Jan Musil

Historical Review of Die Drool Phenomenon in Plastics Extrusion

Die drool phenomenon is defined as unwanted spontaneous accumulation of extruded plastics on die exit face(s) of extrusion die during plastics extrusion process. Such accumulated material builds up on die exit face(s) and frequently or continually sticks onto the extruded product and by this way reduces the quality of the product. When the die drool appears, the extrusion process must be shut down and the die exit face(s) must be manually cleaned, which is time consuming as well as money consuming. Die drool has been observed from the beginning of plastics extrusion and the first published remark about it was made in 1946. For a long time it was considered as only an engineering problem and even if a wide range of suppressing ideas based on modification of extrusion dies, plastics materials, and processing conditions have already been patented, its source remained unclear. During the last few years, a number of experimental as well as theoretical research papers focusing on its fundamental nature have been published. Nowadays, die drool is considered as a fundamental rheological phenomenon. The force which drives the building up of extruded plastics on die exit face(s) is negative pressure (suction) occurring in the die exit edge(s) region where the free surface of the extruded plastics is created. Moreover, two different die drool types (external and internal) exist. The formation mechanisms of external/internal die drool are based on negative pressure occurring in the die exit region together with deformation of free extrudate surface/material separation wherever inside the extrusion equipment, respectively. From the processing point of view, the internal die drool is much more problematic than the external one, primarily due to a higher build up rate and compact shapes of the accumulated drool mass. Two theories of internal die drool formation mechanism based on wall slip theories (flow-induced molecular weight fractionation and stress-induced cohesive chain disentanglement) have been recently proposed and tested. In this historically ordered review, breakthrough works in the field of die drool research are presented, many ways to suppress it are shown, techniques for its quantitative evaluation and experimental methods for its analytical investigation are introduced, external and internal die drool types are explained, and theories of external as well as internal die drool formation mechanism are presented and discussed in depth, which can be especially helpful for plastics extrusion experts as well as for the rheological community.

Characterization of die drool sample produced by HDPE melt extrusion

In this work, flow induced molecular weight fractionation experiment was performed for HDPE polymer on specially designed laboratory extrusion setup. Die drool sample, accumulated at the die exit face, together with virgin pellets were consequently characterized experimentally by gel permeable chromatography, differential scanning calorimetry and rheology as well as theoretically by recently proposed generalized Newtonian model.

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

In this work, two slightly different batches of one commercial linear HDPE polymer melt were rheologically characterized and then extruded through two specially designed dies (annular and slit) in order to investigate die drool as well as slip‐stick phenomena. Flow birefringence stress visualization inside the slit die was also performed. It has been revealed that lower elasticity and both shear and extensional viscosities reduce accumulated drool mass at the die exit face and also correlation between appearance and intensity of both slip‐stick and die drool phenomena was discovered.

Investigation of Internal Die Drool Phenomenon for Different HDPEs

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.

Historical review of die drool phenomenon during plastics extrusion

Die drool phenomenon is defined as unwanted spontaneous accumulation of extruded polymer melt on open faces of extrusion die during extrusion process. Such accumulated material builds up on the die exit and frequently or continually sticks onto the extruded product and thus damages it. Since die drool appears, extrusion process must be shut down and die exit must be manually cleaned which is time and money consuming. Although die drool is complex phenomenon and its formation mechanism is not fully understood yet, variety of proposed explanations of its formation mechanism and also many ways to its elimination can be found in open literature. Our review presents in historical order breakthrough works in the field of die drool research, shows many ways to suppress it, introduces methods for its quantitative evaluation and composition analysis and summarizes theories of die drool formation mechanism which can be helpful for extrusion experts.

Investigation of Vortex Development during Polymer Melt Flows by Flow Birefringence

The vortex development of low‐density polyethylene Lupolen 1840H polymer melt under various processing conditions has been investigated by flow induced stress birefringence technique. It has been revealed that effect of mass flow rate and temperature on the vortex size has non‐monotonic character, which is in good agreement with laser‐Doppler velocimetry based measurements reported for the same polymer in the open literature. This suggests that the flow induced stress birefringence technique can be considered as the quick and reasonably precise tool for vortices shape/size visualization and quantification at the slit die entry region.

Entry flow vortices in polymer melt extrusion: A review

Although, circular or planar abrupt entry flows are geometrically very simple hydrodynamic problem highly viscoelastic polymer melts makes it very complex with extreme differences in velocities and stresses across the geometry. Despite these flows are very common in polymer melt extrusion industry their strongly non-viscometric and transient nature represents exceedingly challenging task for experimental as well as theoretical investigation and consequently complicates their fully understanding. Polymer melts flowing through abrupt entry contractions exhibit several unique features of which the vortices are one of them. Occurrence of infinitesimal stress singularity in the salient corner leads to presence of weak concave Newtonian viscous vortex. Moreover, polymer melts with increasing extensional to shear viscosity (Trouton) ratio as a function of flow rate exhibit strong convex elastic vortex caused by complete reorientation of stress field near the re-entrant corner (infinite stress singularity point) ...

Development of Novel Orifice Die Design for Precise Entrance Pressure Drop Measurements

In this work, novel patent pending orifice die design for precise extensional viscosity data determination from entrance pressure drop measurements has been developed and tested both, theoretically (through Finite Element Analysis) and experimentally. It has been demonstrated that the proposed novel orifice die allows much more precise extensional viscosity measurements for polymer melts in comparison with conventionally used orifice dies.