Two-stage vented extruder

Figure 13.3 Schematic of a vent flow diverter in a two-stage vented extruder...

For example, if a two-stage vented extruder has a screw that was designed for resins with an Mi range of 2 to 20 dg/min, and plant personnel are asked to manufacture a resin with an MI of 100 dg/min, the extruder may not process the resin acceptably. For this case and flood feeding, the extruder is likely to have resin flowing into the vent because the first stage can deliver more resin than the second stage can pump. One practical solution to this problem is to operate the slide valve in a partially open manner such that it controls the rate of the extrusion process. This will allow the extruder to operate at a lower specific rate and allow operation without the flow of resin into the vent. 

The OPS extrusion process typically uses a two-stage vented extruder. The most common size OPS lines use either a 200 or 220 mm diameter extruder. The length-to-diameter (L/D) ratios range from about 24 to 32, the higher ratio extruder... 

FIGURE 2.21 A two-stage vented extruder with a valved bypass. (After Fisher, E. G. 1971. Blow Molding of Plastics. Iliffe, London.)... 

Two stage vented extruders L/D = 24 to 28. Output = 0.8 X output single stage. Twin screw extruder LjD = 12 to 16 compounding capacity is proportional to screw speed conveying capacity = V2 the volume contained in one turn of the screw per revolution. 

FIGURE 14.2 A two-stage vented extruder in which a valved bypass section steps down the pressure developed in the first stage to atmospheric pressure for venting valve is for adjustment. (Data from Van Ness, R. I. et ah, Mod. Plast., 45, 672, October 1968.)... 

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