Volume 20, Issue 3

For a given compounding application the selection of the type of processing equipment, the geometry and the operating conditions is always an important challenge. The outcome of this selection process significantly affects the economics of the process, as well as the microstructural distributions and hence the ultimate properties of the compound. Here mixing indices, defined on the basis of the statistics of the distributions of the concentrations of a plasticized elastomeric binder and graphite filler, were used to quantitatively characterize and compare the degree of mixedness of products of batch mixing, co-rotating and counter-rotating fully-intermeshing twin screw extrusion processes. Two separate experimental methods, involving the wide-angle x-ray diffraction and thermo gravimetric analyses, were used for the characterization of the mixing indices at two different scales of examination. The mixing indices were sensitive to changes in geometry and operating conditions. The resulting electrical properties of the conductive composite samples, mixed with either the batch or continuous, i. e., using either co-rotating or counter-rotating twin screw extrusion, correlated with their degree of mixedness. The presented methodologies should facilitate more accurate comparisons of the performances of different types of processors and better tailoring of the structure and ultimate properties of filled polymers.

Finite element technique is used to model the free surface flow regime representing injection mould filling of elastomeric material such as rubber compounds. The most distinct feature of the constitutive behaviour of elastomers, is the influence of material elasticity on the elongation and shear deformation suffered by the fluid during flow. In the present study this feature has been tackled through the use of Criminale-Erickson-Fibley (CEF) model. A scheme based on the Volume of Fluid technique is developed for free surface tracking. The results represented in this paper have also been compared against the Phan-Thien/Tanner model and are shown to be in closer agreement with the theoretical expectations than those obtained by the P-T/T equation. The results prove the applicability of the developed model to industrially relevant situations.

This study is to investigate the fabrication of a mold insert with negative cylindrical dot micro features using the LIGA-Like process and molding of the component. The effects of molding conditions on the filling of the polymer melt into the negative features were experimentally studied. A parametric study of the effect of the molding conditions on the filling of microstructure in micro molding was conducted. Increase of the mold temperature or injection rate is an effective method to ensure the complete filling of the microstructure. The application of packing pressure was shown to improve the filling in the microstructure also. With the low aspect ratio used in this study (about 0.2), the microdot array can be completely filled with the right selections of injection rate and mold temperature without vacuum.

In our recent work, it was shown that the rate of filling during injection moulding did not significantly affect the tensile and impact properties of recovered and virgin polypropylene, whereas significant changes in the properties were observed after ageing at 100 °C. The purpose of the present work was to extend the former study to include the effects of processing and ageing on surface-appearance-related properties, in particular the gloss. In-mould rheology and gate-seal analyses were performed in order to select the filling and post-filling processing windows, respectively. The influences of filling rate, holding and cooling times on the gloss measured in the gate area and at the far end of the moulded specimens were evaluated. The mould used had an embossed surface giving test plaques with a matte appearance. The results showed that the gloss varied by about 30 to 40% within the filling window obtained by in-mould rheology. The slight gloss difference between the two PP-grades noted at the lowest injection speed vanished with increasing filling rate. Gloss differences were observed between the gate area and the far end of each processed moulding, again at the lowest injection speed. The materials processed at a high filling rate exhibited a fairly constant gloss as the holding time increased regardless of the position of the analysed region, whereas a more significant decrease was noted in the case of the mouldings processed at the low filling rate. The cooling time had a negligible effect on the gloss in comparison to the effect of filling rate. Ageing at 100 °C for 240 h increased the gloss level, especially at the far end of the moulding, but only for specimens manufactured at the lowest filling rate. Roughness measurements with a profilometer showed that the gloss development during processing and ageing was related to the topographical matching between the developing polymer surface and the mould topography.

This work is a part of a study aiming at assessing the processability of recovered polypropylene in relation to the mechanical and aesthetic performance of as-manufactured and heat aged mouldings. The effects of processing and heat ageing on the mechanical properties of injection-moulded recovered polypropylene were studied here. A similar grade of virgin polypropylene was used as a reference. Multipurpose test specimens were manufactured with a two-cavity mould, according to decoupled moulding procedures. The mouldings were aged at 100°C for 170 h and 340 h. The stiffness, the strength, the elongation at break and the Charpy impact resistance of as-moulded and heat-treated materials were investigated. The results show that the recovered polypropylene could be processed as the analogous virgin grade. Insignificant changes in the measured properties due to the different filling conditions were observed for each polypropylene used. The stiffness and strength of the recycled polypropylene was systematically 20% lower than that of the analogous virgin grade, whereas the impact resistance of the recycled grade was on average 40% higher than that for the virgin grade. There was no significant difference in the elongation at break of the two materials. After the short ageing period, an increase in stiffness and strength of the materials was observed, together with a sharp decrease in the elongation at break and a modest increase in the impact resistance. This was related to re-crystallisation. The long ageing period reduced the stiffness and strength of the materials but there was no significant change in the elongation at break or in the impact resistance.

In this paper, we present aspects and examples relating to Rem3D, a 3D finite element software dedicated to the computation of the entire injection molding cycle (filling, packing and cooling). Recent developments are presented, namely mesh generation and adaptation driven by a local natural metric, compressible Stokes and temperature solvers, as well as a multidomain approach. As an example of the application of Rem3D, computations have been compared with well instrumented experiments for a complex mold geometry.

An automatic optimization methodology based on Multi-Objective Evolutionary Algorithms (MOEA) is proposed to optimize the mechanical properties of injection molded products. For that purpose an MOEA is linked to a commercially available mould filling computer simulator code, CMOLD. The developed optimization methodology was applied to a practical case study where the processing conditions are optimized as a function of a desired morphological state (skin thickness, degree of crystallinity and/or level of molecular orientation; these latter are interpreted in terms of thermomechanical indices) and envisaged mechanical performance at two strain rates levels (of the order of 10 –4 and 10 2 s 1 ). Generally, the optimization methodology proposed is able to take into account the influence of the process parameters and produce solutions that are physically consistent. The processing conditions maximizing the morphological state of the moldings and their mechanical performance are identified. This latter is firstly maximized individually for distinct strain-rates and then a simultaneous optimization for both low and high strain-rate response is performed. The optimized sets of processing conditions leading to the maximization of the mechanical performance are analyzed by the computation of two thermomechanical indices aiming at interpreted the resultant morphological state of the moldings.

The relation between PVT and rheological measurements of several polymer melts including polyethylenes, polypropylene, polystyrene, poly(methyl methacrylate), and polycarbonate has been taken into investigation. Pressure-temperature dependent viscosities, determined on rotational and backpressure-modified capillary rheometers, were fitted through the Carreau-Yasuda model. PVT data was analyzed by the help of the Simha-Somcynsky equation of state (SS EOS). The thermodynamical parameters of the SS EOS were connected to constant-stress viscosity (experimental) and zero-shear viscosity (extrapolated). The Doolittle relationship was modified into the form of η = exp(C1 ln(h′h)). The relation was employed and tested for the data evaluation. It proved to be a good tool for linearization of PVT and rheological data.

In PA12 tube extrusion, calibration or sizing is the key step of the process that affects subsequent mechanical properties. The extruded tube is pulled through a cylindrical calibrator under vacuum. A water flow rate is applied at the inner side of the calibrator, creating a lubricant water layer at the polymer outer surface. The scope of this article is to show how this lubrication influences the elongation at break of tubes through drawing kinematics of the polymer. Lubricant water layer thickness measurements and on-line video recording have been employed to monitor the lubrication dependence of the velocity profiles from the extruder die to the end of calibrator. Velocities were measured through three independent innovative methods and thirty-two calibration conditions have been carried out to validate our work. Three main calibration parameters were found to determine the water layer thickness: the level of vacuum applied in the calibration tank, the water flow rate at the calibrator entrance, and the line speed. The influence of each parameter on lubrication level was found out. Simultaneously, the draw ratio in the calibration tank was deduced from velocity profiles. This parameter was found to affect tensile properties and to depend strongly on the level of lubrication during calibration. We showed quantitatively that rising the water layer thickness leads to a diminution of the draw ratio in the calibration tank and an increase of the elongation at break. This implies that we are now able to optimize tensile properties by fitting the main calibration parameters to improve lubrication and restrict draw ratio in the calibration tank.

Elongation at break is one of the major end-use properties of polyamide 12 extruded tubes. It is strongly affected by the tube microstructure and the molecular orientation resulting from extrusion conditions. Molecular orientation was characterized by X-ray diffraction and birefringence evaluation in light microscopy. Measurements were carried out on (r, z) sections obtained by polishing and microtoming. On the other hand, polymer drawing was measured on line by tracer techniques. Calibration stage was determined as the key step of the process that generates orientation in tubes: as the tube is drawn through a cylindrical calibrator under vacuum and cooled from its outside surface, calibration leads to a highly oriented zone in the twenty external microns. Calibration conditions and elongation at break have been connected through orientation level in this region. Molecular orientation was found to strongly depend on the draw ratio in the calibration tank. Finally, birefringence of the tube external layers and elongation at break were successfully correlated. Elongation at break can be enhanced by reducing orientation resulting from calibration conditions.

The description of pressure-throughput behaviour of wall adhering melts in plastifying machines has been issued in a number of publications, e. g. [1 to 4] in details. Nevertheless there are a variety of polymer melts as well as elastomers, polymer suspensions, ceramic materials and food stuff, which do not behave wall adhering in the course of manufacturing. The theoretical discussion of the pressure-throughput behaviour has not sufficiently been described up to now. Worth's [5] case studies include the isothermal one-dimensional flow of a Newtonian medium in an unwinded screw channel. He assumed that after exceeding a critical shear rate the slipping process starts on the barrel wall only – but not on channel wall and screw root surface. Mennig, [6 to 9] took up this statement again and assuming a shear rate on the barrel wall as well as on the screw root surface he derived simple relations for the radial velocity profile from it. Lawal and Kalyon [10 to 12] have developed an analytical model that describes the behaviour of visco-plastic liquids in flat channels for different quotients of wall slippage, which are derived from the wall slippage quotients on screw root surface and on barrel wall. A mathematical model describing the flow behaviour for the two-dimensional Newtonian, isothermal case has been developed for wall slipping materials in order to describe the pressure-throughput behaviour multi-dimensionally. In addition to the development of the analytical calculating model the flow behaviour of wall slipping polymer melts has been analysed by using the finite-element-calculation (FEM). Comparison of the pressure-throughput behaviour results show a high degree of conformity in the calculation results.

An experimental study of mixing of various particulates into synthetic elastomers and polypropylene is presented. For the elastomers, a batch internal mixer and a commercial continuous mixer were used to disperse various small particles including silica, carbon black, and CaCO 3 . For the polypropylene, this same continuous mixer plus a Buss Kneader, co-rotating twin screw extruder and an intermeshing counter-rotating twin screw extruder with different modular screw configurations were used to follow the dispersion of CaCO 3 . The breakage of glass fibers in these machines was also followed. Generally the most severe breakage of agglomerates and glass fiber was by modular intermeshing counter-rotating screws with special Leistritz elements. A model was developed for breakdown of agglomerates and fibers. The agreement with experiment was promising.