Newtonian fluids extrusion with

Chiruvella, R. V., Jaluria, Y., Sernas, V., and Esseghir, M., Extrusion of Non-Newtonian Fluids in a Single-Screw Extruder with Pressure Back Flow, Polym. Eng. Set, 36, 358 (1996)... 

Meskat (M8) has presented a mathematical analysis of the effect of fluctuations in pressure and other variables on the comparative fluctuations in extrusion rates of Newtonian and non-Newtonian fluids. This work indicates the possibility of amplification of such fluctuations under certain circumstances with non-Newtonians rather than the uniform damping predicted for Newtonian behavior. If the validity of this analysis can be proved, it would warrant major attention being given to the problem of unsteady flow of non-Newtonian materials. 

The adhesion and friction characteristics of the plastic material determine the intensity of the flow. In the case of Newtonian fluids this is half of the theoretical conveying (at constant pressure) and even less with counterpressure (extrusion), even down to zero. In the latter case, the product rotates with the shaft and throughput ceases. 

The power-law index plays an important role in melt flow. It is obvious that the high flow rate results in the increase of the die pressure. However, for a Newtonian fluid (such as water, n = 1), a 10 X increase in pressure is accompanied by a 10 X increase in flow rate. For a non-Newtonian fluid with n = 1/2, a 10 X increase in pressure is accompanied by a 100 X increase in flow rate. For n = 1/3, a 10 X increase in pressure results is accompanied by a 1000 X increase in flow rate. For n = 1/4, a 10 X increase in pressure is accompanied by a 10,000 increase in flow rate. For n = 1/5, as in the case of a bad regrind above, a lOX increase in pressure results in a 100,000 increase in flow rate This is essentially applicable for any extrusion die. Hence, the power-law index of a hot melt essentially determines its extrusion behavior. 

The effects of the Reynolds number on the extrusion of Newtonian fluid from square and rectangular dies has been considered. As with planar and axisymmetric jets, extrudates from three-dimensional dies swell at low Reynolds numbers but contract at high ones. Depending on its aspect ratio, limiting die swell from the rectangle varies that of the square (0.7255) and that of two-dimensional planar case (0.8333). Wall slip reduces die swell and in the cases of perfect slip, completely eliminates it. 

With a power law index, = 1, the fluid behaves Newtonian. If 0 < < 1, the viscosity decreases when shear rates increase. This behavior, which applies to virtually all polymers, is called pseudoplasticity or shear thinning behavior. If > 1, the liquid is called dilatant or shear thickening. This behavior, in which viscosity increases when the shear rate increases, has only been observed in materials with a very high concentration of fillers and has no relevance to reactive extrusion. 

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