Melting/plastification

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. 

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. 

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. 

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

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

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). 

The intake behavior is also substantially influenced by the resistance of the downstream plastification section. If discharged at atmospheric pressure, e.g., via screw feeders, the throughput rate is considerably higher than if the same feed section is succeeded by a plastification section (Fig. 11.4). For instance, if metering a product with a bulk density of 0.35 g/cm3 and a melt phase density of 0.7 g/cm3, it is clear that considerable quantities of gas will have to be extracted upstream through the intake hopper. 

The plastification process is initiated the polymer contacts the hot inner wall of the barrel and/or by mechanical stress of the solid particles in the kneading elements. The melting process is continued by the introduction of mechanical energy into the melted product via shear stresses and/or heat conduction from the melt to the as yet un-melted product. The detailed design of the plastification section depends on the product. 

The efficiency of a forward kneading and plastification section depends greatly on the operating conditions of the extruder. The elements in this section may only be partially full, and entrained gas can therefore pass downstream which increases the risk for the polymer to not melt completely (Fig. 11.6). 

As the pitch increases, the conveying efficiency reaches an optimum level before dropping sharply. Conveying elements with a very large pitch, e.g., larger than 6D (Fig. 12.15) are used both as plastification elements in the melt zone and as homogenizing elements further downstream. 

The special elements most commonly used today are for the deliberate application of elongational flows or defined shearing fields, for low-shear plastification, or for retaining un-melted particles. 

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 ... 

The cycle starts with the plastification of the core component in the injection unit. Then the extruder moves to the bottom position, the injection unit moves forward to the extruder nozzle to link the nozzles of the extruder and the injection unit. The extruder starts plastification of the skin component and extrudes the melted skin component into the screw antechamber of the injection unit. Thus the skin and core components are located one after the other in the screw antechamber. After the extruder moved back to the top position, the injection unit moves forward to the mold followed by a conventional filling phase. Due to the fountain flow effect the first injected material forms the skin layer followed by the second component forming the core. Compared to the standard sandwich process the injection phase of the monosandwich process is less complicated as it is identical to the conventional injection molding process. 

Behavior at higher temperatures This test is easily carried out by contact with a hot soldering rod. On thermoset material, the surfaces remain unchanged, on thermoplastic material a plastification, respectively, melting can be observed. 

The melt viscosity is regarded as still too high for injection molding of these materials. However, an additional option for adjusting their processing conditions is their plastification. An effective means for attenuating ionic association within the clusters is the incorporation of various metal acetates, particularly Zn acetate, which supposedly breaks-up divalent sulfonate linkages ... 

With all the varieties of materials available, and their individual grades with many different formulations, it has not yet been possible to design a screw in advance, based on the melt or the rheological/flow physical relationships involved in plastification and conveying. Trial and error, with an observant processor using reliable and reproducible controls, makes the screw perform to its maximum efficiency. 

---