Experimental study and modeling of oscillating flow of high density polyethylenes

Durand, Valérie; Vergnes, Bruno; Agassant, Jean‐François; Benoit, E.; Koopmans, R. J.

doi:10.1122/1.550749

Cited by 47 publications

(50 citation statements)

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“…This seems to indicate that, beyond the oscillatory flow region, the flow is not governed by the viscosity of the polymer melt. This finding is in agreement with the results obtained for HDPE melts by Hatzikiriakos and Dealy [3] and Durand et al [5], who observed that in the high-flow-rate branch most of the flow occurs by wall slip (plug flow) and relatively little by shear flow. Constant-speed long-time experiments, carried out for each examined temperature at various piston speeds within the oscillating flow region, permitted to analyze the effects of temperature, piston speed and reservoir length on the parameters describing the pressure oscillations.…”

Section: Resultssupporting

confidence: 91%

“…In agreement with what described in literature [1][2][3][4][5], the oscillating flow that took place when such a type of instability occurred was characterized by evident fluctuations both in the pressure signal acquired in the reservoir of the rheometer and in the extrudate flow rate; the extrudate appearance was characterized by the alternation of relatively smooth and rough segments. An example of a pressure, P, vs time trace acquired during the oscillating flow exhibited by the HDPE here examined is shown in Figure 1.…”

Section: Resultssupporting

confidence: 86%

“…In particular, by increasing temperature, P o,max increased monotonically with an approximately constant rate, whereas P o,min exhibited an abrupt increase between 175 and 180°C, reaching a value that maintained practically constant at higher temperatures. The experiments showed also that, in agreement with the observations by Bergem [1] and Durand et al [5], at any given temperature, the oscillation period of the pressure signal taken at any fixed reservoir length varied with the imposed flow rate (data not shown). In particular, the time interval of pressure increase from P o,min to P o,max became smaller as the flow rate increased.…”

Section: Figure 3: Values Of Pomin and Pomax Measured In The Constsupporting

confidence: 88%

“…Though the origins of the oscillating flow are not fully understood, it is generally accepted that melt compressibility is one of the essential factors at the basis of this instability [3,5]. For the HDPE here examined, by the application of a simple model, compressibility data were indirectly obtained from the pressure signal recorded in the constant-speed long-time experiments.…”

Section: Figure 3: Values Of Pomin and Pomax Measured In The Constmentioning

confidence: 99%

“…Instability phenomena in capillary flow have been the subject of many theoretical and experimental studies since the beginning of polymer science, but still receive considerable attention because of the limits they impose on die forming processes of polymers (some of the most relevant literature works are cited in the reference list [1][2][3][4][5]). These instabilities, which are commonly referred to as "melt fracture", produce irregularities and distortions at the extrudate surface and thus limit the industrial production rates and the technological opportunities of several polymers.…”

Section: Introductionmentioning

confidence: 99%

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Oscillating Flow in Capillary Dies for a Hdpe Melt: Effects of Extrusion Parameters

et al. 2010

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The study of the instability phenomena of polymer melts in capillary flows is of great technical relevance since, during an industrial die forming operation, these instabilities can give origin to a transition from a defect-free to a highly-defective extrudate. In this work, the instability phenomena exhibited in capillary flow by a blow moulding grade of high-density polyethylene (HDPE) were analyzed. In particular, by means of a capillary rheometer, which mimics polymer extrusion, the effects of specific extrusion parameters (temperature and imposed flow rate) on the oscillating flow, that takes place when "stick-slip" instabilities occur, were investigated.

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Section: Resultssupporting

confidence: 91%

Section: Resultssupporting

confidence: 86%

Section: Figure 3: Values Of Pomin and Pomax Measured In The Constsupporting

confidence: 88%

Section: Figure 3: Values Of Pomin and Pomax Measured In The Constmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Oscillating Flow in Capillary Dies for a Hdpe Melt: Effects of Extrusion Parameters

et al. 2010

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A study of the stick‐slip phenomenon in single‐screw extrusion of linear polyethylene

Kay

Carreau

Lafleur

et al. 2003

Polymer Engineering & Sci

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We present results of a study on the stick‐slip phenomenon observed for two linear polyethylenes (HDPE and LLDPE). Experiments were carried out in a single screw extruder using dies of different diameters and L/D ratios. The pressure and the mass flow rate have been measured simultaneously and the instant mass flow rate was determined using laser Doppler velocimetry. Our results clearly confirm the hypothesis that the flow jumps from one stable branch to the other during oscillations as suggested in the literature. The role of the melt compressibility in the reservoir is also established. The effect of the average mass flow rate imposed by the extruder QIN on the respective compression and decompression phases is in good agreement with effect predicted by the mass balance in the reservoir. We also used different reservoir volumes to confirm that the period of oscillations increases with the compressed melt volume upstream of the die.

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The “sharkskin effect” in polymer extrusion

Koopmans

Molenaar

1998

Polymer Engineering & Sci

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Melt flow instabilities associated with surface defects for polymer extrudates and commonly referred to as the “Sharkskin Effect,” are modeled. The relaxation oscillation model of Molenaar and Koopmans, developed to describe “Spurt” defects is expanded to provide the theoretical framework for the description of surface defects. It is shown that a nonlinear viscoelastic constitutive equation may give rise to a boundary layer, which is unstable. The presence of an oscillating boundary layer is one of the many possible solutions of the momentum equation indicating the need for additional selection criteria to define which one will be selected by the polymer system. The energy balance equation is suggested to provide the extra criterion. The current macroscopic approach may form the basis for the reconciliation of the debate on the origin of extrudate surface melt flow instabilities as either a “slip‐at‐the‐wall” or a non‐monotone “constitutive equation” phenomenon.

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Experimental study and modeling of oscillating flow of high density polyethylenes

Cited by 47 publications

References 0 publications

Oscillating Flow in Capillary Dies for a Hdpe Melt: Effects of Extrusion Parameters

Oscillating Flow in Capillary Dies for a Hdpe Melt: Effects of Extrusion Parameters

A study of the stick‐slip phenomenon in single‐screw extrusion of linear polyethylene

The “sharkskin effect” in polymer extrusion

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