Large-Diameter Pumping Extruders

Process Stream from the High-Pressure Separator (LDPE, ethylene, volatiles) 

Screw design for these large machines is very complex, and their manufacture is very costly and has a long delivery time. Very large screws can take more than six months to fabricate and only a few manufacturers have adequately sized machining equipment. Moreover, since the extruder is a necessary component of a resin pro- 

Barrel Diameter mm Barrel Length, L/D Rate, kg/h Motor Size, kW Specific Rate, kg/(hrpm) 

Early design and simulation of large-diameter, melt-fed extruders were described by Fenner [17]. A numerical simulation of the axial pressure and temperature fora screw similar to that shown in Fig. 15.8 is shown in Fig. 15.10. This simulation was performed using a three-dimensional method using a finite difference approach. The process starts with an LDPE resin (2 dg/min, 2.16 kg, 190 °C) in the low-pressure separator at a pressure of 0.04 MPa (gauge) and a temperature of 230 °C. 

The same analysis was performed using the identical screw design and an LDPE resin with a 25 dg/min (190 °C, 2.16 kg) Mi, as shown in Fig. 15.11(b). For this case, the specific rate was 5 to 20% less than that for the 2 Mi resin shown in Fig. 15.11(a). Moreover, the highest specific rate possible before resin flows into the vent was calculated at 63 kg/(h-rpm), a value that was about 10 % less than that for the 2 Ml resin. For the 25 Ml resin, the optimal clearance for the second blister is about 3.0 to 3.8 mm, providing the highest specific rate while not forcing resin to flow into the vent dome. 

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Figure 12.12 Barrel, discharge, and pump inlet pressures and motor current for stable and unstable extrusion for a large-diameter extruder running HIPS resin...

This chapter will present the three most common uses for melt-fed extruders (1) pumping of resin from a compounding operation to a pelletizer, (2) large-diameter extruders used for finishing sections in resin production plants, and (3) secondary cooling extruders for foam sheet lines. 

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