As far as the mixing performance in single-screw extrusion processes is concerned, it is well known that the deformation measure or stretching which materials undergo due to the regular flows inside a conventional screw channel increases linearly with the extruder channel length. In general, the chaotic mixing is far superior to the regular mixing. Therefore, it would be fascinating if one could make the chaotic mixing possible in single-screw extruders with a special, and yet easily manufacturable screw without sacrificing the pumping performance of single-screw extruders. With this purpose in mind, we have developed a new screw (termed ''Chaos Screw'') for the single-screw extrusion process to enhance the mixing performance via the chaotic flows. The main idea of the Chaos Screw design lies in the spatially periodic barriers inserted in the channel to break closed streamlines in regular flows, which induces the chaotic mixing. The present article describes the basic mechanism of the chaotic mixing in a single-screw extruder and presents experimental evidence of the chaotic mixing using the Chaos Screw. Experimental mixing patterns due to the chaotic flow clearly indicate that the Chaos Screw drastically enhances the mixing performance in a single-screw extruder. It may be mentioned that the accompanying article, Part II, presents numerical investigation which shows that the chaotic mixing was successfully predicted by numerical simulations. (C) 1996 John Wiley & Sons, Inc.
The knowledge of residence time distribution (RTD) in industrial extruders is critical, notably when dealing with easily degradable polymers or when using extruders as chemical reactors. Many methods have been proposed for RTD determination but there are some drawbacks associated with each; they are expensive, hazardous, time consuming, or lacking sensitivity. A novel ultrasonic technique, sensitive to the filler concentration of polymer suspensions, is proposed. Ultrasonic properties (ultrasonic velocity and attenuation) were evaluated with regard to parameters such as linearity of the response, resolution of the measurement, and especially robustness to pressure and temperature variations. The attenuation, chosen for RTD evaluation along with a specific grade of calcium carbonate filler as the tracer, was then monitored to yield the RTD of the material in a twin-screw extruder, for different experimental conditions where quantity of filler utilized, as well as the method used to feed it, were changed in order to optimize the technique. (C) 1996 John Wiley & Sons, Inc.
In the accompanying article, Part I, we have described the basic mechanism of chaotic mixings due to a new ''Chaos Screw'' in single-screw extrusion processes, and presented experimental evidence of the chaotic mixing in a single-screw extruder with a typical Chaos Screw installed. The:present article, Part II, will be focused on the numerical investigations of the chaotic flows via the Chaos Screw in a single-screw extruder. The three-dimensional velocity fields in both the no-barrier and barrier regions were separately calculated via a finite element analysis of the quasi-three-dimensional flow in each region, and were subsequently used in the numerical simulations of chaotic flow behaviors. Extensive numerical simulation results will be presented in terms of particle trajectories, Poincare sections, and mixing patterns for several dimensionless parameters. It was found that invariant manifolds obtained by numerical simulations were in good agreement with those from experiments. (C) 1996 John Wiley & Sons, Inc.
To model fiber spinning and film casting, a boundary condition on the stress at a chosen synthetic inlet is necessary. However, the exact value of the stress for viscoelastic liquids at the synthetic inlet is a priori unknown. In this article, we present the application of the ''free boundary condition'' to the inlet stress, which avoids the necessity of specifying an a priori unknown value of stress at the synthetic inlet. To apply the free boundary condition, the process must be cast and studied as a two-point boundary value problem by finite elements. To verify the admissibility and accuracy of the free boundary condition, the same process is cast and studied as an initial value problem, directly solvable by a DGEAR subroutine. The initial value problem is cast in a matrix form that allows analytic investigation of admissible solutions: With the upper convected Maxwell model, the fiber can only slim monotonically with the downstream distance, whereas with the Giesekus model there may be cases of initially increasing and subsequently decreasing diameter, i.e., extrudate-swelling. (C) 1996 John Wiley & Sons, Inc.
This article describes the outcome of an investigation into the rotational molding of liquid plastics. Three liquid plastic systems were assessed: (1) a nylon block copolymer, Nyrim; (2) two grades of polyvinyl chloride plastisol, Acrol DS409A and Hydro PRC/65/0307/9270; and (3) two grades of polyurethane, Hyperlast 7850506 and Hyperlast 7853184. initially, resin flow behavior was assessed in the uniaxial rotation mode. Characteristic flow regimes were identified for each liquid. The onset and duration of a particular regime depends on the initial viscosity of the liquid, its repooling behavior, and its curing profile. For a particular material, the finished part appearance (hydrocysts, bubbles, fault-free, etc.) depends on the mold rotation speed and material shot size, as well as mold geometry. A fault-free operating criterion is presented for the uniaxial rotation of liquid plastics. Uniaxial rotation tests are useful for predicting general trends in biaxial rotation of liquid plastics. The most important factors in determining the state of a finished part made by biaxial rotation are: (i) resin initial viscosity and curing profile; (ii) mold rotation speed; (iii) shot size; and (iv) mold shape. An ideal viscosity profile for a rotational molding liquid resin, based on information gained from both uniaxial and biaxial rotation tests, is proposed. The rheology of each material was also examined and the profiles observed were related to the molding behavior of each liquid. (C) 1996 John Wiley & Sons, Inc.
Melting in screw extruders has been studied extensively, starting with the experimental studies of Maddock and Street in the 1950s. These workers observed a melting model with the solid particles all clumped together in a solid bed. A thin melt film separated the solid bed from the extruder barrel; most melting took place between the solid bed and the melt film. This melting model is termed the contiguous solids melting model, CSM; it is the model most often observed in single screw extruders. It was first analyzed theoretically by Tadmor in the early 1960s. Another type of melting that has been observed by several workers has separate, solid particles floating in a melt matrix. This melting model is termed dispersed solids melting, DSM; it has been observed in twin screw extruders, reciprocating single screw compounders, and some regular single screw extruders. It has been observed experimentally that melting through DSM occurs much more efficiently than through CSM. However, there has been little work published on a quantitative description of dispersed solids melting. Here the theory of CSM as well as DSM is developed with closed-form analytical solutions. The melting characteristics of the two models are compared. It is shown that dispersed solids melting can be substantially more efficient than contiguous solids melting. This opens up interesting areas of investigation. (C) 1996 John Wiley & Sons, Inc.
The phase morphology of injection molded blends of polypropylene and ethylene vinyl alcohol copolymer (EVOH) changes dramatically throughout the sample and especially across the sample thickness. The development of laminar structure was evaluated by image analysis of scanning electron micrographs. The results indicate that laminar morphology is most likely to develop in the shear zone, as maxima in particle size and aspect ratio are observed in directions parallel and transverse to flow in this region of the sample. The laminar structure is more pronounced at high concentrations of the EVOH minor phase, and at low maleation levels of the matrix resin, as well as for low molding thickness. The influence of processing conditions such as injection speed and mold and melt temperatures appears to be small. Intensive preblending of the components in a twin screw extruder hinders the development of the laminar structure. Toluene permeability measurements show that the presence of laminar phase morphology in injection molded samples produces substantial improvements of the barrier characteristics in the materials. However, impact tests indicate that there is need to optimize the conditions employed in producing the laminar morphology, in order to avoid deterioration of mechanical properties. (C) 1996 John Wiley & Sons, Inc.
The application of poly(ethylene terephthalate) (PET) in extrusion blow molding is limited by its low extensional viscosity, which is related to the low shear viscosity. The suitability of blends of PET/high-density polyethylene (HDPE)/compatibilizer for blow molding is investigated. The rheology of the blends was studied in the context of extrusion blow molding. A digitized video technique is presented for measurement of parison sag and the extent of sag is related to blend composition and processing parameters. Among the blends investigated, blends with a PET content of 70% or above exhibited a high degree of sag. Bottles were blown from the other blends and tested for mechanical properties. On increasing HDPE content, the impact energy of the bottle increases while the bending modulus decreases. A significant increase was noticed in the environmental stress cracking resistance of blends with 20% PET over pure HDPE. (C) 1996 John Wiley & Sons, Inc.
This article discusses the formation of extruded thermoplastic foam sheet due to expansion of supersaturated physical blowing agent while it exists from a high-pressure flow region into atmospheric surroundings. Swarms of tiny gas cavities of unstable nature are formed and gas diffusion virtually promotes expansion, the rate of which has a strong dependency upon degree of supersaturation. However, foam sheet cooling causes increase of rheological properties to prolong the sheet formation period, during which gas molecules close to the sheet surface tend to migrate onto the surface and evaporate away from the surface to reduce foaming efficiency. The viscoelastic ''cell'' growth model was modified to incorporate the gas loss from the sheet surface. Foaming efficiency appears to be dependent upon blowing agent/resin ratio, foam thickness, and nucleation density (#/cc). The simulation results predicted by the model show a good agreement with experimental data for polyethylene foam with low-percentage difluorochloromethane (HCFC-22) blowing agent. (C) 1996 John Wiley & Sons, Inc.
A methodology is presented to estimate cavity melt temperatures and part weight in the injection molding of amorphous thermoplastics. The approach uses measurements of cavity pressure near the gate and surface temperatures at three sensor locations. The surface temperature data with a heat conduction model give estimated temperature profiles across the cavity thickness. These profiles are then used to estimate the average cavity melt temperature. Fitting the cycle-to-cycle values of average mold-cavity temperature and peak pressure to a Tait equation yields a model to estimate part weights. The estimated part weights agree with measured values for different injection-molding conditions. (C) 1996 John Wiley & Sons, Inc.
Debugging and optimization of extruder performances are often complicated by the lack of transient process data. Most production extruders have excellent process controllers, but they generally lack data acquisition systems. Because of these equipment limitations, all transient, unsteady-state data for a process are unavailable for analysis. Highlighted in this article are four case studies where transient data were collected and used (1) for the diagnosis and elimination of extrusion instabilities; (2) to show differences between competing resin processability; and (3) in extrusion research. In all cases, a portable data acquisition system was temporarily connected to the extruder control panel and was used to collect transient process data. For extruder instability problems, the data collected were used to help diagnose and eliminate the problems quickly, bringing the extrusion lines up to standard production in the shortest possible time, minimizing costly recycle, and maximizing the profits for the processors. (C) 1996 John Wiley & Sons, Inc.
The geometric features of screws for both co- and counter-rotating twin screw extruders are developed from kinematic principles. It is shown that in co-rotating twin screw extruders there are more design constraints because the flight flank angle has to equal the angle of intermesh. This is not the case for counter-rotating twin screw extruders. The various geometries possible in the two types of extruders are shown. Although the geometries used for co-rotating extruders are well known, the geometries for counter-rotating extruders are not well known. In fact, some of the geometries possible with counter-rotating extruders have not yet been made commerically available. The implications of the geometric analysis on actual extruder performance is discussed in detail. (C) 1996 John Wiley & Sons, Inc.
A miscibility parameter model, which is based on appropriate combinations of solubility parameters, has been used to estimate polymer miscibilities of a representative number of systems. The model results compare very favorably with literature values and recent experimental results demonstrating the usefulness of this approach as a guide for estimating and investigating polymer-polymer miscibility/compatibility and the importance of group or molecular interactions. Construction of a phase diagram from an estimated miscibility window is demonstrated. (C) 1996 John Wiley & Sons, Inc.