Volume 14, Issue 2

A mathematical model of the mixing process in a two-rotor mixer is proposed. The model developed takes into account the interrelations among the following essential features of the process: non-isothermal effects due to heat exchange and energy dissipation in flow, non-Newtonian behavior of a material and dependence of its properties on temperature and the degree of filler agglomerate destruction, kinetics of changing carbon agglomerate sizes due to mechanical distraction. The system of governing equations is formulated and the procedure of its solution is described. The theoretical model is compared with experimental data obtained in real technological process of mixing of a rubber compound. In all cases quite satisfactory results of comparison were obtained. It means that the model proposed can be used for analysis of real technological processes of mixing and choice of optimal technological regime and geometrical sizes of a mixer (primarily size of the gap between a rotor and the wall of a chamber) as well.

Coated moulds, especially cavity surfaces and ejection parts are state of the art in plastics processing. Regarding the plastification unit i.e. screws and barrels coatings are not well established. Wear in processing remains a problem, especially when no abrasion is permitted e.g. in manufacturing optical elements. The surfaces must resist the abrasion of hard glas fibres as well as the corrosive attack of the polymers. Electroplated chrome is one of the answers to these demands, because of its hardness and corrosion resistance. Another solution are thin hard coatings which are not yet well established. Titanium nitride produced by PVD technique was found to show excellent abrasion resistance but often not sufficient corrosion properties. Chrome nitride was found to have great hardness like TiN, corrosion resistance behaviour similar to electroplated chrome and in contrast to electroplated coatings no tensile internal stress. The present study deals with the spectrum of characteristic properties of Cr x N coatings like hardness, adhesion, internal stress, chemical composition and phases produced by PVD magnetron sputtering. The coatings were deposited on steel, described in characteristics and tested in a model wear test with plasticized moulding material (PA 66-GF50 and others) simulating the tribological situation between screw and barrel of extruders or injection moulds. The tribological behaviour was studied after the complex test procedure. The high temperature of the plasticized material, high abrasion by glas fibres and corrosive influence could only change the coatings properties of Cr x N slightly.

Many everyday engineering applications crucially depend on the mixing efficiency of the appropriate hardware. Chaotic mixing is pertained to be an effective way of attaining high levels of mixing. In this work we report on viscous fluid flow in a cavity, with oscillating boundaries, that exhibits such chaotic motion. Using computational fluid dynamics we explain the transition from order to chaos which can be observed by a cascade of the trajectory orbit of particles as the time period of oscillation is increased. As the system moves from order into chaos it is shown that a particle will be displaced from ordered streamlines to the point where it moves throughout the whole low domain thus showing that good mixing is present. An attempt is also made to characterise the transition to aperiodicity, for the studied configuration.

An analysis of flow fields, pressure fields and screw pumping characteristics is presented for non-Newtonian fluids in various types of screw elements of an intermeshing counter-rotating twin screw extruder. These elements are in turn considered modules of a modular intermeshing counter-rotating twin screw extruder and an analysis of non-isothermal starved low in such a machine is presented. Pressure profiles, fill factor profiles and temperature profiles have been calculated for various modular screw designs of a Leistritz GG modular intermeshing counter-rotating twin screw extruder.

The bulk and dispersed phase polymerizations of urethane were investigated in a co-rotating twin screw extruder. The residence time has predominant influence on the bulk polymerization in twin screw extruders. Furthermore, the polyurethane polydispersity decreases going down the screws, as the result of the extent of the reaction and mixing in the extruder. As expected, the polyurethane polydispersity at the die exit of the extruder is close to the theoretical value (Ip = 2). In the case of urethane polymerization in molten polyethylene matrix, the residence time distributions showed that the two monomers (isocyanate and alcohols) have different behaviors in the extruder in presence of PE due to the solubility of isocyanate and non-solubility of alcohols in PE. Furthermore, this solubility phenomenon PE induces an imbalance stoichiometry which limits the degree of polymerization (molecular weight) and a broadening of the molecular weight distribution during the blending process in extruder. Slip phenomenon was also observed at the die because of the presence of polyurethane, with low molecular weights, at the sample surfaces. An in-situ pre-polymerizing step of isocyanate/alcohols tends to eliminate this phenomenon. Then, the aim of the present work was to study the formation of a polyethylene/polyurethane blend polymer via in situ polymerization of diols and diisocyanate monomers dispersed in a twin screw extruder.

An analytical solution is presented for wire coating by drawdown of an extruded annular melt. The solution is developed with the assumption that the pressures inside and outside the film are equal; a process in which these pressures differ, such as a vacuum inside the film, is treated as a limiting case.

Polypropylene blends, both mechanical blends and a dynamically vulcanized blend, have been melt spun into fibers. Melt spinning was carried out through air and in ice water. Melt spun filaments were characterized by wide angle x-ray diffraction (WAXS), birefringence and differential scanning calorimetry (DSC) measurements. Generally, the polypropylene crystalline orientation-spinline stress relationship is the same for the homoploymer, the blends and the dynamic vulcanizate. However, the blends and the dynamic vulcanizate have much lower birefringence apparently due to a lack of orientation in the rubber phase. It was shown that the PP contribution to the birefringence for all the systems is the same at the same spinline stress.

Numerical simulation of blown-film process of plastics provides a tool for air ring designs and suitable polymer properties. Although cooling-air plays a key role in the process ability and film properties, there have been few research works taking correct account of the cooling-air effect. Numerical analyses of heat transfer from the surface of molten polymer to cooling-air were conducted using precise turbulence models and then simulations of bubble shape of blown-film process were performed under the air pressure and the thermal boundary conditions estimated from the numerical analyses of cooling-air. The film bubble shape predicted has coincided well with experimental data and it is confirmed that the bubble stability is strongly dominated by characteristics of cooling-air flow and heat transfer near the bubble surface.

Modern approaches to the phenomenon of dimensional accuracy in injection moulding link the evolution of shrinkage from the moment of first solidification to a force balance between restraining and constraining forces which sets in inside the mould before ejection. Such an approach needs a complete understanding of what happens inside the cavity during the moulding cycle. In a recent paper [9], a new technique has been presented, by which it is possible to follow the shrinkage development of an injection moulded sample from the moment it starts solidifying to soon after ejection. In this work, with reference to a simple rectangular cavity, shrinkage curves measured by strain gages in different moulding conditions are analysed by means of a simple thermomechanical model recently proposed. Results show that the model satisfactorily predicts the moment shrinkage starts inside the mould and reveal the existence of a restraining force not due to pressure, which builds in gradually in a few seconds from the instant of first solidification. Knowing the value of this force—which can be drawn by Hooke's law from the strain jump at mould opening—the model gives a good description of both shrinkage development inside the mould and ejection shrinkage.

Gas assist injection molding has increasingly become an important process in industry because of its tremendous flexibility in the design and manufacture of plastic parts. However, there are some unsolved problems that confound the overall success of this technique. Weldlines success of this technique. Weldlines form wherever polymer flow fronts meet is one of them. In this report, an L'18 experimental matrix design based on the Taguchi method was conducted to optimize the weldline strength of gas assist injection molded thermoplastics. Experiments were carried out on an 80-ton injection molding machine equipped with a high-pressure nitrogen-gas injection unit. A dumbbell-shape mold cavity was used. After molding, the weldline strength of the parts was measured by a tensile tester For the factors selected in the main experiments, melt temperature and mold temperature were found to be the principal factors affecting the weldline property of gas assist injection molded thermoplastics. Weld strength decreases with the length of gas penetration. In addition the weldline strength of gas assist injection molded parts was found to be generally weaker than that of conventional injection molded parts.

The injection velocity dynamics are found to be both time-varying and non-linear and therefore difficult to control using a conventional fixed parameter controller. In this work, an adaptive injection velocity control system with some special features such as anti-estimation windup, adaptive feed-forward, profile shifting and cycle-to-cycle adaptation is designed, implemented, and experimentally tested. The developed system is demonstrated to have excellent control performance with very good robustness for a wide range of operating conditions.