Key Engineering Materials Vols. 554-557

Abstract: A comprehensive understanding of the inherent link between in-situ growth kinetics of a polymer spherulite and high-pressure constraints under controlled temperature is concerned. As a matter of fact, while the link with temperature is well illustrated, little comprehensive study has been conducted to quantify the effect of pressure. This is yet required to model ‘extreme’ polymer processing conditions.Mainly, the experimental set-ups developed to reproduce the pressure effect can be classified into four families: “simple” cells, dilatometric set-ups, differential thermal analysis and diamond anvil plus in-situ measurement. In this context, an original model experiment, named CRISTAPRESS, has been constructed. The cell design exploits the optical properties of semi-crystalline spherulites. Time-resolved light depolarizing microscopic observations are conducted concomitantly with a fine PVT control, for high pressure up to 200 MPa and temperature up to 300 °C. The physical analysis of isothermal and isobaric holding of a model polymer shows the influence of temperature and pressure on the key kinetic parameters of crystallization, i.e., the growth rate and the number of activated nuclei, as well as on the subsequent morphologies. Simple modeling dealing with the Avrami equation and the Hoffman & Lauritzen theory is established.

Abstract: In the present work, the thickness distribution in a plug-assisted thermoforming process is investigated using finite element (FE) simulations. Numerical simulations have been performed with the FE code ABAQUS/Explicit. The contact between sheet and tools is considered as isothermal. Moreover, the coefficient of friction between plug and sheet is assumed constant. The behavior of the material is described by three hyperelastic laws available in the FE code. The comparison between experimental and FE results highlights that the neglected thermal effects (conduction and convection) and the thermal dependence of coefficient of friction should be considered. For future work, we propose an elasto-viscoplastic model which appears to better describe the behavior of the material than a hyperelastic model.

Abstract: In this paper, a new methodology for the design of effective cooling system of thermoplastic injection tools is proposed. It is named MCOOL® for Morpho Cooling. It allows the design of the cooling channels in the mold with no a priori on the number, the size, the shape of the channels and the temperature of the coolant before performing the optimization. Numerical and experimental results obtained on a mold manufactured thanks to this methodology are compared with those coming from a conventional design. The criteria used to discriminate the results are based on the uniformity of the temperature field in the molded part and on the final warpage of the part.

Abstract: In this work, we present an apparatus associated to a methodology that is able to determine simultaneously and according to temperature (up to 400°C) the specific volume (up to 200MPa), the thermal conductivity and the temperature function of the crystallization kinetics. The PvT-XT is a home-built device that is able to impose and quantify 1D heat transfer through the radius of a sample. This apparatus controls the applied pressure on the sample while measuring its volume variations. The associated moving boundary model takes into account the temperature and crystallinity gradients. Specific volume is determined from direct measurement whereas inverse methods are used to estimate the thermal conductivity and the crystallization kinetics (with cooling rates up to 200K/min). Specific volume measurements are compared with literature results and exhibit a very good agreement. Thermal conductivity identified in the present paper is also very close to literatures values. Finally identification of kinetic function values is consistent with previous studies.

Abstract: Poly-(lactic acid) or PLA is a biodegradable polymer produced from renewable resources. Recently new polymerization routes have been discovered which allows increasing the produced quantity. Hence, PLA becomes of great interest to lessen the dependence on petroleum-based plastics. Due to its good mechanical properties, PLA is a potential substitute to some usual polymers such as PET. Nevertheless the kinetics of crystallization is relatively slow which can be an inconvenient in polymer processing. Thermomechanical history experienced by the polymer during processing affects drastically its relative crystallinity. For example, the flow is known to enhance the crystallization kinetics. Nevertheless, only a few studies were found in the literature about the crystallization of PLA under flow conditions. In the present work we investigate the crystallization of PLA under quiescent and flow conditions. A combination of DSC, rheological and optical measurements is used to identify the crystallization kinetic parameters. Thermal and flow-induced crystallization are then simulated using two sets of Schneider’s differential equations [1] based on a previously developed model Zinet & al [2]. Experimental results are analyzed and compared to the numerical model. New features about the influence of thermal and flow conditions on the crystallization of PLA are discussed.

Abstract: Modeling and optimization of LSR parts require an accurate kinetic modeling of the material crosslinking. Calorimetric and mechanical measurements for different temperature ramps are used in order to calculate the crosslinking extent as a function of time and temperature. These measurements are numerically treated in order to determine the parameters of a representative model. The chosen model will be used to simulate a LSR molded parts and design a LSR controlled molding setup.

Abstract: In this paper, we present an innovative adaptive meshing method developed in the injection simulation software Rem3D^®. This method generates a full optimized mesh with a high accuracy for strong flow/heat/rheology coupling as well for complex interface evolutions, and with a reduced number of nodes. Results show good agreement with analytical solutions, as well as for full industrial process experiments.

Abstract: In the Stretch blow moulding (SBM) process, polyethylene terephthalate (PET)-preforms are biaxially deformed to produce thin walled bottles. Finite-Element (FE)-Simulations are an important tool to optimise this process in terms of material usage and product performance. Thereby, the implementation of the thermo-mechanical material behaviour of PET plays an important role to achieve realistic simulation results. A common approach for this purpose is to calibrate a material model with stress-strain curves from biaxial stretching experiments. Thin PET-sheets are stretched under defined temperatures and strain rates. However, these experiments include process simplifications concerning geometry, heating and deformation parameters. This paper presents a method for extracting temperature dependent stress-strain-curves from experiments close to the production process. PET-Preforms receive thermal treatment with Infrared (IR)-heaters from an SBM-machine and are subsequently inflated in free air (free blow trial). A high-speed-IR-camera is used to image the axial and radial temperature distribution on the preform immediately before blowing. The deformation process is recorded via 3d-high-speed-cameras with a frame rate of 2000/s. The cameras are synchronised with a pressure sensor to consequently calculate reliable stress-strain curves at any point on the preform. In addition FE-simulations of the free blow trials are conducted using a material model calibrated with the simplified stretching experiments of thin PET sheets. Resulting stress-strain-curves from simulations and free-blow-trials are finally compared to evaluate the quality of the material model as well as the underlying testing procedure.

Abstract: Process parameters optimisation has been identified as a potential approach to realise a greener injection moulding process. However, reduction in the process energy consumption does not necessarily imply a good part quality. An effective multi-response optimisation process can be demanding and often relies on extensive operational experience from human operators. Therefore, this research focuses on an attempt to develop a more user-friendly approach which could simultaneously deal with the requirements of energy efficiency and part quality. This research proposes a novel approach using a dynamic Shainin Design of Experiment (DOE) methodology to determine an optimal combination of process parameters used in the injection moulding process. The Shainin DOE method is adopted to pinpoint the most important factors on energy consumption and the targeted part quality whereas the ‘dynamic’ term refers to the signal-response system. The effectiveness of the proposed approach was illustrated by investigating the influence of various dominant parameters on the specific energy consumption (SEC) and the Charpy impact strength (CIS) of polypropylene (PP) material after being injection-moulded into impact test specimens. From the experimental results, barrel temperature was identified as the signal factor while mould temperature and cooling time were used as control factors in the full factorial experiments. Then, response function modelling (RFM) was built to characterise the signal-response relationship as a function of the control factors. Finally, RFM led to a trade-off solution where reducing part-to-part variation for CIS resulted in an increase of SEC. Therefore, the research outcomes have demonstrated that the proposed methodology can be practically applied at the factory shop floor to achieve different performance output targets specified by the customer or the manufacturer’s intent.

Abstract: In injection moulding or in extrusion, plastication is the step during which polymer pellets are melted by the means of mechanical dissipation provided by a rotating screw and by thermal conduction coming from a heated metallic barrel. This step is crucial for melt thermal homogeneity, charge dispersion and fibre length preservation. Although there have been a large number of theoretical and experimental studies of plastication during the past decades, mostly on extrusion and mostly using the screw extraction technique, extremely few of them have dealt with trying to visualise plastication, let alone measuring the plastication profile in real-time. As a matter of fact, designing such an equipment is an arduous task. We designed an industry-sized metallic barrel, featuring 3 optical glass windows, each window possessing 3 plane faces itself to allow for visualisation and record by synchronised cameras and lightening by lasers. The laser can be used in a laser induced fluorescence or in a particle imaging velocity measurement framework. The images recorded can be further analysed by digital image processing. Preliminary results confirm the plastication theory and show a compacted solid bed and a melt pool side by side. The total plastication length is a direct function of screw rotation frequency as it is obvious from results on the melt pool width, which increases when the screw rotation frequency decreases. However, some evidence of solid bed breakage has been recorded, whereby the solid bed does not diminish continuously along the screw but is fractured in the compression zone These experimental findings are compared to predictions by a one-dimensional model of plastication