Barrier flights

Figure 6.1 Schematic for a barrier melting section (courtesy of Jeff A. Myers of Robert Barr, Inc.). The barrier flight is undercut from the main flight to allow molten resin to transfer from the solids channel to the melt channel...

Barrier melting sections are constructed by positioning a second flight (or barrier flight) in the transition section such that the solids are maintained on the trailing side and the molten resin on the pushing side. A schematic of a cross section of a barrier melting section is shown in Fig. 6.22. The resin is melted as discussed in Section 6.3.1 in the solids channel of the device. The resin that is melted near the... 

As shown by Fig. 6.23, the channel geometry is relatively complicated for barrier flight melting sections. The geometric constraint on the channel widths are provided by Fq. 6.33. For most commercial designs, W is constant because the lead length of the primary flight is also constant. 

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...

Figure 6.25 Qualitative shapes of X and Tbed dimensions and melt film thicknesses for a barrier screw model with a transverse barrier flight (a) top view of the solids channel and the entry to the metering section, and (b) side view of the solids channel and the entry to the metering section. The cream color represents molten resin...

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...

Campbell, G.A. and Spalding, M.A., Numerical Analysis of the Melting Process for Barrier-Flighted Single-Screw Extruders Using Screw Rotation Physics, SPE ANTEC Tech. Papers, 56, 418 (2010)... 

Hyun, K. S., Spalding, M.A., and Powers, J.R., Elimination of a Restriction at the Entrance of Barrier Flighted Extruder Screw Sections, SPEANTEC Tech. Papers, 41, 293 (1995)... 

A sled device for measuring the depth of screw channels, barrier flight undercuts, and mixer flight undercuts is shown in Fig. 10.1. To use this device, the sled is positioned on the top of the screw with the micrometer tip contacting the top of the flight. The micrometer is zeroed at this position. Next the sled is slid axially on the screw, and the micrometer is adjusted until the top of the micrometer probe contacts... 

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

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