Square pitch screw

Figure 7.5 Screw element s contribution to extruder rate [5], Data were measured using the free helix with a diameter of 52 mm using a square-pitched screw with a channel depth of 11.1 mm...

The F, correction factors as a function oiH/Wior single-flighted, square-pitched screw geometries are shown in Fig. 7.25. For these cases, only the channel depth was varied. 

Figure 7.30 Experimental temperature increases for a Newtonian fluid (polypropylene glycol) as a function of the shearing time [47]. The data are for a square-pitched screw with a channel depth of 3.78 mm, a barrel diameter of 49.7 mm, and rotations at 55 rpm...

For square pitched screws, Ls = Ds, and the flights are all perpendicular to the screw axis. Extruder screws range from 25 to 150 mm in diameter, with screw lengths typically of 25 to 30 screw diameters. They rotate at speeds from 50 to 150 rpm, and deliver melt at rates from 10 to 1000 kg/h with pressures up to 40 MN/m. ... 

In polymer extrnsion, show that a square-pitched screw has a helix angle of 17.65°. 

Since we are scaling the system with the same material, we can assume that the material parameters remain constant, and for simplicity, we assume that the heater temperature remains the same. In addition, we will fix our geometry to a standard square pitch screw ( =17.65°) and therefore, a channel width proportional to the diameter. Hence, the parameters to be determined are D2, h2 and n2. 

Most screws of SSEs are single flighted, with Ls = Ds, referred to as square-pitched screws. The radial distance between the root of the screw and the barrel surface is the channel depth, H. The main design variable of screws is the channel depth profile that is H(z), where z is the helical, down-channel direction, namely, the direction of net flow of the material. The angle formed between the flight and the plane normal to the axis is called the helix angle, 0, which, as is evident from Fig. 6.8, is related to lead and diameter... 

The value of the helix angle is therefore a function of the diameter. At the tip of the flight it is smaller than at the root of the screw. A square-pitched screw, neglecting the flight clearance, has a helix angle of 17.65° (tan 6 = 1/n) at the flight tip. 

Fig. 6.14 Cross-channel, down-channel, and axial velocity profiles for various Qp/Qj values, in shallow square-pitched screws [Reproduced by permission from J. M. McKelvey, Polymer Processing, Wiley, New York, 1962.]...

Solution This is an open-ended problem, which has many possible solutions. We need to find the diameter of the barrel, the channel depth, and the screw speed needed to generate the head pressure. We shall assume a square pitched screw, and use British units (in, s, Btu, lbf). 

We now take the drag and pressure flow terms in Eq. 9.2-5 and substitute the relevant numerical values. We assume a square pitched screw, neglecting the difference between mean and barrel surface helix angle, and neglecting shape factors and flight clearance. We further assume that flight width is 10% of the barrel diameter. We can make these simplifying assumptions because, at this point, we only wish to select the barrel diameter and the screw speed. The channel width can be expressed in terms of the screw diameter as follows ... 

Fig. 9.14 Values of F(y) for a 6-in-diameter, 20 1 IVD extruder at constant flow rate (500 lb/h) with screw speed as a parameter. Simulation was made for a square pitched screw with a constant channel depth of 0.6 in. [Reprinted by permission from G. Lidor and Z. Tadmor, Theoretical Analysis of Residence Time Distribution Functions and Strain Distribution Functions in Plasticating Extruders, Polym. Eng. Sci., 16, 450-462 (1976).]...

Fig. 10.8 Fully intermeshing, co-rotating elements in configurations matching square-pitch screws, (a) Forward screw conveying elements with different numbers of lobes (b) forward staggered kneading paddles (a.k.a., disk elements) representing leaky screw elements, and (c) neutral configuration. [Reprinted by permission from D. B. Todd, APV (Baker Perkins) Systems, in Plastics Compounding—Equipment and Processing, D. B. Todd, Ed., Hanser, Munich, 1998.]...

The output of a metering screw is fairly predictable, provided that the melt is under control. With a square pitch screw (conventional screw where distance from flight to flight is equal to the diameter), a simplified formula for output is ... 

Figure 7.64 shows the dimensionless throughput versus pressure gradient for a helix angle of 17.66° (square pitch screw). 

Figure 16-34 Kneading paddles in arrays matching square-pitch screws.

LDPE is exfruded in a single-screw extruder having a diameter of 6.35 X 10 m. The square pitched screw (i.e., =... 

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