Band 8, Heft 2

The character of mixing was interpreted through the energy consumption. It was correlated with the quality of mixing which is described by two parameters: average degree of dispersion and coefficient of heterogeneity. Experiments were carried out for polyethylene-based compounds with various fillers and typical rubber compound with carbon black. In the first case the general dependence of degree of dispersion upon the work of mixing for different fillers was obtained. In the second case common dependence for diverse technological regimes of mixing was also observed. In both cases these dependences were very close to exponential functions. It was demonstrated that the quality of mixing influences the performance characteristics (yield stress and deformation at break) of compositions of polyethylene with solid fillers.

A novel in-line rheometer for use with molten plastics has been developed and evaluated. It is a process sensor designed to be installed in a melt flow stream, for example at the exit of an extruder. The entire process stream flows through the rheometer, so there is no output signal delay due to the flow of melt in a sampling line and gear pump. A small fraction of the melt entering the rheometer is drawn into a small gap between a rotating drum and an outer cylindrical wall. The shear stress acting on a small area of the outer wall is monitored by a shear stress transducer. A technique for taking into account the small pressure flow that is superposed on the drag flow in the shearing gap is described. The major problems that must be addressed in designing an in-line sensor arise from the fact that the temperature, pressure and flow rate of melt entering the rheometer are subject to the fluctuations and upsets that are ubiquitous in industrial processes. The new rheometer is able to operate in this environment and yield reliable viscosity values. However, the output signal of the shear stress transducer was found to be affected by the pressure and contained a low frequency noise component that limited its accuracy. An improved version of this transducer is currently under development.

Melting of plastics in intermeshing co-rotating twin screw extruders is known to occur over very short barrel lengths. The melting phenomenon has been studied for both pellets and powder in a readily opened clam shell barrel. The melting mechanism is attributed mainly to the dissipation of the energy imparted in interparticle friction as a consequence of the unique kneading action of this type extruder.

A basic study of blending two different polymers (polyethylene and polyamide 6) in a modular co-rotating twin screw extruder is presented. The development of phase morphology was observed along the axis of the modular screw. Five different modular screw designs are considered. We first look at a machine containing all right-handed screw elements. We then consider a machine with all right-handed elements plus a single left-handed screw element. The study then turns to machines with combined screw and kneading disc elements. The kneading discs play a key role in both melting polymers and in enhancing dispersive mixing. Increasing the number of kneading disc blocks gives enhanced dispersion.

Melt blending, extrusion and drawing of polycarbonate (PC) blends containing a thermotropic liquid crystalline polymer (LCP) are reported for a copolyester PCDT synthesised in this laboratory. A single screw extruder is used at four temperatures. The rheology of the PCDT/PC blend, PCDT and PC was measured by means of a capillary rheometer at these temperatures. The morphology of extruded and subsequently drawn strands was studied with the help of a scanning electron microscope and analysed. In PCDT/PC extruded strands, the morphology of the PCDT dispersed phase, changes from spherical through ellipsoidal to elongated with decreasing processing temperature. In contrast to this, in drawn strands with a draw ratio of 10, the morphology of the PCDT changes from ellipsoidal through elongated to fibrillar with increasing processing temperature. It is believed that the relative viscosity of the two phases is the decisive factor determining the microstructure formation of polymer blends containing thermotropic liquid crystalline polymers. Fibril formation is related to the deformation and coalescence of PCDT melt droplets by the drawing at higher temperatures.

Eyring's theory of liquid viscosity was adapted to the special case of polymer molecules at a solid interface. The resulting model gives the slip velocity as a function of wall shear stress, temperature, pressure, work of adhesion and the molecular parameters of the polymer. The work of adhesion is related, on the one hand, to the critical stress for the onset of slip, and on the other, to the surface tension of the melt on the surface of interest. The predictions of the model are compared with previously published data for the slip of polyethylenes on steel and on steel treated with processing aids, and the agreement is very good. Unlike previous power-law slip models, the new model provides for a smooth transition from no-slip to slip flow at the critical stress.

Recently the use of PMMA films as the top layer of dish membrane solar collectors has gathered popularity. The mechanical properties and surface smoothness of PMMA films are the important factors affecting the durability and optical performance of these stretched membrane solar collectors. In this paper, we present the results of our experimental study on the effect of biaxial stretching on mechanical properties and surface roughness of PMMA films. Biaxial stretching was found to improve both the mechanical properties and surface smoothness of PMMA films.

Mold temperature control in injection molding is very important for control of production rate and product quality. However, mold temperature is a complex function of machine design, process variables, machine settings and ambient conditions. Data were obtained regarding the variability in both space and time (in-cycle and cycle-to-cycle) of mold metal and mold surface temperatures and heat flux. In view of this variability, it is necessary to define variables which may be useful for control purposes. The following variables were evaluated: cycle average temperature, peak temperature, and partial cooling time. Dynamic models were developed, on the basis of experimental data, to determine the process gains, time constants and time delays for the above variables with regard to coolant temperature, coolant flowrate, and melt temperature as the possible manipulated variables. In view of the dynamic studies, the cycle average surface temperature was selected as the controlled variable for control system design and simulation. The regulating abilities of PI, PID, and Dahlin controllers were evaluated. The results indicate that Dahlin control caused the controlled variable to follow the set-point closely, even when the process parameters vary, thus showing good robustness. However, the Dahlin algorithm achieves this advantage via large variations in the manipulated variable. This may cause difficulty in some circumstances.

In recent years composite materials have been widely used, largely because of their superior mechanical properties. Sandwich constructions can be suitably tailored to make use of each material's properties in the moldings, effectively (for example in honeycomb core constructions) in order to improve the mechanical properties of the moldings. In the injection molding process, however, it is quite difficult to fabricate sandwich constructions. A sandwich injection molding process in which the sandwich injection moldings are performed using two melt sources and one nozzle has been developed. At the present, there have been few investigations with respect to the mechanical properties of the sandwich structures produced by the injection moldings. In this study, we tried to examine the sandwich construction made by the injection molding process. Three kinds of materials including polyamide resin, the short and the longer fiber reinforced polyamides were used as layer structural materials for the sandwich injection moldings. The flexural properties for the sandwich injection moldings were measured and their fracture surfaces observed to clarify the mechanical behaviors. Furthermore, the flexural moduli for the sandwich injection moldings composed of the polyamides reinforced with the different fiber lengths were predicted based on two-layer orientation model.

Effects of processing variables on the structural gradients developed in the injection molding of thermotropic wholly aromatic copolyester parts were investigated using optical macro and microscopy combined with image analysis techniques at various sections of the molded LCP parts. The studies indicate the existence of a multi-layer structure from skin to core of the parts. These include: a skin layer, a transition zone composed of a sub-skin layer, a shear layer and a core region. The effects of processing conditions including injection speed, mold temperature, and mold size on the structural features were evaluated. An interesting buckling phenomenon resembling features observed in jetting at core regions near the end of the cavity after the divergence section was observed for the samples molded at the lowest injection speed and mold temperature. Digital image analysis of the images of the sectioned parts revealed that there are more layers than one observes macroscopically. In certain instances as many as 15 layers were detected. Orientation behavior and the positional light reflection intensity appear to correlate with each other particularly away from the skin regions. This may be the manifestation of high visible light absorption anisotropy exhibited by these aromatic polymers.

Rotational molding in two axes has the advantage of producing hollow products. Previously, the inherent difficulty was in the production of objects with uniform thickness since the speed of the two primary axes are fixed and cannot be varied independently to take into account the changes in material properties such as its density and viscosity. With the advent of computers, it is now possible to control the thickness of the object accurately. This is made possible by creating an intelligent software which takes in the viscosity and density time variation of the polymer and using the flat plate withdrawal theory, computes the new speed for each axis. The relationship between rotational molding parameters and polymeric material properties for a solvent – silicone rubber system to produce a hand prosthesis is illustrated in this paper.