Solids conveying in screw extruders

Broyer, E. and Tadmor, Z., Solids Conveying in Screw Extruders Part 1 A Modified Isothermal Model, Polym. Eng. Set, 12, 12 (1972)... 

Chung, C.I., New ideas About Solids Conveying in Screw Extruders, SPEJ., 26, 32 (1970)... 

Fig. 9.30 Graphical representation of the various components of the total power consumption in the solids conveying zone of screw extruders for H/Di, = 0.15 and a constant fs/fb ratio as a function of the angle (f> Pwb, which is the major component of the power, is the power dissipated into heat at the barrel plug interface Pws and Pwf are the power dissipated on the root of the screw and flights, respectively and Pwp is the power consumed for the pressurization of the plug. [Reprinted by permission from E. Broyer and Z. Tadmor, Solids Conveying in Screw Extruders, Part I. A Modified Isothermal Model, Polym. Eng. Sci., 12, 12-24 (1972).]...

A second critical point is the rear vacuum seal around the screw shank. This seal is exposed to sometimes-gritty materials. Leakage of air at this point can cause fluidization of material and adversely affect solids conveying in the extruder. Another method to avoid air entrapment is to use a two-stage extruder screw with a vent port in the barrel to extract air and any other volatiles that might be present in the polymer. 

The early analysis of solids conveying in an extruder evolved through several simple models based on simple concepts such as the frictional forces between the polymer and the metal surfaces is credited to Decker [36] and Maillefer [37] and the concept of the differential torque at the screw and barrel surface was introduces by Simonds [38]. The rigor of the derivation was significantly improved by Darnell and Mol of DuPont and their analysis is most often cited as it gives a good description of the behavior observed. The model is based on the force and torque balance developed on the solid plug, which is assumed to completely fill the screw channel. The basic assumptions used for the derivation of the model are ... 

Hyun, K.S. and Spalding, M.A., A New Model Eor Solids Conveying In Single-Screw Plasticating Extruders, SPEANTEC Tech. Papers, 43, 211 (1997)... 

A number of issues relative to the prediction of solids conveying in smooth bore single-screw extruders are exposed from the theoretical fits to the data in Fig. 5.32. First, the data needed to carry out an effective simulation is difficult to take and is very time consuming, and only a few labs have the proper equipment that is, bulk density measurement, dynamic friction data, lateral stress, and solids conveying data. Moreover, care must be taken to develop an accurate representation of the surface temperature for the barrel and screw as a function of the axial position. This would be quite difficult in a traditional extruder with only a control thermocouple to measure the temperature at the midpoint of the barrel thickness. Second... 

Hyun, K. S., Spalding, M. A., and Hinton, C. E., Theoretical and Experimental Analysis of Solids Conveying in Single-Screw Extruders, SPE ANTEC Tech. Papers, 42, 199 (1996)... 

Solids Conveying of Nylon in Screw Extruders Consider a 1.991-in-diameter screw with 2-in lead, 1.375-in root diameter, and 0.2-in flight width, conveying nylon pellets with bulk density of 0.475 g/cm3 and a coefficient of friction of 0.25. Assuming no pressure rise, calculate the solids conveying rate (g/rev) at the following conditions (a) no friction between the screw and the solids (b) no friction... 

Once the particulate solids have reached the feed port of the extruder, the material will flow down until it is situated in the screw channel. At this point, the gravity induced flow mechanism will essentially cease. In most extruders, the screw and barrel are placed in a horizontal direction and the role of gravity becomes a very minor one. In fact, in most analyses of solids conveying in single screw extruders. 

Metal-to-solid polymer wear occurs in the solids conveying zone of the extruder. The introduction of the grooved barrel extruder has significantly increased the interest and concern about wear in this portion of the extruder. Grooved barrel sections substantially increase the shear and normal stresses between the polymer solid bed and the metal surfaces. As a result, grooved barrel sections are much more susceptible to wear than smooth barrels. The first systematic study of wear in the solids conveying section of extruders was made by Fritz [1 Ij. He used a diamond-shaped specimen that protruded into the screw channel. 

We next consider the solids conveying capacity of a singlescrew extruder. The rectangular channel model cannot be used to describe the solids conveying in a single-screw extruder, because of the presence of deep channels, which... 

All single-screw extruders have several common characteristics, as shown in Figs. 1.1 and 1.2. The main sections of the extruder include the barrel, a screw that fits inside the barrel, a motor-drive system for rotating the screw, and a control system for the barrel heaters and motor speed. Many innovations on the construction of these components have been developed by machine suppliers over the years. A hopper is attached to the barrel at the entrance end of the screw and the resin is either gravity-fed (flood-fed) into the feed section of the screw or metered (starve-fed) through the hopper to the screw flights. The resin can be in either a solid particle form or molten. If the resin feedstock is in the solid form, typically pellets (or powders), the extruder screw must first convey the pellets away from the feed opening, melt the resin, and then pump and pressurize it for a down-... 

The fundamental processes and mechanisms that control single-screw extrusion are presented in Chapters 5 through 8. These processes include solids conveying, melting, polymer fluid flow, and mixing. The analyses presented in these chapters focus on easily utilized functions needed to assess the operation of the single-screw extruder. The derivation of these relationships will be presented in detail in the appendices for those who desire to explore the theory of extrusion in more detail. 

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