Modeling polymer melt‐flow instabilities

Abstract

SynopsisThe long-standing problem of polymer melt-flow instabilities is looked upon from a new and quite general mathematical viewpoint. A single mathematical model is developed allowing the macroscopic description of measurable state variables characterizing the different melt-flow instability regimes. Based on the theory of relaxation oscillations,a set of two coupled first-order ordinary differential equations, relating state variables-pressure and flow rate-provides sufficient degrees of freedom to describe all the relevant flow regimesin shear flow. By means of this model, the appearance of extrudate distortions can be expressed in terms of pressure and output fluctuations without the need to invoke a microscopic theory. The analysis contains the potential for practical melt-flow instability predictions.

Oscillating Flow in Capillary Dies for a Hdpe Melt: Effects of Extrusion Parameters

“…In agreement with what described in literature [1][2][3][4][5], the oscillating flow that took place when such a type of instability occurred was characterized by evident fluctuations both in the pressure signal acquired in the reservoir of the rheometer and in the extrudate flow rate; the extrudate appearance was characterized by the alternation of relatively smooth and rough segments. An example of a pressure, P, vs time trace acquired during the oscillating flow exhibited by the HDPE here examined is shown in Figure 1.…”

“…Instability phenomena in capillary flow have been the subject of many theoretical and experimental studies since the beginning of polymer science, but still receive considerable attention because of the limits they impose on die forming processes of polymers (some of the most relevant literature works are cited in the reference list [1][2][3][4][5]). These instabilities, which are commonly referred to as "melt fracture", produce irregularities and distortions at the extrudate surface and thus limit the industrial production rates and the technological opportunities of several polymers.…”

The effect of die exit curvature, die surface roughness and a fluoropolymer additive on sharkskin extrusion instabilities in polyethylene processing

“…While it is clear that stress concentrations are a crucial factor for the development of the sharkskin instability, they cannot be the only factor. Issues that may also be involved include melt strength [12], the level of wall slip [20], the stability of the partial slip boundary condition [25] and the stability of the point of separation of the melt from the die exit [26][27][28].…”

“…Molenaar and Koopmans [25] proposed a local mechanism of stick-slip or relaxation oscillations in a thin peripheral layer near the die exit, similar to the oscillating defect observed at higher stresses. They modelled melt flow instabilities in terms of cyclic relaxation oscillations, during which potential energy is successively stored and relaxed, and found that above a critical wall shear stress, the increase in the flow curve becomes non-monotonic and successive stick-slip oscillations develop within a localised area at the die exit.…”

Transient behaviour and stability points of the Poiseuille flow of a KBKZ-fluid

“…Another peculiar effect, not yet mentioned, that is observed in experiments as well as in industrial circumstances, is the occurrence of the so-called 'shark-skin' instabilities at the surface of an extrudate (d. [10]). These shark-skins occur in extrusion processes before spurt turns up.…”