Mixing in Screw Extruders

A polymer element that melts early will be exposed to a significant mixing history by the time it reaches the end of the melting zone. On the other hand, a polymer element that melts at the very end of the melting zone will have hardly any mixing history at the end of the melting zone. 

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 

When two viscous liquids are mixed, the interfacial area increases and the striation thickness decreases. Spencer and Wiley [201] have proposed to use the interfacial area as a quantitative measure of the goodness of mixing. Mohr et al. [189] used the striation thickness to describe the mixing process. If a surface element with arbitrary orientation is located in a simple shear flow field, the surface area A after a total shear strain of y can be demonstrated to be [201]  

Angles a, ay, and determine the initial orientation of the surface element under consideration. The three angles are related by  

If the total shear strain is very large (j 1), then Eq. 7.477 becomes A = AjcosaJy 

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We note that the melt circulates around a plane located at exactly two-thirds of the channel height. A fluid particle in the upper one-third of the channel will move in the negative x direction at the velocity determined by its v position. Then, as the particle approaches the flight, it turns around and will move in the positive x direction at location y. The relationship between y and y can be obtained by a simple mass balance. We shall return to this subject and its implication in Chapter 7 when discussing extensive mixing in screw extruders. 

D. Bigg and S. Middleman, Mixing in Screw Extruders A Model for Residence Time Distribution and Strain, Ind. Eng. Chem. Fundam., 13, 66 (1974). 

Extensive Mixing and Residence Time Distribution in Screw Extruders... 

For a variety of reasons the mixing in the extruder may not be uniform and the melt inhomogeneous as a result. With a view to preventing this the root of the screw may be fitted with pins that protrude into the material to help break-up laminar flow or, more usually, the tip of the screw may be modified with mixing elements or sections (in effect the screw is lengthened, so that a section of rings, cams or kneading discs can be fitted to it). 

Extrusion presses and pelleting machines make use of wall friction, causing resistance to the flow of plastic materials through bores or open-ended dies. Particularly in screw extruders extensive mixing takes place and high shear forces... 

Kurian et al. [1993] examined gamma-irradiation of the HDPE/LLDPE blends, at a dose rate of 10 kGy/h (Table 11.9). The blends of HDPE and LLDPE were prepared by melt mixing in an extruder attached to an internal mixer (L/D ratio, 20 screw compression ratio, 3 1 screw speed, 30 rpm extruder heater zones, 140°C die temperature, 150°C). Eor irradiation, the samples were compression-molded at 150°C in a steam-heated laboratory hydraulic press for 5 min. Eor the dicumyl peroxide (DCP) crosslinkable samples, 1 % DCP was added during compound-... 

Flow marker studies in screw extruders for rubber compounds were initiated by Menges and Lehnen [M19] to evaluate the mixing and homogenizing characteristics of various screw designs. These studies feed equal-viscosity... 

When single-screw extruders are starve fed (Fig. 5.12d), plastic particles do not immediately fill the screw channel. As a result, the first few channels of the feed zone lack the pressure required to compact the polymer particles. Particle conveyance in the imfilled channels is not as steady as transport with filled channels. Consequently, metered feeding is seldom used with single-screw extruders. Such feeding can be used to reduce the motor load, limit temperature rises, add several components through the same hopper, improve mixing in singlescrew extruders, control flow into vented barrel extruders, and feed low-bulk-density materials. 

In the bottom of the channel the fluid is exposed to negative shear rates, and in the top of the channel the fluid is exposed to positive shear rates. This has important consequences for the mixing that occurs in screw extruders. At the top of the channel the fluid elements travel in the direction of the barrel, while at the bottom of the channel the fluid elements travel across the channel. The position of a fluid element in the upper portion of the channel (y) corresponds to a complementary position in the lower portion of the channel (yc) as shown in Fig. 7.124. 

In the hottom of the channel, the fluid is exposed to positive shear rates and in the top of the channel, the fluid is exposed to negative shear rates. This has important consequences for the mixing that occurs in screw extruders. 

Deairing pug-mill extruders which combine mixing, densification, and extrusion in one operation are available for agglomerating clays, catalysts, fertihzers, etc. Table 20-55 gives data on screw extruders for the production of catalyst pellets. 

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