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SPE Library content related to rheology
High Strain Rate Testing and Modeling of Polymers for Use in Finite Element Simulations
Finite element analysis plays a crucial role in modern engineering problems, enabling engineers to predict the response of designed parts at any point in the design process. Specifying a constitutive model that accurately captures the mechanical response of a polymer material is paramount to obtaining useful results. In order to understand the capabilities of commercial FE packages used to simulate problems involving polymers, we have tested the uniaxial response of polyamide in tension and compression over six decades of strain rate. We then calibrated four constitutive models to the experimental data: an Abaqus Parallel Rheological Framework model, the LS-DYNA SAMP-1 model, the ANSYS Bergström-Boyce model, and the PolyUMod Three Network model. We compared the performance of the four models in predicting the experimental data; the Three Network model had the lowest error. Additionally, we compared the runtime of a simple test case for each model; the ANSYS Bergström-Boyce model being the fastest.
The Rheology of Concentrated Slurries: Experimental Evaluation and the Effects on Polymer Processing
Highly filled polymer compounds can present processing challenges, including high screw shaft torque, energy consumption, die pressure and melt temperature rise. Previous theoretical development and experimental evaluations of highly filled polymer melts showed that the rheology can be described with a percolation model [1-4]. This paper re-evaluates a batch mixer characterization method used to measure the effects of filler concentration on melt processing. The experimental results are compared with capillary rheometer measurements using several low-density polyethylene resins, calcium carbonate and titanium dioxide. The theoretical treatment of the rheology as a particulate percolating system with power-law behavior is used to analyze rheometer and batch mixer data. The effects of resin molecular weight, filler type and size on rheology and melt processing are described.
Rheological Properties of Polyethylene Blend with Poor Mixing
The effect of mixing condition on flow instability at capillary extrusion was studied using linear low-density polyethylene (LLDPE) blends. Two types of LLDPE with different molecular weights were blended by various mixing devices and conditions. It was found that the onset of flow instability is sensitive to the mixing method even though their linear viscoelastic properties are almost identical. The blend obtained by poor mixing conditions shows shark-skin failure even at a low shear stress, although the blend prepared by intensive mixing provides smooth surface at the same shear stress. This is attributed to the low onset shear stress of shark-skin failure for the blend prepared by poor mixing. Furthermore, a blend by poor mixing is found to show a significantly low value of the maximum draw ratio at hot-stretching. The result suggests the existence of mechanically-weak points, which leads to cohesive failure at strand surface by the abrupt stretching at the die exit, i.e., the shark-skin failure.
Simulative Evaluation of the Temperature Influence on Different Types of Pre-Distributors in Spiral Mandrel Dies
Thermal inhomogeneities in spiral mandrel dies, which occur especially in the pre-distributor, can lead to an uneven flow distribution despite a rheologically optimized design of the die. Against this background an integrative thermal and rheological flow simulation has been developed at the IKV, in which the whole pre-distributor can be modelled non-isothermally. The simulation takes both the non-linear flow behavior of the melt and the thermal phenomena in the die material into account. In this contribution, the developed simulation model is used to evaluate and compare the temperature influence on the melt distribution in three different types of pre-distributors. These are a 23-pre-distributor of a radial spiral mandrel die, a 24-pre-distributor of an axial spiral mandrel die and a star pre-distributor with vertical redirection. The simulations show that in case of the 23- and 24-pre-distributor, both the external tempering of the die and the dissipative shear heating lead to an uneven temperature distribution in the melt and thus cause an inhomogeneous melt pre-distribution. In case of the star pre-distributor, the die tempering has no significant effect on the flow distribution. However, the dissipation leads to an uneven heat-up of the melt in the area of the redirection, which results in an uneven melt flow at the outlets of the pre-distributor. In the next step, thermal design measures are introduced into the pre-distributors in order to homogenize the flow rate distribution at the outlets of the pre-distributors. By integrating heater cartridges, brass inserts and insulating gaps into the die, a more homogeneous flow rate distribution at the outlet of each pre-distributor can be achieved.
Improved Injection Molding of Ultra-High Molecular Weight Polyethylene Using Supercritical Nitrogen
Ultra-high molecular weight polyethylene (UHMW-PE) was injection molded using a microcellular injection molding (MIM) process to introduce supercritical nitrogen (SC-N2) into the melt to decrease the viscosity of the polymer and improve processability while reducing degradation. Solid and foamed parts were produced. Rheological tests indicated that a viscosity reduction during processing decreased the material’s tendency to degrade during injection molding. Although the SC-N2 processing did not improve the tensile strength of the molded parts, it significantly improved the processability of UHMW-PE via injection molding. Micro-computed tomography (µCT) images illustrated the internal structures of the parts and revealed sink marks in the solid formed SC-N2 processed samples, even when packing pressure was applied.
Applied Rheology for Characterization of Nanofiber Based Filters
Full 3D polydisperse particle filtration modeling at low pressures has been performed for a polyurethane nanofiber based filter prepared via electrospinning process in order to more deeply understand the filter clogging and the cake formation. In this work, realistic SEM image based 3D filter model, transition/free molecular flow regime, Brownian diffusion, aerodynamic slip, particle-fiber and particle-particle interactions together with a Euclidian distance map based methodology to calculate the pressure drop have been utilized. Model predictions have been compared with relevant experimental data in order to validate the used assumptions, methodologies and numerical scheme. The effect of particle-particle as well as particle-fiber interactions on the nanofiber based filter efficiency, pressure drop and the quality factor during the filter clogging has been investigated in more detail.
On the Role of Extensional Rheology, Elasticity and Deborah Number on Neck-In Phenomenon during Flat Film Production
In this work, viscoelastic, isothermal extrusion film casting modeling utilizing 1D membrane model and modified Leonov model was performed in order to understand the role of planar and uniaxial extensional viscosities, extensional strain hardening, Deborah number and die exit stress state (captured here via the second to first normal stress difference ratio –N2/N1). It has been found that the neck-in can be expressed via simple set of dimensionless analytical equations utilizing all above mentioned variables, and thus providing detail view into complicated relationship between polymer melt rheology, die design, process conditions and unwanted neck-in phenomenon.
Influences of Molecular Structure on the Rheological Properties and Foamability of Modified Polypropylene
This work combined the grafting maleic anhydride(MAH) onto polypropylene (PP) and the coupling reaction between diamine and MAH grafted PP (PP-g-MAH) into a single step through a twin screw extruder. Detailed molecular weight analysis, rheological characterization and foaming tests were conducted subsequently. The investigation indicated that the concentration of reagents plays a key role in control of the chain structure. By the combination of SEC and rheological analysis, the optimum amount of MAH and diamine for preparing LCB-PPs is decided. However, the optimum peroxide loading during branching modification is not clear and need further evidence. To solve this problem, a foaming test was carried out to assess the performance of the modified PP with different peroxide loading. The results demonstrate that an intermediate level of modification (peroxide concentration, 0.2-0.4 wt%) is already sufficient for the optimization of foaming process.
Development of High Thermal Insuation Polypropylene Foams Blown in Injection Molding with Mold Opening
Polypropylene (PP) foams with a low thermal conductivity (less than 40 mW/m·K) and a low density (0.1-0.2 g/cm3) were fabricated by the foam injection molding technology with mold opening while using CO2 as a blowing agent. PTFE fibrils manufactured by in-situ fibrillation using a co-rotating twin screw extruder were used to improve the melt strength and the strain hardening property. The crystallization behavior and the rheological properties were studied, to demonstrate that the dispersed PTFE fibrils effectively enhanced the crystallinity and, thereby, increased the melt strength, and induced a strain hardening behavior. When foamed in injection molding, the fibrillated PTFE containing PP showed much more improved foaming behavior. The thermal conductivity mainly depended on the expansion ratio of foam, although the quality of the cells (i.e., the size and uniformity) also influenced those properties.
A Revised Approach to Rheological Behaviour and Processing Parameters of Polycarbonate Compound with Dispersion
The objective of this work is to study the rheological characteristics of the compound of polycarbonate resins with different melt flow indexes and the affects of the processing parameters PC1 content (30wt%-pph) of MFI (25gm/10mins) and PC2 content (70 wt. %-pph) of MFI (6.5gm/10mins). By understanding the relationship between shear rate and viscosity, it becomes possible to define the viscosity model and exact color shifts. The temperature was varied at three stages (230°C, 255°C and 280°C) to study its effect on rheological characteristics, colour differences (dE*), pigment size distribution and dispersions.
Toughening polyoxymethylene by polyolefin elastomer and compatibilizer
Glycidyl methacrylate-grafted high-density polyethylene improved the compatibility of polyoxymethylene and polyolefin elastomer, in turn enhancing the properties of polyoxymethylene.
Development of stable polymeric structures for harsh oil-reservoir conditions
Understanding the structure-function relationship in polymeric materials enables the development of stable products to optimize hydrocarbon recovery under harsh conditions.
SPE Applied Rheology Division April 2017 Newsletter
Read the latest issue of the SPE Applied Rheology Division newsletter.
Simulating ultrasonic decrosslinking of crosslinked high-density polyethylene
Process modeling results are validated with experimental twin-screw extrusion measurements.
Crystalline polycarbonate articles fabricated by hot powder compaction
The feasibility of a novel method is demonstrated by producing crystalline polycarbonate with a defined shape from polycarbonate for the first time.
Characterizing the bulk viscosity of polypropylene melt
Uniaxial compression experiments show that the bulk viscosity has a higher magnitude than shear viscosity and should therefore not be neglected during flow analysis of the injection-molding process.
Polytetrafluoroethylene as a suitable filler for poly(lactic acid) composites
The mechanical, rheological, and foaming properties of melt-blended samples were investigated.
Investigating radiation effects in polymethacrylimide sandwich structure composites
Upon radiation exposure, polymer fragmentation causes a sharp degradation in the mechanical and thermal properties of Rohacell foams.
Interfacial nanoreinforcement improves bamboo fiber/polymer composites
A fully biobased composite material, composed of bamboo fibers and a nanoparticle-reinforced bio-epoxy matrix, exhibits improved mechanical properties.
Microcellular injection-molded blends of polylactic acid and poly(e-caprolactone)
Enhancing the complex viscosity of polylactic acid/poly(e-caprolactone) blends leads to better foaming behavior and pore microstructure.
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