Screws, processing zones

The various processing zones of a twin screw extruder are arranged in series, as illustrated in Fig. 4.3. 

In the following, we will describe each processing zone in a co-rotating twin screw extruder. 

Even though there is always a residence time distribution, it is practical to specify a mean residence time. The mean residence time is calculated from the volumetric flow rate V and the free volume Vfree of the screw. In addition, the volumetric degree of fill f must be taken into account [3]. With this in mind, different processing zones are considered individually (e.g., partially filled devolatilization zones and completely filled zones for pressure buildup). 

The polymer or the polymer solution cools down because the vaporization energy required for the vaporization process is taken from the in-fed polymer solution. The required ZSK size depends on the gas quantity to be removed, the gas velocity, the rheological behavior of the polymer melt, and the thermodynamic behavior of the degassed components, the screw speed, and the throughput. Mechanical energy has to be introduced in the processing zone... 

For a majority of applications, the modular design has been accepted for the twin screw extruder. This allows optimum adjustment of wear protection in the respective process zone and material selection for the corresponding process zones to ensure a maximum life span. 

The processing zones in the extruder are the solid zone - feed section, transformation or plastification zone, the pumping or metering section (screw) and the mold. Feeding in the solid zone is the main determinant of machine output. 

In the screw preheat zone in particular, and at the onset of plastification, microwave technology offers promising solutions to the process engineering problems in extruder design mentioned above. Energy transfer in this technology does not involve heat conduction and makes for an efficient transfer of energy into the compacted bulk powder materials with poor heat conductance properties present in the extrude in this phase of the plastification process (For details, see Diemert [20]). 

The WPC pellets were processed in a single-screw, three-zone extruder with a gas injection system [9], The barrel temperature was maintained below 150°C, to suppress the volatile generation. A static mixer, a heat exchanger and a filament die (L/D 0.7/0.038) were attached to the barrel exit. Their temperatures were set at 150°C, 150°C and 150°C, respectively. The die temperature was reduced at an interval of 5°C, from 150°C to the lower temperatures, to prevent gas loss and ensure the formation of the cellular structure in the extruded filament. The processing conditions were the same for both CO2 and N2 except for their contents varied. 

The appearance of air bubbles in the polymer melt may occur under certain circumstances during processing. This phenomenon is rarely related to obvious faults in the polymer, but sometimes gas bubbles can be observed in cases of decreased thermal stability. Gas bubbles appear due to a certain amount of dispersed gas in the polymer matrix. Insufficient removal of gas from the extruder, particularly from the compression zone, can also cause the problem of air bubbles in the melt. An influence of the extruder screw could be established, because gas bubbles can be removed to some extent by using special screws or changing the extrusion conditions, along with the application of a vacuum. 

Fig. 1. Process flow sheet for the continuous conversion of latex in a counterrotating, tangential twin-screw extruder as it might be arranged for the production of acrylonitrile-butadiene-styrene polymer (Nichols and Kheradi, 1982). Polystyrene (or styrene-acrylonitrile) melt is fed upstream of the reactor zone where the coagulation reaction takes place. Washing (countercurrent liquid-liquid extraction) and solids separation are conducted in zones immediately downstream of the reactor zone. The remainii zones are reserved for devolatilization and pumping.

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