Screw characteristics

In Fig. 4.12 these points are shown as the limits of the screw characteristic. It is interesting to note that when a die is coupled to the extruder their requirements... 

As a result of the non-Newtonian behaviour both expressions for the pressure flows bxpirj and cxplrj) are no longer valid. The curve for the die is now curled upward since the apparent viscosity decreases with increasing shear stress. Also the shape of the screw characteristic changes. 

Table 7.16 Comparison of Single- and Twin-Screw Extruder Screw Characteristics...

The pumping capability and characteristic of an extruder can be represented with sets of die and screw characteristic curves. Figure 3.6 presents such curves for a conventional (smooth barrel) single screw extruder. 

Equation (4.38) can be rewritten into the more familiar screw characteristic curve form as... 

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. 

Screw characteristic curves as a function of channel depth. 

Figure 4.11 Nomogram summarizing the screw characteristic curves described in eqn. (4.38).

Through extensive experimental work Pawlowski was able to demonstrate the effect channel depth, h/D, and flight clearance, 5/D have on the screw characteristic curve. These are shown in Figs. 4.9 and 4.10, respectively. The nomogram presented in Fig. 4.11 summarizes the screw characteristic for a specific screw shape with different channel depths and flight clearances. 

For the screw characteristic curves presented in Fig. 4.12 and in Fig. 4.13 for a conventional and grooved fed extruder, respectively, the reduced graphs are shown in Fig. 4.14. As can be seen here, each type of extruder can be represented with a single curve for a whole range of rotational speeds. It is to be noted that in this representation the effect of viscous dissipation was not included, which may explain why some of the points fall somewhat outside the fitted lines. 

Figure 4.14 Reduced experimental screw characteristic curves for conventional and grooved feed single screw extruders.

Plot the screw characteristic curve at 40, 80 and 120 rpm screw speed for a 45 mm square pitch extruder with a channel depth of 4 mm in the metering section and a flight width of 4 mm. The metering section of the extruder is 5 turns long. Assume a Newtonian viscosity n =1000 Pa-s. 

Figures 8.37 and 8.38 [9] present velocity and temperature fields across the thickness, respectively, for various values of Br, and forn = 1 and n = 0.6. Griffith calculated the screw characteristic curves for Newtonian and non-Newtonian shear thinning fluids using various power law indices. Figure 8.39 presents these results and compares them to experiments performed with a carboxyl vinyl polymer (n = 0.2) and corn starch (n = 1).

Figure 8.39 Screw characteristic curves for various power law indeces. Experimental values are shown for a carboxyl vinyl polymer with n = 0.5 and corn starch (Newtonian) with n = 1 [9].

Figure 11.29 Comparison between the dimensionless screw characteristic curve computed using RFM and curves computed using FDM.

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. 

Fig. 6.16 Screw characteristic lines at three screw speeds N < N2 < A , and die characteristic line.

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,... 

Fig. 9.4 Schematic views of screw characteristic lines for Newtonian fluids and isothermal flow. The points where the screw and die characteristic lines cross are the operating points of the extruder. The effect of the screw speed and the channel depth on the operating points is demonstrated.

Distributed Parameter Models Both non-Newtonian and shear-thinning properties of polymeric melts in particular, as well as the nonisothermal nature of the flow, significantly affect the melt extmsion process. Moreover, the non-Newtonian and nonisothermal effects interact and reinforce each other. We analyzed the non-Newtonian effect in the simple case of unidirectional parallel plate flow in Example 3.6 where Fig.E 3.6c plots flow rate versus the pressure gradient, illustrating the effect of the shear-dependent viscosity on flow rate using a Power Law model fluid. These curves are equivalent to screw characteristic curves with the cross-channel flow neglected. The Newtonian straight lines are replaced with S-shaped curves. 

Fig. 10.36 Experimental screw-characteristic operating lines for a 2-in CRNI staggered and matched-screw array at room temperature using polydimethlsiloxane. (a) Comparison of the pumping capabilities of staggered and matched arrays at two different channel depths, H. (b) The effect of the fraction of closed barrel, /, on the matched-array screw characteristics for two fluid viscosities. [Reprinted by permission from R. J. Nichols, Pumping Characteristics of Counterrotating TSEs, SPEANTEC Tech. Papers, 29, 130 (1983).]...

Both staggered and matched screw characteristics show increased sensitivity with increasing channel depth, as expected. Figure 10.36(b) demonstrates the significance of the barrel apex truncation in allowing leakage backflows in this region. 

Engineering Estimates of the Drag- and Pressure-Flow Terms of the Screw Characteristic Lines of Co-TSE Conveying Screws and Kneading Elements... 

Todd presents the calculated values of the At and Bt geometric parameters for conveying screw and kneading elements of Co-TSEs needed for engineering estimates of screw characteristic lines, Eq. 10.3-13, also listed on Table 10.8... 

Continuous extrusion presses As shown earlier (Figure 310), the operating point of a continuous (screw) extrusion press is defined in the extrusion rate/pressure diagram by the point of intersection between lines representing nozzle and screw characteristics. Control of such equipment is accomplished by maintaining the pressure at the optimum level while changing the extrusion rate. 

A plot of the flow G as a function of the pressure P is given in Figure 23.6. For a given screw geometry, the first equation, the screw characteristic, is given for two values of the rpm N, while the second equation, the die characteristic, is shown for two levels of the flow resistance in the die, 1/c. Each combination gives rise to an intersection of the two lines, which is the working point of the extruder, from which the values of Q and P can be read. 

The orientation of dislocations may vary from pure edge to pure screw. At some intermediate point, they may possess both edge and screw characteristics. The importance of dislocations is based on the ease at which they can move through crystals. 

TXS (1999) Non-isothermal, non-Newtonian melt flow with melting, oversimplified screw characteristic curve calculations Canedo [153]... 

Screw characteristic The relationship between volumetric flow rate in a meltmetering extrusion screw and the head pressure. For a melt screw operating iso-thermally, it is given by the equation ... 

Table 8.2 Summary of Barrier Screw Characteristics Advantages of barrier screw Disadvantages of barrier screws...

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