Melt conveying

These simpiifications cannot be made when the fluid is non-Newtonian or when the viscosity is temperature dependent. Most anaiyses on meit conveying can be categorized by the six most important assumptions made about the process  

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- 

To the practicing process engineer, the most important question should be whether the more general analysis will be applicable e.g., are all the boundary conditions well known, and will it result in more accurate predictions. For instance, it probably does not make much sense to use a sophisticated non-isothermal melt conveying analysis to predict the melt temperature at the end of the screw if the actual screw temperature in the process is unknown. These practical considerations do not necessariiy appiy to the academic. On the other hand, it would make sense to perform a non-isothermal analysis to predict the effect of barrel temperature fluctuations on meit temperature or conveying rate. Besides the complications already discussed, there is the additional complication that the polymer melt is not a pure, inelastic power iaw fluid and significant time-dependent effects can occur (e.g., [122-128]). 

The next section wiii start with an analysis of melt conveying of isothermal fluids. This wiii be foiiowed by a non-isothermal analysis of melt conveying of cases that allow exact analytical solutions. More general analyses of the effect of temperature on flow will be discussed in more detail in Chapter 12 on modeling and computer simulation. In the next section, melt conveying of Newtonian fluids and non-Newtonian fluids will be analyzed. The non-Newtonian fluids will be described with the power law equation (Eq. 6.23). The effect of the flight flank will be discussed and the difference between one- and two-dimensional analysis will be demonstrated with particular emphasis on the implications for actual extruder performance. 

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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-... 

Bruker, L, Miaw, C., Hasson, A., and Balch, G., Numerical Analysis of the Temperature Profile in the Melt Conveying Section of a Single Screw Fxtruder Comparison with Fxperimental Data, Polym. Eng. Set, 27, 504 (1987)... 

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... 

S. Syrjala, On the Analysis of Fluid Flow and Heat Transfer in the Melt Conveying Section of a Single Screw Extruder, Num. Heat Trans., Part A, 35, 25-4-1 (1999). 

Fig. 10.38 The examples of Leistritz melt conveying, modular CRNI screw elements studied by White and associates, (a) Thin flighted forward matched and staggered (b) thick-flighted, reverse matched and staggered. [Reprinted by permission from D. S. Bang, M. H. Hong, and J. L. White, Modular Tangential Counterrotating TSEs Determination of Screw Pumping Chararacteristics and Composite Machine Behaviour, Polym. Eng. Sci., 38 485 (1998).]...

The die manifold, which serves to distribute the incoming polymer melt stream over a cross-sectional area similar to that of the final product but different from that of the exit of the melt conveying equipment. 

For simple melt conveying, screw elements with a pitch of 1 D are commonly used. If other components are to be added, e.g., via a side feeder, the pitch should be 1.5 D in order to increase the feed capacity. 

As will be shown in subsequent chapters, melt conveying can be calculated very well provided that sufficient material data is available. 

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. 

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