Open discharge

Consider duplication, locking, temporary blanking of valves on open discharge lines. 

Open discharge valve to discharge mixture into awaiting containers. 

Having established that the barrel and screw rotation pumped the same quantity of fluid when rotated at the same angular velocity, the effect of screw and barrel rotation on dissipation and thus temperature increase can be evaluated. A second set of experiments was designed to determine the effect of barrel and screw rotation on the temperature increase [59]. For these experiments, the extruder was operated at open discharge until it reached steady state at a screw speed of 55 rpm. Then the valve was fully closed and the screw or barrel was rotated at 55 rpm for ten- to sixty-minute intervals. With the valve closed, the extruder was operated such that Omd = Omp At the end of the interval, the valve was opened and the fluid was pumped from the extruder to the reservoir. The temperature difference between the inlet and outlet of the extruder was then recorded. The temperature increase observed was always substantially more than the experimental measurement error. The... 

Figure 20 Product discharge through the side, showing closed and open discharge with overlap gill air distributor. (Courtesy of Atlantic Pharmaceutical Services Inc.)...

If we combine the flow generated by the down channel and cross channel flows, a net flow is generated in axial or machine direction (w ) of the extruder, schematically depicted in Fig. 6.5. As can be seen, at open discharge, the maximum axial flow is generated, whereas at closed discharge, the axial flow is zero. From the velocity profiles presented in Fig. 6.5 we can easily deduce, which path a particle flowing with the polymer melt will take. 

In polymer processing practice, we need to ensure that the particulate gravitational mass flow rate of the hopper exceeds, over the complete operating range, the extruder open discharge rate (i.e., the rate without any die restriction). That is, hoppers must not be the production-rate limiting factor. Second, and more importantly, it is necessary for stable extrusion operations and extruded product quality that the flow be steady and free of instabilities of the particulate flow emerging from the hoppers. Finally, as we will see in Chapter 9, we need to know the pressure under the hopper in order to determine the pressure profile in a SSE. 

The screw extruder is equipped with a die, and the flow rate of the extruder as well as the pressure rise at a given screw speed are dependent on both, as shown in Fig. 6.16. The screw characteristic line at a given screw speed is a straight line (for isothermal Newtonian fluids). This line crosses the abscissa at open discharge (drag flow rate) value and the ordinate at closed discharge condition. The die characteristic is linearly proportional to the pressure drop across the die. The operating point, that is, the flow rate and pressure value at which the system will operate, is the cross-point between the two characteristic lines, when the pressure rise over the screw equals the pressure drop over the die. 

The total power input is at minimum under open discharge conditions (Q = Qd) and is at maximum under closed discharge conditions (Q = 0). However, in addition to the power consumption in the channel, we must account for the power consumption in the flights,5 which is by no means negligible (la). 

Open discharge and close recycle valve keeping pump pressure 140 psi to 150 psi. 

---