Barrier section

As a concrete illustration of the Floquet band structure for a threefold barrier. Section 3.4 of Child [50] contains an explicit analytical form for the matiix u ... 

Figure 6.23 Schematic for the position of the barrier flight via the width of the solids channel [W,) and the depths of the solids and melt channels a) a barrier section design with a continuously decreasing /V b) constant-width solids channel, and c) a hybrid design. The designs all utilize a constant lead length for the primary flight...

As Stated above, both Ws(z) and are fixed by the geometry of the screw. A schematic representation of the solid hed in the channel is shown in Fig. 6.25 for a barrier section geometry similar to that of Fig. 6.23(a). This representation depicts how the four films change In dimension as the solid bed is consumed in the melting section of the screw. 

Figure 6.27 Comparison of melting dynamics for a conventional melting channel and a barrier section with the barrier flight parallel to the primary flight [50], The conventional channel is in red while the barrier melting section is in black...

The iead iength was 124 mm for the main flight of the barrier section and 88.9 mm for all other sections of the screw. The main flight width and clearance were 9 and 0.09 mm, respectively, in all sections of the screw. The first 2.5 diameters of the screw were inside a water-cooled feed casing. The compression ratio was 2.7 and the compression rate was 0.0050. The specific rotational rate was calculated at 2.51 kg/(h-rpm). ... 

Figure 11.20 Photograph showing the modification to the barrier flight at the entry to the barrier section...

Table 11.5 Extrusion Rates and Performance Before and After Modifications to the Entrance of the Barrier Section...

The restriction was mitigated by modifying the screw as outlined in the case study in Section 11.10.1. That is, the depth of the melt channel of the barrier section was increased to that of the solids channel at the entrance, and it was tapered into the depth of the melt channel over 2 diameters. The barrier flight was removed for the first 2 diameters and blended in with the melt channel, the shallower of the two channels in this region. The barrier flight for the next diameter was blended into the original undercut. With this modification the restriction still existed but it was spread over a three-diameter length of the screw instead of over half of a diameter. 

Figure 11.25 Simulated axial pressure profile for the modified 152.4 mm diameter screw. The entry to the barrier section was still restrictive, but it did not control the rate...

The injection-molding press was producing a part and runner system that had a mass of 2.15 kg. The mass was plasticated using a 120 mm diameter, 8L/D screw. The screw used for the process had a barrier melting section that extended to the end of the screw, as shown by the specifications in Table 11.9. That is, the screw did not have a metering channel. Instead, the last sections of the barrier section were required to produce the pressure that was needed to flow the resin through the nonreturn valve and into the front of the screw. The specific rotational flow rate for the screw for the IRPS resin was calculated at 9.3 kg/(h-rpm) based on the depth of the channel at the end of the transition section. The screw was built with an extremely low compression ratio and compression rate of 1.5 and 0.0013, respectively. For IRPS resins and other PS resins, screws with low compression ratios and compression rates tend to operate partially filled. The compression ratio and compression rate for the screw are preferred to be around 3.0 and 0.0035, respectively. The flight radii on the screw were extremely small at about 0.2 times the channel depth. For IRPS resin, the ratio of the radii to the channel depth should be about 1. 

Several mechanisms could cause the specific rate of the screw to be considerably less than the calculated specific rotational flow rate for the screw. These mechanisms include (1) normal operation for a screw with a very short metering section and a low-viscosity resin, (2) the screw is rate-limited by solids conveying, causing the downstream sections of the screw to operate partially filled, and (3) the entry to the barrier section is restricting flow (see Section 11.10.1) to the downstream sections of the screw and causing the downstream sections to operate partially filled. The goal was to determine which of the above mechanisms was responsible for the low specific rates for the plasticator. 

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