Solids channel

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... 

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...

The lead length was 172 mm for the main flight of the barrier section and 114.3 mm for all other sections of the screw. The main flight width and clearance were 11 and 0.11 mm, respectively, in all sections of the screw. The first 2.7 diameters of the screw were inside a water-cooled feed casing. The compression ratio was 2.5 and the compression rate of the barrier section was 0.0030. The depth of the solids channel at the end of the barrier section was 3.2 mm. The specific rotational flow rate was calculated at 5.9 kg/(h-rpm). ... 

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. 

There are many barrier-type screws that differ from each other by the channel depth profiles of the melt and solids channels, by the helix angles, profiles, and the number of flights. We briefly review here just a few if these types of screw. The first barrier-screw design is due to C. Maillefer14 and is shown in Fig. 9.40, in which the auxiliary channel follows roughly the solid-bed profile. Clearly, at certain conditions the auxiliary flight can restrict flow rate, but at all times it prevents solids from leaving the screw. 

Sommerfeld, M. (1995), The importance of inter-particle collisions in horizontal gas-solid channel flows, in Gas-solid Flows , ASME FED 228. 

Life Long Solid, channeled and hollow Brite Manufacturing Polyethylene, wood flour <200... 

At the beginning of the transition (compression), a second flight Is started. This flight Is called the barrier or Intermediate flight, and It Is undercut below the primary flight OD. This barrier flight separates the solids channel from the melt channel. 

The boundary conditions for the above equation are specified potentials at the reservoirs, 4>i = < f appiied> nd zero potential gradient at solid channel walls, n.Vcj) = 0. It should be pointed out that if there is no spatial conductivity gradient (1 is a constant), Eq. 14 takes a simpler format ... 

In addition, a conjugate heat transfer problem has to be solved to simultaneously account for the heat transfer in the solid channel wall, which is governed by the heat conduction equation expressed as... 

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