Optimal control of accelerator concentration for resin transfer molding process

Resin cure following mold filling is an essential element in resin transfer molding. To fabricate a composite part with high dimensional stability and minimize residual stress, uniform resin cure should be achieved. This study considers a three-part resin system composed of epoxy, hardener and accelerator. The cure kinetics can be controlled by the accelerator concentration at the injection gate. A numerical method that can predict degree of cure distribution based on accelerator concentration at the gate was proposed. The degree of cure distribution is obtained by solving the resin flow, heat transfer, accelerator concentration and cure problems sequentially. Utilizing this numerical method, an optimal variation of accelerator concentration during mold filling was sought by solving a constrained optimization problem. The effect of accelerator control on degree of cure distribution was investigated and its validity was examined for two different geometries.     

Parabolic modeling of the pultrusion process with thermal property variation

Pultrusion is a manufacturing method for fiber-reinforced composite with constant cross-section. In this process, a fiber creel is impregnated in a resin bath and passes through a heated die with a constant pulling force and the elevated die temperature induces the curing-resin process. At the present work the effect of variable properties (thermal conductivity and volumetric heat capacity) during the pultrusion process of thermosetting composite materials is numerically studied. The thermal properties are considered as a function of both temperature and degree of cure distributions inside the carbon/epoxy matrix composites. A two-dimensional parabolic model using the finite element method to solve the energy and degree of cure transport equations was used. These two equations are coupled by a source term from resin curing exothermic reaction. The resin cure kinetics and the properties that are temperature-dependent are both modeled by expressions obtained from the literature. The computational domain is discretized using an unstructured mesh with triangular elements and an adaptive refinement. Iterative algorithms are used to solve the algebraic equation system. Results showed that as the temperature and degree of cure along the die extension increase the volumetric heat capacity and the thermal conductivity also elevate. The influence of the pulling speed and the die temperature in the thermal property variation is also analyzed. It is verified that the temperature profile at the pultruded bar centerline for the variable property case is smoother than the constant one, similarly when the pulling speed is increased. The degree of cure development is delayed for the variable property simulation, requiring a larger die length to reach a suitable degree of cure design value. Moreover, the proper knowledge of these characteristics allows a better pultrusion process design.     

Use of non local equilibrium theory to predict transient temperature during non-isothermal resin flow in a fibrous medium

Liquid composite molding processes generally involve injection of a polymeric resin in a fibrous preform previously placed in a closed mold. Resin kinetics largely depend on the temperature cycle applied and, as far as thick composites parts are concerned, they can greatly impact on the temperature profile, especially in the core of the part where high temperature range can be reached, affecting the part mechanical properties. Thermal analysis of the system is usually done at the macro-scale level. However, at micro level and because of resin flow across the fibrous preform, local thermal effects have to be considered. A heat dispersion coefficient for instance will account for the hydrodynamic effects so as to improve significantly the accuracy of the temperature profile prediction at steady state. To improve prediction of transient temperature profiles, local heat transfer between resin and fibers needs to be considered. The characterization of this coefficient is conducted following an inverse method, numerical solutions parametered by this coefficient being derived from a non local thermal equilibrium (or two-equation model) and compared with experimental temperature profiles drawn for several injection velocity cases in a sensored mold. Significant improvement in the prediction of transient temperature profile is then obtained. Correlation between the injection velocity and the local heat exchange coefficient is also shown.     

Using thermally insulated polymer film for mold temperature control to improve surface quality of microcellular injection molded parts

Microcellular injection molding provides many advantages over conventional foams and their unfoamed counterparts, but its applications are limited by visible surface quality problems such as silver streaks and swirl marks. In this study, a mold temperature control method was proposed which a thermally insulated composite polymer film (82%PET + 18%PC) stick on the surface of mold core is used to achieve heat transfer delay at the plastic's melt–mold surface interface during the microcellular injection molding process to improve surface quality of molded parts. Effect of film thickness on the part surface quality, was also investigated using surface roughness measurements and visual inspection of the molded parts. It was found that the surface quality of parts can be greatly improved without a significant increase in cycle time when comparing with parts molded without polymer film. The surface roughness can decreases from 5.6 to 1.8 μm when polymer film thickness increases from 0.125 to 0.188 mm. Meanwhile, the flow marks of gas bubbles on the part surface can be removed completely at film thickness of 0.188 mm. The usefulness by stick a polymer film on the surface of mold core for mold temperature control in improving part surface quality during microcellular injection molding has been successfully demonstrated.     

Autoclave curing of thermosetting composites: Process modeling for the cure assembly

The heat transfer process involved in the autoclave curing of fiber-reinforced thermosetting composites is investigated numerically. In the analysis, the cure assembly is assumed to consist of a tool plate, composite laminate, and bleeder/vacuum bag. Thus, the heat transfer between the composite and the autoclave environment takes place through materials of different thermal properties. The temperature distribution thus obtained is utilized in conjunction with the models formulated by Loos and Springer [2] to calculate the cure parameters such as degree of cure, resin viscosity, and ply compaction. Four important issues arise from the process control and optimization (i.e., the effects of heating rate, laminate thickness, bleeder material, and convective heating) are addressed in this study. Based on the results obtained, recommendations for possible process improvement are made.     

Inverse methodology to determine mold set-point temperature in resin transfer molding process

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.     

Bipolar plate design and fabrication using graphite reinforced composite laminate for proton exchange membrane fuel cells

A bipolar plate is designed to have high electric conductivity, low corrosion and good mechanical strength characteristics. The two most common materials adopted for bipolar plates are carbon and metal. The carbon bipolar plate has good electric conductivity and corrosion resistance but brittle. The metal bipolar plate has good mechanical strength, acceptable electrical conductivity but worse corrosion resistance. The main objective of this paper is to design and fabricate graphite composite laminate based PEMFC bipolar plate. A thermoset type phenolic resin is adopted as the matrix with a plain weave type woven graphite fiber cloth adopted as the composite laminate reinforcement. In the fabrication process, thermoset phenol-formaldehyde resin is first printed onto the plain-weave woven carbon fiber cloth and the waiting until air-dry as prepregs. Several layers of prepregs were then stacked into a mold and heated. The resin contained in the prepregs melted and cured into a composite laminate. The carbonization process is further conducted to increase the electric conductivity. The flow channels are carved and the bipolar plate is completely fabricated. The developed bipolar plates are assembled into a single cell PEMFC and tested. The composite bipolar plate performance with or without carbonization are also studied. The back side bipolar plate electric conductivity would also significantly affect the cell performance. Therefore, increasing the back side conductivity could increase the cell performance.     

HYGROTHERMAL STRESS IN A PLATE SUBJECTED TO ANTISYMMETRIC TIME-DEPENDENT MOISTURE AND TEMPERATURE BOUNDARY CONDITIONS

When the temperature and/or moisture at the surfaces of a composite change suddenly, stresses will arise in the composite owing to the nonuniform diffusion of heat and moisture. Recent investigations have shown that under certain conditions the classical uncoupled theory of diffusion can significantly underestimate the coefficient of diffusion. The coupling between heat and moisture is an inherent part of the diffusion process that cannot be neglected on intuitive grounds. This investigation is an inquiry into the influence of antisymmetric boundary conditions on the magnitude of the hygrothermal stresses in a plate made of T300/5208 epoxy material, commonly used in graphite fiber-reinforced composites. Both moisture and temperature boundary conditions are considered. Because of the nonlinear character of the coupled equations, a finite-difference scheme is adopted. Numerical results involving time-dependent moisture, temperature, and stress distributions in the plate are displayed graphically; they show that the stresses derived from the coupled theory differ appreciably from the uncoupled results, both qualitatively and quantitatively. The hygrothermal stresses with coupling taken into account acquire an oscillatory character when the temperature on the plate is raised suddenly; this factor could contribute to material damage. In addition, antisymmetric boundary conditions can either raise or lower the stress levels, depending on time and the transient nature of the applied temperature.     

Analytical solution for transient temperature and thermal stresses within convective multilayer disks with time-dependent internal heat generation,Part I: Methodology

This work deals with the exact solution for asymmetric transient problem of heat conduction and accordingly thermal stresses within multilayer hollow or solid disks which lose heat by convection to the surrounding ambient. The combination of the separation of variables method (SVM) and Duhamel's theorem is applied to the heat conduction problem which provides a versatile technique. The temperature distribution is obtained by the SVM which concerns the heat conduction problem with time-independent internal heat generation. Applying Duhamel's theorem on the previous solution, temperature distribution with time-dependent internal heat generation can be achieved. Accordingly, assuming plane stress condition, radial and tangential stresses are obtained which are incorporated into the equivalent tensile stress formulation to calculate von Mises stress. The comprehensive methodology described here can be useful addition for many new emerging fields in which both transient and steady-state temperature distributions and thermal stresses for composite disks are important.     

Warpage control of thin-walled injection molding using local mold temperatures

Currently, 3C products are required to be lightweight, portable, and convenient. Injection molding is among the most used techniques for mass production in plastic processing industries; however, producing thinner parts that do not warp is challenging. Although plastic components warp for numerous complicated reasons, warpage primarily is caused by variations in shrinkage during the injection process of plastic part manufacturing. Material properties, part design, mold design, and processing conditions are factors influencing variations in the part shrinkage. For example, inconsistent thickness in component geometry, poor sprue–runner–gate or cooling design in the injection mold, and improper molding condition settings may cause plastic parts to warp excessively. Warpage causes unpredictable component shapes, which may cause poor assembly quality. Although mold cooling achieved by adjusting mold temperatures improves warpage, the conventional single mold temperature setting for each male or female mold plate limits the cooling capability. Therefore, this paper describes local mold temperature settings for a cooling system that can prevent severe warpage in an asymmetric plastic cover for handheld communication devices. The neutral axis theory is introduced to analyze the temperature distribution in the cross section of a part, and then predict the warping trend. Through simulation and experiments conducted in this study, the feasibility of using an effective local mold temperature setting in a cooling system to reduce part warpage was verified.     

Numerical simulation of filling and solidification of permanent mold castings

Filling of a mold is an essential part of the permanent mold casting process and affects significantly the heat transfer and solidification of the melt. For this reason, accurate prediction of the temperature field in permanent mold castings can be achieved only by including simulation of filling in the analysis. In this work we model filling and solidification of a casting of an automotive piston produced from an aluminum alloy. Filling of the three-dimensional mold is modeled by using the volume-of-fluid method. Fluid mechanics and heat transfer equations are solved by a finite element method. Comparisons of numerical results to available experimental data show that the formulated model provides a solution of acceptable accuracy despite some uncertainty in material properties and boundary and initial conditions. This implies that the model can be a viable tool to design permanent molds.     

TRANSIENT THERMAL STRESSES IN A CHEMICALLY HARDENING MATERIAL CAST WITHIN AN ELASTIC CYLINDRICAL MOLD

Abstract

A method is formulated for determining the transient thermal stresses that evolve during the molding process within a material hardening as the result of an exothermic chemical reaction. The formulation is applied to the restricted case in which the shear-to-bulk-modulus ratio for the fully hardened material is small, and where the material is cast in the form of an infinitely long solid cylinder in a thermally thin elastic mold. The heat-generation rate and temperature variation are derived from a first-order chemical reaction. The solidification of the material is described by a transformation from an initial liquidlike state to an elastic solid, with thermoelastic constitutive rate equations during the hardening process based upon a two-component mixture model whose composition is related to the degree of reaction. Graphs for transient stresses and residual stress distributions are shown in terms of dimensionless variables, with hardening rate related to thermochemical constants, and with the elastic properties of the poured material and mold as parameters. Illustrative examples are analyzed to show the existence of tensile stresses that may be associated with cracking during the molding process, and containment stresses in the mold arising from thermal expansion of the hardening material. Material properties are listed to form numerical estimates of the dimensionless variables.     

Thermal stresses in radiant tubes due to axial,circumferential and radial temperature distributions

This paper presents an analysis of the thermal stresses in radiant tubes. The analytical analysis is verified using a finite element model. It was found that axial temperature gradients are not a source of thermal stresses as long as the temperature distribution is linear. Spikes in the axial temperature gradient are a source of high thermal stresses. Symmetric circumferential gradients generate thermal stresses, which are low as compared to the stress rupture value of radiant tubes. Radial temperature gradients create bi-axial stresses and can be a major source of thermal stress in radiant tubes. A local hot spot generates stresses, which can lead to failure of the tube.     

A FINITE ELEMENT COUPLED THERMOVISCOELASTIC CREEP APPROACH FOR HETEROGENEOUS STRUCTURES

Residual stress development in heterogeneous or composite materials is an important problem in manufacture process modeling. Composites possess an intricate microstructure in which the mismatch in thermal expansion coefficients between the fiber and matrix can lead to residual thermal stresses upon part cool-down. It is commonly assumed that prior to cool-down, the entire composite at both micro and macro length scales is at a zero stress temperature. As cool-down initiates, the mismatch in thermostress behavior and the mismatch in viscoelastic time-dependent behavior of the two phases lead to built-in residual stresses in the final product. A novel finite element approach is presented here for the coupled thermovisoelastic analysis of polymer-matrix composite structures containing microscopic heterogeneities. Due to its inherent advantages over other techniques, the asymptotic homogenization approach is employed to obtain the homogenized properties for use in the macroscale problem. For illustration, a simple Kelvin-Voight viscoelastic solid is studied to demonstrate the formulations involved in similar materials for which the time-dependent stress-strain relationship is subsequently homogenized. The formulation accounts for the dissipative corrector behavior for heterogeneous viscoelastic materials. An analytical solution for the degenerative homogeneous viscoelastic material subject to uniform thermal relaxation is employed to verify part of the formulations. Additional examples are shown to further illustrate the approach for more complex scenarios.     

Numerical Investigation on Aeroelastic Behavior of Composite Material Plate Excited by Pulsed Air Jet

Nowadays, carbon fiber composite material is becoming more and more popular in aero engine industry due to its high specific strength and stiffness. Laminate carbon fiber composite material is widely used to manufacture the high load wide chord fan blade, containment casing, etc. The aeroelastic behavior of composite product is critical for the optimization of the product design and manufacturing. In order to explore its aeroelastic property, this paper discusses the coupled simulation of aerodynamic excitation applied on laminate composite material plate. Mechanical behavior of composite material plate is different from that of isotropic material plate such as metal plate, because it is anisotropy and has relative high mechanical damping due to resin between plies. These plates to be studied are designed using 4 different layup configurations which follow the design methods for composite fan blade. The numerical simulation of force response analysis mainly uses single frequency mechanical force input to simulate the electromagnetic shakers or other actuators, which could transmit mechanical force to the test parts. Meanwhile, pulsed air excitation is another way to "shake" the test parts. This excitation method induces aero damping into the test part and simulates the unsteady flow in aero engine, which could cause aeroelastic problems, such as flutter, forced response and non-synchronous vibration(NSV). In this study, numerical simulation using coupled method is conducted to explore the characteristics of laminate composite plates and the property of aerodynamic excitation force generated by pulsed air jet device. Modal analysis of composite plate shows that different ply stacking sequences have a significant impact on the plate vibration characteristics. Air pulse frequency and amplitude in flow field analysis are calibrated by hot wire anemometer results. As the air pulse frequency and amplitude are varied, incident angle of flow and layup configurations of plate can be analyzed in details by the simulations. Through the comparisons of all these factors, air pulse excitation property and the aeroelastic behavior of composite material plate are estimated. It would provide a possible way to guide the next-step experimental work with the pulsed air rig. The new composite fan blade design can be evaluated through the process.     

A NUMERICAL STUDY OF NONISOTHERMAL REACTIVE FLOW IN A DUAL-SCALE POROUS MEDIUM UNDER PARTIAL SATURATION

ABSTRACT

This numerical study investigates the temperature and cure distribution during the flow of a reactive liquid in dual-scale fibrous porous media under partial saturation. An iterative, control-volume approach, based on energy and cure balances, is used for developing discretized equations in the channels and fiber tows of the two-layer model of a dual-scale porous medium. Significant differences in the average temperatures and cures within the channels and fiber tows are observed. The ratio of the channel and fiber-tow pore volumes, the ratio of liquid and fiber heat capacities, the fiber-bundle thermal conductivity, along with the reaction rate are identified as the important parameters for temperature and cure distributions.     

An efficient BEM formulation for three-dimensional steady-state heat conduction analysis of composites

In this work, an efficient boundary element formulation has been presented for three-dimensional steady-state heat conduction analysis of fiber reinforced composites. The cylindrical shaped fibers in the three-dimensional composite matrix are represented by a system of curvilinear line elements with a prescribed diameter which facilitates efficient analysis and modeling together with the reduction in dimensionality of the problem. The variations in the temperature and flux fields in the circumferential direction of the fiber are represented in terms of a trigonometric shape function together with a linear or quadratic variation in the longitudinal direction. The resulting integrals are then treated semi-analytically which reduces the computational task significantly. The computational effort is further minimized by analytically substituting the fiber equations into the boundary integral equation of the material matrix with hole, resulting in a modified boundary integral equation of the composite matrix. An efficient assembly process of the resulting system equations is demonstrated together with several numerical examples to validate the proposed formulation. An example of application is also included.     

Numerical Method for Calculation of Complete Casting Processes—Part I: Theory

This article presents a method for calculation of the complete casting process, including the pouring of the liquid metal into the mold, its solidification, the deformation of the solidified cast, the formation of airgaps between the cast and the mold and their influence on the heat transfer, and the residual stresses. An original phase-change procedure is developed, valid for an arbitrary number of pure metals and/or alloys. A collocated version of a segregated finite-volume method is used to calculate both the liquid metal flow and the deformations and stresses in solids.     

基于温差法及单元生死法的复合材料储能飞轮缠绕预紧分析

在复合材料储能飞轮缠绕制作过程中,给纤维一定的预应力,使之在成型后材料内部产生相应的初始应力,是有效提高飞轮强度的方法之一。基于温差法和单元生死法建立的空心轮毂复合材料飞轮模型,能很好地模拟出飞轮缠绕预紧的过程,通过反复试算,得出较好的预紧缠绕的方案。计算结果表明,没有预紧缠绕的飞轮在高速回转以后存在诸多损伤,而给予合适的预应力,对飞轮回转以后各部分的周向应力分布和径向应力分布都能起到很好的改善作用。     

Effect of asymmetric cooling system on in-mold roller injection molded part warpage

In-mold decoration (IMD) injection molding has been the most promising surface decoration technique in recent years, with the in-mold roller (IMR) injection molding being the most automated production process. During the IMR process, heat transfer in the cavity surface is significantly retarded because of the low thermal conductivity of film. As a result of the asymmetric melt and mold temperature, thermal-induced part warpage easily occurs. To understand the variation in the temperature field of the core and cavity caused by the plastic film, this research uses simulation and experiments to investigate the influence of the mold's (core-and-cavity) asymmetric cooling system temperature on product warpage, and examines the impact of the film's heat retardation effect on the crystallinity, tensile strength, and surface roughness of the treated products. Our results show that the film causes a higher contact temperature between the hot melt and mold during the molding process, resulting in asymmetric temperature in the mold (core and cavity), increasing the crystallinity of the cavity and consequently increasing product warpage. In plastic, the warpage increase is from 0.03 mm to 0.62 mm when the film thickness is 0.175 mm and the temperatures of the mold and hot melt are 50 °C and 230 °C, respectively, a great increase than with steel (P20). With the asymmetric cooling system design, in which the cavity temperature is 50 °C and the core temperature is 65 °C, the warpage can be reduced by 53%. For crystallinity and crystalline size, the film heat retardation effect of the IMR process increases the crystallinity of the cavity by 16%, and the crystallite size by 12%, along with some increase in tensile strength. In addition, the IMR process can also increase the smoothness of the product surface, reducing the surface roughness by 50%.