Extrusion | EPFL Graph Search

Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex cross-sections; and to work materials that are brittle, because the material encounters only compressive and shear stresses. It also creates excellent surface finish and gives considerable freedom of form in the design process. Drawing is a similar process, using the tensile strength of the material to pull it through the die. It limits the amount of change that can be performed in one step, so it is limited to simpler shapes, and multiple stages are usually needed. Drawing is the main way to produce wire. Metal bars and tubes are also often drawn. Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). It can be done with hot or cold material. Commonly extruded materials in

On-chip extrusion of lipid vesicles and tubes through microsized apertures

Philippe Renaud

In this work we present the formation of micrometre-sized lipid vesicles and tubes with perfectly homogeneous diameter and extraordinary length. The method is a novel approach for unconventional fabrication of soft-matter microstructured devices based on the combination of top-down and bottom- up fabrication processes. Photolithography techniques are applied to fabricate microsized apertures that provide the requirements to form lipid structures with predictable size and to align and guide the vesicles and tubes in microstructured channels. The formation is facilitated by self-assembly of polar lipids to a lipid membrane that is afterwards forced to undergo a shape transformation by extrusion through a microsized aperture. Both the geometrical restriction by the small aperture and the pressure difference between the top and bottom sides of the aperture determine the form and length of the vesicles and tubes. A strong pressure difference favors the formation of lipid tubes, while a low pressure difference results in the formation of vesicle bunches with spherical and cylindrical shapes. Potential applications for the formed lipid structures could be as microreactors and transport channels as well as in the construction of flexible microfluidic networks.

2006

Biodegradation and Antimicrobial Properties of Zinc Oxide-Polymer Composite Materials for Urinary Stent Applications

Lorenzo Lucherini

Research advancements in the field of urinary stents have mainly been in the selection of materials and coatings to address commonly faced problems of encrustation and bacterial adhesion. In this study, polylactic acid (PLA) and polypropylene (PP) were evaluated with zinc oxide (ZnO) coating to assess its ability to reduce or eliminate the problems of encrustation and bacteria adhesion. PLA and PP films were prepared via twin screw extrusion. ZnO microparticles were prepared using sol-gel hydrothermal synthesis. The as-prepared ZnO microparticles were combined in the form of a functional coating and deposited on both polymer substrates using a doctor blade technique. The ZnO-coated PP and PLA samples as well as their uncoated counterparts were characterized from the physicochemical standpoints, antibacterial and biodegradation properties. The results demonstrated that both the polymers preserved their mechanical and thermal properties after coating with ZnO, which showed a better adhesion on PLA than on PP. Moreover, the ZnO coating successfully enhanced the antibacterial properties with respect to bare PP/PLA substrates. All the samples were investigated after immersion in simulated body fluid and artificial urine. The ZnO layer was completely degraded following 21 days immersion in artificial urine irrespective of the substrate, with encrustations more evident in PP and ZnO-coated PP films than PLA and ZnO-coated PLA films. Overall, the addition of ZnO coating on PLA displayed better adhesion, antibacterial activity and delayed the deposition of encrustations in comparison to PP substrates.

MDPI2020

Modelling vapour expansion of extruded cereals

Laurent Lach

The vapour expansion of extruded cereals is a versatile technique used in the food processing industry to produce a wide variety of light, crisp & crunchy products such as snacks, breakfast cereals and pet foods. The range of textures that can be produced depends on a complex interaction of a many parameters controlling the expansion phase making the development and optimisation of the process a difficult task involving many trials. This thesis is aimed at developing a numerical model of vapour expansion of extruded cereal to improve our understanding of the physical process and to help speed up the development of new products. Rather than try to model the growth of each bubble explicitly a more economical "micro-macro" approach was developed involving the coupling of a 1D model for single bubble growth with a CFD code for modeling the bulk fluid flow. Several bubble growth models have previously been developed and coupled with simplified models for the flow inside the die, but none of them have succeeded in predicting even the right trend in the observed dependence of the expansion with operating conditions. A microscopic model was first developed along similar lines to those described in the literature with some improvements and then coupled to different macroscopic flow models. Although the process requires at least a 2D coupling to properly capture the full behaviour of vapour expansion, considerable insight was gained by coupling with a 1D compressible macroscopic flow model assuming isotropic expansion in order to predict the evolution of the extrudate outside the die. In particular the 1D model predicted the growth and maximum extrudate diameter in qualitative agreement with experimental measurements and showed for the first time why the expansion is observed to be stronger with lower water content or lower temperature. It showed for example that increasing the water content influences the expansion more by changing the partition between lateral and longitudinal expansion than by changing the degree of volume expansion itself. In other words, the increased water content decreases the viscosity and increases the nucleation pressure so the expansion occurs sooner and more rapidly and hence spends more time expanding inside the die where it can only accelerate. There is thus less bubble growth remaining for lateral expansion by the time it reaches the die outlet. For very low moisture content the model predicts a weak reduction in lateral expansion due to the reduced availability of water for vaporization. The resulting peak was also observed experimentally. Two types of 2&3D coupling with the Fluent CFD code were developed and tested: 1) where the equations for the microscopic model are solved in Eulerian form as User Defined Scalar (UDS) equations and 2) where the microscopic model equations are solved in Lagrangian form along stream lines computed using the Discrete Phase Model (DPM). In both cases the free-surface expansion of the extrudate from the die is simulated with the Volume of Fluid (VOF) capability in Fluent. Unfortunately neither approach could be made to converge stably due to numerical problems that could not be rectified due to the closed source nature of Fluent. A similar problem with the 1D model was solved by deriving a more intimate coupling between the micro and macro equations to reduce their order.

EPFL2006

Modelling vapour expansion of extruded cereals

Laurent Lach

EPFL2006