Experimental investigation of flow induced molecular weight fractionation during extrusion of HDPE polymer melts

Historical review of die drool phenomenon during plastics extrusion

“…"External die drool" is related only to die exit region not to whole flow channel in the extrusion die. This other type is then termed as "internal die drool" (typical for HDPE polymer melts [15][16][17][18]) and in this case dependence of die drool vs. die exit angle can be strongly non-monotonic as shown in [18]. Moreover, rounded die exit edges reducing die drool can be also found in [26].…”

“…Moreover, from recently published papers [16,17] focused on the effect of molecular structure on die drool intensity during HDPE melt extrusion results that increase in HDPE chain branching, and a decrease in its elasticity and shear viscosity signi¿cantly reduce the die drool phenomenon. Further, alpha-olefin polymers prepared using Ziegler-Natta catalysis with addition of metallic stearates of zinc, magnesium, and/or calcium as acid acceptors can also cause die drool problems [45].…”

“…In our recently published series of research papers [15][16][17][18] we have been focused on systematic experimental investigation of die drool during HDPE polymer melt extrusion. We found that first, the die drool phenomenon occurring during extrusion of HDPE polymers can be considered as the result of the flow induced molecular weight fractionation which is initiated under the slip-stick flow instability regime, second, an increase in HDPE chain branching, and a decrease in its elasticity reduces die drool phenomenon and finally, only those die exit modifications which promotes continuous release of low molecular weight species from the die exit region by the moving extrudate suppressing die drool phenomenon for HDPE.…”

Characterization of die drool sample produced by HDPE melt extrusion

“…Unfilled, well-stabilized and virtually linear HDPE polymer melt (HDPE Liten FB 29 E2009 3220 4479, extrusion grade, Unipetrol RPA, Czech Republic) was extruded on specially designed laboratory extrusion line (in detail presented in [8,9]) at following processing conditions. Temperature profile along the screw was:…”

“…Finally, in our recently published papers [8][9][10] focused on flow induced molecular weight fractionation during extrusion of unfilled and well-stabilized HDPEs at very low processing temperature was found that molecular weight distribution curves for HDPE extrudate skin, extrudate core and for virgin pellets are practically identical and the die drool sample represents their low molecular weight fraction, which suggests that in this case the internal die drool phenomenon (spontaneous accumulation of extruded polymer melt on die exit faces) can be considered to be the result of the flow induced molecular weight fractionation taking place only in a very thin layer near the die wall (within less than 8% of the channel radius for the studied processing conditions) and the extrusion process itself has no effect on the polymer bulk. Further, it has been revealed that the low molecular weight polymer chains start to be fractionated from the main polymer melt stream under the slip-stick flow instability regime which consequently then accumulates at the die lips in the form of a low viscosity polymer melt.…”