Pressure Build-Up Zone

Pressure build-up zones are located in the extruder in the discharge zone and upstream of each backward pumping screw element, e.g., mixing elements, re-conveying, or neutral kneading blocks. 

For the same example, Fig. 4.17 shows that pressure build-up ability and energy input depend not only on screw geometry but also on the process parameters (in this case screw 

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Figure 4.16 Energy input in the pressure build-up zone for different conveying elements...

Efficiency during pressure generation is just as important as the back-pressure length for the design of a pressure build-up zone. By efficiency we mean the relationship between the pressure energy that is stored in the flow at the end of the pressure build-up zone and the energy that is introduced into the pressure build-up zone. 

The size of the machine and the throughput (and often the revolution speed as well) are determined by other factors, so the pressure build-up zone should be optimized by selecting the pitch and gap [2]. 

The following numerical example illustrates the effect of pitch selection on energy dissipated in the pressure build-up zone. A nozzle pressure of 50 bar has to be generated, the viscosity of the polymer at the machine s head is 200 Pa s and a double-flighted machine with the following properties should be considered ... 

The second example shows a pressure build-up zone which follows a left-rotating element (Fig. 8.16). The same basic geometry was used for the screw elements as in the first example. Only the pitch, the length, and the direction of rotation vary from element to element ... 

A screw pitch of 1.5 to 2D is considered optimum for the product intake in the feed section. The optimum screw pitch requires knowledge of the material data and can be calculated for the required operating mode of the extruder for the pressure build-up zone at the screw tip. In optimum terms this means building up the required tip pressure with optimum efficiency at the given operating parameters, i. e., the lowest possible energy input. 

Pitch H and screw speed n are available as free parameters for optimization of the pressure build-up zones. Most other process parameters such as throughput, discharge pressure, and material (viscosity) are already set by customer specifications. 

In addition to length of pressure build-up zone, the energy input is an important factor because, among other things, it determines the temperature increase of the melt. Therefore, the energy efficiency e of pressure build-up is actually the more relevant parameter (Eq. 15.6) ... 

The high structural viscosity of bimodal products means that advanced modeling methods are needed to optimize the pressure build-up zones. Calculation and evaluation of dimensionless parameters helps to keep the complexity within limits. 

Conveying zone Pressure build-up zone Modified melt Melt under pressure Spec, heat Frictional heat Frictional heat... 

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