Extruders elementary

A more common situation, however, is that before there is any significant frictional heating, an axial position is reached where the barrel is heated to well above the melting point, forcing the creation of a him of melt. In either case, this marks the end of that portion of the process in the extruder called the solids conveying zone, where only solids are present and the only elementary step that occurs is handling of solids. 

We now discuss the elementary steps of processing as they occur in the twin screw segment of the CRNI tangential extruders melt pumping in the single screw segment was covered in Chapter 9. 

In order to realize the Polyamide-6 scenario problem presented in Subsect. 5.3.2, a process description is defined using ModKit+. The elementary models for the reaction section, the separator, and the extruder, already imported into ROME, are added as submodels of the overall process. Further, a mixer is defined in order to combine feed and recycle streams corresponding mass balances are added to the elementary mixer model. After this modeling activity the model repository ROME contains all necessary models for the overall Polyamide-6 process. The model behavior is partly described by equations (for the mixer) and partly described by model implementations in the form of input files for the modeling tools Aspen Plus, gPROMS, and MOREX. 

So, in the kneader-extruder, peptization can be described as a continuous sequence of breakdown-reconstruction phenomena, the ultimate state of a successful peptization being to allow the above rearrangements to take place between all the elementary boehmite cristallites. 

Fig. 6.2 Schematic representation of a twin-screw extruder and elementary steps...

Disk extruders. There are several types of screwless extruders. These machines employ a disk or a drum to plasticate, mix, and extrude. Most designs are based on viscous drag flow. They include Maxwell s elastic melt extruder, Westover s stepped disk and drum extruders, and Diskpack [39]. The Diskpack extruder has the capability of performing all the elementary steps of plastics processing by combination of differently shaped rotating... 

The flow in many real processing geometries is too complex for us to apply the analytical methods utilized in the preceding chapters. Indeed, even when the flow field is a simple one, the coupled heat transfer problem may not be amenable to a simple treatment the elementary extruder in Chapter 3 is an example of a case in which we are unable to obtain an exact or even approximate solution for the spatial development of the two-dimensional temperature held. 

Polytrifluorochloroethylene can be extruded and injection molded at temperatures of approx. 300 °C, but at these temperatures it already exhibits rapid polymer degradation. It decomposes, like FIFE, into low-molecular components. In contrast to FIFE, elementary chlorine is also cleaved, which acts extremely corrosively and places special demands on the processing machine and the mold [599]. 

Beside the estimation of optimized screw configurations in the melting and homogenization zones, the engineer has to tweak the pressure build up in the final barrels of the extruder. For plastics showing a rather simple flow behavior, this task may be solved using elementary analytical approaches. For bimodal plastics with an explicit non newtonian behavior it is necessary to use sophisticated models or better to employ tools like CFD (computational fluid dynamics) to describe the pressure buildup and energy dissipation. 

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