Nicolas Boyard

In this paper, we present an application of fibres optic sensors to the studies of the Resin Transfer Molding process. We present in a first time our experiments devices. Then, two fibres optics sensors are described and their associated exploitation means too. Finally few experimental results are presented and described. So we show that it is possible to measure temperature, cure degree and material change with a simple sensor. With a more complicated one we can also measure curing reaction stress.

ABSTRACT 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.

The crystallization of polymers is commonly restricted in between the equilibrium melting temperature of crystals and the glass transition temperature. Study of the isothermal crystallization, close to the glass transition temperature, is complicated since nucleation and crystallization must be avoided during the approach of the temperature of interest. It requires cooling at a rate, which is distinctly higher than the maximum rate of nucleation. In fact, the cooling capacity of standard differential scanning calorimetry (DSC) is often insufficient to highly super cool the melt before onset of nucleation or ordering processes. For that reason, the Flash DSC 1 was used, where the cooling rate can reach more than 1000 Kelvin per second, which has allowed studying the isothermal crystallization kinetics in a large temperature range.

We report below about the control of processing of composite materials in particular carbon fibre dyed into epoxy resin. In the aeronautic industry, structures made of such composite materials have a very high "mechanical property to weight" ratio and can for this reason successfully compete with metallic ones. Their utilisation is constantly increasing in this industry and this growth is

ABSTRACT In Liquid Composite Molding (LCM) processes the saturation of the reinforcement by the resin may induce the creation of porosity in the preform affecting the final properties of the composite. The purpose of this work concerns the development of an experimental protocol and the associated modeling to identify the dynamic saturation curve during filling by taking advantage of sharp contrasts of thermal properties existing between dry and fully-saturated reinforcement. To identify saturation, several injections were performed with a laboratory RTM mold for which thermal design allows accurate control of heat transfer. Several heat flux sensors were used to identify the saturation curve. Sensitivity analysis proves the feasibility of the method. The results are compared with a conductometric method with good agreement. Evolution of residual voids identified for several flow rates are also consistent with those expected according to the capillary number.

Injection molding is the most widely used process in the plastic industry. In the case of semi-crystalline polymer, crystallization kinetics impacts directly the quality of the piece, both on dimensional and mechanical aspects. The characterization of these kinetics is therefore of primary importance to model the process, in particular during the cooling phase. To be representative, this characterization must be carried out under conditions as close as possible to those encountered in the process: high pressure, high cooling rate, shearing, and potential presence of fibers. However, conventional apparatus such as the differential scanning calorimeter do not allow to reach these conditions. A PVTalpha apparatus, initially developed in the laboratory for the characterization of thermoset composites, was adapted to identify the crystallization kinetics. The aim of the presented study is to demonstrate the feasibility of the identification. This device allows the molding of a circular sample of 40 mm diameter and 6 mm thick by controlling the applied pressure on the sample and the temperature field on its surfaces. This mold is designed such as heat transfer is 1D within the thickness of the sample. It is equipped with two heat flux sensors to determine the average crystallization rate through the thickness and a displacement sensor for the determination of the volume change. The heat transfer problem between the polymer and the molding cavity is modeled by using a 1D conduction problem with a moving boundary, in which the control volume is a uniform temperature disk with a variable volume, and coupled to a crystallization kinetic model. An inverse method is used to identify the parameters of the crystallization kinetic model by minimizing an objective function based on the difference between the evolutions of the experimental and computed volume of the sample. The first validation of this methodology was to compare the kinetic parameters identified with this apparatus with those obtained from DSC experiments, i.e. without additional pressure and at low cooling rates. A good agreement was obtained between both methods. A second validation was to compare experimental and computed temperatures at the center of the plastic part. In this case also, a very good agreement was found. The feasibility of the methodology is now demonstrated. The device is being adapted to increase the level of applied pressure as well as the cooling rate to achieve injection conditions.

RTM6 epoxy resin curing is usually characterized by the polymerisation degree. We report in this paper on a refractive index measurement technique applied on an experimental mould to control, quantitatively and in situ, the industrial RTM process. For the first time, we determined simultaneously the thermo-optical coefficient, the refractive index evolution, the specific volume and the polymerisation degree of the

ABSTRACT The modeling of thermal behavior of composite parts during their forming requires an accurate knowledge of their thermo-physical properties. Because of the heterogeneous nature of composites, the thermal conductivity tensor appears to be the most tricky to determine experimentally but also to model. A wide range of experimental methods can be found in the literature in order to measure either in-plane or transverse conductivity of composite parts, but very few succeed in performing it on dry preform or uncured laminates. In this study, the effective thermal conductivity tensor of carbon/epoxy laminates is investigated experimentally in the three states of a typical LCM-process: dry-reinforcement, raw and cured composite. Samples are made of twill-weave carbon fabric impregnated with epoxy resin. The transverse thermal conductivity is determined using a classical estimation algorithm, whereas a special testing apparatus is designed to estimate in-plane conductivity for different temperatures and different states of the composite. Experimental results are then compared to modified Charles & Wilson and Maxwell models. The fiber crimping of a ply is also taken into account in modeling. The comparison shows clearly that these models can be used to predict the effective thermal conductivities of woven-reinforced composites provided that the material properties are well known.

ABSTRACT This work presents an efficient method to quickly calculate with good accuracy (to 5%) the solidification time of an injected semi-crystalline polymer slab. Under some hypotheses this polymer can be considered as a phase change material with a constant phase change temperature. We use a noteworthy property established as the ratio between the thickness of a solidifying phase change finite medium and the solidified thickness in a semi-infinite medium. The knowledge of this ratio enables to predict analytically the solidification time in a 1D finite medium. This ratio can be parameterized as a function of characteristic numbers in phase change problems: Stefan numbers and the ratio of thermal diffusivities of both phases. The results are compared with those given by a complete model integrating the physics of the coupling between heat transfer and crystallization kinetics. The solidification times computed from both models are very close, demonstrating the relevance of the simplified model. Finally, we also get a very good accuracy in calculating the total cooling time, from injection to ejection.

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.