Kwabena A. Narh

Spatial variation in viscosity in sheared polymer melts

Measurements have been performed using a Weissenberg Rheogoniometer equipped with parallel plates in order to investigate the rheological properties of a polymer melt, namely polystyrene, at very small plate separations. An appreciable gap dependent torque was observed which is indicative of an inhomogeneous viscosity through the sample thickness. The effect is associated with the macromolecular nature of the melt, as confirmed by subsequent measurements performed with polystyrenes of different molecular weight together with a low molecular weight silicone fluid. The results are discussed in relation to models of the molecular chain conformation at a solid-melt interface.

Phase transition in high molecular weight polyethylene during capillary extrusion: the reversibility of the temperature window

Abstract We present results confirming the existence of discontinuity in extrusion pressure, with temperature, within a very narrow temperature window (150–152°C) during the extrusion of high molecular weight polyethylene melts. This pressure minimum effect, the appearance of which is critical in both molecular weight ( M w ) and shear rate (γ), is now found to be reversible with temperature. Further, our work shows, in a quantitatively documented manner, that die swell is much reduced in the temperature window, corroborating the previously proposed interpretation that the effect is associated with a new phase with liquid crystal characteristics, which is the mobile hexagonal phase. Finally, we report on a structure-related memory effect, observed during the cooling cycle, which is interpreted in terms of structures created during the preceding heating cycle adhering to the flow vessel and persisting in the melt regime up to 180°C.

Experimental determination and numerical prediction of mechanical properties of injection molded self-reinforcing polymer composites

Abstract A novel method to calculate the distribution of tensile modulus of injection molded PET/LCP blends across the thickness of the mold cavity has been developed, based on the generalized Halpin–Tsai composite model, but with a variable fiber aspect ratio. Using this method, we are able to make a number of predictions regarding the effects of melt temperature, mold temperature, injection speed, and LCP volume fraction on the moduli of the injection molded blends. Our predictions show that in order to optimize the reinforcement effect of the in-situ formed LCP fibers in the blends, low mold temperature and low injection speed are required. These results are in good agreement with experimental results.

The effect of liquid gallium on the strengths of stainless steel and thermoplastics

We have investigated the effect of liquid gallium on type 316L stainless steel (as a candidate for the P–V–T pressure vessel), and four thermoplastics: two semicrystalline (high-density polyethylene and polypropylene) and two amorphous (polystyrene and poly(methyl methacrylate)). Specimens were coated with gallium and held at elevated temperatures and reduced pressure for extended periods. Measurements conducted on the plastics include weight change analyses, tensile tests and particle diffusion analysis using dispersion X-ray spectroscopy. For the stainless steel specimens, tensile and corrosion tests were conducted. Scanning electron microscopy was used to determine the level of corrosion. The results are compared with specimens heat treated identically but without gallium contact. After 3 months, the gallium corroded the surface of the steel to a depth of only 12 μm. No penetration path of the gallium into the steel has been observed. The gallium was also found to cause no change in the mechanical properties of the polymers tested, nor was it found to have caused any weight change in the specimens.

Computer Simulation of the Effect of Thermal Contact Resistance on Cooling Time in Injection Molding

Thermal contact resistances (TCR) play an im portant role in heat transfer. TCR is therefore of significance in the simulation of injection mold ing processes where one of the factors determin ing productivity is the cooling time. Simulation software packages either consider TCR as negli gible or use a constant value based on steady- state conduction, whereas in injection molding, TCR is time-dependent. We use simulation of the injection molding process to show that using TCR values within the range obtained from steady state experiments may lead to underpre dicting cooling times by up to 15% and, hence, there is a need to use time-dependent TCR in the heat transfer simulations. Our simulation result indicates that the value of TCR also affects the frozen layer fraction in the early postfilling period.

The effect of temperature dependent thermal properties on process parameter prediction in injection molding

The effect of using temperature dependent thermal conductivity and specific heat data on the simulation of injection molding process is examined. Results of the simulation based on temperature dependent thermal conductivity and specific heat data, are compared with those based on constant thermal conductivity and specific heat. These results are presented for a representative selection of crystalline and amorphous thermoplastics. The results show that the prediction of cooling time and the frozen layer fraction of the part thickness in the context of injection molding simulation, is significantly influenced by the type of temperature dependent thermal data used.

Adsorption-entanglement layers in flowing high molecular weight polymer solutions II. The effect of the nature of the surface

Adsorption-entanglement layers are thick, i.e. multimolecular, layers formed at solid surfaces from solutions of high molecular weight polymers in a state of flow. This publication, the second part of a series of four, is concerned with the effect that the chemical nature of the surface has on formation of these layers. It is shown that for the solutions of polyethylene (PE) and polymethylmethacrylate (PMMA) the layers form most readily on carbon surfaces. Further it is shown that in the case of PE, addition of aluminium stearate can prevent formation of the layers along steel surfaces.These and other effects reported in the paper all bear on actual situations encountered in rheological practice and in particular on the measurement of viscosities. It is concluded that the marked dependence of layer formation on the chemical nature and treatment of the surface is responsible for the differences in viscosity measurements featuring in earlier works by different authors.

Adsorption-entanglement layers in flowing high molecular weight polymer solutions. III: Solution concentration and solvent power

Adsorption entanglement layers are thick, i. e. multimolecular layers formed at solid surfaces during the flow of high molecular weight polymer solutions. The present publication is concerned with the influence of solution concentration and solvent power on the formation of these layers. Three polymers have been used, polyethylene, polymethylmethacrylate and atactic polystyrene in various solvents. The formation and thickness of layers were monitored from their effect on flow behaviour. Two flow geometries were used: Couette flow — where the layers cause peaks in the shear stress during continuous shearing, and capillary flow where the layers reduce the flow rate. The results indicate that there is a critical concentration below which no layers form, and that this critical concentration is an order of magnitude less than the conventionally defined overlap concentration. It is also deduced that the layers form more quickly, and grow to be thicker when better solvents are used.

The effect of nucleation density on the kinetics of crystallization and on the resulting structure and thermal properties of polymers crystallized during cooling

Following up the previous studies on this topic [1] it is verified that in the course of crystallization of polyethylene melts taking place during cooling, the number of predetermined nuclei initiating crystallization influences not only the scale of the spherulitic texture in the fully crystallized sample, but also the thickness of the lamellae and through it the melting behaviour of the final product and this in a predictable manner. Extending the scope of the previous study [1] these results were now obtained in samples in which the nucleation density was systematically altered, with all other relevant variables kept constant. The investigation thus demonstrates the previously unsuspected subtleties which can influence the structure and properties of samples which have been crystallized non-isothermally. It also provides an example for the complementary use of different techniques such as light scattering, optical microscopy, DSC calorimetry and Raman spectroscopy, for the purposes in question.

Parametric Study of Heat Transfer in Injection Molding—Effect of Thermal Contact Resistance

Thermal contact resistance (TCR) plays an important role in the heat transfer during injection molding. However, there is no consensus on the magnitude of TCR to be used in simulation as most of the reported results are based on steady state experiments. A numerical simulation of the heat transfer in injection molding is used in studying its effect and significance. The TCR is shown to attain its maximum magnitude in the postfilling period, and more accurate values than those available in literature are required for a better simulation of the postfilling stage. The effect of interface gap formation between the plastic and the mold on the contact resistance is also studied. This shows that the gap may have contributed to the high magnitude of TCR reported from the one experimental study of TCR in injection molding. However, the gap formation is shown to be dependent on the part geometry as well as processing conditions-in terms of shrinkage and warpage effects. The gap is both a function of time and space (location on the part surface) and this makes any experimental determination of the gap and TCR difficult.