Band 8, Heft 3

In this paper the effect of the inclusion of thermotropic liquid crystalline polymers (TLCP) in thermoplastic polymers is analyzed, with due attention to similar studies reported in the literature. Numerous results relative to TLCP containing blends are considered in terms of structure development in typical processing operations like extrusion and fiber spinning. Several systems have been studied in order to emphasize that different effects can be obtained as regards processing and mechanical properties of the host matrices. As for rheological aspects, it has been verified that TLCP can be considered as effective flow aids, even at low concentrations. The addition of a low percentage (5 to 10 %) of a TLCP with a viscosity lower than that of the matrix produces a significant reduction (25 to 40 %) in the blend viscosity. Moreover, mechanical results have shown that an improvement of the matrix rigidity can be obtained, especially at high percentages of TLCP (30 %), under processing conditions where a highly extensional flow prevails. However, a decrease in the ultimate elongation of the matrices has been found in all systems where a poor adhesion between the two phases exists.

The increased emphasis on the use of recycled feedstocks has brought in the need for continuous on-line monitoring of process conditions. This is done in an effort to detect process shifts and react accordingly to maintain optimal conditions. Time series analysis is ideal for on-line modelling of continuous processes employing recycled feedstocks, as the disturbances present in the feedstock can be quantified using this approach. The models obtained can then be employed for closed loop control of the process. The closed loop control allows for maximizing of the amount of recycled material, yet ensuring an acceptable finished product. This work presents a study of the stochastic dynamics of the extrusion of recycled feedstocks. Both “in-house” and “post-consumer” feedstocks are considered. Process outputs include the extrudate thickness and diameter, the head pressure, the melt temperature and the flowrate. The process inputs are the die gap and the level of recycle material. Process and disturbance transfer function models are obtained. Control simulations are performed on the process and disturbance models to ascertain the feasibility of a closed loop system for maintaining of optimal process conditions. A minimum variance controller (MVC) is employed in the simulation. The response of the closed loop system to various disturbance inputs is monitored. The control scheme results in a much lower offset from the set point for the case with MVC than for the case with no control.

Intermeshing counter-rotating twin screw extruders have played an important role in polymer processing technology in the post World War II period. They operate on basically the same principle as low viscosity multiple screw pumps, i.e. they involve forward pumping of fluid contained in C-chambers, which are regions between the screw root and barrel between the interlocked intermeshing regions. Tangential counter-rotating twin screw extruders with open screw channels have also been manufactured in the same period. A new more general model of flow of a Newtonian fluid in a counter-rotating twin screw extruder with partially intermeshing screws is presented. The model allows transition from fully intermeshing to tangential screw machine systems. Calculations are made to show the effect of level of intermesh on pumping capability. Modular counter-rotating twin screw machines are analyzed. The problem of starvation is discussed.

In this study, we have mainly discussed the dewatering and devolatilizing extrusion behavior of synthetic elastomers using intermeshed twin screw extruders. For these purposes, we utilized various twin screw extruders with long segmented barrels having several slitted sections and venting ports. Furthermore, we selected the optimum operational conditions using either the intermeshed co- or counter-rotation twin screw extruder depending on experimental purposes and raw materials. Some results obtained were compared between the co- and counter-rotation twin screw extruder.

The outstanding economic importance of injection molding in plastics processing and the steadily growing requirements on the molded parts to be produced are leading to constant further developments in the process engineering for injection molding. Computer simulation is a method that has only recently come to be used to an increasing extent in the field of injection molding, and the use of computers has so far been restricted to simulation of the injection mold. One key prerequisite for the quality of the molded parts, however, is that the melt should have been prepared by the plasticating unit in a manner that comes as close to the ideal as possible. The article presents a closed basis for calculation for the simulation of the plasticisation process in injection molding. This makes it possible for a large number of screw layout criteria to be estimated and calculated in advance. The computer simulation serves to make the plasticisation process more or less transparent and also clearer and calculable. The validity of the computation model for any size of plasticating units is demonstrated by examining the operating points of large industrial plants.

With short term shearing at low degrees of supercooling a method has been developed by which nucleation during shearing can effectively be separated from the subsequent crystal growth, which, in these experiments, mainly occurs after the cessation of the flow. In this way the kinetics of nucleation by shear could be unravelled for isotactic polypropylene. The required measurement consists of the monitoring of the optical retardation which increases with the monitoring time and levels off by saturation. From these measurements conclusions could be drawn with respect to the modes of formation of the nuclei and of their type. Model predictions could be confirmed with the aid of electron micrographs.

Two polypropylene (PP)-glass fiber (GF) compounds were injection-molded. One was a polypropylene filled with 0, 10, 20 and 30 wt.-% of an untreated glass fiber. A second was a compound filled with 20 wt.-% of a silane-treated GF plus 5 wt.-% of a maleic anhydride-grafted PP. Flexural test specimens were injection-molded from the filled PPs at cylinder temperatures of 200 to 320 °C. Properties such as flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, and mold shrinkage as well as higher-order structures such as mean fiber length, crystallinity, ß-crystal content, thickness of skin layer, a*-axis-oriented component fraction, and crystalline orientation fraction were measured and structure-property relationships were studied. A simulation of molecular orientation was carried out from the viewpoint of growth of a melt orientation at the gate and its relaxation in the mold cavity.

The structural gradients developed in the injection molding of thermotropic wholly aromatic copolyester parts were characterized using a specially developed micro-beam wide angle X-ray diffraction camera technique and wide angle X-ray pole figure analysis techniques. Wide angle X-ray pole figure analyses indicate that the local macro-symmetry throughout the sample seems to he uniaxial. The polymer chains are generally oriented in the flow direction except in the core regions after the diverging section, where molecular chains tended to orient transversely to the flow direction. The local orientation of chains is determined from the micro-WAXS film patterns using DIA (Digital Image Analysis) technique. This enabled us to map the orientation of the local symmetry axes as well as local chain orientation through the thickness direction. The data indicate that the chain orientation is highest at the skin layer, becomes lower at the sub-skin layer, then increases gradually toward the shear layer and finally decreases to very low levels in the core regions.

Because of their many advantages, long fiber reinforced thermoplastic composites have been used in many industrial applications. However, because material flows during compression molding cause a large scatter in the mechanical properties of moldings, it is necessary to characterize the flow behavior during molding. A new experimental procedure, the “Mark Method”, was used to observe the flow in different layers during molding. Mold flow was found to be characterized by interlaminar slippage, which causes the core layer to flow to the edge of the panel.

This paper presents a patented method [1] for relating the output from individual extruders to measured external extrudate dimensions and then for using these dimensions to control individual screw speeds.