Extrudate nonuniformities

FIGURE 7.3 Irregularities in the extrusion of a sheet with those along the machine direction shown on the left and those along the transverse direction shown on the right. The extrusion direction is in the z direction. 

In the case of the MD, variations in the flow rate due to pressure or temperature variations in the pumping device are the main cause of the irregularities. However, flow instabilities associated with the phenomena of melt fracture and draw resonance can lead to variations in the dimensions of the extrudate. These variations are closely connected to the rheological properties of the melt, but die design can at least alleviate the severity of the irregularities. 

The TD variations are nearly totally due to die design. The first problem is to design a feed system that will distribute the melt uniformly to the shaping portion of the die. (See Fig. 7.4 for definition of parts of a die.) In the event this is not possible, then it must be possible to adjust the die lips in such a way that the fluid will leave the die with a uniform thickness. Part of the thickness variation in the TD is due to the inability to feed the die uniformly from the extmder, while the rest is due to the phenomenon of die swell. Since the degree of swell may vary nonuniformly over the cross section due to variations in the shear rate, the die lips (main shaping section) may have to be designed to compensate for this. 

Before continuing we should note that a lot of the problems concerned with die design are handled empirically. Part 

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One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

The die designs developed or mentioned previously are for a specific polymer and specific processing conditions. Nonuniform sheets of another polymer would result if substitutions were made. The same holds true for the same polymer extruded at a different temperature. 

Profiles are all extruded articles having a cross-sectional shape that differs from that of a circle, an annulus, or a very wide and thin rectangle (flat film or sheet). The cross-sectional shapes are usually complex, which, in terms of solving the flow problem in profile dies, means complex boundary conditions. Furthermore, profile dies are of nonuniform thickness, raising the possibility of transverse pressure drops and velocity components, and making the prediction of extrudate swelling for viscoelastic fluids very difficult. For these reasons, profile dies are built today on a trial-and-error basis, and final product shape is achieved with sizing devices that act on the extrudate after it leaves the profile die. 

Avoid thick and nonuniform extrudate wall thickness to achieve better flow balance control in the die, minimize material use, reduce cooling times, and minimize postextrusion warping of the product. 

The solid phase could be a reactant, product, or catalyst. In general the decision on the choice of the particle size rests on an analysis of the extra-and intra-particle transport processes and chemical reaction. For solid-catalyzed reactions, an important consideration in the choice of the particle size is the desire to utilize the catalyst particle most effectively. This would require choosing a particle size such that the generalized Thiele modulus < gen, representing the ratio of characteristic intraparticle diffusion and reaction times, has a value smaller than 0.4 see Fig. 13. Such an effectiveness factor-Thiele modulus analysis may suggest particle sizes too small for use in packed bed operation. The choice is then either to consider fluidized bed operation, or to used shaped catalysts (e.g., spoked wheels, grooved cylinders, star-shaped extrudates, four-leafed clover, etc.). Another commonly used procedure for overcoming the problem of diffu-sional limitations is to have nonuniform distribution of active components (e.g., precious metals) within the catalyst particle. 

Figures 12.1-12.6 show the radical change in EPR particle morphology from reactor powder to pellets, but the relatively static morphology from pellets to fabricated articles. This is due to the great efficiency of commercial-scale corotating twin-screw pelletization extruders (8). The EPR phase is efficiently dispersed and attains the stationary value of particle size, as described by theoretical treatments of droplet breakup and coalescence (13-15). This droplet breakup and coalescence occurs in the molten state of the viscoelastic iPP and EPR, matrix and dispersed phases, in the extruder under a complex strain held, which is a combination of nonuniform, transient shear and elongational helds. Eurther, a variable temperature prohle is used along the barrel of the extruder causing complex variation in the viscoelastic properties of these components.

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