Hydraulic back pressure

The pressure at the tip of the screw is controlled by setting the hydraulic back pressure at the shank end of the screw. The pressure at the tip is calculated from the hydraulic back pressure and the process intensification factor as follows ... 

Pj hydraulic back pressure for an injection-molding plasticator (Eq. 10.22) maximum power rating for the motor in watts Pr pump ratio for a two-stage screw... 

Sizing, safety relief, 436, 437-441 API liquid valve, 444 Balanced valves, 441 Conventional valves, 438 Critical back pressure, 440 Effects of two-phase flow, 437 Hydraulic expansion, 441 Rupture disks, 434 Sub-critical flow, 449 Slurry flow, process pipe, 142-147 Regimes, 143... 

Facility is provided to bring the equipment up to the desired absolute pressure without subjecting the test unit to excessive pressure difference between its interior and exterior. This is accomplished by first filling the test cell with water by means of a hand operated hydraulic pump(17 in Figure 1) to a suitable value as indicated on the Bourdon dial gauges(P in Figure 1). The pressure thus developed is used also to control the appropriate back-pressure relief valve... 

Since the specific filtration area of the 200/12 configuration is 41.5% greater, the filter length with such a configuration can be 58.5% smaller than that of the filter with the 100/17 cell configuration for identical total filtration areas. In this manner, Eq. (11) helps estimate the back pressure penalty due to smaller hydraulic diameter of 200/12 cell configuration. 

So-called back pressure is also applied to the screw by the hydraulic plunger. Back pressure is purposely applied to hinder the compound fi om moving forward over the flights of the screw. This resistance further enhances homogenization. 

Control is achieved by comparing cavity pressure with the set value if a difference exists, the controller will increase or decrease back pressure (see Figure 7.39) on the next cycle. The correcting parameter is the hydraulic ram pressure. 

General purpose screws can be used, for example, with an L D ratio of about 18 1 and a constant pitch of ID. The feed zone should be approximately 0.5L, the compression zone 0.3L and the metering zone 0.2L a compression ratio of about 2 1 should be sufficient. The screw is usually fitted with a nonreturn valve but valves are not essential on the nozzle unless high back pressures are involved. The best results have been obtained by using spring loaded or hydraulically operated types decompression or suck-back is also used. High speed low torque screw motors are often preferred to achieve short screw recovery times. 

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