Extrusion channel

To accomplish the above, the design of a ram extrusion press must provide a relatively long extrusion channel. However, there are physical limits to this parameter because friction and drive power as well as overall stressing of the equipment increase with channel length. Briquettes may retain a certain elastic deformation which, if suddenly released, will damage or destroy the product. Therefore, in most applications, a gradual release is provided in the channel prior to product discharge. 

The mouth or opening of something, for example an extrusion channel, that forms material into defined shape. 

Press in which a fly-wheel powered reciprocating ram densifies and extrudes particulate solids through a long extrusion channel. Particularly suitable for elastic materials (such as peat, lignite, biomass, etc.). Also ram press, exter press. 

In medium-pressure agglomeration (see Section 8.4.2), the extrusion through perforated die plates (II.2), higher densification can be obtained by two effects. First, the feed, that must have binding characteristics and be somewhat plastic, is predensified prior to extrusion by the screw(s) or between the press roUer(s) and the die. Secondly, the material is passed through holes in the die where pressure builds up due to wall friction. The length over diameter ratio of the hole (= extrusion channel) and its shape as well as the properties of the feed determine final densification. Because of the wall friction, the surface of the extrudate is always denser than the center (see also Section... 

Stronger to withstand these forces which results in somewhat longer extrusion channels and higher frictional resistance that also yield higher densification. 

Sometimes, a space is left between the end of the screw(s) and the die plate. If rheological properties of the material are such that further densification takes place in this transition space, a denser extrudate is produced. In most cases, a large gap is used in high pressure screw extruders (see Section 8.4.3) where the necessary forces to obtain extrusion are solely developed by the screw(s) and high hydrostatic pressure is required to induce hydraulic flow through the extrusion channel(s). In low pressure axial extruders, extrusion blades are commonly attached to the end of the screw shaft (Fig. 8.27). In those cases, the gap is small and the plastic mass is compressed in the... 

Since the extrusion force must be larger than the frictional resistance, considerable pressures develop in front of the extrusion channels. Therefore, the die body thickness must be selected such that it will not break during operation even if, occasionally, overloading takes place. Also, the open area as defined in Section 8.4.1, Fig. 8.33 ( % free area ) is normally relatively small, also to achieve structural integrity that means that the land between the holes is large. 

Fig. 8.37 Sketches of six typical extrusion channel designs for medium-pressure agglomeration. 

In medium-pressure agglomeration equipment (Fig. 8.42), the perforated support (die) can be either flat (a), concave (b), or convex (c). For all three designs, intermeshing, toothed executions have been proposed in the patent literature [B.42] to avoid slippage and improve extrusion as well as extrudate quality, but only the so called gear pelleter , in which the convex dies (Fig. 8.42c) are hollow gears with extrusion channels between the teeth, has reached commercial importance (see below). 

All features depend on the application and the feed properties. Capacities, although strongly influenced by the material as well as the pellet size and shape, may be as low as a few hundred kg/h and, on the high end, exceed 80 t/h. As visible from the dies shown in Fig. 8.50, most of the extrusion channels in pellet mills are cylindrical bores however, as depicted in the center of this Fig. (8.50), square openings (and others) are provided for specific applications (for example, the manufacturing of catalyst carriers). As a typical product of pellet mills, Fig. 8.56 presents some cylindrical animal feed pellets. 

Medium Pressure Axial Screw Extruders Axial screw extruders normalty operate with low (see Section 8.4.1) or high (see Section 8.4.3) pressure. The basic principle of an axial screw extruder was shown in Fig. 6.4b.1 (Chapter 6) and Fig. 8.24 (Section 8.4.1). The pressure that is developed by the screw(s) depends on the power of the drive and the frictional resistance in the extrusion channel or other discharge device. Axial screw extruders that rely solely on the pressure developed by the rotating screw(s) employ hydrostatic pressure as the driving mechanism for extrusion. Such machines generally use high pressure. However, under certain conditions and for specific applications, some can be classified as medium-pressure agglomerators. 

Fig. 8.85 is the schematic representation of the increasing density and the decreasing elastic recovery of a particular briquette during repeated pressing as it moves forward in the extrusion channel. 

The application of ram extrusion presses overcomes the problems of roller presses. Machines with multiple (up to four for example Fig. 6.10-25, bottom) extrusion channels can produce high capacities per unit and, if this is desired or required, comparatively small amounts of binders will produce, for example, water repellant compacts. Since it is preferable to produce a crushed product (below), initial briquette shape is of no concern and can be optimized for capacity. 

The increasing volume of polymer production in the chemical industry and the wide range of items being manufactured require an increase in the capability for the production and quality of the products produced. It has been established that an imderlying problem is the distortions introduced as the melted polymer flows through the extrusion channels. Analysis of the extrusion heads, has allowed identification of certain parameters which can positively be controlled to improved performance reduction of the pressure required leading to a decrease in cost, increase of output of the flow, increase in the hardness of extrudate and decrease the post-extrusion swelling. 

The design improved the continuous extrusion profile process and increased the output of the extrusion head. This improvement is achieved by using an ultrasonic horn within a many stream extrusion head. The magnetostrictive transducer horn is located in the extrusion channel and is cooled. The charmels of the profile section are formed between the profile section and the horn (Figure 5.1). 

---