce products that otherwise would no be cost effective.

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