Screw leakage flow

The simplest section to model can be treated as three sources of flow drag flow due to the relative motion of the screw and barrel pressure back-flow along the channel, as a result of the pressure build up along the channel, often approximated as linear from feed throat to the tip of the screw leakage flow over the flight tips, again as a result of the positive pressure gradient. 

A factor is often required in this equation to allow for eccentricity of the screw in the barrel. Typically this increases the leakage flow by about 20%. 

Figure 4.8 also presents an analytical solution for the screw characteristic curve of a single screw extruder with leakage flow effects. The discrepancies between analytical solution and experimental results arise due to the fact that the screw curvature, the flight angle and the fillet radii are not included in the analytical model. The analytical solution given by Tadmor and Klein [27] was used. 

Now that the physical configuration has been clarified, we can consider in detail the mathematical model for the process. First, we find a relationship between the flow rate and the geometrical and operational variables. Neglecting leakage flow between the screws, as well as that between the screws and the barrel, this is simply given by multiplying the velocity Vi with the cross-sectional area of the melt-filled channel, Am, as shown in Fig. 6.48... 

Thus, as in SSEs, the pressure rises linearly in the directions of the pushing flight and pushing screw, reaching a maximum at the comer between them. However, the absolute pressure cannot be determined from the model unless the chamber is partially empty, where the pressure can be assumed atmospheric. Otherwise, the leakage flow must be considered and the pressure profile along the screw determined. 

Equation 9.2-5 can be represented by plotting the flow rate Qs versus the pressure rise APs. Such plots, called screw characteristics, appear in Fig. 9.4. The intersection with the ordinate gives the drag-flow rate value and that with the abscissa, the maximum pressure at closed discharge. For isothermal flow of a Newtonian fluid in the absence of leakage flow,... 

Qi Leakage flow rate in a counter rotating intermeshing twin screw extruder... 

Figure 9.5 Number of partial flows in a twin screw extruder and mixing of the partial flows as a consequence of the leakage flow over the flight tip and in the intermeshing zone...

Net flow n. The output of an extruder s metering section, being, to a first approximation, the algebraic sum of the drag flow, pressure flow and leakage flow. In most plastics extruders with solid feeds and screws having the conventional three sections - feed, transition, and metering - the net flow may be conservatively estimated from the equation... 

Leakage flow varies with the flight clearance. It is also enhanced by low-viscosity melts and high head pressures. With new screws and barrels, leakage flow is minor and has no apparent effect on extruder output. As the flight clearance increases, leakage flow rises, thereby reducing output. Consequently, the decrease in extruder output over time is used to monitor screw and barrel wear. 

Here N is the rotation rate of the screws, m is the number of thread starts per screw, and V is the volume of a single chamber. In reality the output of a twin-screw extruder is, of course, smaller than the theoretical throughput because the chambers are not completely closed. Four different kinds of leakage flows can be distinguished (4) (see Fig. 2.8) ... 

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