Molecular structure and gross melt fracture triggering

Experimental study of the volume defects in polystyrene extrusion

“…It is now well admitted, and it is largely confirmed by our results, that the periodic volume instability is initiated at the die entry [10][11][12][13][14][15][16][17][18][19][20][21]. An explanation proposed in the literature concerns the destabilization of entry vortices [5,10,20].…”

“…The elongational stresses and deformations developing in the convergent have often been associated with the onset of the upstream instability [12,21,30]. Using capillary rheometry experiments, we can derive approximate values of elongational stresses from entrance pressure losses, according to the Cogswell method [29].…”

“…However, this critical stress depends on the die length, the temperature and also the molecular weight [9]. Moreover, upstream die flow visualizations during the instability, using either flow birefringence [10][11][12][13], tracers [6,[13][14][15][16][17][18], or laser Doppler velocimetry [17,18] proved that the region upstream the contraction was the site of the defect initiation. Nigen et al [19] have recently carried out investigations using particle image velocimetry and confirmed the link between entry flow destabilization and gross melt fracture.…”

Volume defects during extrusion of polystyrene investigated by flow induced birefringence and laser-Doppler velocimetry

“…Flow visualizations using coloured tracers [10][11][12], flow induced birefringence [13][14][15][16] or velocimetry studies [17] have clearly shown a destabilization of the entry flow at the onset of volume instabilities. However, the origin of the instability is not totally understood up to now, although it is clear that the viscoelastic properties of the polymer and the elongational flow in the entry region play a crucial role.…”

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].…”