An accurate description of the heat transfer is essential in the simulation and optimization of p... more An accurate description of the heat transfer is essential in the simulation and optimization of processes, the quality of semicrystalline thermoplastic parts is strongly linked to the thermal history during the formatting. Reliable results require a fine knowledge of the thermophysical properties of the polymer. The objective of this work is to develop experimental devices associated with identification methods to measure the thermal properties and crystallization kinetics of polymer under the material forming conditions. A measurement protocol and the identification of thermal properties and crystallization kinetics of polypropylene by inverse methods under low pressure and low cooling rate has been developed in a first study. Then, a new PvT device running up to 400 ° C and 200 MPa has been designed and experimentally validated. The thermal conductivity and the crystallization kinetics were measured under high cooling rate and high pressure. A luminance sensor combining an optical...
ABSTRACT The quality of thermoplastic parts strongly depends on their thermal history during proc... more 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.
Composite materials are heterogeneous materials which consist of a matrix reinforced by fibers. O... more Composite materials are heterogeneous materials which consist of a matrix reinforced by fibers. One of the most promising manufacturing processes are Liquid Composite Moulding (LCM) in which a stationary fibrous preform is impregnated by a liquid. It is essential to monitor two key-steps in order to ensure high quality composite parts: the mould-filling stage, during which the liquid resin flows through the fibrous preform and the solidification. Two examples related to these issues are detailed here, the common point being the use of the same sensor to identify the different properties. In a first part, an experimental device has been designed to quantify the saturation profile of a preform. This set-up consists of a mould with heat flux sensors and an injection device reproducing the actual forming process. The saturation curve can be obtained by exploiting the wall heat fluxes. In a second part, this study emphasizes the use of the same heat flux sensors to monitor the consolidation phase. Indeed heat transfer and temperature during the forming process are strongly coupled with the phase change through the variation of thermal properties. These methods clearly open up new perspectives regarding the online control of composite manufacturing processes at an industrial scale.
Injection molding is the most widely used process in the plastic industry. In the case of semi-cr... more 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.
Volume 2: Applied Fluid Mechanics; Electromechanical Systems and Mechatronics; Advanced Energy Systems; Thermal Engineering; Human Factors and Cognitive Engineering, 2012
Injection molding is the most used process for thermoplastic pieces manufacturing. This process i... more Injection molding is the most used process for thermoplastic pieces manufacturing. This process is commonly divided in four steps: injection, packing, cooling and ejection. During packing step, an amount of material gets in the molding cavity to compensate shrinkage of the polymer mainly due to the crystallization. Once gate is frozen, polymer is subjected to an isochoric cooling while pressure of the polymer is higher than atmospheric one. Improving quality of injected pieces requires prediction of shrinkage, warpage and residual stress and pressure impacts deeply on morphology and consequently on the shape of the final piece. Pressure decrease during isochoric phase also determines the ejection time. However, description of the behavior of the polymer during packing and isochoric steps needs an accurate model that considers coupling between heat transfer and crystallization and also a good knowledge of the behavior (specific volume and crystallization kinetics) of polymer under high pressure. Some studies have already underlined the influence of shear rate on kinetics of crystallization. Here, based on a pressure analysis and a experimental-numerical comparison, we confirm crystallization is strongly coupled to flow history.
The recently developed fast scanning differential calorimetry is used for the first time to deter... more The recently developed fast scanning differential calorimetry is used for the first time to determine the crystallization kinetics of Poly(EtherEtherKetone) (PEEK). In our experiments, crystallization is studied in isothermal conditions over a large temperature range from 170°C to 310°C. Two different measurement protocols were employed. Between 200 °C and 300 °C the heat flow was directly measured during isothermal crystallization. Outside this temperature range we measured the heat of fusion on heating after interrupted isothermal crystallization. We show that data can be analyzed with the Avrami approach incorporating into it a term describing secondary crystallization. The crystallization half-times are measured. The Avrami kinetic coefficient K Av associated with primary crystallization is evaluated from isothermal crystallization between 170°C and 310°C where data were not previously available. The kinetics of crystallization of PEEK has only one maximum located around 230°C and its Avrami exponent is close to 3, suggesting instantaneous nucleation with subsequent spherical growth. The whole isothermal crystallization process is modeled in terms of Hillier's model since it takes secondary crystallization kinetics into account.
ABSTRACT The control and optimization of heat transfer during the forming of thermoplastic parts ... more 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.
Quantification and understanding of the evolution of chemical shrinkage of thermoset polymers is ... more Quantification and understanding of the evolution of chemical shrinkage of thermoset polymers is of crucial importance for modelling of residual strains and stresses. Thermal properties of resin and the strong coupling between chemical reactions and thermal fields lead to non-negligible thermal and curing degree gradients in the part. In this paper, modelling of the volume chemical shrinkages of an unsaturated epoxy vinylester 2 resin and associated glass fibre composites is proposed, by taking into account the coupling between volume variation and thermal gradients. Modelling is also compared with the measurements done with a home-made instrument (PVT-). Results suggest that chemical shrinkage is non linear as a function of degree of cure. Moreover, for an equal mass of resin, chemical shrinkage of resin carrying fibres is lesser than the neat resin.
An accurate description of the heat transfer is essential in the simulation and optimization of p... more An accurate description of the heat transfer is essential in the simulation and optimization of processes, the quality of semicrystalline thermoplastic parts is strongly linked to the thermal history during the formatting. Reliable results require a fine knowledge of the thermophysical properties of the polymer. The objective of this work is to develop experimental devices associated with identification methods to measure the thermal properties and crystallization kinetics of polymer under the material forming conditions. A measurement protocol and the identification of thermal properties and crystallization kinetics of polypropylene by inverse methods under low pressure and low cooling rate has been developed in a first study. Then, a new PvT device running up to 400 ° C and 200 MPa has been designed and experimentally validated. The thermal conductivity and the crystallization kinetics were measured under high cooling rate and high pressure. A luminance sensor combining an optical...
ABSTRACT The quality of thermoplastic parts strongly depends on their thermal history during proc... more 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.
Composite materials are heterogeneous materials which consist of a matrix reinforced by fibers. O... more Composite materials are heterogeneous materials which consist of a matrix reinforced by fibers. One of the most promising manufacturing processes are Liquid Composite Moulding (LCM) in which a stationary fibrous preform is impregnated by a liquid. It is essential to monitor two key-steps in order to ensure high quality composite parts: the mould-filling stage, during which the liquid resin flows through the fibrous preform and the solidification. Two examples related to these issues are detailed here, the common point being the use of the same sensor to identify the different properties. In a first part, an experimental device has been designed to quantify the saturation profile of a preform. This set-up consists of a mould with heat flux sensors and an injection device reproducing the actual forming process. The saturation curve can be obtained by exploiting the wall heat fluxes. In a second part, this study emphasizes the use of the same heat flux sensors to monitor the consolidation phase. Indeed heat transfer and temperature during the forming process are strongly coupled with the phase change through the variation of thermal properties. These methods clearly open up new perspectives regarding the online control of composite manufacturing processes at an industrial scale.
Injection molding is the most widely used process in the plastic industry. In the case of semi-cr... more 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.
Volume 2: Applied Fluid Mechanics; Electromechanical Systems and Mechatronics; Advanced Energy Systems; Thermal Engineering; Human Factors and Cognitive Engineering, 2012
Injection molding is the most used process for thermoplastic pieces manufacturing. This process i... more Injection molding is the most used process for thermoplastic pieces manufacturing. This process is commonly divided in four steps: injection, packing, cooling and ejection. During packing step, an amount of material gets in the molding cavity to compensate shrinkage of the polymer mainly due to the crystallization. Once gate is frozen, polymer is subjected to an isochoric cooling while pressure of the polymer is higher than atmospheric one. Improving quality of injected pieces requires prediction of shrinkage, warpage and residual stress and pressure impacts deeply on morphology and consequently on the shape of the final piece. Pressure decrease during isochoric phase also determines the ejection time. However, description of the behavior of the polymer during packing and isochoric steps needs an accurate model that considers coupling between heat transfer and crystallization and also a good knowledge of the behavior (specific volume and crystallization kinetics) of polymer under high pressure. Some studies have already underlined the influence of shear rate on kinetics of crystallization. Here, based on a pressure analysis and a experimental-numerical comparison, we confirm crystallization is strongly coupled to flow history.
The recently developed fast scanning differential calorimetry is used for the first time to deter... more The recently developed fast scanning differential calorimetry is used for the first time to determine the crystallization kinetics of Poly(EtherEtherKetone) (PEEK). In our experiments, crystallization is studied in isothermal conditions over a large temperature range from 170°C to 310°C. Two different measurement protocols were employed. Between 200 °C and 300 °C the heat flow was directly measured during isothermal crystallization. Outside this temperature range we measured the heat of fusion on heating after interrupted isothermal crystallization. We show that data can be analyzed with the Avrami approach incorporating into it a term describing secondary crystallization. The crystallization half-times are measured. The Avrami kinetic coefficient K Av associated with primary crystallization is evaluated from isothermal crystallization between 170°C and 310°C where data were not previously available. The kinetics of crystallization of PEEK has only one maximum located around 230°C and its Avrami exponent is close to 3, suggesting instantaneous nucleation with subsequent spherical growth. The whole isothermal crystallization process is modeled in terms of Hillier's model since it takes secondary crystallization kinetics into account.
ABSTRACT The control and optimization of heat transfer during the forming of thermoplastic parts ... more 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.
Quantification and understanding of the evolution of chemical shrinkage of thermoset polymers is ... more Quantification and understanding of the evolution of chemical shrinkage of thermoset polymers is of crucial importance for modelling of residual strains and stresses. Thermal properties of resin and the strong coupling between chemical reactions and thermal fields lead to non-negligible thermal and curing degree gradients in the part. In this paper, modelling of the volume chemical shrinkages of an unsaturated epoxy vinylester 2 resin and associated glass fibre composites is proposed, by taking into account the coupling between volume variation and thermal gradients. Modelling is also compared with the measurements done with a home-made instrument (PVT-). Results suggest that chemical shrinkage is non linear as a function of degree of cure. Moreover, for an equal mass of resin, chemical shrinkage of resin carrying fibres is lesser than the neat resin.
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Papers by Xavier Tardif