Papers by Keyword: Coextrusion

Abstract: When using the coat hanger die method for co-extrusion, the biggest challenges often involve maintaining the uniformity of the velocity distribution at the outlet of the die and ensuring the stability of the interface plane. This paper investigates the effect of different cross-section of feed channels connected to the coat hanger die on the velocity and pressure distribution of the flow at different parts of the die. Co-extrusion of LLDPE (Linear Low Density Polyethylene) and HDPE (High Density Polyethylene) polymers is simulated using ANSYS software 2020 R2 for coat hanger die design with rectangular and circular cross-sections inlet geometry; the results are compared for Carreau-Yasuda model to observe the result differences between rectangular and circular coextrusion channels connected to coat hanger die. Our results showed that rectangular cross-section feedblock generated higher values for pressure in comparison with the pressure generated by the circular cross-section feedblock. The maximum velocity generated in the circular feedblock is lower than that generated in the rectangular one, nevertheless there is more uniformity in velocity distribution in circular than rectangular cross-section.

Abstract: A process when different materials are combined to produce a product with multiple layers is called co-extrusion. During this process, polymers are melted in separate machines and then extrudate from different die channels. Once these channels converge, the polymers meet and flow through a single channel. The surface where the two fluids face is called “interface”. It is crucial to maintain the interface's uniformity and stability in order to achieve the desired multi-layered structure. Most of the issues in co-extrusion are related to issues that can be classified into two categories such as polymer encapsulation/interfacial distortion and die swell. To solve these problems, designers focus on improving the interface's stability. This paper examines effects of cross-section modification of the two-channel feedblock on the interface location and velocity and pressure distributions of the flow. The ANSYS software was used to simulate the co-extrusion of polymers, LLDPE and HDPE, in two-channel feedblock with rectangular, circular, and straight slot cross-sections. The results show that sharp corners increase the thickness of dead zones, while rounding them decreases the thickness. Additionally, stadium-shaped (or straight-slot) cross-section channels can move the flow with a higher maximum velocity and thinner boundary layer combining the results of rectangular and circular feedblocks.

Abstract: To enhance the mechanical properties of fused filament fabricated parts, the process integrates continuous fibers. Currently, fibers are impregnated either with thermoplastics or with thermoset material, which is completely cured before printing and later combined with thermoplastic filament during the coextrusion process. A major problem about using cured thermoset matrix for the fibers is an insufficient bond between the fiber matrix and the thermoplastic material. A new approach proposed by the authors combine uncured thermoset matrices with thermoplastic filaments to form a substance-to-substance bond. To investigate the material and bonding behavior, a test bench is constructed. Its main purpose is to replicate the coextrusion of thermoplastic filament and thermoset impregnated continuous fibers. Parameters, such as temperature, tension and extrusion speed can be adjusted within the setup to accurately simulate the additive manufacturing process. Aluminum blocks including heater cartridges and thermocouples act as hot ends and impregnation units. Heated blocks compact the fiber strands. We tested different heating blocks containing flat and curved geometries including actual additive manufacturing nozzles to evaluate the impregnation behavior of the dry carbon fiber filaments. Approaches with additive manufacturing nozzles show the most promising results regarding fiber impregnation with thermoplastic material.

Abstract: The fire retardancy of coextruded wood-plastic composites (WPCs) containing melamine, ammonium polyphosphate (APP), natural graphite, expandable graphite and carbon nanotubes (CNTs) in the shell layer was characterized with a cone calorimeter test. A coextruded composite manufactured without any fire retardant (FR) in the shell layer was used as a reference. The incorporation of different combinations of FRs in the shell layers of WPCs reduced the peak heat release rate by 3-43%, depending on the FR combination. Other studied parameters, such as ignition time, total heat release and mass loss rate were improved after FR systems loading. The best improvement of flammability characteristics was observed with melamine/natural graphite combinations, whereas the melamine/expandable graphite system resulted only in slight improvement of the studied parameters. However, it should be noted that the amount of expandable graphite loading was 2-4 times lower than the amount of natural graphite loadings. Incorporation of 2 wt.% CNTs in the shell layer did not show any significant improvement in the studied parameters. The total smoke release and carbon monoxide production were increased with melamine/APP loading in the shell layer.

Abstract: Several polymers can be combined in one multilayer structure by reactive coextrusion. Tie-layers are often used to compatibilize adjacent layers and may reduce or suppress interfacial instabilities and defects in multilayer coextrusion flow. However, a new additional defect defined as “grainy” defect can be observed. In our best of knowledge, no study in literature has been dedicated to understand its origin. The phenomena are quite complex due to the coupling of the effects of flow and the physico-chemical mechanisms at the interface. The aim of this work is to understand the relationship between the instabilities and defects encountered in multilayer coextruded films and the role of the copolymer formed in-situ between tie and barrier layers. Polyamide 6 (PA6) and ethylene-vinyl alcohol copolymer (EVOH) were used as barrier layers sandwiched in polypropylene (PP) with or without tie-layer based on polypropylene grafted maleic anhydride (PP-g-MA). Influence of process parameters and nature of the polymer pair on the generation of “grainy” defect has been assessed and related to the rheological and the physico-chemical properties of layers. These experiments showed that this defect appeared mainly in the compatibilized EVOH system and could be distinguished from the usual coextrusion instabilities. Interfacial properties between tie and barrier layers have been investigated. Shear stress relaxation experiments have been carried out on reactive tie/barrier bilayers. Due to the interphase generated in-situ, the relaxation behavior was altered by extending the relaxation time. Investigation of interfacial morphology highlighted that the copolymer architecture significantly affected the interface/interphase development and interface roughness. Hence, relationships between relaxation process, interfacial morphology and copolymer structure were correlated with the generation of grainy defects in coextrusion.

Abstract: The three-dimensional numerical simulation model of two polymer melts flowing through the traffic circle section path was founded. The coextrusion process of composite pipe was simulated used the finite element method. The stream line method was used to simulate the extrudate swell. Such simulated results as the location and shape of coextrusion interface, the shear stress profile were analyzed. It is found that the maximum shear rate occur near the convergence section of the outer polymer. The interface excursion is less than 1mm in the die path. As the melt flow out of the die, the interface excursion increase distinctly, the value is up to 6mm. The extrudate swell rate is 21% along the radial direction.

Abstract: The eccentricity coextrusion flow of polymer melt was analyzed based on finite element simulations. Such simulated results as the fields of flow velocity, pressure and shear stress were obtained. Through the analysis of the results, the mechanism of the column interface forming in the axis-symmetry coextrusion flow path was obtained. For the coextrusion flow, if the low viscosity polymer flows near the die wall, the flow would be steady. Whereas, if the polymer with low viscosity is in the core and the high viscosity polymer at the outer region, Which is disadvantage in terms of energy, and the instability flow would occur. This is also accord with the least energy consume theory.

Abstract: Due to a changing environmental awareness and the improved use of resources, the application of light-weight materials, such as aluminium and magnesium, becomes increasingly important and partially substitutes the utilisation of conventional materials such as steel. However, a widespread application demands a better understanding of these materials. This concerns production processes of semi-finished products and the products itself. Coextruded aluminium-magnesium compounds are investigated in the subproject B3 (“Experimental and numerical investigations of the interface behavior of Al-Mg compounds”) which is part of the special research area 692 – HALS at the Chemnitz University of Technology. These compounds are characterized by a very good weight-strength-ratio and allow a wide field of application, for example in automotive industry. The compounds are manufactured in extrusion processes. The interface which is developed during the production is of special interest and the investigation of it is a shared aim of the Department of Experimental Mechanics and the Department of Virtual Production Processes. The two main tasks are the extrusion process optimisation and the microstructural, mechanical and thermal examination of the semi-finished product. The following paper gives an overview of the performed investigations in this subproject.

Abstract: Extrusion of composite materials can offer big advantages. In this work the manufacturing of a hybrid metal profile in a single production step was investigated. A porthole die was used, thus producing profiles with extrusion seams. Along the seams a material mix up was visible. The extrusion process was simulated with the Finite Element Method to investigate the material flow in die and welding chamber in order to understand the cause for the defects at the seams.

Abstract: Co-extrusion processing is a powder-based forming method that uses the simultaneous ram extrusion of two or more materials to form multi-phase systems with functionally designed architectures. A functionally designed material has functional properties that are specifically tailored for an application by changing the macrostructure (100’s of microns) with little or no change in overall composition. Thus, co-extrusion can be used to improve the functional properties of composite materials, with or without a concurrent boost of its intrinsic properties. An example of a functionally designed material with a coaxially co-extruded architecture, known in the ceramic field as a “fibrous monolith”, has elongated cells of a major (70-90 vol.%) phase surrounded by cell boundaries of a minor (10-30 vol.%) phase. A variety of strong-phase/weaker-phase combinations have been demonstrated, including Si3N4/BN, SiC/graphite, SiC/BN. Several wear resistant-phase/ductilephase combinations have also been studied, including Diamond(Co)/WC(Co) for use in the petroleum drilling industry. The presentation will cover co-extrusion processing as a technology, applications that are being considered or are in production for most of the latter materials combinations, and a discussion of the functional properties that can be achieved using co-extrusion as a means to create functionally designed materials.