Volume 37, Issue 4

John Vlachopoulos (JV) started his polymer processing career with the process of calendering. In two landmark papers with Kiparissides, C. and Vlachopoulos, J. (1976). Finite element analysis of calendering. Polym. Eng. Sci. 16: 712–719; Kiparissides, C. and Vlachopoulos, J. (1978). A study of viscous dissipation in the calendering of power-law fluids. Polym. Eng. Sci. 18: 210–214 he introduced the Finite Element Method (FEM) to solve the governing equations of mass, momentum, and energy based on the Lubrication Approximation Theory (LAT). This early work was followed by the introduction of wall slip (with Vlachopoulos, J. and Hrymak, A.N. (1980). Calendering poly(vinyl chloride): theory and experiments. Polym. Eng. Sci. 20: 725–731). The first 2-D simulations for calendering PVC were carried out with Mitsoulis, E., Vlachopoulos, J., and Mirza, F.A. (1985). Calendering analysis without the lubrication approximation. Polym. Eng. Sci. 25: 6–18. In the intervening 35 years, other works have emerged, however our understanding has not been drastically improved since JV’s early works. Results have also been obtained for pseudoplastic and viscoplastic fluids using the general Herschel-Bulkley constitutive model. The emphasis was on finding possible differences with LAT regarding the attachment and detachment points of the calendered sheet (hence the domain length), and the extent and shape of yielded/unyielded regions. The results showed that while the former is well predicted by LAT, the latter is grossly overpredicted. More results have been obtained for 3-D simulations, showing intricate patterns in the melt bank. Also, the transient problem has been solved using the ALE-FEM formulation for moving free-boundary problems. The results are compared with the previous simulations for the steady-state and show a good agreement. The transient simulations capture the movement of the upstream and downstream free surfaces, and also provide the attachment and detachment points, which are unknown a priori . Finding these still remains the prevailing challenge in the modeling of the calendering process.

A one-step ultrasonic film casting process to manufacture nanocomposite films was developed, in which polycarbonate (PC) was mixed with multi-walled carbon nanotubes (CNT) and cast into films in one process. Numerical and experimental investigations of necking phenomenon were carried out for film casting of PC/CNT composites. Experimental results revealed that the necking along film line decreased with imposition of ultrasound and increasing CNT content, indicating that incorporation of CNT and imposition of ultrasound restrained the elongational flow behavior of melt, resulting in film of a larger width. Isothermal and nonisothermal numerical simulations of the process were performed. In isothermal simulations, the polymer melt was assumed to be maintained at the die temperature. In nonisothermal simulations, the temperature change along the film line was determined from heat transfer calculations with the WLF temperature-dependent viscosity. The simulated and experimental results on normalized film width, defined as a ratio of cast film width to die width, as a function of the distance from the die at various extension ratios were compared. The comparison indicated that changes in film width and thickness along the stretching direction in the nonisothermal process were in better agreement with experimental results than that in the isothermal process. Both experimental and simulated results showed a decrease of film width with take-up speed. Due to the presence of edge effect, the film width in experiment was lower than the simulated one. With incorporation of CNT, a better agreement between experimental and simulated results was obtained, due to a reduced edge effect in the film.

In this study, properties of polypropylene/graphite nanoplatelets (PP/GNP) composites and corresponding micromoldings were systematically studied in terms of filler loading concentrations and mixing methods. PP of different forms, i.e., PP pellets and powders, were adopted to fabricate PP/GNP composites. Additionally, a comparative study of precoating GNP and PP powders using solvent-based solution blending and ultrasonication-assisted mixing was performed. Results showed that PP/GNP composites prepared using powder form PP resulted in at least one order of magnitude higher electrical conductivity than using pellet form PP and further reduced the percolation threshold from 12.5 to 10 wt%, which was related to the state of filler distribution within corresponding moldings. Morphology observations revealed that microparts prepared with powder-PP/GNP composites exhibited less preferential alignment of GNP particles along the flow direction when compared with those molded using pellet-PP/GNP counterparts, which was helpful in improving the overall electrical conductivity for PP/GNP micromoldings.

This paper aims at a better understanding of the polypropylene (PP) physical extrusion foaming process with the objective of obtaining the lowest possible foam density. Two branched PPs were compared to the corresponding linear ones. Their shear and elongation viscosities were measured as well as their crystalline properties. Trials were conducted in a single screw extruder equipped with a gear pump and a static mixer cooler to adjust the melt temperature at the final die. The effect of decreasing this temperature on the PP foamability and on the pressure drop in the die was analyzed. The foam density of branched PPs varies from high to low values while decreasing the foaming temperature. In the same processing conditions, the foam density of linear PPs does not decrease so much, as already evidenced in the literature. The foamability transition coincides with an increase of the pressure drop in the die. The originality of the work lies in the thermomechanical analysis of the polymer flow in the die which allows the identification of the relevant physical phenomena for a good foamability. The comparison of the experimental pressure drops in the die and the computed ones with the identified purely viscous behavior points out the influence of the foaming temperature and of the PP structure. At high foaming temperature the discrepancy between experimental measurements and the computed pressure drops remains limited. It increases when decreasing the foaming temperature, but the mismatch is much more important for branched PPs than for linear ones. This difference is analyzed as a combination of the activation energy of the viscosity, the elongational viscosity in the convergent geometry of the die which is much more important for branched PPs than for linear ones, and the onset of crystallization which occurs at higher temperature for branched PPs than for linear PPs.

With the aim of improving the thickness gradient of thermoformed parts, and thus increase their performance and/or reduce their weight, the concept of hybrid thermoforming is introduced, whereby local thickness differences in extruded sheets are created prior to thermoforming. Material is removed by CO 2 ablation or 3D printing of an over-thickness at specific locations of sheets previously extruded. The feasibility and potential usefulness of the approach is explored experimentally for the production of a truncated conical cup, since this is a well-characterized application. The conventional thickness distributions obtained by conventional vacuum forming are significantly changed with the new strategy and can be tuned by adequately selecting the locations and amount of material do be removed and/or added.

The application of 3D printing of thermoplastics by Material Extrusion (MatEx) has commonly been limited by their poor mechanical strength that results from voids and weak interfaces between printed layers. Here, we demonstrate that core–shell structured filaments made of polycarbonate-based thermoplastics can achieve synergistic improvement in their interfacial bonding from the combination of high-glass transition temperature ( T g )/high-viscosity core and low- T g /low-viscosity shell. Tensile strength along the printing direction was enhanced with the core–shell filaments. Layer-interfacial bonding strength as determined by Izod impact tests of the 3D printed parts is significantly improved by using filaments either with only a core–shell T g mismatch or both T g /viscosity core–shell mismatch. The mechanical behavior can be rationalized in terms of improved inter-layer molecule diffusion by a low T g /viscosity shell, better printability at higher temperature due to the core with higher melt strength, and better bulk mechanical strength of high-viscosity/ T g core.

The production of thermoplastic polyurethane-based porous heterostructures combining physical foaming with fused deposition modeling is detailed in this contribution. The choice of combining these two techniques lies in the possibility of creating objects endowed with a dual-scale structure at millimeter scale by fused deposition modeling and at microscopic scale by gas foaming. Thermal stability and rheological properties of the neat polymer were studied prior to foaming to design a suitable processing protocol and three different combined techniques are proposed: pressure quench, temperature rise and direct 3D foam printing. Foam morphologies were evaluated by SEM and foamed samples were characterized by thermal and mechanical analyses to highlight the differences among the combined processing techniques. Samples foamed via pressure quench exhibit the highest degree of crystallinity and a uniform cell morphology, also resulting in the largest stiffness. The results presented in this contribution open up the possibility of producing objects with complex geometry and porosity architecture at the dual scale.

A surfactant-free oil-in-oil emulsion-templating method is presented for fabrication of monolithic polyimide aerogel foams using monomer systems that produce fast sol–gel transition. An aerogel foam is a high porosity (∼90%) material with coexisting meso- and macropores inherent to aerogels with externally introduced micrometer size open cells (macrovoids) that are reminiscent of foams. The macrovoids are introduced in polyimide sol using surfactant-free emulsion-templating of droplets of an immiscible liquid that are stabilized against coalescence by fast sol–gel transition. Three immiscible liquids – cyclohexane, n -heptane, and silicone oil – are considered in this work for surfactant-free emulsion-templating. The aerogel foam monoliths, recovered by supercritical drying, exhibit smaller size macrovoids when n -heptane and cyclohexane are used as emulsion-templating liquid, while the overall porosity and the bulk density show weak dependence on the emulsion-templating liquid.

Flow erosion and part deformation are unsolved molding problems that restrict the overall success of film insert molding. This work investigated, both experimentally and numerically, the factors that affect flow erosion and part deformation in film inset molded products. Three plate-with-thickness-variation geometries, namely flat, thin-to-thick, and thick-to thin, were molded for the products. Polystyrene films and polyethylene terephthalate (PET) resins were employed in the experiments. It was found that the thin-to-thick specimens exhibited the most severe flow erosion. Increasing the injection pressure or melt temperature worsened flow erosion. Meanwhile, for the processing parameters adopted in the experiments, part deformation generally increased with melt temperature and hold time, while it decreased with injection pressure and hold pressure. Additionally, a numerical software (Moldex ® 3-D) was employed to simulate the temperature and shear stress distributions in molded products. The calculated results suggested that part deformation in insert molded products results mainly from the non-uniform temperature profile during the cooling stage, owing to the product configuration and the insert film, while flow erosion is induced by the high shear stress of the polymer melt in the filling stage.

External gear pumps are used in fluid transport systems because of their tight clearances and accurate flow control. These tight clearances are a challenge for numerical studies in terms of spatial discretization. In earlier work, the flow of a viscous fluid in an external gear pump is computed using the finite element method (FEM). An element size based on the respective distance between boundaries is proposed. In this study, results based on the earlier work are compared to extrusion experiments of EPDM. The aim of this study is not only to validate the numerical simulations, but also to determine what material characteristics need to be taken into account for an accurate output prediction of the external gear pump. Especially the introduction of shear-thinning behavior results in an improvement of the amplitude of the pressure difference fluctuation. Taking into account compressibility, alters the torque fluctuation in such a way that it mimics the experiments. Unfortunately, the fluctuation in torque still has a too high amplitude. Eventually, simulations are performed including shear-thinning behavior, a temperature- and pressure-dependent viscosity, and compressibility. The effect of measuring the material behavior using oscillatory or shear experiments is shown. Furthermore, the simulations are applied to a second EPDM. Finally, different processing conditions are tested. For the simulations, only qualitative agreement is found, possibly as a result of the no slip boundary condition.