Plastification Zone

Comprehensive investigations of the polymer melting process in co-rotating twin screw extruders by Bastian/Gabor [9] yielded the following results  

In addition, for polymer blends, the ratio of melt viscosities r = r)dKp(.r / of the blend partners has a major influence on the melting process. In this case, r dlspen is the viscosity of the blend partner present in the dispersed phase and r)rn is the viscosity of the blend partner present in the continuous phase. The smaller the value of r, the sooner the melting process begins, but also the longer it takes [9]. In unfavorable process conditions, as yet unplasticized particles may be encountered in the compounded pellets (see Fig. 4.5). 

The addition of low-viscosity additives can impede the formation of the melt film that is essential for melting. On the other hand, high-viscosity or solid additives can cause increased energy dissipation due to internal and external friction, which accelerates the formation of the melt film but can also lead to overheating of the viscous phase. 

Along with the actual melting, part of the key tasks of dispersion and homogenization also takes place in the plastification zone. 

Kneading block assembly Solid conveying zone 

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Each processing step is linked to the next. Therefore the different steps cannot be considered independently from each other. For example, coloring processes already take place in the plastification zone, the incorporation of fibers added to the melt takes place not only in the designated dispersing zone but also in the discharge zone and in partially filled screw channels. 

When conveying solids with a low bulk density, high fill rates in the intake zone can cause fluidization, if the air that is drawn in cannot escape from the extruder. This can be avoided by a partially filled downstream plastification zone (e. g., with conveying kneading blocks) and a vent opening installed downstream of the plastification zone. 

Varying the barrel temperature in the plastification zone makes hardly any difference to the melting process. 

Depending on the subsequent process steps, achieving 100% melting at the end of the plastification zone may not be mandatory. Solid particles still present after the plastification zone can be melted in subsequent zones, e. g., in the dispersion or discharge zones. Figure 4.6 illustrates a typical melting profile along a plastification zone. 

Figure 4.6 Melting profile along a plastification zone [10]...

Up to 80 % of the overall mechanical energy input in the twin screw extruder takes place in the plastification zone. Unfortunately, currently only rudimentary modeling capacities for the melting process are available. 

Figure 4.9 Energy input as a function of throughput for polyamide melting with kneading block and MPE plastification zones [13]...

The energy input via the screw elements and kneading discs results in splaying forces, particularly in the plastification zone, which can cause wear to the cylinders (see Fig. 4.10). This can be reduced by an appropriate design of the plastification zone and by heating the cylinder wall, which results in the formation of a melt film [8]. 

In the split feed process, the additives are fed into the polymer melt via a side feeder downstream of the plastification zone. Here, gravimetric feeding is always used (see Fig. 4.32). 

An exception to this are very fine powders that can be pneumatically entrained by returning interparticle air in front of the left-handed plastification zone, drastically reducing the potential throughput. Air speeds decline with increasing flight depth, which explains why the ZSK MEGAvolume has stand the test for these products. 

If the plastification zone is too strong, the viscosity in the matrix may decrease so that it is impossible to break up high-molecular weight particles in the homogenization zone. Plastification should preferably take place at a high shear rate and short residence time. 

Different types of wear occur in the plastification zone, in the additive intake zone, or in the discharge zone of the twin screw extruder. These include ... 

Here, the harder partner penetrates the surface of the softer partner resulting in a microcutting process. This problem is particularly pronounced when processing plastics because the fillers and reinforcements that cause wear are added in their pure form, i. e., not coated in melt. The problem applies both for the plastification zone (not in the case of rework materials) as well as for solid fillers. The amount of abrasive wear is primarily determined by ... 

Figure 16.2 shows abrasive wear on screw elements from the plastification zone when... 

A significant heating-through of the material begins after it comes from the transportation zone into the plastification zone, because of contact with the hot surface of the gate channel, as a result of compaction and the appearance of dissipative heat release [38], The process of conversion into the melt (plastification stage) of the composition must be carried out in such a manner that chemical transformation at this stage are practically absent. 

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. 

Like Bartilla et al. (2), Cartier and Hu (15) identified the plastification zone as the focal point of the reactive extrusion as a function of material, machine design, and process variables ... 

Experimental observation of what happens inside the extruder including the melting/ plastification zone. This allows us to obtain qualitative/quantitative information about where and how morphology develops along the screw axis. Both uncompatibilized and compatibilized blends are studied. 

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