Volume 14, Issue 1

There are a variety of problems in polymer processing. In this paper those problems which arise from non-rheological causes are described. The examples were taken from the period from 1960 to 1970's. Even though remedial steps were taken at the time when problems occurred, similar problems may occur in the future. The problems are well-known among those who worked in the area, but may not be known widely, because very few have ever been published. Some of the problems may be eliminated in the course of improving polymerization and fabrication (processing) techniques. Some problem-solving will become easier with the improvement of sensitivity of analytical techniques. Some problems merit further scientific studies. Many problems provide seeds for inventions.

The effects of wood flour and plasticizer on the rheology and extrusion of polyvinyl chloride (PVC) based wood—plastic composites are investigated. The intention of the study was to determine an optimal pair of PVC based wood—plastic composites that should exhibit substantially different rheological behaviors so that patterns similar to grain of wood can be developed inside and on the surface of the product when two selected wood—plastic compounds are extruded at once. The rheological study reveals that the shear viscosity and shear thinning behavior of the composites can be controlled by varying the contents of wood flour and plasticizer. It is also found that the depression of glass transition temperature due to the addition of plasticizer is substantially reduced by the loading of wood flour. In addition, various wood—plastic composites were compounded into different colors, and several pairs of the compounds with different rheological properties were extruded in single and twin screw extruders to see whether any wood-patterns are developed. When the differences in the shear viscosity and the glass transition temperature of the two compounds were too large, the incomplete plasticization of the higher viscosity component was observed due to the premature plasticization of the lower viscosity component. It is shown that distinct wood-patterns were developed both inside and on the surface of the extruded products only for the pairs of the composites with an optimal differences in both viscosity and plasticizer content.

In this contribution the computation of the drive power of single screw machines with smooth feed sections for extruders and injection molding machines will be presented. The drive power will be determined by means of the shear stresses at the barrel wall for the completely melt-filled channels as well as for the polymer melting section of the screw. In order to determine the shear stress and thus the power consumption the correct velocity profiles in the screw channels must be known, taking into account the non-Newtonian flow behavior of the polymer melt. For analytical solutions of this problem extreme simplifications are necessary. For this reason approximation equations are set up, which are based on numerical tests and by means of which a reliable determination of the screwdrive power is possible, knowing the throughput, the temperature development across the screwas well as the melting proile.

Mixing is one of the main functions of screw extruders. In the analysis of mixing, there are two mixing mechanisms that need to be considered: distributive and dispersive mixing. It is well known that single screw extruders generally have poor dispersive mixing capability, even when dispersive mixing elements are incorporated into the screw design. This paper will discuss the requirements for dispersive mixing, current dispersive mixing elements used in single screw extruders will be analyzed, and the lack of efficient dispersive mixing will be explained. A new generation of dispersive mixing elements will be introduced for use in both single and twin screw extruders and internal mixers. A two and three dimensional flow simulation will be used to analyze the mixing performance of these new mixers. Experimental work on the new mixing elements will be presented in a follow-up paper; the results demonstrate that it is possible to achieve dispersive mixing on single screw extruders that is as good as what can be achieved on twin screw extruders. The implications of these results on the compounding industry will be briefly discussed.

A 45 mm diameter single-screw “transparent” extruder was employed for an investigation on the dynamic performance of screws of different configurations. The extruder had eight glass windows installed along the entire metal barrel which enabled visual observation to be easily made. It was demonstrated, with this extruder, that pressure profile appeared to be an important factor for screw performance such as throughput rate, residence time, mixing characteristics, break-up of solid-bed, etc. The full-flight screw had the lowest pressure profile among all the screws tested, and its performance was found to be generally inferior to that of the mixing screws. The Darnell and Mol solid plug flow theory was shown to be applicable only to the mixing screws for the solid flow behaviour in the entire solid conveying section. The secondary flight of the barrier screw was found to effectively suppress solid bed break-up, but only at low screw speed. A screw with a Maddock mixing device was found to have the highest throughput rate. Other findings were believed to be of practical significance in the screw design.

Residence time distributions were measured for a Buss Kneader, Kobelco Nex-T continuous mixer as well as intermeshing co-rotating and counter-rotating twin screw extruders using polypropylene with aluminum flakes as a marker. Various modular screw configurations, screw speeds and feed rates were considered. Increasing feed rate and increasing screw speed the mean residence to shorter values and narrows the residence time distribution. Generally, in comparison of the different machines, the intermeshing counter-rotating twin screw extruder has the shortest mean residence time and narrowest distribution of residence times, while the Buss Kneader has the longest mean residence time and broadest residence time distribution.

A novel ‘Rotating Tip’ die has been designed and built that offers the tubing processor the ability to produce a wide variety of tubing ID's and OD's from a single fixed geometry die. Furthermore the device permits, amongst its many other advantages, the production of variable ID tapered tubing, increased hoop stress tubing and ease in production of microbore tubing.

3- and 4 parameter models (Carreau-Winter, Sabia, Yasuda) are used for fitting capillary rheometer tests. These fitted data are then compared with rotational visco-simeter test data in the region of small deformation speeds. The results clearly demonstrate that the extrapolated capillary rheometer data acc. to the Sabia and Yasuda models yield good practice-relevant results. A good correlation was achieved between the calculated “zero viscosity” and the mean molecular mass and between the model curvature parameters and the width of the molecular mass distribution. For quality assurance in laboratory rheometery and on-line rheometry these obtained good results are of high importance.

As-spun polypropylene (PP) fibres, obtained under a one-step statistical experimental design using orthogonal arrays and involving all the common spinning parameters were studied with wide angle X-ray scattering (WAXS). The WAXS results revealed that the micro-structure of PP fibres can be altered progressively from less-ordered para-crystalline states to highly-ordered crystalline states with an α-monoclinic lattice. The degree of structural order has been evaluated quantitatively in terms of (W ½ ) −1 , the reciprocal of the half-height width of the first WAXS reflection peak. On the basis of such quantitative evaluation, which is shown to be consistent with the conclusions derived from “traditional” qualitative assessment, comprehensive quantitative analysis of the effects of processing parameters and their interactions on the degree of structural order is accomplished, using systematic but simple statistical methods. Parameters significantly affecting structure, including types of raw materials, winding speed, spinning temperature, application speed of spin finish and metering pump speed are identified, and correlation between structure and parameter setting is established in a concise statistical model. The model indicates clearly the combination of parameter levels for increasing or reducing the degree of structural order over the whole range from essentially amorphous to highly crystalline forms. Comparisons with the “conventional” approach of “one factor at a time” demonstrated the validity of the new approach and its significant advantages. The latter includes the economy in experiment scale, efficiency in result analysis and a comprehensive overview of the process being studied, which covers effects arising from synergistic interactions as well as from individual parameters.

Melt spinning is a polymer processing technique that makes great demands on the extensibility of the polymer melt in the distance between die exit and solidification point [1]. The polymer material is exposed to a rapidly growing deformation rate over a large range of deformation within a short time of about 100 milliseconds. Simultaneously an extreme cooling occurs with cooling rates of about 1000 K/s. For this reason only a few polymer materials are usable for this kind of polymer processing with sufficient take-up speeds. Most polymers show a fiber break in the molten state either by brittle cohesive rupture or ductile failure when approaching critical conditions of deformation. The rheological behaviour of a polymer melt at the critical conditions of deformation in the fiber forming process can not be predicted by means of usual rheological material functions. This paper reports the attempt to find out material functions, which describe the critical deformation states of the melt spinning process. The established material functions are compared with the results of spinning experiments to estimate their practicality.

Extruded sheets were prepared from homo-isotactic poly-propylenes (PP) with 0 to 10 ppm of quinacridonequinone which is a β-crystal nucleator for PP and the characteristics of drawing behavior and properties of drawn sheets were studied. The sheet without the nucleator consisted of usual α-crystals (hereinafter called α-PP), whereas the sheets with the nucleator contained much β-crystals (hereinafter called β-PP), whose content slightly increased with increasing nucleator content. While the tensile yield stress of the β-PP is lower than that of the α-PP, the drawing stress of the former is higher than that of the latter at the higher strain region after yielding. Even after drawn, the β-crystals remain in the α-crystals with low crystallinity at low drawing temperatures. However, the β-crystals and amorphous phase gradually decrease with increasing drawing temperature, and the α-crystals dominate above 80°C. The resultant tensile modulus and strength of the drawn sheets show maxima at drawing temperatures of 40 to 80°C on a balance between the effects of crystallinity and molecular orientation. The tensile modulus and strength of drawn sheet prepared from the β-PP are higher than those prepared from the α-PP by about 10% at those temperatures. The tensile elongation of drawn sheet prepared from the β-PP is slightly higher than that prepared from the α-PP at low drawing temperatures due to the remained β-crystals.

A mathematical analysis model of multilayer viscoelastic fluids flowing down an inclined plane for any arbitrary wave numbers has been established to improve multilayer product quality in the coating processes. In the stability study of the free surface with small elastic force, Reynolds number is the primary parameter to induce surface instability for long and intermediate waves. When the elastic force in either layer is large, elasticity can affect the stability of long and intermediate waves. In addition, viscosity stratification across the interface can also affect the stability of the free surface. For the interfacial stability of two-layer flow system, it is found that purely elastic instability occurs in most depth ratios for the whole spectrum of waves when the fluid with elasticity is placed as the inner layer next to the solid wall. In the study of stability for viscoelastic fluids it demonstrates that arranging the more elastic and less viscous fluid as the outer layer leads to interfacial stability. In the more than three-layers investigation we found that placing the most viscous and least elastic liquid as the innermost layer next to the solid wall and the least viscous and most elastic liquid as the outermost layer next to the passive air can ensure the stability for the whole spectrum of waves.

Thermoforming of polypropylene/CaCO 3 composite sheets has become an important process in industry because of their low cost and good formability. However there are some unsolved problems that confound the overall success of this technique. Nonuniform thickness distribution caused by inappropriate mold design and processing condition is one of them. An L'18 experimental matrix design based on the Taguchi method was conducted to optimize the thermoforming process of CaCO 3 filled polypropylene composites. For the factors selected in the main experiment, the assist plug's displacement and velocity were found to be the principal factors affecting the thickness distribution of thermoformed composites.

The cooling time of an injection molded part directly affects the production cost and efficiency. In order to accurately estimate the cooling time, it is necessary to define the ejection condition. One common criterion for part ejection is when the mean or maximal temperature is equal or less than an estimated ejection temperature, T e [1]. But in industrial practice, the ejection condition is determined by the part's deformation or warpage due to ejection forces and subsequent cooling [2]. This research uses a combination of experimental, analytical, and statistical methods to discuss different ejection criteria and their rationality, thereby developing a more effective criterion based on part stiffness. This stiffness criterion facilitates more accurate estimation of cooling time at early stages of product development. This knowledge will help the designer to improve the design and reduce the cost by increasing the production efficiency while simultaneously ensuring injection molded part quality.