Extrusion Functional Zones

This chapter covers two main sections extruder hardware systems and extrusion functional zones. The first section identifies the primary components of the extruder and provides detail into their capabilities and size ranges. The second section describes how the hardware interacts with the material as it passes through the system. In other words, it provides a look at what is happening inside each zone of the extruder. The descriptions here are purposefully brief, serving as a foundation to support later discussions specific to blown film extrusion. If the reader is interested in pursuing this topic further, there are many books devoted entirely to the extruder and extrusion processes. For example. Polymer Extrusion by Rauwendaal [3] provides an excellent in-depth analysis. Other references are listed in the back of this book [4-6]. 

Finally, the poljmier is processed in an extruder, where polymer properties (such as the chain length distribution) are adjusted by means of a sequence of different functional zones of the extruder. Further modifications of the polymer properties can be achieved by adding additives to the polymer melt. The extrusion step can be simulated by a special purpose tool called MOREX [146, 394]. Given geometry data of the extruder, it calculates the required energy demand and the properties of the resulting polymer. In addition, it can be used to calculate the vapor stream of -caprolactam which can be stripped of if a considerable amount of e-caprolactam is present in the polymer melt. 

Chapter 7 covers the actual analysis of the extrusion process. The process is analyzed in discrete functional zones with particular emphasis on developing a quantitative understanding of the mechanisms operational in each zone. 

Pipe haul-off units form a key part of the extrusion process since they have the function of exerting a steady, uniform pull back through the cooling and calibration zones to supplement the extrusion pressure at the die head. The haul-off unit effectively creates a tensile stress in the solid material that balances the hydrostatic and viscous forces in the liquid state of the extrudate and overcomes the resistance of external friction. Haul-off units may be belt driven or multi-pad caterpillar systems. Large diameter pipes require multiple caterpillar units and thin wall pipe also require more units to avoid distortion. 

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 ... 

If the extruder is fed with a liquid monomer, the first part not necessarily needs to be partly empty. However, as will be explained later, for reasons of stability it is advisable to create a zone where the chambers are not fully filled. Especially the fully filled zone is very important for a proper functioning of the extruder. In this zone the pressure is built up, mixing and kneading mainly takes place, and the major influence of viscous dissipation also occurs. In reactive extrusion, changes in the length of the fully filled zone are associated with changes in reactor holdup and residence time. 

The hand-held portable extrusion welding device or apparatus also consist of three functional elements preheat system for the hot air, which is used to preheat the welding zone the plasticising system with temperature control, a small extruder, in which the extrudate pellets or an extrudate rod are melted, homogenised and brought to a controlled mass temperature and from where the melt strand is discharged at a certain discharge speed... 

The changes of the characteristics of the cold crystallization range of the extruded polymer in function of the capillary length are presented in Table 3.85. The decrease of is a proof of the appearance of orientation during the extrusion through the capillaries of the melted PET, [766,767]. The orientation induced in the polymer in the entrance zone of the capillary is more accentuated in the case of extrusion through long capillaries. 

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