Search

International audience; Additive manufacturing techniques based on liquid resins, such as digital light processing (DLP), demonstrate higher resolution and accuracy than other printing techniques, but their applications have been hindered by the limited materials selection. Overall, there have been numerous successes in printing acrylic-epoxy resins but the final parts generally suffer from relatively poor mechanical properties. In this study, hybrid printable formulations that are based on acrylic-epoxy resins and ceramic filler are introduced and their printability on DLP machines is demonstrated. Here, we present an ultraviolet (UV) printed acrylic-epoxy with high Young modulus via a two-stage curing approach. Herein, different experiments are used to investigate the evolution of the materials properties during the post-process. Indeed, we conduct experiments to characterize the physico-chemical features and the evolution of mechanical properties before and after post-process. Finally we report the preparation of polymeric rapid tooling inserts with tailored mechanical and physical properties, which were used as injection molding cavities for prototyping and low volume production.

Foldable composite structures are bellows-like structures that can be stored in a flat state and deployed to a finite volume after folding. These composite structures are being increasingly demanded for numerous engineering applications including aerospace, aeronautics, robotics and medical. In this study, we report an original processing route for obtaining foldable composite laminates. These composite laminates were obtained by selectively curing pre-impregnated materials that were made of three layers of plain weave fabrics impregnated with an UV-curable epoxy-acrylate resin system. After selective curing, the composite laminates were made of rigid domains (cured zones) connected by flexible domains (uncured zones) that played the role of hinges upon folding. The selective curing of the photocurable resin system was performed using two different approaches. The first approach consisted in depositing UV-blocking masks designed numerically onto the surface of pre-impregnated materials before placing them inside the chamber of an UV-curing machine. The second one consisted in using a Digital Light Processing (DLP) 3D printing machine. The UV-curable epoxy acrylate resin exhibited fast curing kinetics characterized by a gelation time on the order of 1 s. Hence, both types of approaches enabled foldable composite laminates with very short time cycles (≈ 5 to 10 s) to be obtained. The as-obtained pre-impregnated sheets were folded into 3D composite structures and then irradiated with UV-light. This original fabrication method is versatile enough to provide a wide diversity of composite part geometries that are promising for many engineering applications. The observations of the microstructure of processed samples revealed that the reinforcement fabrics were rather well impregnated by the epoxy acrylate resin. These observations also tended to show that both types of approaches enabled a complete and efficient in-depth curing of the resin system.

In order to consider thermal conditions involved in laser sintering (LS), the effect of temperature on the flow properties of a polyamide 12 powder was assessed. Shear tests were carried out at temperatures from ambient up to values close to the melting temperature of the polymer. The measurement method provides quantitative descriptors of powder flowability. Experiments indicate that flowability significantly deteriorates when temperature rises and approaches a value of about 20 °C lower than the melting temperature. The spreading process may be hindered by natural agglomeration of the material. A simple analysis based on the extrapolation of the bulk flow properties at zero consolidation shows that agglomeration is excluded in the temperature range between 100 and 140 °C. This turns out to be the range in which it is known that the material can be used safely for LS applications.

This work investigates the capacity of a particular class of flows, namely drag and pressure driven flows, to propagate and distorts a transient sinusoidal temperature disturbance occurring at the inlet of the metering zone of a polymer processing screw channel. The integral transform method is applied to the forced convection heat transfer problem in a parallel plate channel with a general drag and pressure driven flow velocity profile for a Newtonian fluid, with the pressure gradient as an adjustable flow parameter. We follow closely the procedure outlined by Cotta and Ozisik in their seminal paper [5]. We found that the knowledge of the lowest eigenvalue in the solution is sufficient to characterise the dampening efficiency of an inlet temperature fluctuation by the flow and that a higher level of positive pressure difference is indeed improving the thermal homogeneity in the flow. Moreover, we found that for any given flow, the higher frequency disturbances are dispersed more efficiently than the lower frequency disturbances.

International audience; An innovative liquid rotational moulding process aiming at using standard injection-moulding polymer grades, shortening cycle times and at accessing more higher-end markets is studied. In this new process, the polymer is melted beforehand in a conventional injection unit and injected into the mould rotating around two axes. This work focus on modelling the liquid polymer flow occurring in this process, as well as providing guidance on the choice of process parameters. First, a stability criterion for rimming flow is derived, defining the process and material parameters space available for a functional process. From this criterion, it appears that initial thickness, and density have a destabilising effect, while liquid viscosity and angular velocity have a stabilising effect. Next, using Lax-Friedrichs scheme, the one-dimensional transient gravity driven liquid film is numerically modelled, linking the total extent of spreading to the frequency of orientation reversals and to the major and minor angular velocities ratio : the closer this ratio to one, the larger the extent of spreading. Then, Computational Fluid Dynamics (CFD) simulations are carried out to take into account three-dimensional features in the moulding of a cube part. Finally, comparison with trials performed on the moulding of the same cube part validates entirely this approach. In particular, some peculiar features of spreading can only be explained and reproduced when employing three-dimensional CFD simulations.

International audience; Single-screw plastication, used in extrusion and in injection moulding, is a major way of processing commodity thermoplastics. In injection moulding, a high level of reliability is usually achieved that makes this process ideally suited to massmarket production. Nonetheless, process fluctuations still appear that make moulded part quality control an everyday issue. Duringthe plastication phase, prior to the injection phase, the polymeric material is melted by the combined effects of shear -inducedself heating (viscous dissipation) and heat conduction coming from the barrel. A combined drag and pressure difference flow isimparted on the molten polymer along the screw channel. The low thermal diffusivity of molten polymers implies that convectionis dominant over diffusion and leads to very large Peclet numbers. However, the screw channel length is large enough for the flowto present significant diffusion effects. Therefore, the objective of this work is to study the capacity of this particular class of flowsto dampen any transient inlet temperature disturbances occurring at the beginning of the metering zone of the screw channel. Theintegral transform method is applied to the transient energy conservation equation with the laminar velocity profile obtained fordrag and pressure difference flow of a Newtonian fluid. We found that the knowledge of a single eigenvalue is sufficient to charac-terize the dampening efficiency of a flow and that a higher level of back-pressure is indeed improving the thermal homogeneity inthe flow.

International audience; A regular perturbation method in which the small parameter is taken as the characteristic frequency of variation of a two-dimensional slit wall height is applied to the creeping flow of a viscoelastic fluid. The Upper-Convected Maxwell model has been chosen as the constitutive equation. The equations of motion are expressed in terms of the stream function as the only unknown. The stream function is expressed then as a regular perturbation expansion up to the fourth order in the small parameter. When these expansions are written for slits with slowly varying walls of any shape, curvature and higher-order derivatives appear in the expansion solution. From the stream function solution, we distinguish two different types of viscoelastic higher-order effects: one is symmetric and the other is a loss of fore-and-aft flow symmetry due to the convected derivatives appearing in the constitutive equation. The expansion is formally validated with a ratio between two consecutive higher-order terms. It shows that the accuracy of the perturbation will depend on four factors: the small parameter ϵ, the Weissenberg number We0, the local depth h and the shape of the slit. In some instances, Padé approximants can restore a diverging perturbation expansion and extend the range of viscoelastic levels for which the expansion is usable. Results for diverging and for converging flows in a wedge, as well as flows in corrugated slits, are checked against Finite Elements Method calculations. At the same level of viscoelasticity, the stress profiles are completely different in a diverging flow from a converging flow. In particular, the shear stress component of the stress tensor becomes non-monotonic in divergent flows.

International audience; Single stage injection blow molding process, without preform storage and reheat, could be run on a standard injection molding machine, with the aim of producing short series of specific hollow parts. The polypropylene bottles are blow molded right after being injected. This implies that the preform has to remain sufficiently malleable to be blown while being viscous enough to avoid being pierced during the blow molding stage. These constraints lead to a small processing window, and so the process takes place between the melting temperature and the crystallization temperature, where the polypropylene is in its molten state but cool enough to enhance its viscosity without crystallizing. This single stage process introduces temperature gradients, high stretch rate and high cooling rate. Melt rheometry tests were performed to characterize the polymer behavior in the temperature range of the process, as well as Differential Scanning Calorimetry. A viscous Cross model is used with the thermal dependence assumed by an Arrhenius law. The process is simulated through a finite element code (POLYFLOW) in the ANSYS Workbench framework. The geometry allows an axisymmetric approach. The transient simulation is run under anisothermal conditions and viscous heating is taken into account. Sensitivity studies are carried out and reveal the influence of process parameters such as the material behavior, the blowing pressure and the initial temperature field. Thickness measurements using image analysis are performed and the simulation results are compared to the experimental ones. The simulation shows broad agreements with the experimental results. An optimization loop is run to determine the optimal initial thickness repartition. Design points are defined along the preform and the optimization modifies the thickness at these locations.

International audience; To be able to predict the throughput of a single-screw extruder or the metering time of an injection moulding machine for a given screw geometry, set of processing conditions and polymeric material is important both for practical and designing purposes. The model is based on viewing the entire screw simply as a pump, conveying a solid and a molten fraction. The evolution of the solid fraction is the essence of the plastication process, but under a particular hypothesis on solid bed acceleration, its influence on the throughput is nil. This allows getting a good estimate on the throughput and pressure development along the screw. Calculations are compared to a large set of experiments available from the literature. Consistent agreement with these published results is obtained, both for throughput and pressure along the screw. The effect of the plasticating process on the throughput is non-existent if the plastication length is short, and more visible if the plastication length takes a good part of the screw length (for instance, at higher screw rotation frequency). This diminishes the throughput value and widens the pressure peak. The model also shows that the screw geometry is the most important parameter, followed by polymer rheology and processing conditions. Melting properties and length seem to intervene to a lesser extent. Finally, the model is used for screw design, highlighting the influence of the compression zone on throughput.

Thanks to the single stage injection blow molding process, specific hollow parts can be produced with standard injection molding machine. A specific mold is designed, enabling the preform injection molding, as well as its blow molding on the same machine. The preform is blow molded right after its injection molding. This particular process does not involve preform storage and reheating before the blow molding stage. This implies that the preform has to remain sufficiently malleable to be blown while being viscous enough to avoid being pierced during the blow molding stage. The polymer used is a polypropylene random copolymer. Therefore the process takes place between the melting temperature and the crystallization one. As the preform is blow molded right after being injected, this single stage process involves high temperature before the blowing. The polymer rheological behavior is investigated through dynamic rheometry in its molten state at various temperatures. A viscous Cross model is used with the thermal dependence assumed by an Arrhenius law. The process is simulated through a finite element code (Polyflow) in the ANSYS Workbench framework. The geometry allows an axisymmetric approach. The transient simulation is run under anisothermal conditions and viscous heating is taken into account. Two geometries are simulated. First, the flask geometry has been used to show the process efficiency. The yoghurt pot geometry has been designed later for industrial application in a more sophisticated mold than the flask mold. Blow molded parts are analyzed by tomography and the thickness has been measured by the tomography data image analysis. The simulation results are compared to the experimental ones. The simulation fits well with the experimental results concerning both geometries. The simple numerical model proposed proves to be robust enough to predict well the blow molded parts final thickness repartitions provided that the relevant temperature and pressure conditions are well defined.

International audience; The elaboration of composites with natural fibers and polymeric matrices has to face two major difficulties: natural fibers present a hydrophilic character (absorption of water and possible deterioration of the properties) and thermal sensitivity (the fibers can be degraded during composite elaboration and/or in certain applications). In addition, common polymers are generally hydrophobic, which induces poor compatibility at the polymer/fiber interface. It has been demonstrated that the quality of the interface between the natural fibers and the polymeric matrix is a key issue which should be controlled in order to improve the mechanical properties, the durability and the recyclability of such composites. In this purpose, our approach is to develop a new method of fibers/polymer coupling consisting in a unique operation of reactive compounding, based on an original chemistry of organosilanes in the polymer melt. Our objective is to enhance the fibers/polymer interface by using two functionalized organosilanes simultaneously. The method is then to graft covalently one of these functions on the polymer chains and the other one onto hemp surface. In parallel, formation of Si-O-Si network between the two grafted organosilanes occurs during reactive extrusion. The achievement of this grafting of hemp fibers and thermoplastic matrix during a single compounding operation constitutes a new route for the elaboration of bio-based composites. Mechanical properties (tensile and impact) and thermal behavior of these new composites were compared to those of fibers/polymer composites without coupling agent or with a single classical one (Vinyltrimethoxysilane). Results showed that the presence of two organosilane coupling agents enhances considerably the tensile strength (+40%) of the newly developed composite. Their thermal decomposition, as shown by TGA, was shifted to higher temperatures and their water uptake at room temperature was decreased. Morphological characterizations by SEM suggest better adhesion at the interface, which can explain the properties enhancement.

cited By 35; International audience; The microstructure and properties of plastic parts made by laser sintering are known to be affected by a number of process and material parameters, whose influences are not totally elucidated yet. This study addresses the influence of the energy supplied to the powder on the microstructure and on the physical properties of sintered parts. Two different polyamide 12 powders were used. The energy density, which is derived from the laser characteristics, was shown to have a great impact on the residual porosity and on the amount of nascent particles in parts. The study of the pore size and of the porosity distribution was carried out by X-ray tomography. It is shown that the particles size distribution and the crystallisation temperature are the key parameters in the porosity formation. Finally, the influence of the microstructure and crystalline features on the mechanical properties of sintered parts is discussed. © 2012 Elsevier Ltd. All rights reserved.

Blow moulding is a process whereby a cylindrical parison is extruded first, then pinched between the two halves of a mould and, finally, blown into the product. Parison size and shape result from complex interactions between mandrel and die geometries, processing parameters and viscoelastic properties of the polymeric material. Moreover, parison size changes with time due to sag. An innovative, contactless and online measurement technique of the parison is shown to be an effective tool to measure precisely parison diameter and thickness and to capture dimensional changes with time. This technique employs laser lighting of the parison and hinges on the refractive properties of molten polymer. Images taken with a digital camera are processed to give a precise measurement of diameter and thickness, at different time step during extrusion. Thus, parison swell and sag have been recorded for a commercial HDPE. Influence of processing parameters such as the rotational screw speed or die gap width can be brought forward. From the measurements, thickness swell is found to possess a different behaviour from diameter swell. Moreover, sag has been measured and can be modelled from a Newtonian perspective using one dimensional convected coordinates. Parison sag is shown to be governed by two parameters: an extrusion velocity and a a single coefficient of sagging susceptibility which value has been deduced from experiments.

An organo-montmorillonite (Cloisite® 30B) was modified with fluorescent dyes to obtain photo-functional inorganic–organic complexes for real-time monitoring of polymer-clay nanocomposite processing. The photo-functionalization with 9-anthracenemethanol (anth) was tested using adsorption from solution, as well as dry and melt compounding. Rhodamine 6G Perchlorate (RhP) and Nile Blue A Perchlorate (NBAP) were adsorbed by cation exchange. XRD, TGA and spectrofluorimetry showed that cation exchange in water/ethanol at 80 °C, with an added amount of RhP corresponding to 25% of the initial surfactant content, followed by washing, recovering and drying, yielded the best results. The intercalation of the fluorescent dyes rearranged the paraffinic structure of the surfactant molecules by increasing the tilting angle from 37° to 49°.

Blow moulding is a process whereby a cylindrical parison is extruded first, then pinched between the two halves of a mould and finally blown into the product. Parison size and shape are resulting from complex interactions between mandrel and die geometries, processing parameters and viscoelastic properties of the polymeric material. Moreover, parison size changes with time due to sagging effect. An innovative, contactless and on-line measurement technique of the parison as it is extruded is shown to be an effective tool to measure precisely parison diameter and thickness and to capture dimensional changes during time. This technique employs laser lighting of the parison and hinges on the refractive properties of molten polymer. Images taken with a digital camera are processed to give a precise measurement of diameter and thickness, at different time step during extrusion. Thus, parison swell and sag have been recorded for a commercial HDPE. Influence of processing parameters such as the rotational speed screw speed or die gap width can be brought forward. Moreover, numerical simulations of the parison formation process have been carried out, with an integral viscoelastic constitutive equation.

Blow moulding is a process whereby a cylindrical parison is extruded first, then pinched by the two halves of a closing mould and finally blown into the product. The actual size and shape of the parison is the result of complex interactions between the die and mandrel geometries, the viscoelastic properties of the polymer and the processing parameters. In practice, obtaining a satisfactory parison is a time-consuming trial and error process. A novel technique using laser lighting of the parison at different angles, during extrusion, gives access to the size and thickness along the entire length of the parison and to the shape of the cross-section at a given location. Results are given for a polypropylene an a polyethylene materials, together with extrusion simulations of a parison with an elliptic cross-section. Increasing the flow rate is increasing the swelling of the parison and its thickness. This effect is more marked with Polypropylene than with Polyethylene.

In injection moulding, long glass fibre reinforced thermoplastics (LGFT) are an attractive way to produce large parts at low cost. However the strength of the part depends chiefly on the average fibre length, fibres which are subjected to considerable attrition during processing in conventional three stage screws.

Due to increasing expectations from the market, the aspect of molded parts has to be improved constantly. Some of the defects observed on these parts such as weldlines are related to the filling stage. To limit this, we investigated the influence on weldlines using various surface deposits on the mold surface, mainly PVD and PACVD deposits : Chromium nitride (CrN), Titanium nitride (TiN), Diamond like Carbon (DLC), Chromium and polished steel (PG) on an instrumented plate mold. Injection campaign was led on three polymers which differ in terms of nature (amorphous, semi‐crystalline, copolymers).We studied the evolution of the dimensions of weldlines appearing on the plate using the same injection parameters for a given polymer, but with various deposits and thicknesses. Another aspect that had been investigated is the morphology of the weldline through the thickness of the part, depending on polymer nature. Adhesion of polymer at the flow front with the mold surface proved to change. The modification of t...

International audience; The flow of a viscous Newtonian and of a viscoelastic (upper-convected Maxwell) fluids in the helical rectangular channel of a screw have been solved using a series expansion on dimensionless torsion and curvature. At any given power of curvature or torsion, the velocity and stream function are solved using series of orthogonal eigenfunctions. Back pressure driven flow and drag driven flow have been both considered for Newtonian fluids, while only pressure driven flow can be studied for viscoelastic fluids. Contrary to common knowledge in polymer processing based on the Parallel Plate Model, we found that, in the case of cross-sections with large aspect ratio, torsion effects can be significant. Specific viscoelastic effects triggered by curvature and torsion have also been modelled. Our analytical model has been precisely validated by three-dimensional finite elements calculations