Investigation of Internal Die Drool Phenomenon for Different HDPEs

Abstract

Abstract. In this work, two different HDPEs were investigated from rheological as well as die drool phenomenon point of view. It has been revealed that long-chain branching and low polymer melt elasticity significantly reduce die drool phenomena at the die exit region.

Experimental Investigation of Die Drool and Slip-stick Phenomena during HDPE Polymer Melt Extrusion

“…From the comparison of drool tendencies of both batches it is clear that HDPE Liten FB 29 E2009 5498 7937 generates much lower level of drool than HDPE Liten FB 29 E2009 3220 4479. It is clearly visible that the polymer with low elasticity and shear viscosity generates lower level of drool, which is in good agreement with the previously published results [11]. Moreover, the build up ratio vs. shear rate dependence for HDPE Liten FB 29 E2009 5498 7937 and the second batch is linear and non-linear, respectively (after first scalable drool appearance at apparent shear rate of 216 s -1 ).…”

“…Further, three recently published works are focused on die drool phenomenon during HDPE extrusion. In the first [11] influence of long chain branching on drool mass is discussed, in the second, [12] thermal degradation in extruder as other possible die drool source is investigated and in the third [13], flow-induced low molecular species fractionation inside the extrusion die as a fundamental source of die drool in the case of HDPE extrusion is proved.…”

Characterization of die drool sample produced by HDPE melt extrusion

“…Extrudate mass flow rate was 731 g.hr -1 (corresponding apparent shear rate 652 s -1 ) and die drool mass flow rate was 0.3462 g.hr -1 (corresponding dimensionless buildup ratio BR [11] defined as drool mass flow rate divided by extrudate mass flow rate was 0.0004733427). These conditions were set due to highly pronounced fractionation effect inside the die and high die drool mass accumulation on die exit face [9]. Die drool mass was collected after 10 minutes of extrusion and total weight was approximately 1.5 g. Then, samples for rotational and extensional rheology were prepared by compression moulding and special cutters.…”

“…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.…”