Roller presses

Antioxidants. Widely used antioxidants are eugenol, ionol, and the like. They retard premature oxidation of inks on the press rollers when used at low concentrations. 

Glueglis. TM for a composition containing chiefly glue and glycerol used in printing-press rollers. Has soft, rubbery consistency but is easily decomposed by heat. 

Water-tight main compartment 2, Barometric columns 3, Main drum 4. Presssing roller 5. Pair of pressing roller 6. Cooling tank 7. Device to arrange fabrics in laps. 

Figure 89 depicts sketches of typical pelleting machines.With the exception of the first method (a), which applies a flat die, all other machines feature cylindrical dies and operate in a very similar manner as roller presses. The general principle is modified such that the material is pulled into the nip but then is extruded through bores in the dies and often scraped off with a knife . Equipment types (b) and (d) represent directly the modified roller press principle. Machine type (c) uses a press roller located within the cylindrical die and therefore resembles ring roll presses (see Section 4.2.2.4.4). To improve the grip and increase densification force gear pelleters (e) are applied which are said to produce pellets of superior quality. 

Figure 89. Schematic representation of different pelleting machines, (a) Flat die and edge rollers, (b) two hollow cylindrical dies, (c) one hollow cylindrical die with internal solid press roller, (d) one hollow cylindrical die with external solid press roller, (e) gear pelleter...

Figure 180. Schematic representation of pelleting equipment, (a) screw extruder, (b) pelleting machine with flat die and muller-type press rollers, (c) pelleting machine with one solid and one hollow roll, (d) pelleting machine with two hollow rolls, (e) pelleting machine with internal press roll, (f) gear-type pelleter...

Theory of rolling The basic principle of compaction of particulate solids between two counter-currently rotating rollers (Figure 230) is similar to that used in calenders for plastic foils or in rolling mills for metals. The first can be adjusted to extremely narrow gap tolerances across press rollers with face widths of up to 2 meters and production speeds of approximately lOOm/min in the latter enormous pressing forces can handle ingots of more than 35 tons weight. 

Press frame designs Two different frame designs exist which are distinguished by the location of the press rollers in respect to the frame. 

Figure 278. Overlay welding of a press roller. (Courtesy of Koppern, Hattingen, FRG)...

The quasi-continuous extrusion of plasticized materials, mostly mixtures of steam-conditioned animal feeds and binders, such as molasses, through bores in differently shaped dies by press rollers (see Figure 180), was developed during the first part of this century. Today, the process, which is called pelleting, is used for the manufacture of cylindrical extrudates from many pasty, plastic, or deformable materials. 

Figures 322(a) and (b) show again the conditions in a pelleting machine with concave die. Figure 322(a) depicts the mechanisms of compression and extrusion in the work area , the material volume wedged in between the press roller and die. Figure 322 explains the phenomena. Feed deposited in a layer on the die is pulled into the space between the roller and die and compressed. Neither the roll force nor the force from the die resisting extrusion (flow) are constant. The roll force increases with progressing densification while the flow-resisting force remains constant until a threshold pressure, defined by the static friction in the die holes, is surpassed. After extrusion (movement in the die holes) has started, both the resisting and the roll forces decrease.

Figure 323. Patent drawing showing a pelleting machine featuring toothed press roller and concave die ...

Figure 325. Patent drawing depicting a pelleting machine with a toothed circular die and conical press rollers ...

Customary controls in pelleting plants normally comprise feed and product characteristics, such as feed rate and steam or liquid addition as well as product temperature, moisture, and strength. As shown in Figure 321, permanent or temporary overfeeding or excessive slip between the roller(s), die, and material— all resulting in too much material in front of the press roller(s)—may lead to choking and, ultimately, plugging of the equipment. Therefore, measurement of the power consumption of the press motor is often monitored and interlocked with the result that, if a maximum is exceeded, the plant stops. 

Schematic representation of the most commonly used machines are shown in Figures 88 and 89. While, because of the large cross section of its extrudates, the reciprocating piston extrusion press (Figure 88a) is not normally used for pharmaceutical applications, screw extruders (Figure 88b and c) may find varied applications. However, the most commonly utilized equipment features differently arranged press rollers and perforated dies (Figure 89). If the extrusion bores are long and without relief counter-bores, relatively high densification...

Special roller press with one press roller within a large ring-shaped die. (No longer used.)... 

Hollow, perforated cylinder(s), feed from the outside Hollow, perforated cylinder, feed from the inside Flat, perforated die plate with press roller(s) Gear-shaped press rollers, feed from the outside Medium pressure axial screw extruders... 

Fig. 8.32 Photograph of a small, low pressure flat die extruder in operation. The front half of the extrusion chamber is transparent to make the press rollers visible (courtesy LCI Corp., Charlotte, NC, USA).

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,...

Nevertheless, machines with concave die rings and internal press rollers do have advantages. For example, if the feed material exhibits a certain elastic behavior, because the forces in the relatively long and slender nip increase slowly, a more complete conversion of temporary elastic into permanent plastic deformation takes place. Fig. 8.41b is another presentation of the forces at work. Feed, ideally deposited in a uniform layer on the die, is pulled into the space (nip) between roller and die and compressed. Friction between roller, die, and material as well as interparticle friction in the mass are responsible for the pull of the feed into the nip and for densification. Smooth surfaces of roller and/or die may result in slip. Axial grooves in the roller, which may also favor build-up of a thin layer of material, and the above mentioned residual layer of densified feed on the die effectively reduce slip. Low interparticle resistance to flow or a distinct plasticity result in a more or less pronounced tendency of the mass to avoid the squeeze" (back-flow), thus reducing densification and potentially choking the machine (see above). 

Fig. S.44 Pholograph or the two operating parts (die and press roller) of an Alexanderwerk "moist granulator" (courtesy Alexanderwerk. Remscheid. Germany).

Fig. 8.49 is another schematic representation, published by one of the manufacturers (CPM), showing the typical design of a pellet mill with cylindrical die and two internal press rollers. The execution suggested by Fig. 6.4b.5 (Chapter 6), 8.40, and 8.41 with one press roller is only used in laboratory and small production machines. For structural and process reasons, the perforated concave die can not be very wide (Fig. 8.50). Therefore, to increase the capacity of a given press and more uniformly distribute the forces acting on the ring, up to three rollers (see below. Fig. 8.52) are installed. With two rollers (Fig. 8.49) the capacity of a machine doubles as compared with a single roll pellet mill and triples with three rollers from a ring loading point of view, the latter also results in the best distribution of the pressing forces.

Fig. 8.49 Schematic representation by one of the manufacturers (CPM) of the operating principle of medium pressure extrusion in a pellet mill" with ring die and internal press rollers.

Fig. 8.50 A selection of typical concave die rings for pellet mills with internal press roller(s) (courtesy Sprout-Matador, Muncy, PA, USA).

It is clear from Fig. 8.49, that pellet mills with ring dies and internal press roller(s) have the great advantage over the previous design, in which the material passed from the outside to the inside of the cylindrical die, that the pellets exit on the outside periphery of the ring. Therefore, they can be easily cut and discharged and no leakage of feed material occurs. 

However, to obtain uniform pellet quality, minimize pellet length variations, avoid uneven die wear, and ascertain constant power demand, it is necessary to distribute the feed evenly across the entire working width (= perforated area) of the die. Since the particulate feed can only enter the operational area of the machine from the open front of the die ring and, additionally, the interior is to a large extent occupied by the press rollers, this requirement is not easily met. 

Fig. 8.55 Ring die and three press rollers with different surface configuration (courtesy CPM-Roskamp Champion, Waterloo, lA, USA).

Fig. S.57 Photograph depicting several flat dies and different press roller assemblies (courtesy Amandus Kahl, Hamburg, Germany),...

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