Volume 17, Issue 1

This paper describes work carried out in order to match experimental processing flows to numerical simulation. The work has brought together a consortium that has developed reliable experimental methods by which processing flows can be achieved in the laboratory and then ranked against numerical simulation. A full rheological characterisation of a selected range of polymers was made and the results compared from different laboratories. The data was fitted to a number of rheological models. Multi-mode parameter fitting was universal for the linear viscoelastic response. Particular attention was paid to the non linear response of the material. Prototype industrial flow experiments were carried out for a number of geometries in different laboratories and the flow birefringence technique was used to map out the experimentally observed stress fields for different polymers in a range of complex flows that contained both extensional and shear flow components. Numerical simulation was carried out using a number of algorithms and a range of constitutive equations. In order to make a quantitative comparison between experiment and simulation, an Advanced Rheological Tool (ART) module was developed that was able in some cases to quantify the level of fit between the numerically predicted and the experimentally observed stress patterns. In addition the ART module was able to optimise certain non-linear parameters in order to improve the quality of fit between experiment and simulation.

The flow of plastic pellets into the first section of a single screw is frequently neglected when analyzing the solids conveying process in single screw machines. In order to get a better insight into the complex correlations that exist between pellet properties, barrel and screw geomtry a physical-mathematical model is presented. As a result the pellet inflow behavior and the degree of filling in the first channel of the screw can be determined.

An attempt has been made to estimate the resident time distribution (RTD) in the KE and EZ elements of the Buss Ko-Kneader. The effects of both rotating and reciprocating motions of the screw were taken in to account in the flow analysis as well as in a simplified mathematical model developed for the RTD estimations. The basic function employed in the present work was F(t) = ∫ f(t) dt, where f(t) dt defined as dq/q was evaluated as a function of the channel depth. Due to the periodic nature of the system averages of minimum resident time was considered as t min in the above equation. The computation of the model was carried out to evaluated the F(t) as a function of time for both KE and EZ elements. These results demonstrate that the residence time distribution in KE element is slightly narrower than that in EZ element.

The flow behavior of a Newtonian fluid through special mixing enhancers in a modular intermeshing co-rotating twin screw extruder has been examined. The mixing enhancers are slotted screws and gear mixing elements. Particular attention has been directed to drag and pressure flow characteristics and residence time distribution in such devices. The data obtained are useful for guidance in the selection of appropriate mixing elements and validation of flow simulation models for twin screw extruders.

Bulk polymerization of ε-caprolactone has been carried out both in a laboratory internal mixer (Brabender Plasticorder) and in a modular intermeshing co-rotating twin screw extruder. Various initiators, such as titanium n-butoxide, aluminum triisopropoxide, and sodium hydride were used to polymerize ε-caprolactone in an internal mixer. The continuous polymerization of ε-caprolactone was performed using aluminum triisopropoxide under a range of processing conditions, including barrel temperature profiles, throughput, and screw speed. This polymerization was investigated for various ratios of monomer to initiator. GPC analysis demonstrated that significant quantities of oligomers were produced together with high molecular weight polymer at different reaction temperatures. H NMR detected the hydroxylation of growing chain end group for bulk polymerization at high barrel temperature. The molecular weight was reduced along the screw axis once the maximum molecular weight was obtained after the first kneading disk block. Increasing screw speed caused further reductions in molecular weight.

Flow instabilities of linear polyethylenes have been studied in capillary dies. The effects of shear stress, temperature, molecular weight and material of construction of the dies on both the critical shear stress for the onset of slip and slip velocities have been evaluated. Flow curves were obtained using capillary dies with constant L/D and different diameters. Slip velocities were calculated using the Mooney technique [1] as a function of shear stress. The onset of slip and slip velocity are affected by the material of construction of the capillary die. Flow instabilities were eliminated when brass dies were used under specific conditions. Instabilities persisted when naval brass and copper capillary dies were used.

Mechanical and rheological properties of filled polymers are very sensitive to the filler dispersion. Classical methods for evaluating dispersion are based on optical or scanning electron microscopy observations on extruded specimens. They cannot be used in-line and are generally time consuming. We present here an in-line characterisation technique to evaluate the dispersion of calcium carbonate, CaCO 3 , in polypropylene. The technique is based on the monitoring of velocity and attenuation of ultrasounds in the flowing material. These properties are measured in a specially instrumented die placed at the end of a single screw extruder. Measurements were made for filler concentrations up to 60 wt.%. In-line data are compared to dispersion indexes measured by an optical method on 25 micron thick films.

An experimental study of the characterization melt spinning cyclopolyolefins is presented. The shear and complex viscosities were measured and the data was found to fit the Cox-Merz relation. The birefringence of the melt-spun filaments was determined and correlated with spinline stress. These were found to be much lower than polycarbonate and polystyrene filaments melt spun under the same conditions. The birefringence was represented in terms of polymer chain orientation. The uniaxial mechanical behavior was also measured. The Young's modulus and tensile strength generally increased with spinline stress and orientation for most cyclopolyolefins. The elongation to break decreased with measured polymer chain orientation for most cyclopolyolefins. For others it was enhanced.

Gas-assisted injection molding of thermoplastic materials has become an important process in industry, because of its capability of producing parts that have thick and thin sections with a good structured rigidity. However, there are some unsolved problems that confound the overall success of this technique. Surface blush of the molded parts caused by glossy difference of the materials is one of them. An L'18 experimental matrix design based on the Taguchi method was conducted to investigate the surface blush of gas-assisted injection molded thermoplastics. The material used was black-pigmented polypropylene. Experiments were carried out on a 4.1 oz reciprocating injection-molding machine equipped with a high-pressure nitrogen-gas injection unit. A plate cavity with gas channels of different geometry was used to mold the parts. After molding, the surface quality of the parts was characterized by a gloss-meter. For the factors selected in the main experiments, mold temperature and gas pressure were found to be the principal factors affecting the surface quality of gas-assisted injection molded parts. The surface glossy difference increases with the content of the pigment in the materials. In addition, a trapezoidal gas channel of small width was found to mold the parts with the least glossy difference. An aluminum mold helps reducing the surface blush in molded parts.

The prediction of shrinkage and warpage of crystalline polymers is quite difficult. This is because of the complications of the crystallization process and associated material property changes. Particularly, the pVT behavior is dependent on the cooling rate during injection molding. Traditionally, the pVT data were measured in a device where the cooling rate is very slow compared to that in the actual injection molding process. This cooling-rate dependent pVT data is considered to affect the shrinkage and warpage calculation significantly. In the present study, a method has been developed to obtain pVT behavior at the same cooling rate as in the actual injection molding process. Obtaining other material data such as viscosity, thermal conductivity and heat capacity at the cooling rate of the actual injection molding process has also been considered. Injection-molding experiments have been conducted to measure the shrinkage of a part using semi-crystalline polymers. The polymers used in this study are polyamide 66 (PA 66) and polybutylene terephthalate (PBT). Simulation has been done to calculate the shrinkage using the cooling-rate dependent material properties. Using the fast-cool pVT behavior, the agreement between the simulation and the experiment improved significantly compared to the traditional approach.

In the first part of this paper, impregnation was positioned in terms of consolidation in composite processing and theoretical model for impregnation behaviors was presented. Impregnation behaviors were classified using the parameters contained in the model. Calculation for some simple cases was conducted. In the second part, impregnation behaviors of two material groups were experimentally estimated by on-line measurement of tool gap displacement during compression molding for making UD composites. First material group is composed of 4 kinds of thermoplastic composite materials. The other is thermoplastic film stacking with opened and unopened reinforcing fiber tows. Microscopic observation on the cross-section of parts fabricated at various molding time and measurement of transverse bending strength of these parts were also conducted. Based on the theoretical model proposed in the first part and these experimental data, impregnation behaviors of these materials were systematically discussed. The results suggest basic technological ways for attaining good impregnation in thermoplastic composites.