Screw-rotation speed control

WPC samples were compounded using a Leistritz Micro-27 co-rotating twin-screw extruder with K-Tron powder and pellet loss-in-weight feeders. The PP-g-MA or SEBS-g-MA pellets were hand mixed with PP pellets in a bag before loading into the feeders. The wood flour was pre-dried in an oven at 100°C for 7 hours to ensure that the moisture content was less than lwt%, while clay was pre-dried at 80°C for at least 16 hours before compounding. The barrel temperature was controlled at 165 - 180°C from hopper to the die, while the screw rotation speed was fixed at 150 rpm. The total feeding rate was controlled at 4.8 kg/hr, and all of the samples contained 50 wt% wood flour the experimental matrix is listed in Table 1. 

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-... 

Figure 3.7. Solids feeders with variable speed drives, (a) Rotary vane (star) feeder with variable speed drive, (b) Horizontal screw feeder, (c) Belt feeder taking material from a bin with an adjustable underflow weir, (d) Rotary plate feeder Rate of discharge is controlled by the rotation speed, height of the collar, and the position of the plow, (e) Continuously weighing feeder with variable speed belt conveyor.

Reactive extruders and extrusion dies of different designs can be easily included in standard technological scheme of polymer production plants, such as those for polycaproamide synthesis, as shown in Fig. 4.39. In this case, a reactive material premixed in a tank 1 is fed into a static device 2 for prepolymerization, where part of the polymerization process takes place. Then the reactive mixture enters the extruder-reactor 3. The necessary temperature distribution is maintained along the extruder. Transfer of the reactive mass proceeds by a system of two coaxial screws mounted in series in a common barrel. Controlling the relative rotation speed of both screws provides the necessary residence time for the reactive mass in the extrader, so that the material reaching the outlet section of the die is a finished polymer. 

The controlling geometric parameters that characterize the extruder are the screw diameter D and the length to diameter ratio UD. Extruders are categorized according to diameter, from which the output and the power demand for a given rotating speed of the screw are derived. 

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