Band 22, Heft 1

Coating like polymer processing contributes greatly to the production of thin films using polymers in solutions or in a melt form. From a fluid mechanics perspective, coating operations share common features with polymer processing film operations and it is the purpose of this review to highlight the similarities and help foster a common approach for analysis, design and operation of these processes.

Two important instabilities associated with carrier layer formation on the inclined plane of slide-fed coating processes are explored, with a two liquid layer prototype configuration employed to mimic the slide-bead process as used by industry. The work relates to both slide-bead and curtain coating methods. Experiments first reveal how broad diffuse bands, that destroy the quality of the final coating, are formed as the carrier layer flow rate is reduced. The bands are found to be exacerbated by either decreasing the viscosity or flow rate of the carrier layer or by increasing the viscosity of the upper layer. As the flow rate of the carrier layer is reduced further, the upper layer begins to invade the carrier layer delivery slot. Flow visualisations reveal that a parallel sided delivery slot is superior to a chamfered one in terms of robustness against invasion of the interface and that reducing the slot width increases robustness further. Experiments and complementary numerical simulations confirm the existence of a recirculating eddy near the top of the downstream wall as the carrier layer flow rate is reduced and this insight is used to propose an instability mechanism for the onset of the bands. Further experiments expose the mechanism for the formation of cross bars. These can develop downstream of the carrier layer delivery slot and arise because the merging of the layers there becomes increasingly influenced by the flow, demonstrating that the sensitivity of cross bars to pump induced disturbances grows rapidly as the critical carrier layer flow rate for instability is approached and then collapses as the flow rate is reduced to a value where the layer is no longer effective in lubricating the flow of adjacent layers.

This paper presents a simulation of 3D crystallization of colloidal nanoparticles on a substrate during drying. The translational motion and the rotational motion of nanoparticles are modeled by Langevin equation and the law of angular momentum conservation, respectively. Contact force, capillary force, Brownian force, van der Waals force, electrostatic force and fluid drag force are taken into consideration. The drying of colloid is expressed as a decrease of the colloid thickness with time. The drying process of a water solution of polystyrene nanoparticles on a flat substrate is investigated, so that a self-ordered 3D crystal of polystyrene nanoparticles is formed after drying. The crystallization is visualized with time, and vertical and planar structures of nanoparticles are evaluated temporally and quantitatively. The result indicates the primary mechanism of 3D crystallization of colloidal nanoparticles during drying, in which both layering in the vertical direction and planar crystallization take place at the same time.

We present a lubrication model of thermo-capillary flows in moving volatile liquid film coatings. The forced air impingement from a slit nozzle onto the moving coating imposes an axi-symmetric surface temperature distribution, and thus the local surface-tension gradients on the liquid surface. Despite the symmetric temperature profile, local thickness variations became asymmetric and exhibited a particular ridge in downstream and a depression in upstream. The competing feature between the surface-tension-driven and the pressure-driven flows gives a characteristic growth and decay in the surface roughness as the temperature profile travels in the opposite direction to the moving coating. The model prediction showed that the surface roughness was first enhanced and then suppressed with increasing the impinging air velocity, suggesting some directions for achieving more uniform coatings at higher speeds.

Coating of functional materials is the basis of many electronic device manufacturing processes. Screen printing, roll coating and other methods have been used for decades, permitting low cost, high production rates of Printed Circuit Boards (PCBs), where conductive pathways are printed before components are integrated. Nowadays, the PCB industry in developed countries is suffering a revision of all manufacturing processes, to cope with severe market constraints: short runs and fast time-to-market. Inkjet technology has arrived to complement the traditional, analogical production, with new tools that makes possible to produce goods with great versatility. In this work we will state the problem of inkjet printing functional materials in the frame of PCB manufacturing. The use of numerical methods to asset the formation of conductive pathlines using digital techniques will be presented. Also, the use of resistive and dielectric inks will be discussed.

Management of moisture penetration and hydrolytic degradation of polylactide (PLA) is extremely important during manufacturing, shipping, storage, and end-use of PLA products. Crystallinity in PLA was measured with Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD). Moisture sorption isotherms in PLA films were measured with Quartz Crystal Microbalance (QCM) and Dynamic Vapor Sorption (DVS) experiments with samples of varying crystallinity. A surprising result is that crystalline and amorphous PLA films exhibit nearly identical sorption isotherms, within the accuracy of the experiments. The effect of hydrophobicity of PLA end groups on degradation was evaluated by synthesizing PLA with hydrophobic and hydrophilic end groups.

This paper provides a general introduction to welding fundamentals (section 2) followed by sections on a few selected welding processes that have had significant developments or improvements over the last few years. The processes that are discussed are friction welding (section 3), hot plate welding (section 4), ultrasonic welding (section 5), laser/IR welding (section 6), RF welding (section 7) and hot gas/extrusion welding (section 8).

The free-surface flow of fully melted material in the conveying section of a co-rotating twin screw extruder has been simulated numerically with the Finite Volume Method and an Eulerian multiphase model. In this model the interface is captured in an implicit manner with the help of the local volume fraction in a cell. The results presented here reveal the power and conveying characteristics of the twin screw configuration. Some interesting findings are discussed for the power characteristic in the fully filled state as well. The flow field and derived quantities are discussed with regards to the axial transport, symmetries in the flow field and locations of high energy dissipation. All results are compared to the fully filled state. Additionally, a new term, the volume flux based “conveying degree”, is introduced in this paper. The difference to the volume-based degree of fill is pointed out.

A homogenous melt is the prerequisite to achieve the demanded product quality in single screw plasticising technology. At higher screw speeds and mass flow rates solid particles can occur in the melt. Dispersive mixing elements are often used to improve the melt quality. Calculation models are necessary for designing mixing elements in the right way. A new model for the often used fluted mixing sections like axial and spiral Maddock-elements and Z-elements is presented in this paper. The calculation model is reduced to a single pair of inlet and outlet flute, connected by the shearing gap. Especially in spiral Maddock mixers the longitudinal flow in the flutes is a combined pressure and drag flow but in axial Maddock mixers pure pressure flow occurs. In the shearing gap a combined pressure and drag flow can occur. The leakage flow through the flight clearance is also considered. Geometry functions are introduced for considering the rectangular or circular segment shaped cross section of the flutes. The viscosity is shear rate and temperature dependent, the channel height as well as the channel width can vary along the mixing element. The model allows the calculation of the longitudinal pressure profile in the inlet and outlet flutes, the total pressure drop and the shear stress distribution in the shearing gap. Mixing sections sometimes show a too high pressure consumption and insufficient shearing efficiency. Therefore a geometry optimization method for axial Maddock mixing heads is presented in this paper.

Water-assisted injection molding technology has been used to manufacture plastic tubes in recent years, due to its light weight, relatively lower resin cost per part and faster cycle time. The residual wall thickness around dimensional transitions and curved sections can significantly affect the strength of molded parts and is of great concern. This research investigated the uniformity of the residual wall thickness distribution in water-assisted injection molded circular tubes with dimensional variations and curved sections. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection system, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The material used was semi-crystalline polypropylene. It was found that the wall thickness was not uniform near transitions. With the addition of fillets with proper angles around transitions, the uniformity of residual wall could be greatly improved. The residual wall thickness in curved sections was thick around the outer wall and thin around the inner wall. In addition, the experimental results suggest that water-assisted molded parts exhibit a more uniform thickness distribution at transition and curved sections than in gas-assisted molded parts.

In packaging industry, many structures are produced by extrusion coating. In this process, a polymer film is extruded through a slit die, then stretched in air, coated on a substrate (paper, aluminium or steel foil) between a chill roll and a flexible pressure roll, and finally cooled on successive chill rolls. Due to their non-polar character, polypropylenes are not suitable for extrusion coating on metallic surfaces. Adhesive properties can be improved by grafting on the polymer chain a polar group like maleic anhydride which may react with the aluminium surface. Our purpose was to develop a general model in order to predict the temperature field in the thickness of the multilayered structure along the stretching, laminating and cooling steps, and especially near the polymer/metal interface. This model includes crystallization kinetics and accounts carefully for the heat transfer coefficient with the successive rolls and surrounding air. Its predictions have been successfully compared to experimental temperature measurements along the coating line for various processing conditions (velocity, roll temperature, etc). Moreover, thermal history has a real impact on structure and morphology in the film. All these aspects are revealed by microscopic observations of thin microtomed sections of the film and X-ray diffraction experiments. Finally, adhesion properties of the laminate have been tested for the same process conditions. A good correlation has been established between adhesion properties and the thermal history experienced by the grafted polypropylene near the interface with aluminium before crystallization.

The objectives of this work were to identify the injection molding processing variables with the greatest effect on the morphology and mechanical properties of an injection molded blend made of poly (p-phenylene sulphide), PPS and block copolymer styrene-ethylene-butadiene-styrene ‘SEBS’. Artificial Neural Networks, ANNs, are used as an alternative method to constitutive and empirical models, to predict morphological features and mechanical properties from the injection molding conditions, and to predict mechanical properties from the morphological features. The quantification of SEBS dispersion in the PPS matrix was done using a dispersion function. Mold temperature and flow rate were the processing variables with the highest influence on the entire morphology, while the holding pressure influenced mainly the inner layers. Impact strength and toughness were most influenced by mold temperature, holding pressure and the outer layers. The flexural modulus was influenced by all processing variables and the intermediate layers. Three different ANNs were evaluated: one (ANN-1) to predict morphology from processing conditions and another two to predict mechanical properties from morphology and from processing conditions (ANN-2 and ANN-3, respectively). These latter ANN models had similar results, indicating that both inputs could be successfully used to predict mechanical properties, as the mean residuals were close to experimental errors. On the other hand, ANN-1 showed a lower performance, with a mean error smaller than the experimental error, suggesting that ANNs could overtake some inherent uncertainties. In this case, it was concluded that the distribution of data along output domain was more important than a high number of training data in the ANN's performance.