Plasticating Single-Screw Extruders

Figure 1.2 Schematic of a typical plasticating single-screw extruder. The extruder is equipped with four barrel heating and cooling zones and a combination belt sheave gearbox speed reduction drivetrain (courtesy of William Kramer of American Kuhne)...

The plasticator on an injection-molding machine is a specialized plasticating single-screw extruder. The plasticator has two main differences there is a nonreturn valve on the tip of the screw, and the screw retracts as molten material accumulates between the nonreturn valve and the end of the barrel. Pressure is maintained on the accumulated material by a constant force applied to the shank of the screw via the drive system. This force is typically measured as a pressure applied to the shank and is referred to as the back pressure. During the injection step of the process, the screw is forced forward, the nonreturn valve closes, and the material is injected into the mold. Additional information on the injection-molding process can be obtained elsewhere [Ij. 

Altinkaynak, A., Gupta, M., Spalding, M.A., and Crabtree, S.L., An Experimental Study on Shear Stress Characteristics of Polymers in Plasticating Single-Screw Extruders, Polym. Eng. Set, 49, 471 (2009)... 

Plasticating single-screw extruders will typical range in size from 15 mm up to 250 mm in diameter. Larger diameter plasticating machines are very rare and were likely built years ago. Plasticating rates above about 4000 kg/h are more economically processed using twin-screw machines. 

Figure 3.4 Schematic of a plasticating single screw extruder.

Melting Zone in a Plasticating Single Screw Extruder... 

Solid bed profile prediction for a plasticating single screw extruder. In this example we would like to use Tadmor s model to predict the solid bed profile of a low density polyethylene in a plasticating single screw extruder, based on experiments published by Tadmor and Klein [27], In their experiments they used the following screw geometry ... 

Plasticating single-screw extruder (from Osswald and Hernandez-Ortiz, 2006). 

PLASTICATING SINGLE-SCREW EXTRUDERS 251 where y is given by... 

Tadmor, Z., Fundamentals of Plasticating Extrusion, Polym. Eng. Set, 6, 185 (1966) Potente, H., Single-Screw Extruder Analysis and Design, Chapter 5 in Screw Extrusion, White, J. L. and Pontente, H. (Eds.), Hanser, Munich (2003)... 

The size of flight radii on the screw channel is important for the proper performance of a single-screw extruder. The specifications of the flight radii are commonly made using a combination of personal experience and guidelines [16] from the Society of the Plastics Industry, Inc. (SPl). These guidelines state unless otherwise specified the root radius will not be less than one-half of the flight depth up to... 

In polymer processing, compaction is an important and necessary step in order to reduce the interparticle, unoccupied spaces and thus eliminate air. It is essential for melting in both single-screw extruders as well as for twin-rotor processors, as we shall see in Chapters 5 and 10. In twin-rotor devices, such as Co-TSEs, for example, the large and repeated deformation of compacted particulates by the kneading elements, which induces large plastic deformation of particulates, is the dominant melting mechanism. 

Fig. 10.7 Schematic view of the CM/single screw extruder combination used in polymer compounding operations. The chute, connecting the two processing machines, transfers the molten and mixed stream at low pressures to the throat of the melt pump SSE, which generates the pressure needed for pelletization. [Reprinted by permission from E. L. Canedo and L. N. Valsamis, Farrel Continuous Mixer Systems for Plastics Compounding in Plastics Compounding—Equipment and Processing, D. B. Todd, Ed., Hanser, Munich, 1998.]...

In an extruder of this type a system of intermeshing gears is used to force the plastic towards and through the die. A gear pump can be fitted to a single-screw extruder in order to obtain steadier pressure and smoother output, or it may be used to extrude fibres from polymers which at processing temperatures are liquids of low viscosity. 

Figure 8-1. Main components of a single-screw extruder (courtesy of the Association of Plastics Manufacturers in Europe).

The action that pushes solid particles forward in the feed section of a single screw extruder, blow, or injection machine has always been a potential for one of the weakest features of these machines. This forward feeding force near the feed hopper is often weak and erratic and is classified as non-positive. It can be so tenuous that a specific screw/barrel combination will feed virgin but little or no additions of regrind, or one feedstock shape but not another, and often one family of plastics but not another. This action results in non-uniform feed that will in turn result in poor production rates, non-uniform output (surging), and poor product quality.147... 

The machinery has been borrowed from the plastics industry. An early single screw extruder was used in the 1940s to produce a snack product from maize grits but the operational advantage of twin screw machines was demonstrated later. It appears that they are more efficient in mass transport, less heat is dissipated, and therefore heat transfer can be better regulated by barrel heating (Lo et al. 1998). 

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