A simple way to suppress surface defects in the processing of polyethylene

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

“…While Ramamurthy [17] attributed this result to increased adhesion of the melt to the die wall, Piau et al [38] proposed that the coating material promotes slip at the die wall, thereby reducing the exit stresses. Recently, Kulikov and Hornung [40] found that coating the die land with a rubber compound produced defect-free extrusion, for flow rates 10 times higher than those in Teflon dies. Coatings of PTFE or rubber that are localised just at the exit of the die wall, have also been found to be effective at delaying the onset of sharkskin, though to a lesser extent than a coating along the whole die surface [38,40].…”

Historical review of die drool phenomenon during plastics extrusion

“…Other very interesting solution for suppression not only die drool but also other polymer melt flow instabilities is suggested in [32]. It is based on silicon rubber coating of the inside die exit wall.…”

“…The second group contains low molecular weight fractions, volatiles, fillers, poor dispersion of pigments [16,17,21,[40][41][42][43][44][61][62][63][64][65], die swell [11,14], dissimilar viscosities in blends [90], wall slip [32], slip-stick [16,17,32] and shark skin [13,24]. The last group represents high melt temperature [90], processing near degradation temperature [15,91], and draw down [21].…”

“…This negative pressure causes suction effect which is the driving force to build-up polymer on die lips. However, it is crucial to mention that die drool is always connected and influenced by other phenomena (die swell [11,14], shark-skin [13,31], slip-stick [16,17,29], degradation [14,15,91], fractionation of polymer chains [16][17][18], phase separation of polymer matrix/filler [61,62] or polymer blends [60] inside the die and wall slip [32]) occurring during polymer melt extrusion. Thus, due to this, die drool is very complex problem and it is not possible to solve it only by reduction of negative pressure magnitude at the die exit.…”

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

“…This instability appears at very narrow processing window (depending on temperature and flow rate for given polymer melt) in which pressure in extrusion die oscillates between two extreme values, although the imposed flow rate is kept constant. A lot of works focused on this instability have been already published [14][15][16][17] however, only one small notice about correlation between slip-stick and die drool phenomena can be found in [18].…”