Roller compaction roll diameter

The simplest approach during scale-up would be to maintain the applied roller compaction force as the roller compactor roll diameter and width increases on scale-up. This technique assumes that the applied force is linear at the point where the powder sees the maximum force. Dehont s team defined this as the neutral angle, gamma. This approach although simple to understand and theoretically correct has been shown to be successful only in a limited number of cases in the author s experience and is very much dependent on the formulation s composition. 

Several authors have experimentally checked the relationship (equation 97) for the compaction of metal powders between smooth rollers (strip rolling) and expressed the strip thickness as a percentage of the roll diameter. However, these results, as well as the correlation found by Spinov and Vinogradov that the strip thickness also depends on the roll or strip width /, may be influenced by so many variables that their validity is very limited. 

Expansion of the strip after pressure release is influenced by the physical characteristics of the material to be compacted (plasticity, brittleness, particle size and distribution, particle shape, etc.), the roll diameter, the speed of rotation, and the surface configuration of the rollers. With increasing roll diameter and/or decreasing speed the expansion of compacted material is reduced due to better deaeration during densification and a more complete conversion of elastic into permanent, plastic deformation. 

In contrast, the briquetting or compacting of some other materials demands a degree of compaction that cannot be achieved by a single pass in a choke-fed roller press, irrespective of the ratio of pocket size (or gap width) to roll diameter. In addition, redistribution of material (which may be extensive) from the nip against the flow of material or from the rear of cups into following cups, e.g. due to the flow of displaced air, may further reduce the efficiency of compaction. 

Dehont et al. assumed that the material in the compaction area remains horizontal and moves at the peripheral speed of the rollers. They also considered that the angle a is independent of the roller diameter size and noted that the flake thickness ei depends on the roller speed, the roller surface, and the compaction pressure. All these parameters influence the density of the flake, d. Dehont et al. concluded that if the same flake thickness was obtained with different roller diameters, the flake density would be greater with larger diameter rollers. This is due to the greater nip angle formed, with the larger rolls allowing more material to be compacted. 

Wheel tracking test specimens were prepared by compacting asphalt mixtures into a mold of 300 by 300 mm, and 50 mm in depth, with a roller compactor. The test apparatus is a machine in which a loaded wheel with a solid rubber tire of 200 mm diameter and 50 mm wide is rolled to and fro on a specimen of an asphalt mixture. A photograph of the apparatus is given as Fig. 2. 

A product as described under operation 1 is most easily obtained with a small roller diameter or narrow roll gap (small sheet thickness) and low circumferential speed (both resulting in small capacity) as well as little compaction ratio. Then, only a relatively small amount of air is expelled which can escape partly to the top and partly to the sides of the rollers. 

In machines with fixed rolls the optimum compaction force is obtained by adjusting the feed rate and density to the nip. Roller diameter, speed, and gap, as well as characteristics of the raw feed such as friction between material and rolls and particulate size, shape, and brittleness, all greatly influence the respective operating conditions. 

Fig. 8.123 describes the compaction of a particulate solid in the nip between two gravity fed counter rotating rolls. For clarity, the roller diameter D and the distance between the rolls are not to scale. In reality the roll gap is much smaller as compared with the roller diameter (e.g. D/Iia 100/2 to 100/5). Compaction between two smooth rolls may be explained by dividing the nip area into three zones The feed zone, the compaction zone, and the extrusion zone. 

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