Extrudate Swell Research Articles

Influence of gas layers length on the gas-assisted extrusion molding of plastic microtubule

The influence of gas layers length (GLL) on gas-layers assisted extrusion molding (GAEM) for plastic microtubule (PMT) was investigated by the numerical method in this paper. Meanwhile, 3D models with different GLLs of die’s channel were established. Based on the same flow rate imposed on inlet and other boundaries, the influences of the GLL on the extrusion swell of PMT were studied. The extrusion swell ratios of the different GLLs were obtained. Results show that with the increasing of the GLL, the extrusion swell phenomenon of PMT is reduced. To clear the mechanism for the influence of GLL on the GAEM of PMT, the pressure, stresses distributions were also obtained. Numerical results show that with the increasing of GLL, the decreased pressure, stresses of melt are the main reason of eliminating the extrusion swell of GAEM for PMT.

Influence of interfacial condition on rheological instability behavior of UHMWPE/HDPE/nano-SiO2 blends in capillary extrusion

In this study, rheological behaviors and capillary extrusion flow instabilities of ultra-high molecular weight polyethylene (UHMWPE)/high-density polyethylene (HDPE)/SiO2 composites containing modified nano-SiO2 or pure nano-SiO2 are investigated. Effects of interfacial conditions between the dispersed nano-SiO2 phase and PE matrix on rheological behaviors are analyzed. The results show that modified nano-SiO2 in the PE matrix has a relatively strong interfacial interaction with the polymer chains compared with pure nano-SiO2, thus causing a more pronounced shear thinning behavior and reducing extrudate swell during capillary extrusion. Nanocomposite extrudates experience the transition of smooth- sharkskin- oscillating distortion—overall melt fracture with the increase of shear rate (or shear stress). The interfacial interaction allows a more storage of elastic energy as the melt flow through the die, resulting in sharkskin distortion and oscillating distortion to occur prematurely at critical shear rates. It also restricts the elastic recovery of the melt after leaving the die, thus delaying sharkskin distortion under shear stress. At high shear rates, the modified nano-SiO2 particles begin to roll, and some of the unmodified particles move and embed to the wall. It is believed that the interfacial adsorption effect and the wall-slip effect of nanoparticles exist simultaneously in the whole extrusion process.

Polymer extrudate-swell: From monodisperse melts to polydispersity and flow-induced reduction in monomer friction

This paper describes finite-element simulations and associated experimental studies of extrudate swell for near-monodisperse and polydisperse polystyrenes. The tube-model based Rolie-Poly constitutive model, when extended to include a reduction of monomeric friction at high extension rates, makes much-improved predictions of extrudate swell at high Weissenberg number. This is especially significant for near-monodisperse polymers where rheological features are unchanged by the effects of polydispersity. Extension of this molecular rheology scheme to a polydisperse constitutive model addresses extrusion experiments on polydisperse polystyrenes inside a multipass rheometer, accounting for experimental data up to Rouse Weissenberg numbers of 50. We, therefore, show that from a measurement of polymer molecular weight distribution, it is possible to predict extrudate swell over a broad range of processing conditions for polydisperse polymers and realistic extrusion processes. Small changes in the capillary length to diameter ratio have little effect on extrudate swell in this range of Weissenberg number. This is because the capillary residence time is sufficiently long for a steady state to be reached within the polymer stretch relaxation time, which controls the most decisive physics responsible for extrudate swell.

Numerical solution of the Giesekus model for incompressible free surface flows without solvent viscosity

We present a numerical method for solving the Giesekus model without solvent viscosity. This paper is concerned with incompressible two-dimensional free surface flows and employs the finite difference method to solve the governing equations. The methodology involves solving the momentum equation using the implicit Euler scheme and an implicit technique for computing the pressure condition on the free surface. The nonlinear Giesekus constitutive equation is computed by a second order Runge–Kutta method. A novel analytic solution for channel flow is developed and is used to verify the numerical technique presented herein. Mesh refinement studies establish the convergence of the method for complex free surface flows. To demonstrate that the technique can deal with complicated free surface flows, the time-dependent flow produced by a fluid jet flowing onto a rigid surface is simulated and the influence of the parameter α on the jet buckling phenomenon is investigated. In addition, the simulation of the extrudate swell of a Giesekus fluid was carried out and the effect of the parameter α on the flow was similarly examined.

Extrudate swell ratio characteristics of CaCO3 added linear low density polyethylene

Four CaCO3 added LLDPE polymer systems were compounded in a 24 mm diameter, co-rotating, closely intermeshed twin-screw extruder. The compounds differed from each other by the weight percent of CaCO3 added. A laser beam system installed at the die exit of a capillary rheometer was used to determine the diameter of the swollen extrudate. Extrudate swell ratio, B∗, was found to decrease with increasing temperature, L/D ratio of the capillary die and the weight percent of the added CaCO3. Increasing apparent shear rate was, however, seen to cause B∗ to increase. An empirical correlation for predicting extrudate swell ratio was developed at apparent shear rates similar to those found in industrial processing conditions. The validity of this correlation was verified by measuring the extrudate swell ratios of the same polymer systems on a 36 mm diameter twin-screw extruder.

Numerical Studies for the Effects of Viscoelastic Constitutive Parameters on the Extrudate Swell of Plastic Micro-Tubes

The effects of four difference viscoelastic constitutive parameters, i.e., viscosity, relaxation time, ε and ξ on the extrudate swell of plastic micro-tubes were studied by using the numerical method. Numerical results show that the extrudate swell of plastic micro-tube increases with the increase of the relaxation time, but decreases with the increase of ε and ξ. In addition, the extrudate swell ratio of plastic micro-tube is not changed with the increase of the viscosity of melt. To ascertain the effect of four different viscoelastic constitutive parameters on the extrudate swell of plastic micro-tube, the physical field distributions, i.e., flow velocity, shear rate, shear stress, and first normal stress difference distributions of melt were obtained, respectively. Results show that the extrudate swell phenomenon of plastic micro-tube is closely dependent on the elastic energy storage of melt induced by the above mentioned physical field distributions, especially at the outlet of die.

Origin of Reduction in Extrudate Swell of Molten Polymer by Adding of Rigid Fillers

Origin of Reduction in Extrudate Swell of Molten Polymer by Adding of Rigid Fillers

Numerical simulation on the influence of geometric size on the swell for traditional and gas-assisted extrusion of plastic micro-pipe

In this paper, to study the influence of geometric size on the swell of extrusion forming of plastic micro-pipe, we performed the numerical simulation based on the finite element software Polyflow. The geometric models of plastic micro-pipes with different inner diameters but same wall thickness were established. Under the same boundary conditions and material parameters, the extrusion forming and swell ratios of plastic micro-pipes with different sizes were obtained. At the same time, the numerical results of traditional extrusion forming were compared with that of the gas-assisted extrusion forming. Numerical results show that the swell ratio of outer diameter and wall thickness increase with the decreasing of the geometric size for the traditional extrusion, but the extrudate swell of plastic micro-pipe is well eliminated by using the gas-assisted method.

Surface treated bagasse fiber ash on rheological, mechanical properties of PLA/BFA biocomposites

Abstract The use of silica based bagasse fiber ash (BFA) as a reinforcing filler in polylactic acid (PLA) biocomposites was examined. The effects of surface-treated BFA on the rheological, swelling behavior, and mechanical properties and water absorption of biocomposites were studied. BFA was treated using a silane coupling agent (Bis-[3-(triethoxysilyl)-propy]-tetrasulfide). Composites with BFA were varied from 5 to 25wt.%. The PLA/BFA composite melts were pseudoplastic non -Newtonion fluid and exhibited a shear thinning behavior. The viscosity of the surface-treated BFA biocomposites was higher than that of the untreated. The viscosity of the composites decreased with increasing BFA content and shear rate. The extrudate swell of the composites increased with increasing shear rate, whereas it decreased with increasing BFA content and die temperature. The extrudate swell tended to be suppressed when surface-treated BFA was used. Silane treated BFA composites showed improvement in their mechanical properties, and exhibited significantly reduced water absorption.

Pinch-off masses of very viscous fluids extruded from dies of arbitrary shape

The extrusion of slender very viscous fluid cylinders from dies of arbitrary geometry is modeled to approximate the mass of the first drop to pinch off. The model neglects inertia, which, although important to the dynamics near pinch-off, does not have a significant impact on the drop mass. Extrudate swell is also assumed to be negligible. The model is able to compute the pinch-off mass for fluid cylinders with cross-sectional geometries of any connectivity. By way of illustration, pinch-off masses are computed for epicycloidal cross sections, two touching circular rods, and circular cylinders with a single circular hole placed both centrally and off-centre. It is shown that the drop mass may be controlled by altering either the extrusion conditions or the die geometry.

Melt die-swell behavior of polyoxymethylene blended with ethylene-vinyl acetate copolymer and high-density polyethylene

The influence of extrusion conditions (temperature ranging from 170 to 220 °C, loads varying from 1.2 to 12.5 kg) and high-density polyethylene (HDPE) content on swelling behavior of polyoxymethylene (POM)/ethylene-vinyl acetate copolymer (EVA)/HDPE blend melts was examined. It was found the melt die-swell ratio of the blends decreased approximately linearly with rise in temperature, while it increased as a quadratic function with increasing shear rate or shear stress. The melt die-swell ratio increased with increasing HDPE weight fraction under constant test temperature, and the effect of temperature on the sensitivity of the melt die-swell ratio to the HDPE weight fraction was insignificant, while the effect of load on the melt die-swell ratio was significant. The correlation between the melt die-swell ratio of the blends and the HDPE weight fraction followed a quadratic relation. The melt extrudate swell behavior mechanisms of the blends during the die extrusion flow are discussed.

Prediction of the Fiber Orientation State and the Resulting Structural and Thermal Properties of Fiber Reinforced Additive Manufactured Composites Fabricated Using the Big Area Additive Manufacturing Process

Recent advances in Fused Filament Fabrication (FFF) include large material deposition rates and the addition of chopped carbon fibers to the filament feedstock. During processing, the flow field within the polymer melt orients the fiber suspension, which is important to quantify as the underlying fiber orientation influences the mechanical and thermal properties. This paper investigates the correlation between processing conditions and the resulting locally varying thermal-structural properties that dictate both the final part performance and part dimensionality. The flow domain includes both the confined and unconfined flow indicative of the extruder nozzle within the FFF deposition process. The resulting orientation is obtained through two different isotropic rotary diffusion models, the model by Folgar and Tucker and that of Wang et al., and a comparison is made to demonstrate the sensitivity of the deposited bead’s spatially varying orientation as well as the final processed part’s thermal-structural performance. The results indicate the sensitivity of the final part behavior is quite sensitive to the choice of the slowness parameter in the Wang et al. model. Results also show the need, albeit less than that of the choice of fiber interaction model, to include the extrudate swell and deposition within the flow domain.

Numerical studies of temperature effect on the extrusion fracture and swell of plastic micro-pipe

Temperature is a key factor that impacts extrusion forming quality of plastic micro-pipe. In this study, the effect of temperature on extrusion fracture and swell of plastic micro-pipe was investigated by numerical method. Under a certain of the melt’s flow volume, the extrusion pattern, extrusion swelling ratio of melt are obtained under different temperatures. Results show that the extrusion swelling ratio of plastic micro-pipe decreases with increasing of temperature. In order to study the reason of temperature effect, the physical distributions of plastic micro-pipe are gotten. Numerical results show that the viscosity, pressure, stress value of melt are all decreased with the increasing of temperature, which leads to decrease the extrusion swell and fracture phenomenon for the plastic micro-pipe.

A technique to remove the tensile instability in weakly compressible SPH

When smoothed particle hydrodynamics (SPH) is directly applied for the numerical simulations of transient viscoelastic free surface flows, a numerical problem called tensile instability arises. In this paper, we develop an optimized particle shifting technique to remove the tensile instability in SPH. The basic equations governing free surface flow of an Oldroyd-B fluid are considered, and approximated by an improved SPH scheme. This includes the implementations of the correction of kernel gradient and the introduction of Rusanov flux into the continuity equation. To verify the effectiveness of the optimized particle shifting technique in removing the tensile instability, the impacting drop, the injection molding of a C-shaped cavity, and the extrudate swell, are conducted. The numerical results obtained are compared with those simulated by other numerical methods. A comparison among different numerical techniques (e.g., the artificial stress) to remove the tensile instability is further performed. All numerical results agree well with the available data.

Effect of geometric scale on the extrudate swell of plastic micro-tubes

In this study, the effect of geometric scale on the extrudate swell of plastic micro-tubes was numerically studied by using the finite element method. The four geometric models with same inner diameter but different wall thicknesses were used. Under the same boundary conditions, material parameters, and numerical algorithms, the extrudate swells and flow field distributions of two plastic micro-tubes were obtained. Study results show that the extrudate swell effect of plastic micro-tubes increase with the decreasing of geometric scale. Finally, the mechanisms of the geometric scale on the extrudate swell for the plastic micro-tubes were analyzed.

Numerical study on the effect of the die compression angle on extrusion forming of plastic pipe

The effect of compression angle of die on the extrusion forming of plastic pipe was numerically studied by using the finite element method. Based on the geometric models of plastic pipes with four different compression angles, the extrudate swell results of plastic pipe under the different compression angles are numerically simulated. Numerical results show the extrudate swell phenomenon for the little compression angles are larger than that of the large compression angles. To eliminate the extrudate swell effect and keep the wear-resistant strength of the plastic pipe, the gas-assisted extrusion method was used in this paper. The numerical results fully show that the shear stress of melt was greatly eliminated and the high pressure was still maintained by using the gas-assisted method.

Numerical simulation of the planar extrudate swell of pseudoplastic and viscoelastic fluids with the streamfunction and the VOF methods

We present an Eulerian free-surface flow solver for incompressible pseudoplastic and viscoelastic non-Newtonian fluids. The free-surface flow solver is based on the streamfunction flow formulation and the volume-of-fluid method. The streamfunction solver computes the vector potential of a solenoidal velocity field, which ensures by construction the mass conservation of the solution, and removes the pressure unknown. Pseudoplastic liquids are modelled with a Carreau model. The viscoelastic fluids are governed by differential constitutive models reformulated with the log-conformation approach, in order to preserve the positive-definiteness of the conformation tensor, and to circumvent the high Weissenberg number problem. The volume fraction of the fluid is advected with a geometric conservative unsplit scheme that preserves a sharp interface representation. For the sake of comparison, we also implemented an algebraic advection scheme for the liquid volume fraction. The proposed numerical method is tested by simulating the planar extrudate swell with the Carreau, Oldroyd-B and Giesekus constitutive models. The swell ratio of the extrudates are compared with the data available in the literature, as well as with numerical simulations performed with the open-source rheoTool toolbox in OpenFOAM®. While the simulations of the generalized Newtonian fluids achieved mesh independence for all the methods tested, the flow simulations of the viscoelastic fluids are more sensitive to mesh refinement and the choice of numerical scheme. Moreover, the simulations of Oldroyd-B fluid flows above a critical Weissenberg number are prone to artificial surface instabilities. These numerical artifacts are due to discretization errors within the Eulerian surface-capturing method. However, the numerical issues arise from the stress singularity at the die exit corner, and the unphysical predictions of the Oldroyd-B model in the skin layer of the extrudate after the die exit, where large extensional deformations occur.

Controlling the extrudate swell in melt extrusion additive manufacturing of 3D scaffolds: a designed experiment

Tissue engineering using three-dimensional porous scaffolds has shown promise for the restoration of normal function in injured and diseased tissues and organs. Rigorous control over scaffold architecture in melt extrusion additive manufacturing is highly restricted mainly due to pronounced variations in the deposited strand diameter upon any variations in process conditions and polymer viscoelasticity. We have designed an I-optimal, split-plot experiment to study the extrudate swell in melt extrusion additive manufacturing and to control the scaffold architecture. The designed experiment was used to generate data to relate three responses (swell, density, and modulus) to a set of controllable factors (plotting needle diameter, temperature, pressure, and the dispensing speed). The fitted regression relationships were used to optimize the three responses simultaneously. The swell response was constrained to be close to 1 while maximizing the modulus and minimizing the density. Constraining the extrudate swell to 1 generates design-driven scaffolds, with strand diameters equal to the plotting needle diameter, and allows a greater control over scaffold pore size. Hence, the modulus of the scaffolds can be fully controlled by adjusting the in-plane distance between the deposited strands. To the extent of the model’s validity, we can eliminate the effect of extrudate swell in designing these scaffolds, while targeting a range of porosity and modulus appropriate for bone tissue engineering. The result of this optimization was a predicted modulus of 14 MPa and a predicted density of 0.29 g/cm3 (porosity ≈ 75%) using polycaprolactone as scaffold material. These predicted responses corresponded to factor levels of 0.6 μm for the plotting needle diameter, plotting pressure of 2.5 bar, melt temperature of 113.5 °C, and dispensing speed of 2 mm/s. The validation scaffold enabled us to quantify the percentage difference for the predictions, which was 9.5% for the extrudate swell, 19% for the density, and 29% for the modulus.

Transient Swell of a High Density Polyethylene Using Adjustable Gap Slit Die

Abstract A novel slit die with an adjustable gap is designed to perform transient swell measurements while changing the die gap dynamically. A generalized control model is developed to predict time-dependent swell using characteristic relaxation time and corresponding steady state swell as model parameters. Further, a series of slit dies are designed and used to measure steady state thickness swell of a high-molecular-weight blow-molding grade high density polyethylene resin under various operating and geometrical conditions. A generalized expression for steady state thickness swell is obtained by applying multi-variable nonlinear regression on thus obtained steady state thickness swell data, and is used in the empirical control model to predict transient extrudate swell.

Study on the Viscoelastic Fluid Behavior in Hollow Fiber Membrane Fabrication Process Using Giesekus’ Model

In the present study, attempts are made to study the behavior of a viscoelastic fluid in hollow fiber membrane production process. For this purpose, the fluid is considered to obey the Giesekus’ model and flow is taken as steady, two-dimensional and isothermal flow. The flow field is obtained using DEVSS/SUPG scheme combined with a free surface tracking method. The obtained results are compared to the case of Newtonian fluid. It is shown that the extrudate swell is 20% more in the case of viscoelastic fluid. Moreover, the elastic behavior of the viscoelastic fluid significantly affects the velocity field in the extrudate outside the die. It causes the fluid flow to become reversed in the middle part of the extrudate as the direct result of the stress relaxation process, whereas such a behavior is not seen for Newtonian fluid. Also, the first normal stress difference (FNSD) distribution in the extrudate is obtained and its variation is discussed. Furthermore, the effect of operational parameters including die back pressure and take-up velocity are investigated. Finally, the predicted extrudate swell for various operation conditions is compared to experimental results showing a good agreement between experimental and simulated data.