Hopper forces

Figure 20 shows an arrangement which is unaffected by air pressure fluctuations, because any force appHed to the material is canceled by an equal and opposite force appHed to the iaside top surface of the hopper. It may be desirable or necessary to vent the hopper for efficient material handling. 

Fig. 13. Injection molding machine A, hydiaulic motoi foi turning the screw B, hydraulic cylinder and piston allowing the screw to reciprocate about three diameters C, hopper D, injection cylinder (a single-screw extmder) E, no22le F, fixed platen G, tie rods H, mold I, movable platen , hydraulic cylinder and piston used to move the movable platen and supply the force needed to keep the mold closed and K, machine base.

If most of the particles are less than ca 0.6 cm in size, flow obstmctions can occur by physical, chemical, or electrical bonds between particles. This cohesiveness is characterized by the bulk material s flow function. The forces acting to overcome a cohesive arch and cause flow are described by a hopper s flow factor, which can be obtained from the design charts (see Fig. 7). The minimum opening size required to prevent a cohesive arch from forming can be calculated from the comparison of the flow factor and flow function. 

For fine powders that tend to bridge or stick and are of low bulk density, some form of forced feed, such as the tapered screw feeder shown in Figure 9, must be used to deaerate, precompact, and pressurize the feed into the nip. Large machines are available with up to five screw feeders to spread the flow across the roUs, and vacuum hoppers are also used to remove air when densifying low density feeds. 

Dust lood Plote design Hopper design Gos velocity Gos distribution Rapping force Popping intervol... 

Electrodes in the center of the flow lane are maintained at high voltage and generate the electrical field that forces the particles to the collector walls. In dry ESPs, the collectors are knocked, or "rapped," by various mechanical means to dislodge the particulate, which slides downward into a hopper where they are collected. 

The earliest injection moulding machines were of the plunger type as illustrated in Fig. 4.30 and there are still many of these machines in use today. A predetermined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heaters. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel as shown. 

In injection molding, the polymer is fed from a hopper into a heated barrel where it softens and becomes a viscous melt. It is then forced under high pressure into a relatively cold mold cavity where the polymer has sufficient time to solidify. Then the mold is opened and the fabricated part is ejected. The cycle of operation is then repeated. 

Dust lood Plate design Hopper design Gas velocity Gas distribution Rapping force Rapping interval... 

For heights greater than Lc the mass of additional solids is supported by frictional forces at the walls of the hopper. It may thus be seen that hoppers must be designed to resist the considerable pressures due to the solids acting on the walls. 

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