Inlet instabilities in the capillary flow of polyethylene melts

Perez‐González, José; Pérez-Trejo, Leonor; Vargas, Lourdes de; Manero, Ο.

doi:10.1007/bf00367364

Cited by 26 publications

(4 citation statements)

References 20 publications

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“…Relationships between extensional stress and chaotic de-20 Intern. Polymer Processing XXX (2015) Combeaud et al, 2004) fects have also been reported by other authors (Hürliman and Knappe, 1972;Perez-Gonzalez et al, 1997;Meller et al, 2002;Den Doelder et al, 2005;Muliawan et al, 2005). This critical elongational stress increases as the molecular weight decreases or long chain branching increases.…”

Section: Mechanisms and Interpretationssupporting

confidence: 66%

Extrusion Defects and Flow Instabilities of Molten Polymers

Vergnes

2015

International Polymer Processing

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International audienceWhen certain critical conditions are exceeded, the flow of a polymer melt becomes unstable, giving rise to particular phenomena that are the subject of this review. These instabilities are a vital industrial problem, as they are the key to productivity of extrusion processes. Indeed, their onset leads to unacceptable products and is thus the upper limit of the processing conditions. They are also a fascinating scientific problem because, although studied and discussed for more than half a century, they are still not fully understood and their mechanisms are still sometimes controversial. In the present literature review, we present an overview of the studies carried out for fifty years, as well as our own opinion on the underlying mechanisms.3-2

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Section: Mechanisms and Interpretationssupporting

confidence: 66%

Extrusion Defects and Flow Instabilities of Molten Polymers

Vergnes

2015

International Polymer Processing

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“…An example of such an approach would be associating sharkskin to the existence of slip at the die exit (no slip, no sharkskin). [17,23] A similar reasoning can be made based on whether the polymers show monotonic or non-monotonic flow curves [25] or whether the balance of tensile and shear stresses at the die exit in the presence of swelling can be linked to surface cracks. Complementary, geometrical arguments can be used to distinguish between surface and volume distortions based on how the amplitude of the extrudate distortions compare to the extrudate height, in the case of slit dies.…”

Section: Introductionmentioning

confidence: 99%

“…In addition, at the time there was conflicting evidence on the occurrence of pressure oscillations inside the die during the extrusion of LCB polymers. [17] In this context, Perez-Gonzalez et al [17] showed that despite the absence of slip and a monotonic flow curve, pressure oscillations, having at least two characteristic frequencies, could be detected during the (capillary) extrusion of LDPE. Accordingly, their extrudates clearly showed evidence of both surface and volume MFEI.…”

Section: Introductionmentioning

confidence: 99%

“…In addition, at the time there was conflicting evidence on the occurrence of pressure oscillations inside the die during the extrusion of LCB polymers. [ 17 ] In this context, Perez‐Gonzalez et al. [ 17 ] showed that despite the absence of slip and a monotonic flow curve, pressure oscillations, having at least two characteristic frequencies, could be detected during the (capillary) extrusion of LDPE.…”

Section: Introductionmentioning

confidence: 99%

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An Intriguing Array of Extrudate Patterns in Long‐Chain Branched Polymers During Extrusion

Pashazadeh,

Aulova,

Georgantopoulos

et al. 2023

Macro Materials & Eng

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The present study highlights a range of surface and volume extrudate patterns that can be detected during the extrusion flow of long‐chain branched polymers. Thus, four linear low‐density polyethylenes (LDPEs) have been extruded using a single‐screw extruder coupled to an inline optical imaging system. The selected LDPEs are selected to outline the influence of molecular weight and long‐chain branching on the types of melt flow extrusion instabilities (MFEI). Through the inline imaging system, space–time diagrams are constructed and analyzed via Fourier‐transformation using a custom moving window procedure. Based on the number of characteristic frequencies, peak broadness, and whether they are surface or volume distortions, three main MFEI types, distinct from those typically observed in linear and short‐chain branched polymers, are identified. The higher molecular weight, low long‐chain branching LDPEs exhibited all three instability types, including a special type volume instability. Independently of the molecular weight, higher long‐chain branching appeared to have a stabilizing effect on the transition sequences by suppressing volume extrudate distortions or limiting surface patters to a form of weak intensity type.

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Flow through a convergence. Part 1: Critical conditions for unstable flow

Meller

Luciani

Sarioglu

et al. 2002

Polymer Engineering & Sci

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The critical conditions leading to fracture in elongation and different types of flow instabilities were examined in uniaxial elongation and in a capillary rheometer equipped with dies having different entry profiles. Either ductile or brittle fracture may be observed, ductile being related to necking of material. The critical stress approach was used to predict fracture in elongation. All linear polymers studied in this work exhibited ductile fracture in elongation, but the transition to brittle fracture is discussed in relation to existing experiments with other materials. In a ductile h c t u r e regime, critical stress and work both increase with an increasing rate of deformation, whereas in a brittle regime the critical values remain constant. The converging flow studies indicated that two types of flow instability that have been previously related to each other, namely, pressure oscillations and volume distortions, are of different origins. The critical flow rate for pressure oscillations is independent of entry profile, and the origin for this type of instability lies along the wall of the capillary. On the other hand, the critical flow rate for volume distortions increased with a decreasing entry angle, indicating that volume distortions are not a consequence of pressure oscillation, nor are their origin at the cap& lary wall. Numerical simulations were used to determine the stress profies within the flow, and it was shown that the onset of volume distortions is directly related to the magnitude of elongational stress and work, and may therefore be considered to be caused by fracture in elongation. In dies with 90" entry profile, volume distortions were observed simultaneously with pressure oscillations, making it difficult to distinguish between the two phenomena. a maximum of four different types of flow instability 61 1 Towfiornmmsponduurshouldbeaddressed. a 1 2 t z h / HDPE and LLDPE Flow Rate, Qlet PP and LDPE r Flow Rate, Q&

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Inlet instabilities in the capillary flow of polyethylene melts

Cited by 26 publications

References 20 publications

Extrusion Defects and Flow Instabilities of Molten Polymers

Extrusion Defects and Flow Instabilities of Molten Polymers

An Intriguing Array of Extrudate Patterns in Long‐Chain Branched Polymers During Extrusion

Flow through a convergence. Part 1: Critical conditions for unstable flow

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