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Latent heat storage is a very efficient technique because of the high energy density of Phase Change Materials (PCM). However, these materials have low thermal conductivities which limit their use for industrial applications requesting a large power density. The purpose of this study is to present an experimental thermal energy storage using a phase change material associated with various configurations of conductive structures: finned exchangers, graphite powder and Expanded Natural Graphite (ENG) matrix. The experimental thermal conductivity enhancer setup consists of a heat exchanger tube. The outer annular portion of the exchanger is filled with the PCM. The Overall Heat Transfer Coefficient (OHTC) between the fluid and the PCM is measured for each configuration. PCM embedded in an ENG matrix appears to be the best configuration with an OHTC of about 3000 W m −2 K −1 and confirms the interest of such a storage device for industrial processes. A study of the thermal properties of this composite material is detailed. The measured thermal conductivity of the ENG/PCM composite is about 100 times greater than the conductivity of the PCM alone. A numerical model for the simulation of the heat transfer within such a material is developed by an enthalpy method in an axisymmetric geometry. It is used for the identification of the thermal contact resistance between the tube and the composite material. The numerical simulations are also compared to the experimental results and used to investigate the impact of the thermal contact resistance on the heat transfer process of the LHTS system. [ABSTRACT FROM AUTHOR]

A composite mold was developed to manufacture pieces by the resin transfer molding process. This mold can be instrumented with different sensors which allows us to obtain great amounts of information during realistic conditions of production. The main features of the mold, as well as the used metrology, are detailed. The results of different experiments and the influence of temperature on the molding are presented. The experimental heat fluxes are compared to the predictions of a kinetic model. A thermal optimization of the curing cycle is proposed. [ABSTRACT FROM AUTHOR]

The aim of analytical models presented in this paper was to provide a first set of data for choosing thermal conditions during thermoplastic injection molding. Two different models allowed the quick determination of the cooling/solidifying time, the mold surface temperature variation and the heat flux densities exchanged between the polymer and the mold. The first model specific to amorphous polymers was based on the 1D heat conduction equation. The second model dedicated to semi-crystalline polymer was based on the adaptation of the solution of the Stefan problem for part with a finite thickness. For each case, the influence of the thermal contact resistance between polymer and the mold on the cooling/solidifying time was highlighted. Then, the description of heat transfer in the mold allowed to determine the time needed to produce reliable parts when the mold was in periodic steady state. The attenuation of the periodic temperature variation through the thickness of the mold was evaluated through the penetration depth. Parameters calculated from these analytical models were compared with experimental results obtained with an instrumented mold or with data computed with a coupled model. The good agreement between them validated the interest to these models to get quickly reliable characteristic parameters of injection molding. [ABSTRACT FROM AUTHOR]

Heat transfer simulation in thermoplastics processing is crucial to predict the quality of parts in terms of geometry and mechanical properties. The results accuracy depends on the knowledge of thermo-physical properties, crystallization kinetics and boundary conditions, which have to be determined in representative process conditions. In this work, we focus on the impact of high cooling rate on heat transfer and crystallization at the surface of a thermoplastic part. For this purpose, a home-made device named “Lagardère apparatus” is presented. It can reproduce cooling rate encountered in injection moulding by the sudden contact between a cold surface and a molten polymer. The first objective of this paper is the direct measurement of the thermal contact resistance used to model the imperfect contact between the polymer and the mould. The mould surface temperature and the heat flux at the interface were provided by a heat flux sensor. The specificity of this methodology was to obtain the polymer surface temperature with an optical fibre associated with a photodetector. A particular attention was made to avoid the invasiveness of the sensors on heat transfer. The second objective of the paper is the crystallization study at the surface of the polypropylene part. The experimental heat flux was compared with computed ones for which crystallization kinetics comes from Fast Scanning Calorimetry measurement and a standard model commonly used in the literature. It seemed that a very fast crystallization occurred at the surface of the part with a lower released enthalpy compared to the bulk. [ABSTRACT FROM AUTHOR]

In this paper, we present the isothermal crystallization kinetics study of a new generation of polyamides 66 (PA 66) over a large temperature range [100–234 °C] using differential scanning calorimetry (DSC) to reach low crystallization temperature at very high cooling rates. This PA 66 with a low viscosity is of high industrial interest since it is dedicated to composite part manufacturing at low pressure (≤0.15 MPa) using new thermoplastic for Resin Transfer Molding (RTM) process with high fiber volume fraction (≈50 vol%). The low viscosity (≈15 Pa s), is one of the major difference between the new generation of PA 66 and the others. A crystallization model based on Avrami's theory is proposed for further simulation of this industrial process in commercial softwares. However, industrial process conditions require to know the crystallization kinetics over a large temperature domain. For this purpose, isothermal crystallization is quantified from Flash DSC 1 experiments using two methodologies. The measurements were compared to standard DSC data to validate the results. Then, the effects of the molar mass, additive and the sample size of the Flash DSC 1 on the crystallization kinetics were studied. As a result, crystallization of PA 66 without additive exhibits a bimodal temperature dependence. The molar mass has only an effect on the crystallization kinetics in the high temperature range, suggesting a change in the nucleation mechanism. However, the presence of additives suppresses this bimodal behavior. Finally, the effect of the sample mass on the Avrami parameters was also discussed. The results obtained in this study can be used to accurately compute heat transfer coupled to solidification during the manufacturing of PA 66 parts including processes with high cooling rates. [ABSTRACT FROM AUTHOR]

The quality of thermoplastic parts strongly depends on their thermal history during processing. Heat transfer modelling requires accurate knowledge of thermophysical properties and crystallization kinetics in conditions representative of the forming process. In this work, we present a new PvT apparatus and associated method to identify the crystallization kinetics under pressure. The PvT-xT mould was designed for high performance thermoplastics: high temperature (up to 400 °C), high cooling rate (up to 200 K/min) and very high pressure (up to 200 MPa). Specific volume measurements were performed at a low cooling rate to avoid a thermal gradient. The crystallization kinetics under pressure can be identified for a wide range of cooling rates by an inverse method taking into account the thermal and crystallinity gradients. Since identification is based on volume variations, the proposed methodology is non-intrusive. Furthermore, the enthalpy released by the crystallization was measured during the experiment by a heat flux sensor located in the moulding cavity. [ABSTRACT FROM AUTHOR]

Abstract: The control and optimization of heat transfer during the forming of thermoplastic parts is of primary importance since they impact on the quality of final parts. The modelling of this transfer requires accurate knowledge of the polymer thermo-physical properties, and also of the parameters describing the crystallization, this latter data being very sensitive to the thermal history for semi-crystalline polymers. The experimental determination of these parameters requires the use of many instruments, which is time consuming. To address this issue, a home-built instrumented mould was designed to measure and identify several properties from a single experiment. Specific volume, transverse thermal conductivity in amorphous and solid states can be estimated as a function of the temperature. Parameters of a crystallization kinetics model are identified with a non invasive procedure. Our methodology is illustrated on a well-known semi-crystalline thermoplastic. Identified parameters are compared with literature results. [Copyright &y& Elsevier]

Abstract: This study consists of determining by inverse method the set-point temperature of the fluid flowing through heating plates in a Resin Transfer Molding (RTM) process tool so as to reach a predetermined thermal history in the composite part. Although the described methodology is applied in a specific mold in this paper, it remains general and may be transposed to a large scale of molding configuration. The considered mold is metallic and composed of several parts. Assembling these parts is not possible without introducing imperfect contacts that perturb heat transfer between them. The heat transfer at the interface is modeled by thermal contact resistances (TCR) whose values are unknown. In the case of metallic molds TCR are of the same order of magnitude than the equivalent thermal resistance of the mold. Therefore they cannot be neglected. The influence of these TCR is then a key-point on heat transfer since a bad knowledge of their values implies a wrong estimation of the temperature field. Then before being able to estimate the set-point of the temperature of the thermoregulated fluid, it is necessary in a first stage to evaluate the most influent TCR that are spatially and time dependent. Their determination is achieved by an optimization approach and carried out on a 2D transverse cut of the mold. Experimental temperature measurements in the mold are matched to the computed responses of the heat conduction model. A least square criterion is minimized by using the conjugate gradient algorithm. The gradient of the criterion is determined by solving a set of adjoint equations. After the identification of these parameters, the same optimization method is used to compute the mold set point temperature. It is notable that the same set of adjoint equations is used to solve both problems. [Copyright &y& Elsevier]