Plastics extruders theory

Z. Tadmor, I. Duvdevani, I. Klein, Melting in Plasticating Extruders Theory and Experiments , Poly Eng Sci,l, (1967), 19... 

When an initially isotropic polymer is drawn or extruded to a high deformation ratio under suitable conditions, it develops appreciable anisotropy which is apparent in mechanical tests at all stresses up to and beyond the yield stress. A typical example of the anisotropy of yield observed is shown in Fig. 2 in which the tensile yield stress of oriented polyethylene terephthalate (PET) sheets is shown as a fiinction of the an e 6 between the tensile axis (TA) and the initial draw direction (IDD). This large anisotropy is somewhat similar to that observed in cold-rolled metal sheets, for which theories of anisotropic plasticity were suggestedby Hill, Yoshimura and others. A modification of the theory... 

The choice of a plasticizer is mainly ruled hy its compatibility with the polymer material involved. Indeed, as explained by the plasticization theories, plasticizer molecules have to deeply penetrate into the macromolecules network and remain stable inside. Thereby, the initial emulsion during the mixing process using an extruder or roll mixers requires a thermodynamically favourable plasticizer/polymer pair. In order to quantify the compatibility, several approaches have been developed. The most complex ones are the QSAR (Quantitative Structure-Activity Relationship) or UNIFAP (Universal Functional Activity coefficient for Polymers),which are highly effective, but need extensive amounts of data and consequently might be uncomfortable to employ in a first approach. 

A condition of plasticity has been proposed (49) that makes allowance for a changing of the density of powder polymer during the deformation under pressure. It was the basis for the elaborated SSE theory to predict the density of extrudates (50). It follows, in particular, that during the SSE a nonmonotonic dependence of the density of extrudates on the extrusion ratio is possible. The presence of the effect has been verified experimentally. 

More comprehensive equations for stiff paste rheology have been developed from Griffiths original concepts [4,9,10]. The pressure required to extrude a paste body from a barrel of diameter Dq, through a die of cross-sectional diameter and length L at an extrudate velocity can be considered in two parts the pressure in convergent flow at the die entry (Pi) and the pressure required to push the paste along the die land in parallel flow P2). From plasticity theory of metals it can be shown that... 

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