Volume 17, Issue 2

A new method for simulating the solid conveying process in plasticating machine is proposed in this paper. The new simulation method is superior to previous ones by having the ability in modeling the movements, in any direction, of solid pellets of any geometry or whether it is densely or sparsely packed. New normal and tangential contact force models are introduced. With the new method the solid conveying process are simulated more realistically. In the simulation, the effects of velocity of the barrel, the sliding frictions of the pellets against the barrel and the screw walls with respect to the solid conveying speed are studied, which coincide with empirical results.

A numerical study of three-dimensional fluid flow and heat transfer in the metering section of a single screw extruder has been performed. The screw channel was simplified by unwinding the channel and the coordinate system was fixed on the screw root in the present model. The pressure distribution over the inlet region was to be extrapolated from the solution of interior pressure. The prescribed mass flow rate was imposed on the outlet of the unwinding screw channel. The three-dimensional velocity fields, temperature, viscosity, and pressure profiles in the screw channel were calculated by using the commercial code, STAR-CD, based on the finite volume method. Some of the results of the present full 3D model were compared with the results from the marching-type 3D model. It is shown that the present work can capture the back flow by the pressure gradient near the screw root in the cases of small mass flow rates. There exist significant discrepancies in temperature between the marching-type 3D model and the present 3D model. The present work well describes the effects of heating-up through the circulation flow and viscous dissipation. The cross-section contours of velocities, temperature, viscosities and pressures along the down-channel direction are also shown in this study.

The mathematical models so far available for describing the throughput behavior of plasticising extruders were developed with the assumption of a wall-adhering melt. There are, however, a series of plastic melts, and also elastomers, polymer suspensions, ceramic materials and food products that display wall slippage during processing. A mathematical model has been developed for this material behavior, which describes the flow behavior for the unidimensional, Newtonian, isothermal case. Apart from the development of the analytical model, the flow behavior of wall-slipping polymer melts was also analysed with the aid of finite element calculations (FEM). A comparison of the results for the pressure/throughput behavior shows that the calculation results tally very well for the two methods. It is thus possible to develop a procedure which makes it possible to describe the phenomenon of wall-slippage for the non-Newtonian, multi-dimensional, non-isothermal case.

An attempt has been made to predict the velocity field in the conveying element of a Ko-Kneader using CFD method. The effects of both rotating and reciprocating motions of the screw have been taken in to account in three dimensional flow analysis. It has been demonstrated that such computational method can provide a great insight into visualization of the flow field in this element of a Ko-Kneader. Among the variables the reciprocating action was found to play a significant role on enhancing the mixing performance of the Ko-Kneader by means of periodical changing the flow field and shear rate distribution in the channel.

There has been little effort to quantitatively understand the simultaneous graft copolymerization of two monomers in batch and continuous mixers. In this study maleic anhydride/styrene and methyl methacrylate/styrene monomer pairs were grafted onto polypropylene in both an internal mixer and a twin screw extruder. There is little information on the evolution of grafting reactions of dual monomers with respect to time in an internal mixer and along the screw axis in a twin screw extruder. The influence of reaction time in an internal mixer and residence time in a twin screw extruder on degree of grafting was measured. The dependence of the degree of grafting with respect to ratio of comonomer concentration was determined. A kinetic scheme was proposed and experimentally tested.

Poly(ethylene terephthalate) (PET) fiber was heated by carbon dioxide laser radiation during the continuous drawing process. Numerical calculation shows that the PET fiber can be heated much more rapidly and uniformly by heat radiation than by convective heat transfer through the fiber surface. During CO 2 laser heated drawing, temperature in the vicinity of a neck-like deformation can be measured on-line with high precision, because the neck-like deformation is located within a range of 0.5 mm. We measured the fiber temperature profiles on the drawing process by IR thermometer that has a range resolution of 0.355 mm. The temperature at which neck-like deformation of the fiber initiates is higher than T g when draw ratio is less than 4.5, but lower than T g when draw ratio is more than 5.5. The maximum fiber temperature in the drawing process increases with draw ratio, up to 208°C for a draw ratio of 6.0. The rate of orientation-induced crystallization in the drawing process was estimated by comparison of measured temperature profiles with calculated temperature profiles.

The simulation of the advancing flow front during mould filling in the injection moulding cycle is important as a way of anticipating manufacturing defects, particularly different types of welds, air traps, cold spots and hot spots. The purpose of the present work was to evaluate a model (the distance model) simulating the advancing flow front and predicting potential issues related to the progression of the flow front such as welds and air traps. The distance model is based on a mathematical theory of Hele-Shaw flow for strongly shear-thinning fluids, i. e. fluids with a power-law index, n, equal to 0,3 or preferably less. Two grades of general-purpose polystyrene were selected according to their abilities to shear thin (weakly or moderately strong) at the selected processing temperature and in the typical shear rate range of the injection moulding process. The two grades were injection moulded into twelve mould configurations derived from two similar single-gated moulds. The flow length from the gate to the weld was measured in the mouldings obtained at three distinct rates of filling (conventional, slower and faster) and compared to the flow lengths obtained by the distance model. Good agreement was found between the predicted flow lengths and the experimental flow lengths at the conventional and faster filling conditions. Larger deviations between simulation and experiment were found at the slower filling rate, particularly for the weakly shear-thinning polystyrene grade. Some comparisons were also made with the predictions obtained using the commercial simulation code Moldflow. A comparison between the advancing front predictions of the distance model and experimental short shots of a commercial polypropylene grade in a lure-box type of mould geometry showed that the distance model, despite its simplicity, could probably be used to detect welds and air traps in more complex and practice-related mouldings.

The process design of the thin-walled injection molding is investigated in this study. It was realized that the behavior of warpage in injection molding parts was affected by different process conditions such as melt temperature, mold temperature, packing pressure and injection rate. Experimental design of Taguchi method was used to determine the process conditions. Both the numerical simulations and experiments were conducted to study the effects of the process conditions on the warpage of a cap model. The proper processing parameters were also identified by the experimental design. The packing pressure was found to be the most important factor on the warpage of a thin-walled part. The experimental design done by numerical simulations also provided proper results in terms of warpage reduction.

The mechanical properties of hybrid (commingled) yarn based reinforced thermoplastics depend on many parameters of the fiber components as well as of the different technological conditions during the commingling and the hot press molding. Among the fiber parameters the diameter ratio of the matrix and the reinforcing yarn components are most important to obtain best mechanical properties of the reinforced thermoplastics. The paper treats theoretical considerations in order to reach a high probability of coating each reinforcing filament with the thermoplastic material during the hot press molding. The calculations include the filament ratios and the achieved volume ratios of both hybrid yarn components of the finished reinforced thermoplastics. It is demonstrated that under different optimization criterias (same filament numbers for both components or the same surfaces of the reinforcing yarn filaments and the matrix yarn filaments before the hot press molding) a more or less small range of filament ratios seem to be valid for manufacturing a homogeneous faultless reinforced thermoplastic. Quantitative calculations for two hybrid yarn systems PEEK/carbon fibers and PP/glass fibers, presented in diagrams, are demonstrated. Technological conditions for the spinning of micro fibers like PEEK-filaments complete the presentation.

Rheological measurements on materials with fibre-like inhomogeneities will be significantly affected by sample size when the in-plane dimensions of the sample are comparable with the length of the fibres. In this paper, a 310 t hydraulic press has been modified for squeeze flow rheology studies so that large tools that will minimize the influence of sample size can be used in combination with high closing velocities. Isothermal squeeze flow rheology experiments of GMT have been performed for closing velocities up to 30 mm/s corresponding to maximum closing forces of 1000 kN. Successful power law fits of closing velocity versus closing force have been performed for the velocity interval. Four different analytical models have been fitted to the experimental data and the accuracy in these has been investigated. The force prediction of the biaxial extension model is acceptable for the major part of mould closure, but better fits during the last part of mould closure were obtained using an additive model incorporating both shear and extensions flow.

The inverse analysis method is applied in the evaluation of heat flux at the interface between the molten polymer and the mold during the injection molding process. In addition, the dependence of cooling behavior on both the molding conditions and the material properties of the resin are investigated. During low-velocity polymer flow in the thick regions of the mold cavity, the heat flux in the later stage of the process evaluated through experiment correlates well with the theoretical results of equations describing heat conduction at the interface of two semi-infinite objects. In contrast, in the thin region of the mold cavity, the analyzed heat flux is greater than the theoretical result for high-velocity polymer flow, and the difference expands with an increase in the shear rate at the wall of the mold. Based on these results, a simple empirical equation applicable to a wide range of molding conditions is proposed for the estimation of heat flux at the polymer-mold interface.