Melt-conveying channels

The axial pressure and temperature distributions for the molten resin in the melt-conveying channel are calculated using the control volume method outlined in Section 7.7.5. For this method, the change in pressure and temperature are calculated using the local channel dimensions, HJ z) and FK (z), and the mass flow rate in the channel using Eq. 7.54 for flow and the methods in Section 7.7.5.1 for energy dissipation and temperature. The amount of mass added to the melt chan-... 

Two constraints are placed on the solids-conveying channel. That is, the sum of the widths and thicknesses for the components must equal the width and depth of the solids-conveying channel. Here, the melt film thicknesses 6t, and 6 for Zones B and D were found by difference ... 

In Equation 9.3-34, for 5 we face a difficulty with the density, whose value is a function of pressure and temperature. The pressure varies with the down-channel location, which couples the melting with melt conveying. This is a weak coupling, however, and we shall use a constant density at a mean temperature of T = (0.7)(149 — 110) + 110 137°C and estimated mean pressure of 6.89 x 106 N/m2(= 1000 psi). Thus with pm = 791 kg/m3, Eq. 9.3-34 results in... 

Melt conveying is the forward motion of the molten polymer through the extruder, due to the pumping action of the rotating screw. This simple drag flow Md is proportional to melt density, down-channel velocity, and cross-sectional area of the screw channel. In most cases, however, there is also a pressure gradient as the melt moves downstream, either... 

Devolatilization can be used to remove up to 5 percent of volatile impurities from the plastic melt. The first melt conveying (metering) zone builds up melt pressure. Then channel depth is increased abruptly in the vent zone, the melt is decompressed, and volatiles escape through the vent. After this the melt enters a second metering zone, which builds up melt pressure again, and feeds it to the die. 

Material drops In the axial gap between relatively thin disks mounted on a rotating shaft. The material will move with the disks almost a full turn, then it meets a channel block. The channel block closes off the space between the disks and deflects the polymer flow to either an outlet channel, or to a transfer channel in the barrel. The shape of the disks can be optimized for specific functions solids conveying, melting, devolatilization, melt conveying, and mixing. A detailed functional analysis can be found in Tadmor s book on polymer processing (Chapter 1, [32]). 

The infinite channel width assumption applies to shallow channels, channels with a width-to-depth ratio higher than 10 (W/H > 10). If the depth of the channel is large relative to the width of the channel, the effect of the flight flanks on the down-channel velocity profile has to be taken into account. Several reviews of the work on melt conveying in extruders have been written [101-106]. 

In addition to the six main assumptions of the analysis of melt conveying, there are a few more that are sometimes considered, such as elastic effects, the influence of an oblique channel end, etc. After the Newtonian analysis had been mostly worked out, non-Newtonian fluids were analyzed In the early 1960s. This adds significantly to the complexity of the analysis, and generally the equations cannot be solved ana-... 

This represents the volumetric throughput of the melt conveying zone. The first term after the equal sign in Eq. 7.198 is the drag flow term. It represents the flow rate in pure drag flow, i.e., without a pressure gradient in the down-channel direction ... 

The power consumption in the melt conveying zone is an important parameter to consider in screw design and in the analysis of actual extrusion operations. The power consumed for pumping in the channel is ... 

The direction of shear stress x and veiocity v is determined by Xy and Xyx- At this point, there are three unknowns the cross-channei pressure gradient g, the crosschannel shear stress at the screw surface x o, and the down-channel shear stress at the screw surface x . At the time of writing, no analytical solutions to this problem are known. In fact, an analytical solution does not seem possible. Therefore, some numerical scheme has to be used in order to determine the unknowns. Because this problem is of considerable importance to the proper analysis of melt conveying, it will be discussed in some detail. 

Similar problems occur in the melt conveying zone. A polymer element at about 2/3 of the height of the channel will have no cross-channel velocity component and as a result will have a short residence time in the melt conveying section and little mixing... 

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