Vincent Sobotka

Evolution of chemical and thermal curvatures in thermoset-laminated composite plates during the fabrication process

Residual deformations and stresses formation in the thermoset-laminated composite is a frequently studied subject in the recent years. During fabrication, the laminated composites undergo chemical deformation during cross-linking and thermal deformation while cooling. In thin laminates, due to large displacements and complex evolution of shape, these deformations can only be explained by using nonlinear strain–displacement relationship. In the present article, we calculated together for the first time, the thermal and chemical deformations occurring in carbon/epoxy laminates by considering a nonlinear geometrical approach to understand the evolution of shape and hence residual stresses induced during fabrication process. The effect of fibre fraction on the chemical and thermal deformations is studied as well.

A Device to Measure the Shrinkage and Heat Transfers during the Curing Cycle of Thermoset Composites

Residual stresses development during manufacturing of composites depends mostly on the shrinkage behaviour of the polymer matrix from the point where stresses cannot be relaxed anymore. The matrix shrinkage may have a thermal and/or chemical origin and can leads to dimensional instability, ply cracking, delamination and fibre buckling. The approaches for measuring cure shrinkage can be classified as volume and non-volume dilatometry. Each technique has corresponding advantages and drawbacks but volume dilatometry is the one that is mostly used. In the present article, we report a home-built apparatus, named PVT-a mould, on which temperature, volume change and reaction conversion degree are measured simultaneously for an applied pressure. It can also be used to study the composite during curing and for the bulk samples having several millimetre thicknesses. The instrument is preferred over other techniques as it works in conditions close to the industrial ones. This device was used to measure cure shrinkage of resin and thermoset composite material with different fibre fractions as a function of temperature and reaction conversion degree. The heat of cure of the resin measured by PVT-a mould was compared to the results obtained by DSC.

In situ characterization of in-plane chemical shrinkage of thermoset laminated composites using a simple setup

Cure shrinkage in the thermoset matrix is the major source of cure-induced defects in composite parts for industrial applications. Thus, its correct determination is very important to optimize the composite fabrication process. In general, volume chemical shrinkage of resin is tested and assuming it is isotropic, rule of mixture or a homogenization technique is used to model the linear chemical shrinkage of composite. Some studies are also found in the literature on the measurement of linear chemical shrinkage of very small composite samples under atmospheric pressure. In the present article, a new setup is presented for the measurement of evolution of in-plane chemical shrinkage of thermoset laminated composite during curing. Using this setup, characterization of mass scale samples was done under pressure and heating ramp conditions. Degree of cure of composite during the test was determined using differential scanning calorimeter. Results show that chemical shrinkage in the composite appears from gel point and its evolution with the degree of cure is nonlinear. Experimental results also led to conclusion that most of the chemical shrinkage occur along the thickness direction.

Analysis and Control of Heat Transfer in an Industrial Composite Mold in RTM Polyester Automotive Process

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.

Uniform Cooling and Part Warpage Reduction in Injection Molding Thanks to the Design of an Effective Cooling System

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.

Optimization of the Incident IR Heat Flux upon a 3D Geometry Composite Part (Carbon/Epoxy)

The main purpose of this study is to cure a 3D geometry composite part (carbon fiber reinforced epoxy matrix) using an infrared oven. The work consists of two parts. In the first part, a FE thermal model was developed, for the prediction of the infrared incident heat flux on the top surface of the composite during the curing process. This model was validated using a reference solution based on ray tracing algorithms developed in Matlab®. Through the FE thermal model, an optimization study on the percentage power of each infrared heater is performed in order to optimize the incident IR heat flux uniformity on the composite. This optimization is performed using the Matlab® optimization algorithms based on Sequential Quadratic Programming and dynamically linked with the FE software COMSOL Multiphysics®. In a second part, the optimized parameters set is used in a model developed for the thermo-kinetic simulations of the composite IR curing process and the predictions of the degree of cure and temperature distribution in the composite part during the curing process.

Experimental Determination and Modeling of Thermal Conductivity Tensor of Carbon/Epoxy Composite

In this study, the effective thermal conductivity tensor of carbon/epoxy laminates was investigated experimentally in the three states of a typical LCM-process: dry-reinforcement, raw and cured composite. Samples were made of twill-weave carbon fabric impregnated with epoxy resin. The transverse thermal conductivity was determined using a classical estimation algorithm, whereas a special testing apparatus was designed to estimate in-plane conductivity for different temperatures and different states of the composite. Experimental results were then compared to modified Charles & Wilson and Maxwell models. The comparison showed clearly that these models can be used to accurately and efficiently predict the effective thermal conductivities of woven-reinforced composites.

A New PvT Device for the Thermoplastics Characterization in Extreme Thermal Conditions

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.

Identification of the crystallization kinetic parameters of a semi‐crystalline polymer by using PVTα measurement

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 PVTα 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...

Inverse method for the cooling system design in injection moulding – application to a ‘T-shaped’ piece

The designing of the cooling channels in the thermoplastic injection process is one of the most important steps during mould design. Indeed, inappropriate cooling will lead to defects in the piece and a low production rate. In this paper, a new approach for the design of cooling channels is presented. Based on morphological concepts, the idea of regulation by cooling surface is considered. The first part of the methodology leads to the optimal determination of fluid temperature distribution along the cooling surface in order to minimise a cost function composed of two terms linked to the quality of the piece and the productivity of the process. The conjugate gradient algorithm coupled with a Lagrangian approach is implemented for the determination of the optimal fluid temperature distribution. The method is applied to design the cooling system of a ‘T-shaped’ piece, and numerical results are then compared with those available in the literature for the same piece shape and the same injection moulding conditions.