Twin-screw extruders positive-displacement

The industrial use of twin-screw extruders for this purpose revolves extensively, but not exclusively, around intermeshing co-rotating variants. Closely in-termeshing counter-rotating designs are widely used for profile extrusion of UPVC dry-blends since they permit close temperature control and exhibit a high conveying efficiency due to the positive displacement of material where the screws intermesh [150]. 

In contrast, in twin-screw extruders with intermeshing screws the relative motion of the flight of one screw inside the channel of the other pushes the material forward almost as if the machine were a positive displacement gear pump which conveys the material with very low friction. In twin-screw extruders, heat is therefore controlled independently from an outside source and is not influenced by screw speed. This fact becomes especially important when processing a heat-sensitive plastic like PVC. 

In the extrusion process, a polymer melt is pumped through a shaping die and formed into a profile which can be, for example, a plate, a film, or a tube (Osswald and Herndndez-Ortiz, 2006). For this purpose, ram and screw extruders are used. The ram extruder uses a positive displacement pump based on pressure. In this process, the volume is reduced, the fluid is moved from one point to another and the pressure rises. A screw extruder is a viscosity pump based on the pressure gradient and fluid deformation. Currently, single-screw extruders are the most common equipment used in the polymer industries. Twin-screw extruders are widely used as mixing and compounding devices. 

A closer look at the two types of extruders shows that the closely intermeshing counterrotating twin-screw extruder has a better ability to build up pressures, even in an environment with low-viscosity liquids. This is due to the nearly closed C-shaped chambers, which provide a positive displacement action that enhances stability. Self-wiping extruders, on the other hand, are generally capable of much larger throughputs at comparable residence times. This leads to a preference of counterrotating extruders for... 

Displacement efficiency in twin-screw extruders depends on the flight geometry, the extent of conjugation (intermesh) between the screw flights (generally greatest for trapezoidal screws and channels) and the number of thread starts. Screw design and positive-displacement action will also be influenced by the direction of screw rotation. 

The main difference in the method of operation between single and twin screw extruders is the way in which the material is conveyed within the machine the single screw relies on friction between the material processed and the cylinder to move the material, whereas the configuration of two screws can produce positive displacement. This means that, for example in the case of unplasticised PVC, the formulation that can be processed on a twin screw machine is not as critical as for a single screw extruder, since the quantity and type of lubricant does not affect the material feed. Furthermore, it also means that less work may need to be done (by virtue of less frictional heat being generated) on the polymer and consequently degradation may be avoided. 

Simple intermeshing coimter-rotating twin-screw extruders are positive displacement pumps. This was realized by many early investigators [150, 151, 160, 161] and is clearly discussed in various books [4,162]. Generally, discussions are given in terms of C-chambers, which are forwarded by screw rotation. The output of a machine with i thread starts, C-chamber volume and screw speed N is... 

By starve feeding of a twin screw mixer, ttie amount of mechanical work input to the material can be varied. The materiab to be mixed have to enter the twin screw mixer continuously in their proper proportions because the twin screw mixer, owing to the fact that the material passes through in almost a plug flow fashion, does not intermix in the axied direction. Since the material in this extruder is transported by positive displacement and not by hictional and viscous forces, the melt temperatures do not rise and, in fact, generally heat has to be applied externally. 

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