Extrusion flow mechanism

Figures 322(a) and (b) show again the conditions in a pelleting machine with concave die. Figure 322(a) depicts the mechanisms of compression and extrusion in the work area , the material volume wedged in between the press roller and die. Figure 322 explains the phenomena. Feed deposited in a layer on the die is pulled into the space between the roller and die and compressed. Neither the roll force nor the force from the die resisting extrusion (flow) are constant. The roll force increases with progressing densification while the flow-resisting force remains constant until a threshold pressure, defined by the static friction in the die holes, is surpassed. After extrusion (movement in the die holes) has started, both the resisting and the roll forces decrease.

Zheng J, Carlson WB, Reed JS (1993) Flow Mechanics on Extrusion through a Square-Entry Die. J Am Ceram Soc vol. 75 No 11 pp 3011-3016... 

Mechanical or flow-induced degradation of polymers may occur under extreme shear or extrusion flow conditions giving rise to unfolding of macromolecules in extensional flow with subsequent chain scission. 

White, J.L. and Kim, E.K. (2010) Flow Mechanisms and ModeUng of Intermeshing Co-Rotating Twin Screw Extruders in Tvin Semv Extrusion Tedtndogy and Prirtdples, 2nd edn, ch. 5, Hanser, Munich, 93-122. 

Cast material is stated to have a number average molecular weight of about 10. Whilst the Tg is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx. 170°C). There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. For injection moulding and extrusion much lower molecular weight materials are employed. Such polymers have a reasonable melt viscosity but marginally lower heat distortion temperatures and mechanical properties. 

Although only two protons are pumped out of the matrix, two others from the matrix are consumed in the formation of H2O. There is therefore a net translocation of four positive charges out of the matrix which is equivalent to the extrusion of four protons. If four protons are required by the chemiosmotic mechanism to convert cytosolic ADP + Pj to ATP, then 0.5 mol ATP is made for the oxidation of one mol of ubiquinol and one mol ATP for the oxidation of 2 mols of reduced cytochrome c. These stoichiometries were obtained experimentally when ubiquinol was oxidized when complexes I, II, and IV were inhibited by rotenone, malonate, and cyanide, respectively, and when reduced cytochrome c was oxidized with complex III inhibited by antimycin (Hinkle et al., 1991). (In these experiments, of course, no protons were liberated in the matrix by substrate oxidation.) However, in the scheme illustrated in Figure 6, with the flow of two electrons through the complete electron transport chain from substrate to oxygen, it also appears valid to say that four protons are extmded by complex I, four by complex III, and two by complex 1. 

The flow toward the surface is caused by the pressure under the indenter. It is analogous to the upward flow around a sphere dropped into a liquid. It is also analogous to inverse extrusion. A model of the flow has been proposed by Brown (2007) in terms of rotational slip. This model reproduces some of the observed behavior, but it is a continuum model and does not define the mechanism of rotational slip. 

The basic principle is somewhat similar to cast film extrusion but the flow of the polymer is obtained by dissolving it in a solvent instead of heating it during mechanical shearing in an extruder. 

The fundamental processes and mechanisms that control single-screw extrusion are presented in Chapters 5 through 8. These processes include solids conveying, melting, polymer fluid flow, and mixing. The analyses presented in these chapters focus on easily utilized functions needed to assess the operation of the single-screw extruder. The derivation of these relationships will be presented in detail in the appendices for those who desire to explore the theory of extrusion in more detail. 

Real polymers are more complex than these simple mechanical models. Qualitatively, when a real polymer is forced to flow through a contraction or expansion in an extrusion screw, it will exhibit viscoelastic behaviour. The polymer molecules will be elongated if forced through a contraction, or they will retract when they flow into an expansion. The effect of viscoelastic behavior in a capillary rheometer is observed in the form of recirculation flow just before the polymer enters the... 

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