Volume 24, Issue 5

Real-time, non-intrusive and non-destructive ultrasonic technology has been used to monitor the melting process in an internal mixer. Visual observation, mechanical torque measurement, and ultrasonic signatures, such as amplitude and time delay of transmission and reflection echoes were used for the diagnosis of the melting process of low density polyethylene (LDPE). Phenomena during the melting process, including phase change from solid to melt, partially melted pellets, air bubbles inside the melt, were successfully monitored by ultrasound. The ultrasonic signatures were able to determine when the polymer has melted completely. The method of moving standard deviation (MSD) was applied to establish the melting completion timing accurately. Higher temperature of mixing chamber and faster rotation speed of blades reduced melting completion period, indicated by MSD of ultrasonic signatures. The presented ultrasonic technique can be utilized to optimize the melting process.

Three-dimensional (3-D) computational modeling approach of mold insert injection molding is developed to build a more robust and complete simulation solution of polymer melt flow in a mold cavity considering the effects of process conditions on non-isothermal, viscous, compressible and non-Newtonian flow behavior. The changes in viscosity and density in the polymer melt flow are successfully modeled within a volume of fluid (VOF) method coupled with a finite volume approach to generate more realistic melt flow physics during filling stage of injection molding under different process conditions. The Pressure Implicit with Splitting Operators (PISO) pressure-velocity coupling algorithm is employed to enable higher degree of approximate relation between corrections for pressure and velocity, and a comprehensive high-resolution differencing scheme (CICSAM) is successfully utilized to capture moving interfaces. The present developed numerical approach is verified for a box shape mold insert injection molding process of melt flow polypropylene (PP) and comparisons are made with a well-known commercial software program and the experimental data available in the open literature in terms of basic flow features. The numerical results are also compared with each other for each process condition to predict the evolution of a few flow parameters such as temperature and pressure field distributions in the selected regions of the mold cavity to optimize the polymer melt flow during the filling stage of injection molding. The present study is later extended to simulate the filling process of the injection molding for a mold insert PP pipe fitting model for further assessment of the modeling capabilities of the present numerical modeling methodology in injection molding of complex geometrical configurations.

Microcellular foamed polymeric products have found various applications depending upon their cell morphology. The microcellular injection molding process parameters have effect on the cell size, cell density and cell distribution. In order to develop desired cell morphology for a particular application, the set of injection molding parameters is required to be optimized for achieving a correct process window. In this paper an effort is made to study the effect of various injection molding processing parameters on cell morphology. Series of experiments were conducted for each of the processing parameter and the samples were observed under Scanning Electron Microscope (SEM). The trends of cell size and cell density with respect to the processing parameters were qualitatively analyzed. The results are useful for the processors who are interested in building up an understanding of this novel process and also to enable optimization of the set of processing parameters so that the desired microstructure of a foamed polymer can be obtained.

The processing conditions and the mould design are important parts of the powder injection moulding (PIM) process. In the present work, the mould filling phase of the process has been studied in situ with a special mould equipped with a sight glass using a high speed camera. Effects of features such as the mould temperature, the melt temperature, the surface roughness of the mould cavity, the gate dimensions, the mould design and the flow rate on the filling behaviour were demonstrated. In particular it was noted that a rougher mould surface to some extent facilitated the filling of the mould. The mould temperature, the nominal melt temperature and the flow rate affected the position of weld lines via the cooling of the feedstock and the corresponding change of the melt viscosity. The experimentally determined weld-line positions were compared with predictions from numerical simulations performed with the commercial software Moldex3D. The agreement between the experiment and the prediction was in most cases quite fair or good. The in situ observation of the mould filling also provided the possibility to study how the gate dimensions and the flow rate influenced the jetting behaviour.

Wood polymer composite is an important material class for industries. Microbiological properties can be an important aspect to enlarge its application base. The biocidal properties of wood polymer composite (WPC) can be aggregated by incorporation of biocidal additives on its composition. But, an important aspect is the evaluation the influence of the fabrication process on its biocidal properties. In particular, the temperature and screw velocity are important parameters for extrusion processes, and the mass proportions of the compounds are important factors to define the WPC properties. Different combinations of process conditions and mass proportions of the composite can promote variation in the magnitude of the bactericidal effect. The bactericidal properties are aggregated in the composite by organic bactericidal additive and the high temperature and high shear rate during the extrusion process can affect the effectiveness of the bactericidal additive function. This paper investigates the effects of the extrusion process parameters associated with different percentages of bactericidal additives on the WPC. The results showed significant effects on the bactericidal properties that depended only on the bactericidal content.

Computational simulation model has been used extensively in studying the filling flow in injection molding process. Most of the models are based on the assumption of a no-slip boundary condition and are verified by the experiments of filing flow in macro scale. This study is to investigate the flow behaviors during the micro channel filling of an injection molding process. Flow distances were measured on products from series of short shots to understand the relationship between the injection pressure and flow distance. It was found that cavity size variation due to mold deformation is important in the micro injection molding. By adjusting the cavity size due to mold deformation, the C-Mold simulation provides reasonable results compared to the experimental data for cavity thickness in 150 μm or 200 μm. On the other hand, the simulation will not give accurate prediction of filling flow for a micro cavity that has a depth below 100 μm. The wall slip effect is believed to the major reason and must be considered in the analysis for a cavity size below 100 μm.

The effect of sparse long chain branching, LCB, on the film-casting process is analyzed using a molecular model for the rheological behavior for a series of well-characterized, high-density polyethylene (HDPE) resins and a low-density polyethylene (LDPE) resin at various drawdown ratios. A full rheological characterization in both shear and shear-free flows is also presented. At low drawdown ratios, the LDPE exhibited the least degree of necking at distances less than the HDPE frostline. The sparsely-branched HDPE resins films had similar final film-widths that were larger than those of the linear HDPE. As the drawdown ratio was increased, film width profiles separated based on branching level. Small amounts of LCB were found to reduce the amount of necking at intermediate drawdown ratios. At higher drawdown ratios, the sparsely-branched HDPE resins of lower LCB content had film-widths that mimicked that of the linear HDPE, while the sparsely-branched HDPE resins of higher LCB content retained a larger film width. Molecular structural analysis via the Pom-Pom constitutive model suggested that branching that was distributed across a larger range of backbone lengths served to improve resistance to necking. As the drawdown ratio increased, the length of the backbones dominating the response decreased, so that the linear chains were controlling the necking behavior of the sparsely-branched resins of lower LCB content while remaining in the branched regime for higher LCB content HDPEs. Other processing variables such as shear viscosity magnitude, extrudate swell, and non-isothermal processing conditions were eliminated as contributing factors to the differences in the film width profile.

Fountain flow is the phenomenon of deceleration and outward motion of fluid particles as they approach a slower moving interface. Numerical simulations have been undertaken for the flow of viscoelastic fluids, obeying an integral constitutive equation of the K-BKZ type, capable of describing the behavior of polymer melts. Two polyethylene melts are considered, a branched LDPE and a linear HDPE. Their rheology is well captured by the integral model. The flow simulations are performed for planar and axisymmetric geometries and show the shape and extent of the free surface, as well as the stresses and pressures in the system. The semicircle is a good rough approximation for the free surface of fountain flow, but detailed computations show the effect of elasticity on the free surface, which is non-monotonic for the LDPE as the elasticity level (or apparent shear rate) increases. The less elastic HDPE shows a monotonic decrease in the extent of the flow front as elasticity increases. In all cases, the excess pressure losses (front pressure correction) increase with increasing flow rate. The effect of a nonzero second normal stress difference is to extend the flow front and increase the pressure losses.

Over 85% of the articles manufactured via rotational molding employ Polyethylene (PE) resins. PE resins used for rotomolding have usually particle sizes less than about 0.8 mm and used as powders or micro-pellets. During heating in a rotating mold, these particles get fused and merge into one piece. A disadvantage of the rotomolding process is long cycle times that affect not only the production rate but also increase thermal degradation of the polymer due to its long exposure to heat. One of the problems in rotomolding is bubbles of gasses trapped during sintering of the PE powders which reduce mechanical strength of the article produced. We propose to use reacting mixtures of Polyethylene Glycol with citric acid as an additive to rotomolding grades of PE. The additive accelerates sintering of the PE particulates and greatly reduces number of bubbles in the melt but at high concentrations (>0.4 wt.%) it impedes flow of the particles. Fine powders with particles of irregular shape and wide distribution of sizes are characterized by low flowability while micro-pellets of oval shape with smooth surface have too high flowability for rotomolding. To improve cost-efficiency and quality of the rotomolding process we propose to use micro-pellets instead of powders and to fabricate micro-pellets by extrusion at reduced temperatures and cutting the produced strands in conditions of air-cooling. Amazingly, the same processing additives improve extrusion of PE resins and rotomolding of the PE micro-pellets. The additives also adjust flowability of micro-pellets in the direction to optimum. Silica fume, vinyl-silanes and stearates can be ingredients of the additive package. Mechanisms of the observed improvements are discussed.