Extruder venting

Ext-501 to Ext-509 9 Extruder, vented single screw, D=8in, L=16ft, stainless steel, 180 hp, insulated. 

In recent years, extruder screw architecture has become highly sophisticated, with special configurations for each type of extrudate. Venting in the barrel contributes much to product quality and throughput. As with most high-speed polymer processing systems, closed-loop control systems monitor and regulate process parameters for optimum production. 

FIG. 18-50 Intermeshing corotating twin screw extruder (a) drive motor, (h) gearbox, (c) feed port, (d) barrel, (e) assembled rotors, (f) vent, (g) barrel valve, (h) kneading paddles, ( ) conveying screws, (/) splined shafts, (k) blister rings, (APV Chemical Machinery, Inc.)... 

Barrels and Heaters These are also similar to those in extruder machines. In recent years, vented barrels have become available to facilitate the moulding of water sensitive plastics without the need for pre-drying. Water sensitivity in plastics can take several forms. If the plastic absorbs water then dimensional changes will occur, just as with wood or paper. The plastic will also be plasticised by the water so that there will be property changes such as a reduction in modulus and an increase in toughness. All these effects produced by water absorption are reversible. 

Film recycling is particularly troublesome because of the high surface area of the scrap. Moisture, air, and dust can easily become entrained in the material, introducing potential problems in the extrudate. To address these issues, vent ports and fine mesh screens are often incorporated to the extrusion system to release volatiles and trap solid contaminants. 

Vacuum extruders can be single- or multi-vented. Versions also exist which have an added pin barrel processing extension. 

Many types of equipment have been developed to improve the evaporation of solvent in order to provide energy savings. The most widely used techniques for devolatilization are the falling strand devolatilizer (FSD), the thin-film evaporator and the vented extruder [113]. 

Single-screw and double-screw extruders are normally used for polymer melts to accomplish the deaeration or devolatilization of residual volatiles. Devolatilization in an extruder is effected through formation of the venting zone inside the chamber by carefully designed upstream and downstream screw sections. 

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.

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