Experimental studies on extrudate swell behavior of PS and LLDPE melts in single and dual capillary dies

Extrudate swell behavior of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts was investigated using a constant shear rate capillary rheometer. Two capillary dies with different design configurations were used, one being a single flow channel and the other being a dual flow channel. A number of extrudate swell related parameters were examined, and used to explain the discrepancies in the extrudate swell results obtained from the single and dual flow channel dies, the parameters including output rate and output rate ratio, power law index, wall shear rate, wall shear stress, melt residence time, pressure drop induced temperature rise, flow channel position relative to the barrel centerline, and the flow patterns. It was found in this work that the power law index (n value) was the main parameter to determine the output rate ratio and the extrudate swell between the large and small holes for the dual flow channel die: the greater the n value the lower the output rate ratio and thus decreased extrudate swell ratio. The differences in the extrudate swell ratio and flow properties for PS and LLDPE melts resulted from the output rate ratio and the molecular chain structure, respectively. The extrudate swell was observed to increase with wall shear rate. The discrepancies in the extrudate swell results from single and dual dies for a given shear rate were caused by differences in the flow patterns in the barrel and die, and the change in the melt velocities flowing from the barrel and in the die to the die exit. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1713–1722, 2003     

A study of bicomponent coextrusion of molten polymers

An experimental study has been carried out of coextruding polystyrene with lowdensity polyethylene and polystyrene with high-density polyethylene, using both slit and circular dies. Two melt streams were separately fed to the die entrance and forced to flow side by side through a die. When using the slit die, wall normal stresses were measured with three melt pressure transducers flush-mounted on each of the rectangle's long sides, directly opposite each other. When using the capillary die, three different capillary length-to-diameter (L/D) ratios were employed: 4, 11, and 18. Wall normal stresses were measured for dies having L/D ratioes of 11 and 18 only. The measurements of wall normal stresses permitted one to determine the pressure gradient, and hence the viscous property. For each set of extrusion conditions (L/D ratio, flow rate, and component ratio), extrudate samples were collected. These were later carefully cross sectioned and photographed in order to examine the shape of the interface between the two components. At the phase interface of the polystyrene/low-density polyethylene system, it has been observed that the polystyrene, which is more viscous and yet less elastic than the low-density polyethylene, has a convex surface. However, at the phase interface of the polystyrene/high-density polyethylene system, the high-density polyethylene, which is more viscous and also more elastic than polystyrene, is seen to be convex. This then appears to indicate that the viscosity ratio of two components is primarily responsible for the final shape of the interface.     

Effects of the actual diameters and diameter ratios of barrels and dies on the elastic swell and entrance pressure drop of natural rubber in capillary die flow

The effects of the actual diameters and diameter ratios of barrels and dies on the elastic swell and entrance pressure drop of natural rubber compounds in an extrusion capillary rheometer were investigated. Either the barrel diameter or the die diameter was altered so that different barrel‐diameter/die‐diameter (D_B/D_D) ratios were obtained, both the barrel and die diameters also being varied simultaneously. The extrudate swell and entrance pressure drop were dependent not only on D_B/D_D but also on the actual diameters used. For fixed D_B/D_D ratios, the change in the extrudate swell was linearly influenced by the entrance pressure drop at low actual barrel and die diameters (D_B/D_D = 20/4–30/7 mm/mm) but was associated with a change in the material viscosity at high barrel and die diameters (D_B/D_D = 35/7–40/8 mm/mm). When the die diameter was fixed, the relationship between the entrance pressure drop and the extrudate swell was linear up to a certain value of the barrel diameter greater than 30 mm. Beyond this critical barrel value, the relationship became nonlinear and associated with the shearing stress generated by the formation of semipluglike flow patterns and the residence time of the material. For a constant barrel diameter, the smaller the die diameter was, the greater the extrudate swell was because of the increases in the extensional deformation and wall shear rate coupled with a reduction in the material residence time. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1762–1772, 2002     

Extrudate swell and texture of PS,LDPE, ABS,PVC melts and their blends in extrusion capillary flow using a magnetic die

The extrudate swell behavior and extrudate texture of various thermoplastic melts, namely, polystyrene (PS), low‐density polyethylene (LDPE), acrylonitrile‐butadiene styrene (ABS) copolymer, poly(vinyl chloride) (PVC), and their blends, were examined weith a magnetic die system in a constant‐shear‐rate capillary rheometer at a shear rate range 5–28 s^?1 and a temperature range 170–230 °C. The extrudate swell results obtained from the magnetic die were then compared with those produced by a nonmagnetic die. The results showed that the extrudate swell increased with shear rate, but decreased with temperature. In a pure polymer system, up to 25% increase in the extrudate swell was observed with the application of the magnetic field to the PS melt, and the effect decreased in the order ABS > LDPE > PVC. The extrudate swell changes were associated with the changes in rheological properties of the melts. The extrudate textures of the ABS and PVC melts were improved by the magnetic field. In PS/LDPE or PS/ABS blend, it was found that the magnetic die resulted in higher values of the extrudate swell than the nonmagnetic die for all blends, the magnetic effect being less as the LDPE or ABS content was increased. For PS/LDPE system, the extrudate swell of the PS melt did not change much with addition of 20% LDPE, but slightly decreased at the LDPE loading of 40%. At higher LDPE loadings, the extrudate swell increased towards the value of the pure LDPE melt. For PS/ABS system, the extrudate swell ratio progressively decreased with increasing ABS content. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 509–517, 2002     

Extrudate swell behavior of polypropylene composites filled with microencapsulated red phosphorus

The microencapsulated red phosphorus (MRP) filled polypropylene (PP) composites were prepared using a twin‐screw extruder. The effects of load and temperature on the extrudate swell behavior of the PP/MRP composite melts were investigated by means of a melt flow indexer. The test temperatures and loads were varied from 180 to 205°C and from 2.16 to 12.5 kg, respectively. The results showed that the die‐swell ratio (B) of the composite melts increased roughly linearly with increasing load while decreased slightly with a rise of test temperature. The sensitivity of the die‐swell ratio of the composite melts to load was significant. When the test temperature or load was constant, the values of the B of the composite melts decreased slightly with increasing MRP weight fraction. The findings can provide useful information for processing of these composites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Foam extrusion characteristics of thermoplastic resin with fluorocarbon blowing agent. I. Low-density polyethylene foam extrusion

An experimental study was conducted to investigate the foam extrusion characteristics of low-density polyethylene resin. For the study, we used dichlorotetrafluoroethane and dichlorodifluoromethane as blowing agent and talc as nucleating agent. In the study, we investigated the effects of processing and material variables on the foam extrusion characteristics, namely extrudate swell behavior, foam density, and cell morphology. It was found that an inverse relationship exists between the extrudate swell ratio and the foam density. Also investigated was the effect of die geometry (theL/D ratio, D_R/D ratio, and entrance angle) on the foam extrusion characteristics of low-density polyethylene resin. Suggestions are made on the experimental technique that may be useful in selecting resins for foam extrusion operation. Also suggested are guidelines for selecting an optimum die geometry that would produce good quality foams of low-density polyethylene.     

Effects of die temperature on extrudate swell in screw extrusion

Extrusion of a hot polymer melt through a cooler die zone substantially increases the extrudate swell of some thermoplastics. This effect was examined for commercial samples of low-density polyethylene, polypropylene, and polystyrene. Two conflicting effects come into play during extrusion of a thermoplastic. Colder melt temperatures promote increased extrudate swell, but the same conditions also facilitate molecular disentanglement and reduced melt elasticity and die swell. Since the extrusion process itself may affect the relation between die swell and melt temperature, laboratory-scale measurements for the design of processes like blow molding are better carried out with small-scale screw extruders than with capillary rheometers. For some applications it may be advantageous to use a polymer whose die swell is particularly responsive or unresponsive to die temperature variations. The procedure described in this article can be used effectively to monitor this characteristic.     

Study of the relationship between shear modification and melt fracture in extrusion of LDPE

Successive passes through an extruder can modify the melt morphology of low-density polyethylene (LDPE) by applying a shearing process. The major effects of shear modification are to decrease the elastic properties, as evidenced by the decrease in extrudate swell at the exit of a capillary and in melt viscosity at a low-frequency region. The effect of shear modification is also shown by the delay in the onset of melt fracture upon extrusion. The critical shear stress resulted from extrudate appearance, and apparent discontinuity in the flow curve of LDPE shows a similar value with polypropylene rather than those of LLDPE and HDPE. The shearing histories experienced by these materials did not result in any measurable change in molecular weight, so that the chemical modification process such as degradation and crosslinking may be ruled out. These behaviors were also confirmed to the fact that the extrudate swell was fully reversible by annealing in a molten state. The effects of shear modification on rheological properties could be explained by the changes in melt morphology owing to the disentanglement of temporary couplings between long branches. Also, a reduction in melt elasticity by shear modification of LDPE can be used as an effective tool to improve the surface roughness of extrudates in the cable-making process. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2187–2195, 1998     

LLDPE／LDPE共混物熔体挤出胀大行为的研究

应用毛细管流变仪，考察了实验条件下线性低密度聚乙烯与低密度聚乙烯共混物（ＬＬＤＰＥ／ＬＤＰＥ）熔体挤出胀大行为及其影响因素。结果表明，挤出胀大比Ｂ与剪切应力近似呈幂律关系，而随口型长径比的增加呈指数衰减，当共混比为５０／５０时，Ｂ产生局部极涉值现象。     

Experimental studies on radial extrudate swell and velocity profiles of flowing PS melt in an electro‐magnetized die of an extrusion rheometer

This article investigates the radial extrudate swell and velocity profiles of polystyrene melt in a capillary die of a constant shear‐rate extrusion rheometer, using a parallel coextrusion technique. An electro‐magnetized capillary die was used to monitor the changes in the radial extrudate swell profiles of the melt, which is relatively novel in polymer processing. The magnetic flux density applied to the capillary die was varied in a parallel direction to the melt flow, and all tests were performed under the critical condition at which sharkskin and melt fracture did not occur in the normal die. The experimental results suggest that the overall extrudate swell for all shear rates increased with increasing magnetic flux density to a maximum value and then decreased at higher densities. The maximum swelling peak of the melt appeared to shift to higher magnetic flux density, and the value of the maximum swell decreased with increasing wall shear rate and die temperature. The effect of magnetic torque on the extrudate swell ratio of PS melt was more pronounced when extruding the melt at low shear rates and low die temperatures. For radial extrudate swell and velocity profiles, the radial swell ratio for a given shear rate decreased with increasing r/R position. There were two regions where the changes in the extrudate swell ratio across the die diameter were obvious with changing magnetic torque and shear rate, one around the duct center and the other around r/R of 0.65–0.85. The changes in the extrudate swell profiles across the die diameter were associated with, and can be explained using, the melt velocity profiles generated during the flow. In summary, the changes in the overall extrudate swell ratio of PS melt in a capillary die were influenced more by the swelling of the melt around the center of the die. Polym. Eng. Sci. 44:2298–2307, 2004. © 2004 Society of Plastics Engineers.     

A method for the measurement of the true die swell of polymer melts

A new device has been designed for the measurement of the die swell of extruded polymer melts. According to the proposed procedure, samples can be collected, annealed, and photographed. The device may be used in conjunction with any capillarytype rheometer. It can accommodate simultaneously as many samples as it is needed. The samples are suspended in a thermostated liquid, carefully selected for each polymer. The liquids must be thermally stable and of proper density and thermodynamic and interfacial properties. The device was used in conjunction with the Instron capillary rheometer, ICR. Three types of polymer were tested: polystyrene (PS), polyethylene (PE), and a semirigid poly(vinyl chloride) formulation (PVC). The swelling of the extrudates was followed for ca. 40 min; the equilibrium dimensions were usually reached within the first 2 min. Parallel with these measurements, the samples were tested in the Weissenberg rheogoniometer (WR) recording both shear and normal stresses. For PS and PE, the flow curves determined in these two rheometers overlapped, while they differed for PVC. The swell ratio, B_exp = D/D₀ (where D and D₀ are the equilibrium diameter of the extrudate and diameter of the capillary, respectively), was converted to recoverable shear strain, s, as follows. First, B_exp and s were determined in ICR and WR, respectively, for a PS sample over wide and overlapping ranges of rate of shear. This experimental dependence was found to follow Tanner's theoretical relation. Consequently, this relation was used to compute s from B_exp for all the other samples. Excellent agreement was observed between the s values calculated from B_exp and s values determined in WR.     

Die–swell behavior of glass bead‐filled low‐density polyethylene composite melts at high extrusion rates

The extrudate swell behavior of glass bead‐filled low‐density polyethylene (LDPE) composite melts was investigated using a constant rate type of capillary rheometer at high extrusion rates and test temperatures varied from 140 to 170°C. The results show that the die swell ratio (B) of the melts increases nonlinearly with increasing apparent shear rates for the system filled with the surface of glass beads pretreated with a silane coupling agent, while the B for the system filled with uncoated particles remains almost constant when the true wall shear rate is greater than 2000 s⁻¹ at a constant temperature. The values of B for both the pure LDPE and the filled systems decreases linearly with an increase of the temperature and an increase of the die diameter at fixed shear rates, and the sensitivity of B on the die diameter and temperature for the former is higher than that of the latter. Furthermore, the effect of the filler content on B is insignificant, while the values of B decreases, obviously, with an increasing glass bead diameter (d) when d is smaller than 50 μm; then B varies slightly with d. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 419–424, 2000     

Influence of reservoir diameter on the melt elasticity in capillary flow

An experimental study has been carried out to investigate the influence of the reservoir-to-capillary diameter (D_R/D) ratio on the elastic properties of polymer melts flowing through capillary tubes. For the study, a capillary rheometer, described in an earlier paper by Han, was used with D_R/D ratios of 3, 6, 9 and 12. Measurements were taken of wall normal stresses in high density polyethylene along the axis of the capillary, and the axial pressure profile was used to obtain the “exist pressure,” which is believed to be an elastic phenomenon. The analysis of the experimental data indicates that, for a given capillary length-to-diameter (L/D) ratio, the exit pressures first increase and then level off, as the D_R/D ratio is increased. This behavior of the exit pressure is in accord with that of extrudate expansion which has also been determined in the experiment. This dependence of the exist pressure and extrudate expansion ratio on D_R/D ratio is explained with the concept that the amount of recoverable elastic energy stored in the melt flowing through the capillary depends on the strain history, which the melt experiences on entering the capillary.     

Compression creep of molten polystyrene

Viscoelastic parameters of polystyrene (PS) melt in compression creep have been measured in an lnstron capillary-rheometer. Bulk compression creep compliance B(t) shows plateau regions in the molten state (B₁) and the glassy state (B_g), both decreasing with increasing stress. Shifting of B(t) curves provides master curves suitable to analysing the total (superposed elastic and viscous deformations) bulk compression behaviour. The steady-state creep compliance (B^s) allows to describe the recoverable elastic energy (B_e) and seems to be related to the extrusion die swell (B/B). Volume viscosity (η_k) decreases with decreasing stress (P), increasing compression rates (k) and decreasing temperature (T).     

Effect of shear modification on the rheological behavior of two low-density polyethylene (LDPE) grades

Shear modification treatment represents a special shearing history affecting mainly the elastic behavior of polymer melts. This process has been attributed to reversible physical changes in the entanglement structure of the polymer chain network. Shear modification studies were performed for two well-characterized low-density polyethylene (LDPE) grades differing in molecular weight distribution and degree of long chain branching. The shear working of the material was carried out using a specially designed shearing unit producing definable amounts of pure shear in a continuous process. Measurements of the dynamic storage modulus, G′, steady-state shear compliance; J_e, extrudate swell, melt flow index, and the extensional behavior (Rheotens test) indicate that primarily properties associated with the elasticity of the melt are reduced in value. The observed reduction is found to correlate with the mean specific energy dissipated during sample preparation. Comparing the two LDPE grades showed that higher degrees of modification can be obtained at lower energy input levels for the more highly branched grade. Reversibility tests were performed and complete recovery of the initial material behavior was observed. Comparison of measurement results for samples prepared using the shearing unit presented here and a Brabender Plasticorder indicates that the degree of modification depends not only on the molecular structure of the polymer but also on the manner in which the shearing history is imposed upon the material.     

数值模拟黏弹流体通过椭圆环模具的挤出胀大

The numerical simulation of extrudate swell is significant in extrusion processing.Precise prediction of extrudate swell is propitious to the control of melt flow and the quality of final products.A mathematical model of three-dimensional(3D)viscoelastic flow through elliptical ring die for polymer extrusion was investigated.The penalty function formulation of viscoelastic incompressible fluid was introduced to the finite element model to analyze 3D extrusion problem.The discrete elastic viscous split stress(DEVSS)and streamline-upwind PetrovGalerkin(SUPG)technology were used to obtain stable simulation results.Free surface was updated by updating the streamlines which needs less memory space.According to numerical simulation results,the effect of zero-shear viscosity and elongation parameter on extrudate swell was slight,but with the increase of volumetric flow rate and relax time the extrudate swell ratio increased markedly.Finally,the numerical simulation of extrudate swell flow for low-density polyethylene(LDPE)melts was investigated and the results agreed well with others’work.These conclusions provided quantitative basis for the forecasting extrudate swell ratio and the controlling of extrusion productivity shape.     

Numerical investigation of spinneret geometric effect on spinning dynamics of dry‐jet wet‐spinning of cellulose/[BMIM]Cl solution

In this study, the effect of spinneret geometry, including the entrance angle α of the entrance channel, the length L_s, and the diameter D₀ of the exit channel, on the spinning dynamics of dry‐jet wet‐spinning of cellulose/1‐butyl‐3‐methylimidazolium chloride ([BMIM]Cl) solution was simulated by using finite element method. Based on the mathematical model of dry‐jet wet‐spinning established in our previous work (Xia et al., Cellulose 2015, 22, 1963) the radial and axial profiles of velocity, pressure, and shear rate in the spinneret and the profiles of diameter, temperature, and tensile stress in the air‐gap region were obtained. From the simulated profiles, the effect of spinneret geometric parameters on the flow behavior and the pressure drop of polymer solution in the spinneret and the die‐swell ratio near the spinneret was discussed. The entrance angle α of the entrance channel mainly influences the flow behavior of polymer solution in the spinneret and the die‐swell effect near the spinneret. As the decrease of the entrance angle α of the entrance channel, the vortices in the spinneret could be removed and the die‐swell ratio decreases. The increase of the length L_s of the exit channel results in the increase of pressure drop in the spinneret and the decrease of the die‐swell ratio. It is also found that the increase of the diameter D₀ of the exit channel reduces the flow velocity of polymer solution and decreases the pressure drop in the spinneret at a constant mass flow rate. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43962.     

High weissenberg number simulation of an annular extrudate swell using the differential type constitutive equation

Annular extrudate swell simulations at high Weissenberg numbers were made using a differential type constitutive equation. The streamline-upwinding method with a sub-element for extra stress components, which is called SU4 × 4, is one of the best mixed finite element methods for computation of viscoelastic flows. Planar and capillary extrudate swell calculations at high Weissenberg numbers (We > 1000) were accomplished by SU4 × 4. However, annular extrudate swell simulations at high We by SU4 × 4 were not successful. The calculated We was less than about 4. A new calculation technique using a Newton-Raphson discretization of the equation of motion was developed. This technique is called a “new under-relaxation method.” The calculated We of annular extrudate swell simulation by the new under-relaxation method with SU4 × 4 was about 6～250 times larger than those by SU4 × 4. Reasonable calculation results were obtained in an annular flow and a capillary extrudate swell by this method, and the reliability and the utility of the new under-relaxation method are shown. It is now possible to consider the swell shapes of annular extrudate under industrially useful conditions. The calculated swelling ratios were also compared with experimental ones.     

Melt extrudate swell behavior of multi‐walled carbon nanotubes filled‐polypropylene composites

The extrudate swell behavior of polypropylene (PP) composite melts filled with multi‐walled carbon nanotubes (MWCNTs) was studied using a capillary rheometer in a temperature range from 190 to 230°C and at various apparent shear rates varying from 50 to 800 s⁻¹. It was found that the values of the extrudate swell ratio of the composites increased nonlinearly with increasing apparent shear rates, while the values of the extrudate swell ratio decreased almost linearly with increasing temperature. The values of the melt extrudate swell ratio increased approximately linearly with increasing shear stress, while decreased approximately nonlinearly with an increase of the MWCNT weight fraction. In addition, the extrudate swell mechanisms were discussed with observation of the fracture surface of the extrudate using a scanning electronic microscopy. This study provides a basis for further development of MWCNTs reinforced polymer composites with desirable mechanical and thermal properties. POLYM. COMPOS., 38:2433–2439, 2017. © 2015 Society of Plastics Engineers