Effect of Flight Clearance

The melting rate for a non-zero clearance can be determined from Eq. 7.107 it can be written as  

Equation 8.62 expresses the melting rate per unit length z, where i is the direction of the relative velocity between the solid bed and barrel. The melting rate per unit down-channel distance z can be written as  

Wear in the compression section of the screw is often caused by large compression ratios. This will be discussed next. 

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Klein, I., The Effect of Flight Clearance on the Performance of Plasticating Extruders, SPE ANTEC Tech. Papers, 20, 237 (1974)... 

The average temperature rise is directly proportional to the consistency index m and the tangential flight width w/sincp. The temperature rise is strongly dependent on the radial clearance S, the power law index of the polymer melt n, and the screw speed N. Figure 11.24 shows the effect of flight clearance 6 and the power law index n for a 114-mm (4.5-in) extruder running at 100 rpm the specific heat is 2250 J/kg°C, the melt density is 900 kg/m, and the consistency index is 10 Pa - s . 

The heat transfer coefficient between the polymer melt and the frozen surface depends on the clearance between the flight and the frozen layer [1-3]. Analytical means are presented for estimating heat transfer coefficients [2,] and effects of flight clearance have been shown [3]. 

When the clearance is less than about 1/2 mm (0.020 in), the pressure drop increases quite dramatically. When the clearance is larger than about 3/4 mm (0.030 in), the effect of the clearance becomes quite small. In fact, when the clearance is larger than 1 mm (0.040 in), the pressure drop starts to increase because of the reduced drag flow in the inlet channel. It should be noted, however, that changes in the barrier flight width and barrier clearance directly affect the dispersive mixing capability of the mixing section. 

Clearly, the effect of calender clearance Is similar to the effect of flight flank angle. The effect of the radial flight clearance Is shown in Fig. 10.36 when AP° = 1E4. 

The output starts to drop off rapidly when the flight clearance becomes larger than 0.005 D. The radial flight clearance normally ranges from 0.001 to 0.002 D. Figure 10.37 shows the effect of side clearance when AP° = 1E4. Increased side clearance causes a strong reduction in output. 

So far we have neglected the effect of the flight clearance. As small as the clearance is, polymer melt is being dragged across the clearance by the barrel surface and the pressure drop may pump melt across the flight width. This creates a continuous leakage flow from downstream locations to (one turn back) upstream locations, reducing net flow rate. 

Next we turn to Eq. 9.2-5 and derive the optimum channel depth for the maximum pressure rise at fixed screw speed and barrel diameter. We rewrite Eq 9.2-5, neglecting the effect of the flight clearance and the shape factors, as follows... 

Moreover, flight clearance and curvature effects were also accounted for. Figure 9.37 indicates that, in this particular case, the simple Newtonian model provides a reasonable estimate, although it overestimates the rate of melting. Note that the predicted curve should approach the closed circles and triangles, which are the measured solid bed width at the melt film, rather than the open circles and triangles, which are the corresponding values at the root of the screw. As observed experimentally, the width near the root of the screw is reduced as a result of melt pool circulation. 

Modihcations of this model, including a nonlinear temperature prohle in the melt him, channel curvature effects, and an approximate method to account for the flight clearance effect, are presented by Tadmor and Klein (1), together with expressions for power calculations. Numerous other improvements of the melting model have been suggested in the literature (33,41 -6). A detailed discussion of these, however, is beyond the scope of this text. 

Another source of error that can become quite important is the leakage flow through the clearance between the flight and the barrel. A normal design clearance (radial) is 0.001 D, where D is the diameter of the screw. When the clearance is normal, the flow through the clearance will be quite small. However, if the screw and/or barrel is subject to wear, the actual clearance can increase substantially beyond the normal design clearance. This can cause a considerable reduction in output and it is important to know how to evaluate the effect of clearance flow. 

Equations 7.401 through 7.403 allow a more realistic calculation of the power consumption. The melt temperature rise in the flight clearance is generally small because the heat is effectively conducted away due to the fact that the clearance... 

The bracketed term on the right-hand side of Eq. 7.404 can be used to as a correction term in Eqs. 7.381 through 7.400 to incorporate the effect of the power dissipated in the flight clearance. Obviously, for polymers with a relatively large value of the power law index, the correction can be significant. 

Lastly, it was assumed that the details of the screw geometry do not affect the heat transfer to the barrel this is not completely true. The number of flights, the flight clearance, the flight helix angle, and the flight width all affect the heat transfer from the polymer melt to the barrel. If we want to study the effect of these parameters in detail we have to use a more complicated, numerical analysis. 

In most cases, the best way to avoid binding problems is to reduce the screw diameter in the feed section by at least 0.002 mm per mm (0.002 per in) of screw diameter. Because most plastics are fed in pellet form, increasing the flight clearance in the early part of the feed section is most likely not going to have an effect on the performance of the extruder. On the other hand, an increased flight clearance in the feed section will substantially reduce the chance of the screw locking up in the extruder barrel or feed throat. 

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