Extrusion metering section

The baseline extrusion process was numerically simulated using the processing conditions in Table 9.4 and the method described in Section 9.2.1, that is, with a rate of 77 kg/h, a screw speed of 27 rpm, and a discharge pressure of 10.6 MPa. The iterative calculation process was used to estimate a bulk temperature of 160 °C and a pressure of 13.1 MPa at the entrance to the meter section. The axial pressure and temperature profile for the simulation is shown in Fig. 9.5. 

Determining the effect of viscous dissipation in the metering section of a single screw extruder. Consider a 60 mm diameter extruder with a4 mm channel depth and a screw speed of 60 rpm. The melt used in this extrusion system is a polycarbonate with a viscosity of 100 Pa-s, a thermal conductivity of 0.2 W/m/K and a heater temperature of 300°C. To assess the effect of viscous heating we can choose a temperature difference, AT of 30K. This simply means that the heater temperature is 30K above the melting temperature of the polymer. For this system, the Brinkman number becomes... 

Fig. 9.20 Cross sections obtained from cooling experiments of a 2.5-in-diameter 26.5 length-to-diameter ratio screw extruder. Material rigid PVC. Operating conditions are listed in the figure Tb is the barrel temperature, N the screw speed, P the pressure at the die, and G the mass flow rate. Numbers denote turns from the beginning (hopper side) of the screw. The screw was of a metering type with a 12.5 turn feed section 0.37 in deep, a 9.5 turn transition section, and a 4.5 turn metering section 0.127 in deep. [Reprinted by permission from Z. Tadmor and I. Klein, Engineering Principles of Plasticaling Extrusion, Van Nostrand Reinhold, New York, 1970. The experiments were carried out at the Western Electric Engineering Research Center, Princeton, NJ.]...

Fig. 9.37 Simulated and measured pressure profiles for an LDPE extruded in a 2.5-in-diameter, 26.5 length-to-diameter ratio extruder, with a metering-type screw having 12.5 feed section with channel depth of 0.37 in and 4.5 turns of metering section of depth of 0.1275. The flow rate is 136 lb/h, the screw speed 60 rpm, and the barrel temperature was set at 300°F. The SBP is shown in Fig. 9.24. The screw geometry is shown at the top of the figure. Simulation was carried out by the first computer simulation package for plasticating extrusion developed by the Western Electric Princeton Engineering Research Center team (17). [Reprinted by permission from Z. Tadmor and I. Klein, Engineering Principles of Plasticating Extrusion, Van Nostrand Reinhold, New York, 1970.]...

The channel depth decreases in the extruder compression section. Screws are designed with different numbers of turns of flight, and different compression ratios (the ratio of the channel depth in the feed section to that in the metering section) to suit the rheology of the polymer being extruded. The pressure generated here either squeezes out any gas bubbles, or causes gas to dissolve in the melt. Unless a foamed extrusion is required, bubbles must not be allowed to reform in the melt when it returns to atmospheric pressure after the die. Consequently, there may be a vent to the atmosphere or to a vacuum line just before the compression section to aid degassing. 

Metering screw n. An extruder screw whose final section, from four to ten flights, has a shallow chaimel of constant depth and lead. As its name suggests, the metering section of such a screw is intended to regulate the amount delivered per rotation of the screw. It also provides time for the equalization of melt temperature and helps to control the steadiness of the extrusion rate. 

Shear flow n. The flow caused by the relative parallel or concentric motion of the surfaces confining a liquid, as in an extruder screw or caused by a pressure drop, in the direction of flow, from the entrance of a flow passage to its exit, as in a die. Sometimes the two basic driving modes coexist, as in the metering sections of most extrusion screws and in wire-covering dies. In the direction crosswise to any laminar flow, successive layers slide past each other in shear. 

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