Volume 18, Issue 4

Residence time distribution is among the important parameters of the Buss Ko-Kneader which allows one to better understand the flow conditions inside the system and to get information on mixing efficiency. However it has always been a difficult task to predict the RTD of complex geometries such as Buss elements. The aim of the present work was to estimate the residence time distribution in EZ and KE element of the Buss Ko-Kneader by utilizing an elemental tracing method performed on the flow fields in the same elements which were analysized by CFD method. The effects of rotating and reciprocating motions of the screw have been taken into account in three dimensional flow analysis. It was demonstrated that these results can provide valuable information on mixing performance of these two elements.

Attempts were made to predict the velocity field and shear rate distribution of a Newtonian type fluid in the KE element of the Buss Ko-Kneader by means of CFD method. The role of both rotating and reciprocating motion have been considered in three dimensional flow field. The computed results showed appreciable changes in the velocity counters during a period of reciprocating motion, and these changes were greater for the transverse flow compared to the down channel flow. The predicted results also showed pronounced variations in the velocity profile of the transverse flow regarding both with direction and extent of drag and pressure flow as a result of reciprocating action, while the down channel velocity profile was found to be almost unaffected. The extent of shear rate and hence shear rate distribution were found to be dominantly control by the down channel flow while transverse flow has a greater influence in determining the residence time distribution.

Effects of material feed rate, water addition amount, screw speed and sets of kneading elements on sweet bread dough handling properties and bread qualities were studied by Response Surface Methodology in twin screw extrusion. Results showed that dough stickiness was significantly affected by water addition amount and screw speed (p < 0.001, R 2 = 0.9227). Firmness of bread crumb and spread ratio, which is an index for bread appearance, were both significantly affected by water addition amount, screw speed, and sets of kneading elements (p < 0.001, R 2 = 0.8940; p < 0.001, R 2 = 0.8884). The optimal conditions for sweet bread dough processing are water addition amount ranging from 45 to 50 %, screw speed ranging from 180 to 280 min −1 , with material feed rate and kneading element set fixed at 40 min −1 and ten, respectively.

The traditional method of making yuba has several shortcomings, including complicated processes, longer processing time, requirement of many equipments, creation of uncomfortable working environments, and production of wastes. To solve these problems, extrusion may be a promising alternative. The techniques of making yuba-like films by using a twin-screw extruder with special forming dies were investigated in this study. The effects of raw materials, plasticizers, operation parameters, and forming dies on twin-screw extrusion of yuba-like films were also analyzed. The results showed that yuba-like films could be made successfully by using a twin-screw extruder with T-die. To achieve a good performance of yuba extrusion and to get a good quality of films, the heating temperature of the raw materials should be over the denaturation temperature of soy protein, and the screw speed/feed rate ratio should be taken into account. Finally, a twin-roller system should be used to extend the films.

It is often observed in industrial practice that the thickness uniformity of blown film obtained with a given die varies with the type of polymer. We have shown in previous works the importance of the wall slip of polymer melt in the film uniformity. Currently available models of polymer melt flow through spiral mandrel dies neglect the wall slip. The present work considers the slip in flow analyses and predicts the effects of rheological properties on the uniformity; the decreases in melt viscosity and wall slip and the increases in shear thinning and the stress dependence of wall slip reduce the uniformity. The predictions agree well with the experimental results obtained in our previous studies.

The flow of a low-density polyethylene in a flat die is simulated using the axisymmetric and planar elongational viscosities estimated in an earlier publication by Beaupre and Gupta. Elongational viscosity is found to have only a limited effect on the velocity distribution at the die exit. However, the predicted pressure drop in the die and the temperature distribution at the die exit changed significantly when the effect of elongational viscosity was included in the simulation.

A dual-sensor system is developed to measure the density and viscosity of polymer melts, simultaneously, during extrusion process. The sensory system utilizes a 2.25 MHz longitudinal ultrasound transducer in conjunction with a 2.25 MHz shear type ultrasound transducer, placed in a modified extrusion die. The acoustic impedance of the polymer melt, corresponding to the longitudinal ultrasound waves (UTL), is used to measure the density of the polymer melt using established physical models. While, the acoustic impedance of the shear waves (UTS) is used to measure the viscosity of the polymer melt, utilizing the density measurements obtained by the longitudinal ultrasound transducer. The system was evaluated using polyvinyl chloride compounds containing various levels of foaming agent and processing aid at fixed processing conditions. The system measurements of the melt density was compared to the density of the extrudate, while the viscosity measurements were compared to the calculated values using slit-rheometry equations. The correlation coefficients between the ultrasound sensor and actual measurements were in the range of 92% and 96% for the melt density and viscosity; respectively.

The analysis of scale-up rule for double bubble tubular film process of PA6 was investigated. The scale-up rule previously proposed for a blown film process by the authors was applied to a double bubble tubular film process with a small-scale plant for research (pilot plant) and a large-scale plant for commercial production (production plant). According to the scale-up rule based on the equivalent stretching stress between the pilot and production plants, the equivalence of stretching stability, birefringence pattern and deformation pattern were confirmed. It has been confirmed that the proposed scale-up rule is applicable to predict the physical properties such as film impact strength and the bubble stability for the large-scale tubular film process on the basis of the experimental results obtained from the pilot plant.

The aim of this work was to observe the development of the phase morphology of the PBT/ABS blends during their extrusion mixing and injection molding steps. The evolution of the blend morphology during the mixing stage was analyzed using a specially designed co-rotational twin-screw extruder with a collecting device located along the barrel. Blend samples were collected in-line along the length of the extruder barrel during the blending process. Samples obtained from the extruded pellets and from injection molded specimens were also used to observe the blend morphology. All the samples were observed by transmission electron microscopy (TEM). They were prepared by cryo-ultramicrotomy and the rubbery phase of the ABS contained in the blends was stained with osmium tetroxide (OsO 4 ). A special function was established and is proposed to analyze the blend morphology trends through image analysis. In binary blends, the ABS phase showed better dispersion after the mixture passed through the extruder die. However, a coalescence phenomenon was observed during the next molding step. Compatibilized PBT/ABS blends showed better dispersion than binary blends, even after injection molding. The special function used to quantify the dispersion throughout the mixing and molding steps showed an excellent performance.

Mechanical properties, flow characteristics and chemical resistance of polycarbonate (PC)/poly(butylene terephthalate) (PBT)/impact modifier (IM) blends were investigated over the wide composition range of PC and PBT. Mechanical properties of the PC/PBT/IM blends for two different IMs, butadiene based IM and butyl acrylate based IM, were studied. For the study of chemical resistance of the PC/PBT/IM blends, test specimens were dipped in solvent, thinner, and then variations of mechanical properties were analyzed. Tensile and flexural strengths were increased linearly and heat distortion temperature (HDT) also increased as PC content in the blends increased. Impact strength increased drastically as PC content in the blends increased up to 50 wt.-%, and then it leveled out for over 50 wt.-% of PC. Flowability of the blends decreased as PC content increased. Impact strengths of the blends were different from IMs specially at low temperatures. The blends with butyl acrylate based IM showed slightly higher impact strength than the blends with butadiene based IM at 0°C and room temperature. However, the blends with butadiene based IM showed remarkably higher impact strength than the blends with butyl acrylate based IM at the temperatures below 0°C. After the chemical treatment of test specimens, tensile and flexural strengths decreased, and impact strength increased as PC content in the blends increased. PC in the blends would become mild and ductile when it contacted with solvent, thus the impact strength increased while tensile and flexural strengths decreased.

We make a comparative study of structure development in injection molding of isotactic polypropylene, polypropylene blends/compounds and dynamically vulcanized polypropylene thermoplastic elastomers (TPVs) WAXS 2θ scans were used to determine crystalline forms and the distribution of relative amounts of crystalline forms through the cross-section of injection molded parts. WAXS film patterns, polarized optical light micrographs and birefringence were used to qualitatively indicate polymer chain orientation variation through the cross-section. We also used WAXS pole figures to investigate polymer crystalline orientation and filler orientation in different layers of the injection molded parts. It was found that a low or modest orientation layer exists on the part surface. There is an intermediate highly oriented layer in the parts of isotactic polypropylene and most polypropylene blends/compounds. The interior of the parts was spherulitic and increasingly isotropic as one moved to the center. The thickness of the surface layer increases as the injection pressure decreases. The layer was not observed in parts of dynamically vulcanized polypropylene thermoplastic elastomers. This would seem to be associated with the TPVs possessing a yield stress and not exhibiting substantial fountain flow in mold filling.

The processing variables that control the occurrence of breakthrough phenomenon in sandwich injection molding were investigated. Particular attention was paid to the conditions that would yield an optimum sandwich molded part. The effects of variation of core volume ratio that would yield optimum core shot size, injection speed and the skin/core viscosity ratio were examined. The flow morphology at the various stages of flow was also explored. The study was conducted with a bar-flow mold mounted on a twin-barrel injection machine fitted with a common nozzle. It was noted that the core material flow distance depends on the core cylinder screw metering position (core shot size). At the stage at which the core material completely penetrated through the skin material (breakthrough), four distinct regions of flow could be identified. The flow distance of the core material is controlled by the injection speed of the core material while the injection speed of the skin material is of little significance. In order to increase the flow distance of the core material, it is effective to increase the core material injection speed albeit limited by an optimum value of screw metering position. It was found that the flow distance of the core material was longer when the core material viscosity was lower than that of the skin material but breakthrough occurred much earlier so core volume is limited. Breakthrough failed to occur when the viscosity of the core material was higher than that of the skin material. It was found that optimum core volume and core distribution occurred when skin/core viscosities were intermediate. The morphology of the skin material leading to breakthrough was also analyzed.

The processing conditions that exert influence on the occurrence of breakthrough during sandwich injection molding were investigated. A spiral flow mold mounted on a twin-barrel injection machine fitted with a single nozzle was used. The effect of mold cavity thickness and the effects of skin material cylinder temperature on skin material viscosity, melt strength, and drawing ratio were considered. Scanning electron microscopy was used to study the morphology of the skin material at the flow front. It was found that the flow length at core expansion region increased and breakthrough phenomenon was delayed as the thickness of the mold cavity increased. When cylinder temperature is increased, the viscosity is lowered and the flow front of the skin material extends with greater ease when pushed by the core material, thus, a greater core volume can be accommodated. As the flow front progresses, the tendency is that the skin material becomes thinner. Polishing and etching followed by SEM examination of the skin material at the flow front revealed that the skin material is oriented perpendicular to the flow direction and the extent of orientation increases with increasing cylinder temperature. This further supported the observation that the deformation ratio and drawing ratio increased with increasing cylinder temperature.