Compression press, typical

Figure 18 shows a typical line of compression presses fitted with vacuum chambers. This type of press can be purchased with platen sizes in the range 355 x 355 mm to 710 x 710 mm. The choice of press size is dependent on product size, product design, and volume. 

Figure 18 A typical compression press molding line.

A second flow measurement to be considered during scale-up is the ability of the granulate/powder to fill the dies. This can most efficiently be monitored by punch force variability and individual core weight measurements. Acceptable weight control (<3% RSD) and force (<5% RSD) may be masked at slower compression speeds typically used for development or when a tablet press is not fully tooled. Production operations will tend to run at the high end of any validated range, so flow must be consistent from batch to batch. 

Filtration. Filtration can include filter presses, rotary drum vacuum filters (RDVF), belt filters, and variations on synthetic membrane filtration equipment, such as filter cartridges, pancake filters, or plate and frame filter presses. These processes typically operate in a batch mode when the filter chamber is filled up or the vacuum drum cake is exhausted, a new batch must be started. This type of filtration is also called dead-end filtration because the only fluid flow is through the membrane itself. Due to the small size of cells and their compressible nature, typical cell cakes have low permeability and filter aids, such as diatomaceous earths, perlite, or other mined materials are added to overcome this limitation. Moreover, the presence of high solids and viscous polymeric fermentation byproducts can limit filtration fluxes without the use of filter aids. 

Precompression is often used to tamp or apply a small compression force prior to the main compact compression cycle. Rotary tablet presses are often equipped with a separate precompression station, which is positioned between the die-filling feed frame and the main compression station. Typically, precompression is used to improve the quality of tableted products, where it increases the strength of the compact and/or decreases the incidences of capping and lamination. The compact strength is enhanced through the increase in the effective contact time in which the powder particles are in contact under an applied force. During this extended contact time, stronger interparticulate bonds form and stress relaxation occurs. 

The process/quality control strategy for the manufacture of detergent tablets is largely based on the approach developed for pharmaceutical tablets. The tablet weight is controlled by the fill cam position. In modem presses, this will be a motorized adjustment and is linked by a feedback control loop based on the compression force. Typically, this can achieve a weight variation of less than +/ 4% (three relative standard deviations), since for a given material the compression force is... 

Figure 14.3 Schematic view of the major components of a typical compression press.

The press cycle, or the time taken to perform a series of operations until they are again repeated, varies according to the material being moulded and the requirements, such as the need to load loose cores and metal inserts into the tool. A brief description of the necessary operations on a semi-automatic press (Fig. 9) will now be given to acquaint the reader with the normal sequence of operations for a typical moulding on a compression press. 

Filtration and compression take place with the press closed and the belt stationary the press is then opened to allow movement of the belt for cake discharge over a discharge roUer of a small diameter. This allows washing of the belt on both sides (Fig. 15). Cycle times are short, typically between 10 and 30 minutes, and the operation is fully automated. Si2es up to 32 m are available and the maximum cake thickness is 35 mm. 

Flows are typically considered compressible when the density varies by more than 5 to 10 percent. In practice compressible flows are normally limited to gases, supercritical fluids, and multiphase flows containing gases. Liquid flows are normally considerea incompressible, except for certain calculations involved in hydraulie transient analysis (see following) where compressibility effects are important even for nearly incompressible hquids with extremely small density variations. Textbooks on compressible gas flow include Shapiro Dynamics and Thermodynamics of Compre.ssible Fluid Flow, vol. 1 and 11, Ronald Press, New York [1953]) and Zucrow and Hofmann (G .s Dynamics, vol. 1 and 11, Wiley, New York [1976]). 

DisK Presses Figure 18-157 shows a disk press. The two disks, or press wheels, converge to a veiy narrow space at the bottom. This is the point of maximum compression, which can be more than 14 times the feed pressure. The press wheels have channels to cany the hquid from the dewatered product, and they are covered with a screen plate. Wheels 1.5 m (5 ft) in diameter are used on a large press that requires about 80 connected horsepower and produces just under 1 tonne/h (0.9 ton/h) of sohds (diy basis). Typical apphcations are fibrous materials such as coffee grounds, pineapple and citrus peels and wastes, alfalfa, and brewers spent grain. 

A typical method of preparation of 5- and 7-ply plywood panels is by pressing at room temperature and the simultaneous use of a one-step exothermic heat gluing/ monomer curing technique. Prior to monomer impregnation, all veneer samples were conditioned at room temperature. The assembly was compressed to 1.03 MPa (150 psi) between steel plates in a hydraulic press, and the steel plates fastened with a pair of bolts and nuts at both ends. 

Molding is the process where a pattern is pressed or embedded into special sand to the desired shape or form. Alternatively, the pattern can be placed on a molding board, and the sand rammed or compressed around the pattern. Figure 4.4 shows the typical molding flow. The sand and binder are first... 

The parts produced were in specification and did not have the splay defect. When the press was switched between different colored resins, the purge time was very short and typically required less than 20 parts. The higher compression ratio and compression rate screw combined with channels that had large radii provided a constant plastication rate and no detectable degradation of the resin. 

For most ceramic pressing, a CR < 2.0 is desired since it rednces both the punch displacement and the compressed air in the compact. As indicated in Eq. (7.15), a high fill density leads to a low CR. For comparison, the CR in metal powder pressing is typically much greater than 2.0 dne to the dnctility of the particles. 

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