Pressurization extruder

In the extruder the flour and water will be subjected to intense agitation and will be heated to above 100°C under pressure. Extruders are constructed to work at elevated pressures, indeed the barrel of the extruder is constructed in the same way as a gun barrel. Under these conditions the starch will gelatinise but the water cannot boil because the high pressure elevates the boiling point. 

There is one piece of equipment that approaches the process engineer s dream of a machine that takes in raw materials at one end and finished product emerges from the far end. That machine is the high pressure extruder. Unfortunately, while high pressure extrusion can make crisp-bread it has a limited application in the bakery sector. 

In the Redaimator, a bigb pressure extruder, fiber-free mbber is heated to 175—205°C with oils and other ingredients. High pressure and shear between the mbber mixture and the extruder barrel walls effectivdy devulcanize the mixture in one to three minutes. In the Lancaster-Banbury method, bigb temperature, pressure, and shear are applied to the mbber in a batch process that is otherwise similar to the Redaimator process. In another high pressure process, scrap mbber is devulcanized at 5.5—6.9 MPa (54—68 atm) for ca five minutes. The product is milled, baled, or pelletized as in other processes. 

The A-A extruders are equipped with a flat perforated plate or screen, combined with two flat extrusion heads. The wet mass is pressed axially through the screen parallel to the feed screws. The extrusion forces are the highest, compared to other low pressure extruders, resulting in a hard and dense extrudate. It is commonly used in the extrusion of food, thermoplastics, and other industries, where a large pellet diameter and thermosetting properties are desired. 

The physical transport of the reactor products may also need sufficient processing. Bulk polymerizations in high-pressure extruders may be followed by dicing the product into small pellets suitable for pneumatic transport. Emulsion and solution products are usually transported as obtained and frequently used in the same mode. The design of suitable containers and transportation protocols is very important to avoid harming the product during transportation and storage. For example, special containers may be required for air-sensitive materials, or to avoid solvent or water loss from solutions and emulsions, respectively. 

Extruders that rely solely on the pressure developed by the rotating screws employ hydrostatic pressure as the transport mechanism and are generally high pressure extruders. Those extruders that utilize dragging or rolling motion (for the latter see Section 4.2.2.6.6) feature a localized drag flow transport mechanism and, consequently, the rate of work performed and internal pressure developed are lower. 

Axial extruder Low, medium, or high pressure extruder with a flat die plate at the end of a barrel, the material is extruded in the same direction as it is transported by the screw (sj. 

Basket extruder Low pressure extruder in which the die plate resembles a basket, using rotating or oszillating extrusion blades. 

The ratio of the volume of loose particulate matter in a die to the volume of the compact made from it. Synonymous v/ith Jill ratio. In low and medium pressure extruders, the total thickness of material that is under compression in a die (including any inlet chamfer) divided by the nominal hole diameter. 

Axial, low pressure extruder, most often with two screws, in which the die plates resemble domes. 

In low pressure extruders, the flat, curved, or engineered blade that pushes material through the openings of a die plate-, it is the part closest to the die plate. 

Low pressure extruder in which part of the barrel consists of a screen or perforated thin sheet through which moist, plastic material is passed by extrusion blades to form ex-trudates the material is extruded radially to the direction in which it is transported. 

Fig. 8.22 Conicai. vaned and intermeshing extrusion biades for radiai iow pressure extruders (courtesy LCi Corp., Chariotte. NC, USA).

Fig. 8.23 Photograph of the extrusion section of a low pressure extruder with radial discharge (courtesy LCI Corp., Charlotte, NC, USA).

The dome extruder, also called extended die extruder, is a hybrid between the axial and radial low pressure extruders (Fig. 8.29, see also Chapter 6, Fig. 6.4a.4). This design was developed while trying to find a way to make the flat die plates of axial extruders last longer. The latter tend to bend out when they are overloaded if, for example, difficult to extrude materials are processed. Geometrically, the best balance of forces happens on a sphere and the extrusion stresses are spread over a three-dimensional area. An additional advantage is the increase in production capacity even if compared to radial extruders. Although the extrusion area is smaller than that of... 

Scale-up of low pressure extruders usually begins in the laboratory with testing on smaller equipment. After extensive experimentation with the formulation and equipment, an optimal set of parameters is defined which includes information on the material s bulk density (before and after extrusion), the extrusion rate, the power consumption during extrusion, and the product s temperature rise. An efficiency factor is then determined by ratioing the actual extrusion rate obtained on the small equipment to the calculated theoretical maximum extrusion rate. Efficiency factors are in the range of 5-35 % for axial, 15-55 % for radial, and 35-85 % for dome extruders. This efficiency factor is then applied to the theoretical extrusion rates of the industrial extruder. Many manufacturers of extruders will also include an application related experience factor for the determination of a safe but reasonable expected extrusion rate. Fig. 8.35 depicts relative levels of extrusion pressure and shear that are applied by the various low pressure extrusion equipment. 

Fig. 8.41 Sketches depicting the nip area of a medium pressure extruder with concave die and press roller explaining the forces at work [B.42]. For explanations see text,...

Fig. 8.82 Photograph of the open barrel of a double shafted high pressure extruder showing the processing elements for mixing and conveying (courtesy Readco, York, PA, USA).

Other low-, medium-, and high-pressure extruders are also sometimes applied for the agglomeration of food products. Many of the materials mentioned in Tab. 6.4-6 are processed in such equipment however the relationship between machine vendor and food processor is often a close one and governed by secrecy agreements. 

Extrusion blade In LOW PRESSURE EXTRUDERS, the flat, curved, or en-... 

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