Extrusion processes single-screw extruders

Polystyrene foam sheet can be made by a variety of extrusion processes (single screw, twin screw) but is most commonly made using a tandem extrusion process as shown in Figure 11.6. This process uses two extruders in series, which effectively optimize the melting, mixing, and cooling unit operations. The diameters of the extruders currently used in this process are usually 115 mm for the primary extruder and 150 mm for the secondary extruder, with an output capability of 400 kg/h. 

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. 

Fig. 2.15. Reactors for producing these materials include batch, continuously vented, tray reactors, twin-screw extruders, and vented single-screw extruders. These production devices will not be covered in this text because they are of more interest to the manufacturing engineer than the extrusion process engineer.

Bruin, S., Van Zuilichem, D.J., and Stolp W., A Review of Fundamental and Engineering Aspects of Extrusion of Biopolymers in a Single-Screw Extruder, . Food Process Eng., 2, 1 (1978)... 

In a final RTD experiment, a sheet of dye was frozen as before and positioned in the feed channel perpendicular to the flight tip. The sheet positioned the dye evenly across the entire cross section. After the dye thawed, the extruder was operated at five rpm in extrusion mode. The experimental and numerical RTDs for this experiment are shown in Fig. 8.12, and they show the characteristic residence-time distribution for a single-screw extruder. The long peak indicates that most of the dye exits at one time. The shallow decay function indicates wall effects pulling the fluid back up the channel of the extruder, while the extended tail describes dye trapped in the Moffat eddies that greatly impede the down-channel movement of the dye at the flight corners. Moffat eddies will be discussed more next. Due to the physical limitations of the process, sampling was stopped before the tail had completely decreased to zero concentration. 

When the rheometer is retrofitted with a single-screw extruder one can measure rheological properties and extrusion processing characteristics to differentiate lot-to-lot variance of polymer stocks. It also enables the process engineer to simulate a production line in the laboratory and to develop processing guidelines. 

Simulation of the extrusion process in the laboratory is one of the most important applications of the torque rheometer in conjunction with single-screw extruders. Figure 13 illustrates simulations of widely used extrusion processes in the industries. 

After mixing, preconditioned materials are discharged into an extruder. The final product characteristics are affected by screw and barrel profiles, screw speed, processing conditions (temperature, moisture content, etc.), raw material properties, and die/knife type. A single-screw extruder is used in 85%-90% of all semimoist pet food production. All equipment requires stainless steel construction due to low pH (4.0-4.5) of most formulas. The extrusion temperature is maintained... 

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