Polymer Melt Flow Instabilities in Extrusion: Investigation of the Mechanism and Material and Geometric Variables

Abstract

A comprehensive study of the effect of material, geometric, and operating variables on the onset of melt flow instabilities in extrusion was made. Primary objectives were to compare such melt instabilities for a variety of materials and to correlate instability criteria with independent rheological parameters. Seven polymer melts were studied: two polyethylenes, polypropylene, polystyrene, two polybutadienes, and an SBR copolymer. Two distinct extrusion apparatuses were used: a die fed by a screw extruder system and a die fed by an Instron rheometer. The variables studied were temperature, entry cone angle, length/diameter ratio of the die, and flow rate. Independent rheological measurements were performed. These consisted of the evaluation of normal stresses on the Weissenberg rheogoniometer, entrance pressure drop measurements on the Instron rheometer, and viscosity measurements on both instruments and the screw extruder. In a companion study, flow visualization studies were made in the die entry region of the Instron rheometer, for most of the above materials and also a solution of polyisobutylene and glycerine. Distinctively different patterns were observed for the various materials, ranging from severe channeling with corner eddies to nonchanneling flow. Interrelationships between the onset of instabilities with other measurements were noted. In comparing a variety of materials, a decrease in the critical shear rate is generally associated with an increased tendency of the material to channel, an increased normal stress/shear stress ratio and an increased entrance pressure drop/shear stress ratio. It is argued that the instability phenomena are strongly associated with the elastic properties of the materials.

Wall slip of molten high density polyethylenes. II. Capillary rheometer studies

“…It has also been observed that the sharkskin effect is more pronounced in short capillaries of small diameter (Ballenger et al 1971). It can be seen from Fig.…”

Experimental study of the volume defects in polystyrene extrusion

“…In other words, it seems that there is no attenuation of the instability along the capillary, what was first reported by Bergem [24] using flow visualizations. Consequently, we may imagine that the attenuation of the instability for long capillaries, largely cited in the literature [3,7,8,20] could be a visual artifact: as the pitch is higher, the extrudate looks much less distorted, even if the volume affected by the instability is larger.…”

“…Critical parameters usually used to quantify the onset of the upstream instability are mostly shear flow parameters, such as apparent shear rate or wall shear stress. According to Ballenger et al [8], volume defects occur for a critical wall shear stress around 0.1 MPa. However, this critical stress depends on the die length, the temperature and also the molecular weight [9].…”

Two-Dimensional Viscoelastic Flow of Polymer Solution at Channel Junction and Branch

“…5 are for PAA 0.8 % solution and the dextrose syrup respectively and for the exit channel width of 3 mm. As [5] and [6] show clearly effects of viscoelasticity on flow pattern become more pronounced when the exit channel is narrow in width. Observed flow directions are shown in Fig.…”

“…The flow field shows no circulating secondary flow and the bending of stream lines are more gradual than the viscoelastic flows in [1J- [3]. Flows [5] and [6] in Fig. 5 are for PAA 0.8 % solution and the dextrose syrup respectively and for the exit channel width of 3 mm.…”