Backpressure is the amount of hydraulic pressure applied to the back of the screw as it rotates. Varying the amount of backpressure alters the pressure exerted on the polymer in front of the screw. Increasing backpressure also changes the amount of internal energy transmitted to the melt by the shearing action of the rotating screw. An increase in backpressure raises the melt temperature without requiring an increase in heating cylinder temperatures and improves mixing and plasticating. Unfortunately, increasing backpressure also reduces screw recovery rates and can add unnecessary shear (heat) to the polymer which may lead to polymer degradation. Typically, backpressure is set at a minimum unless additional mixing is required.
Two-stage systems, also called screw preplasticators, are available (Figure 17) in which the plasticating unit feeds a separate injection cylinder called an accumulator. Melt is injected into the mold using a ram in the accumulator. Machines equipped with accumulators can be used for molding parts requiring very large shot sizes, for the highspeed injection needed to fill long and narrow mold cavities, and for molding parts requiring better control of shot size and injection pressure.
Figure 15. Schematic cross-section of a typical plunger (or ram or piston) injection molding system
 

Figure 16. Schematic cross-section of a typical screw injection molding machine, showing the screw in the retracted (A) and the forward (B) position
 
Figure 17. In this 2-stage injection molding machine, the screw-type preplasticizer is atop and parallel to the horizontal plunger injection cylinder and chamber.
 

Plasticator specifications
Injection capacity is defined as the maximum shot size in ounces (oz.) of general-purpose polystyrene (PS). In equating this to polyolefins, use approximately 90% – 95% of the capacity stated for PS. The plasticating rate is usually given in pounds/hour or ounces/second for PS. Because of differences in melting characteristics and different sensitivities to screw design variables, it is not possible to easily convert or apply this value to polyolefins. Injection rate is the maximum rate at which the plasticized material can be injected through the nozzle in cubic inches/minute at a stated pressure.
Injection pressure is generally expressed as the hydraulic pressure in psi (pounds/square inch) applied to the screw during injection. The maximum injection pressure available varies and the actual pressure required depends on the resin, melt temperature, mold cooling, part design and mold design. Most plasticating units have a chart which relates the hydraulic pressure to the pressure actually applied to the polymer.
Screw designs
Numerous plasticating screw designs are available for injection molding polyolefins (Figure 18). However, since it is impossible to have a screw designed for every molding job, general-purpose screws are most commonly used. The shallower the screw channels, the smaller the resin volume conveyed to the tip of the screw. On the other hand, while deep screw channels accommodate larger shot sizes more quickly, they do not heat and plasticate the melt as efficiently as a screw with shallower channels. The three basic screw sections are described in Table 3.
Table 3. Functions of the three sections of an extrusion screw
CECTION    CHANNEL
DEPTH    FUNCTION
Feed    Deep
(Constant)    Cool resin pellets are moved forward into hotter barrel zones and begin melting.
Transition    Decreasing
(Tapered)    Resin is compressed, melted and mixed. Air carried along slips back to the feed section to be vented out the hopper.
Metering    Shallow
(Constant)    Sufficient back pressure is created to make the melt homogenous (uniform), make its temperature uniform and meter it into the injection chamber. 注射成型工艺英文文献和中文翻译(2):http://www.youerw.com/fanyi/lunwen_6568.html