Volume 15, Issue 3

Taking into consideration the extensive literature we conducted our own studies of polymer flow velocity in the helical channel of the plasticating system. They included the following subjects: studies conditions in rectangular and helical coordinate systems—a model with a rotating barrel and a model with a rotating screw (theoretical principles); computational verification on the concrete example for different screw channel depths; comparison of the models with regard to drag flow velocity, pressure flow velocity, longitudinal flow velocity (resultant), shear rate and polymer viscosity. The thesis that both models of a screw-based plasticating system with a rotating barrel or a rotating screw differ fundamentally was confirmed in the same conditions in two coordinate systems—rectangular and helical. In connection with this theoretical investigations at a rotating barrel and next their experimental verification at a rotating screw, which is a common practice, are unsuitable.

A model for non-isothermal Newtonian flow startup in an initially empty modular self-wiping co-rotating twin screw extruder is developed. A viscous Newtonian fluid is considered to gradually fill a modular co-rotating twin screw extruder with viscous heating and heat transfer to the barrel (and screw). The build up of the axial cup mixing temperature profiles with time was calculated for various modular screw configurations.

Modeling of batch and continuous twin rotor mixers, widely used in polymer processing, is a challenging computational problem. One approach to understand the mechanics of flow in twin rotor mixers is to simplify the geometry of the rotors by using cylindrical rotors. This geometry is referred to as the double-Couette geometry. Advantages of the double-Couette geometry are a simple symmetrical mesh design and no time-dependent flow boundaries. In this work, we used the double-Couette geometry to study the mechanics of flow and mixing efficiency in laminar and turbulent flow regimes. Flow visualization experiments utilizing a fluorescent dye were carried out in a transparent flow cell. A fluid dynamics analysis package—FIDAP, based on the finite element method, was used for the flow simulations in laminar and turbulent flow regimes. Numerical results showed good agreement with the experimental data. We attempted a qualitative comparison for distributive mixing efficiency in laminar and turbulent flow regimes in light of the spreading of a tracer line (dye) in the matrix. The analysis pointed out to differences in the mixing mechanisms encountered in different flow regimes.

Melting is one of the most important process in plastics processing. Admit the extensive use of plastic injection moulding machine in industry, most studies were on extruders which were simpler to model. This paper studies the melting process and derives its transient (dynamic) model for predicting the characteristics of melting in feeding stage of injection extruder. Based on this transient model, factors affecting the melting speed in injection extruders are exposed. Simulation results agree with the results from experiments.

A detailed simulation of the cooling process in the production of plastic pipes has been used to gain insights into the problem of gravity flow or sag, which can lead to troublesome wall thickness variations in thick walled pipes. It is shown how the simulation can be used in an iterative, forward-backward calculation, based on the time reversibility of the creeping gravity flow problem, to calculate the initial extru-date wall thickness profile that the die must deliver to precisely compensate for sag. For pipes with particularly thick walls, this calculation shows that no compensating initial profile exists whilst conventional cooling arrangements are used. Reduction of the cooling rate of the lower part of the pipe promotes wall thickness uniformity and the existence of a compensating profile. Means for achieving the required initial profiles are discussed, and the need for an integrated view of the design and operation of the die and cooling line in understanding and overcoming the effects of sag is emphasised.

In a Rheotens test, the tensile force needed for elongation of an extruded filament is measured as a function of the draw ratio. In this way, the melt strength can be assessed, which is an important property in many polymer processes like fibre spinning, film blowing, and blow moulding. Several LDPE grades with different melt index and polymerised by both tubular and autoclave reactor technology were investigated. It is demonstrated that to assess melt strength, Rheotens tests have to be compared at constant extrusion pressure, while comparison at constant mass flow rate can be highly misleading. At constant extrusion pressure, Rheotens curves are invariant not only with respect to temperature but also with respect to average molar mass, and polymer grades with similar branching structure and molar mass distribution fall onto a common mastercurve. Two distinct and different Rheotens mastercurves were found for grades produced by tubular versus autoclave reactor technology, indicating distinct differences in the branching structure. Irrespective of the processing conditions, all LDPE melts investigated fail by brittle fracture, and a true rupture stress in the range of 1 to 2 MPa was found.

An analysis of flow and heat transfer in Liquid Composite Molding (LCM) is presented. Analytical solutions for the velocity profiles including the boundary effects and temperature distributions are derived for constant viscosity fluids. The heat transfer during mold filling is also analyzed numerically. It is found that the boundary effect has a significant influence on heat transfer when the Reynolds number, Re s , is greater than 1. When the Reynolds number is less than 1, the boundary effect on heat transfer can be negligible and Darcy's law is applicable. The results also show that the inertia effect can be neglected during mold filling in LCM. Experiments on heat transfer with a non-reactive fluid lead to measurements of the dispersion effect and test of the boundary effect on heat transfer. Several non-isothermal mold filling and curing experiments on two rather different reactive systems lead to an assessment of the factors a model must include to capture the curing behavior.

The phenomenon of shrinkage in injection moulding is particularly evident in semi-crystalline polymers. During cooling, these materials experience a transition from a completely amorphous to a partially crystalline status, which results in a severe change of all material characteristics, including volumetric parameters. In this study an iPP was injection moulded in a rectangular impression (120 mm × 30 mm × 2 mm) with a 1.5 mm thick line gate. The influence of holding pressure and time, and geometry constraints on linear shrinkage was explored. In-mould shrinkage was measured by means of a recent method based on strain gauges. Experimental results are compared with predictions for shrinkage obtained by C-Mold and a code developed at the University of Salerno, which takes into account crystallisation kinetics. The solidification criterion resulted to be extremely relevant for shrinkage predictions.

The importance of polymer flow properties during processing has required the development of new on line measurement systems, able to work under rates encountered during industrial processes. This paper will describe a rheometer which is able to study the thermorheological behavior of polymers within industrial injection or extrusion machines. Its main feature is to work at high shear rates which are experimented during production processes; it is equipped with dies of different diameters and lengths, allowing shear rates between 10 2 to 10 7 s −1 . Some results concerning viscosity and first normal stress differences are presented for high density polyethylene (HDPE), polypropylene (PP) and polypropylene/polyarylamide blends (PP/PA). The plot of viscosity and first normal difference against shear rate for these materials are not linear. Two non Newtonian regions and a third one similar to Newtonian behavior were observed in the wide range of shear rates up to 8.10 6 s −1 .

Gas assist injection molding has become an increasingly 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. Unstable gas penetration in molded bifurcation parts caused by inappropriate mold/part designs and processing conditions is one of them. This report was to study the gas penetration behavior in gas assist injection molded bifurcation parts. Experiments were carried out on an 80-ton injection molding machine equipped with a high pressure nitrogen-gas injection unit. A bifurcation cavity with a part thickness of 20 mm was used for all experiments. The resin used was general purpose polystyrene to help visualize the gas penetration behavior in the parts. Various processing variables were studied in terms of their influence on the stability of gas penetration in the parts: gate position, mold layout, melt temperature, mold temperature, short shot size, gas pressure and gas injection delay time. The purpose of this study was to investigate the parameters affecting the stability of gas penetration, so that steps can be taken to optimize the gas penetration behavior. This would provide significant advantages in improving product quality.

An analytical and experimental study of the transient flow behavior of polymer melt in injection molding is presented. The polymer volume flow rate into a rectangular mold cavity during the filling stage was determined using the Reynold's transport theory. The analysis, which neglects melt elasticity, shows that the flow rate is transient during the initial filling of the cavity. Moreover, the cavity flow rate was found to depend on material compressibility and expansivity, total volume upstream of the gate as well as time rate of change of melt pressure and temperature. Experimental results obtained through direct flow visualization are in good agreement with the theory. The effect of gate size on the cavity flow rate was also examined. A small gate gives rise to a greater degree of melt compressibility in the runner system. The prediction of actual flow rates into the cavity not only enhances determination of the interrelationship of flow rate, morphology, and properties in injection molding but also aids in the avoidance of molding defects. The results of this study can be used to improve the accuracy of CAE analysis.

We developed a method to determine the characteristics of a press machine required for the injection-press molding process. In this paper, a model based on a serial spring connecting two different springs was used. One was spring represented apparent machine rigidity and the other the resistance to compression of the resin. Using this model, we calculated the flow between parallel plates and flow behavior in a “tray” mold.

This work was concerned with evaluating the properties of injection molded composites consisting of polyphenylene sulfide (PPS) reinforced with various loadings of a melt processable phosphate glass. During injection molding, both the PPS and melt processable glass were deformable, facilitating the processing of higher glass loadings than are possible in conventional solid glass fiber reinforced materials. The stiffness of the composite blends was found to increase with glass concentration with a composite containing 60 wt% (44 vol%) phosphate glass showing moduli over 4 times greater than that of the neat PPS. Composites possessing stiffness comparable to a conventional E-glass reinforced material (40 wt% fiber) could be produced utilizing higher phosphate glass loadings on the order of 60 wt%. While the addition of the phosphate glass led to significant enhancements in the stiffness of the composites, the strength of the blends of ten fell to values below that of the neat resin. Although the use of phosphate glass typically resulted in strengths 2–3 times lower than those found for the E-glass filled material, these composites offered several advantages including lower anisotropy in mechanical properties, lower viscosities, and smoother surfaces.

For the purpose of process optimization of the injection-molding process a robust processing point is determined, at which specified characteristics of the parts are met and the process is insensitive to disturbances. The idea of the new optimization concept is to ensure that also around the robust processing point, within the so-called robust processing window, the part geometry is in tolerance and a low standard deviation respectively a high SN-ratio is achieved. Instead of reducing the optimization task to a one-dimensional problem by combining the target functions, in this approach one defines lower and upper bounds for the characteristics to be optimized. These bounds must be fulfilled in the range of the robust processing window and, if necessary, might be sharpened or relaxed during the optimization procedure. The corresponding response surfaces are generated by means of new Scattered Data Interpolation-methods.