Experimental study of the volume defects in polystyrene extrusion

Experimental evidence of a helical, supercritical instability in pipe flow of shear thinning fluids

“…This strongly suggests that both systems develop instabilities of different origins. This is confirmed in [6] by the direct visualization of the velocity field in the reservoir, upstream of the inlet, which clearly indicates that emergence of the extrudate instability coincides with the destabilization of the contraction flow, a phenomenon which has been extensively studied by McKinley et al for non shear-thinning Boger fluids [7]. Correlatively, as the die aspect ratio is reduced, the extrudate undulation becomes more pronounced for PS, whereas it vanishes for ALG (Fig.12).…”

“…Among these, an ubiquitous one, usually referred to as "melt fracture", results in a distortion of the extrudate into a more or less pronounced and regular helical shape. Visualization of the velocity and stress fields in the reservoir [3][4][5][6] has lent strong support to the idea that melt fracture was ascribable to the well documented destabilization of the elongational flow upstream of the sudden contraction at the entry of the die [4,7]. Due to viscous damping, such velocity oscillations are expected to decay over a finite length as the fluid is advected through the pipe.…”

“…Despite of its intriguing nature, clearly suggesting two distinct types of instabilities, this phenomenon has not been studied systematically until recently. Here we focus on the extensive study by Combeaud et al [6,25] of the helical extrusion instability of linear polystyrene (PS). Their polymer melts and our entangled polymer solutions exhibit a common rheological characteristic since the flow viscosity of PS is strongly shear thinning [25] with a power law exponent n 0.2, a plateau viscosity η 0 5 × 10 4 Pa.s and a cross-over shear rateγ 0 1 s −1 .…”

Molecular physics of a polymer engineering instability: Experiments and computation

“…The first two are "sharkskin" instabilities, which develop due to free surface effects, and "stick-spurt" or "stick-slip" instabilities, which result from material compression and stick-slip at the wall. While the mechanisms underlying the formation of these first two are relatively well understood [2,3], the third class, termed "volume instability", is less so [4]. This instability in converging flows for extrusion and injection moulding has been known for many years, and an empiricism has been developed.…”

An experimental evaluation of the formation of an instability in monodisperse and polydisperse polystyrenes

“…There are a number of reviews on viscoelastic instabilities [1][2][3][4][5][6] and a recent review [6] highlighted three forms of polymer instability observed in extrusion which occur at increasing levels of flow deformation. These are the "Sharkskin" instability [7][8][9][10][11], the "stick-spurt instability" [12][13][14], and "upstream extrusion instabilities", also termed "gross melt fracture" or "volume instabilities" [15][16][17][18][19][20][21][22][23][24].…”