LCM Process Research Articles

Online monitoring technology for liquid composite molding based on Ultrasonic method

Aiming at the problem that it is difficult to monitor the resin flow state in the process of Liquid composite molding, this paper proposes an online monitoring technology based on the non-contact ultrasonic pulse reflection method. The characteristics of LCM process as well as the monitoring principle of ultrasonic method is introduced. A prototype for online monitoring resin flow state in liquid composite modeling is built. The test results show that the characteristics of the ultrasonic signal changes significantly before and after the arrival of the resin flow, which demonstrates that the proposed monitoring technology is effective.

Equation for modelling energy transfers in multi-phase flows through porous media, optimised for liquid composite moulding processes

Liquid Composite Moulding LCM processes are increasingly popular in the aerospace and related industries for the production of high quality composite parts. Filling simulations are often used in the design of moulds for LCM processes for efficient and complete filling of the mould. This paper proposes an energy balance equation for modelling energy interactions in a typical LCM process for implementation in Computational Fluid Dynamics (CFD) tools such as Open Field Operation and Manipulation (OpenFOAM). The derivation of the energy balance is first described in detail, followed by a brief description of an additional equation to be solved to find the degree of cure for simulations involving a chemically reactive resin. For a preliminary validation, a simplified version of the energy equation is implemented in OpenFOAM and simulation results are compared to experiments. The temperatures predicted by the simulations are seen to closely match the experimentally observed temperatures.

Modeling the reactive PA6 flow for LCM processes

Fiber reinforced thermoplastic composites have shown to be attractive for industry as they can be reused, reshaped, welded and repaired, while keeping mechanical properties on par with thermoset composites. Since thermoplastics usually have high melt viscosities unsuitable for liquid composite molding processes, in-situ synthesis of PA6 from ε-caprolactam is considered. Its reactive mix has low viscosity which allows impregnation. However, the coupled crystallization and polymerization affects the resin viscosity and its flow is altered by the dual-scale permeability of the fiber preform. Thus, to predict the local differences in the thermoplastics properties, a coupled polymerization crystallization model needs to be integrated in the LCM processing simulation at representative scales. This study aims to propose a reliable simulation of the resin flow through a fibrous preform. Hence, viscosity measurements on the reactive mix are achieved using a rheometer with parallel-plate geometry, aiming to associate a viscosity model with the Hillier coupled polymerization-crystallization model previously determined by Vicard. The full chemorheological model will then be integrated into a simulation of LCM process in OpenFOAM®, an open source CFD software in order to follow the extent of the synthesis in the resin flow during the process. As a future work, simulations including microscale tow information extracted from a real textile specimen will permit to investigate the effect of permeability and double scale porosity in fibrous preforms on the final polymerization rate and crystallinity.

Liquid Composite Molding: A Widely Used Group of FRPC Processing Techniques, but still a Challenging Topic

Liquid Composite Molding techniques are widely used technologies in order to manufacture fiber reinforced plastic composites using near-net-shaped preforms consisting of single reinforcements, e.g. woven textiles or multiaxial fabrics. All LCM process variants have in common to impregnate and saturate dry reinforcing structures with a liquid thermoset resin system. The challenge during LCM process development and mold designing is the prevention of potential error sources for safe in-spec FRPC production. Race tracking zones and air inclusions are two major issues which need to be avoided in order to ensure an excellent FRPC quality. The knowledge about preform transmissibility, i.e. permeability, of the dry reinforcing structure to the liquid flow during the saturation phase is of major importance. The knowledge about the filling and flow behavior during FRPC processing is responsible for the process efficiency and process success. In-plane and out-of-plane permeability characterization is of great interest. Especially industry is interested in precise permeability values for numerical mold filling simulations in order to support the process development and the mold design. Industrial work is also carried out for filling strategies and textile development as well as textile improvement. The paper presents different LCM processing techniques and discusses the advantages and disadvantages as well as the linked challenges during FRPC processing. Furthermore, the in-plane permeability characterization of reinforcing structures and moreover influencing factors on the filling behavior are presented. Finally the significance of accurate and reliable permeability values according to numerical filling simulations and their validity are discussed.

Towards a 2.5D geometric model in mold filling simulation

Resin Infusion (RI) process is frequently used for large composite parts production. This Liquid Composite Molding method uses vacuum pressure to shape a plastic bag as a counter mold. Once a complete vacuum is achieved, the resin is sucked into a dry preform textile laminate via placed tubing. In this note we introduce a 2.5D model for a Liquid Composite Molding LCM process starting from a recent one introduced by Besson and Poussin in Besson and Pousin (2005) for the Resin Transfer Molding RTM process. Moreover for 2.5D models defined over quadrilateral reference domains, we show the effectiveness of the use of the Proper Generalized Decomposition. Finally, we propose a procedure, for a particular class of 2.5D model, in order to perform a numerical simulation of a mold filling process.

The Resin Radial Flow Numerical Simulation in Dual-Scale Porous Fibre Media at Constant Pressure

In this paper, setting up a mathematical model about LCM process based on the theory, which contains a sink term in the mass balance equation of the fluid dynamics. In two-dimensional mold, the finite element/control volume method is used to simulate the flow front and pressure distribution of the flowing resin in single-scale and dual-scale porous media at constant pressure.

Monolithic Approach of Stokes-Darcy Coupling for LCM Process Modelling

Monolithic Approach of Stokes-Darcy Coupling for LCM Process Modelling

Monitoring LCM Process by FBG Sensor Under Birefringence

Abstract: Optical fibre Bragg gratings (FBG) provide accurate and non‐intrusive strain and temperature local measurements. FBG sensors can be embedded into fibrous preforms to monitor the flow and the cure of the resin and deliver real‐time information on the ongoing process. The paper concentrates on the utilisation of the strain‐induced birefringence of the FBG to derive information on the effective stress–strain state of the composite at the end of the process cycle (Vacher 2004, Optical fiber sensors to monitor the processing and the mechanical characterization of composites. PhD thesis). During the cooling phase, the reflection spectrum from the FBG splits into two peaks because of the birefringence of the glass fibre owing to the residual stress. The paper shows that this effect can be utilised to estimate the residual stress and strain in composites manufactured by Liquid Composite Moulding technologies. The birefringence effect arising from the cooling of a [06,903]S CFRP laminate is first characterised, and then the determination of the strains along the principal axes inside the laminate is completed by modelling the local stress–strain state because of the interaction of the optical fibre and its environment within the framework of orthotropic thermo‐elasticity and the Classical Laminated Plate Theory.

Coupled visco-mechanical and diffusion void growth modelling during composite curing

Most critical processing step during long fiber reinforced epoxy matrix composite laminate manufacturing is the polymerization stage. If not optimized, it gives birth to defects in the bulk material, such as voids. These defects are considered as possible sources of damage in the composite parts. The aim of this work is to model the evolution of void growth in thermoset composite laminates after ply collation (autoclave processes) or resin impregnation (RTM, LCM process). A coupled mechanical and diffusion model is presented to better predict the final void size at the end of polymerization. Amongst the parameter investigated, onset of pressure application and diffusive species concentration where found to have a major effect on void size evolution during curing process.

An efficient solver of the saturation equation in liquid composite molding processes

A major issue in Liquid Composite Molding Process (LCM) concerns the reduction of voids formed during the resin filling process. Reducing the void content increases the quality of the composite and improves its mechanical properties. Most of modeling efforts on process simulation of mold filling has been focused on the single phase Darcy’s law, with resin as the only phase, ignoring the formation and transport of voids. The resin flow in a partially saturated region can be characterized as two phase flow through a porous medium. The mathematical formulation of saturation in LCM takes into account the interaction between resin and air as it occurs in a two phase flow. This model leads to the introduction of relative permeabilities as a function of saturation. The modified saturation equation is obtained as a result, which is a non-linear advection-diffusion equation with viscous and capillary phenomena. In this work, a flux limiter technique has been used to solve a modified saturation equation for the LCM process. The implemented algorithm allows a numerical optimization of the injected flow rate which minimizes the micro/macroscopic void formation during mold filling. Some preliminary numerical results are presented here in order to validate the proposed mathematical model and the numerical scheme. This formulation opens up new opportunities to improve LCM flow simulations and optimize injection molds.

A new computational tool for liquid composite moulding process design based on configuration spaces

This paper aims to define a new configuration space, called flow pattern configuration spaces (FPCS), as a new computational tool for LCM process design. The most relevant aspect when using these spaces is the definition of a new coordinate system which relates the process parameters to the flow, instead of to the traditional Cartesian coordinate system. These spaces are commonly used in mobile robots which use wheeled turning radius, path length, velocity, etc. as parameters, enabling a better understanding of the process and inherently reducing the computational costs in decision tasks. The proposed configuration space defines a mould mesh discretization using an alternative coordinate system based on two variables. One of these coordinates is based on the radial flow behaviour. Hence, the angle defined between an interest point, such as the nozzle injection or the vacuum vent, and the location of the evaluated point is selected as a fixed parameter of the FPCS. This liberates the other parameter so that it can be selected depending on the application of the FPCS. The first FPCS proposed in this paper is based on the node to node distance criterion, which has been extensively used in the literature. The resulting space is called flow pattern distance space (FPDS). The second space is based on the node filling time. Then, through Finite Element simulation, the normalized filling time is used as a criterion for the FPCS development. The resulting space is called flow pattern time space (FPTS). When we apply the normalized unidirectional flow model equations to different filling techniques, constant flow rate or pressure, the flow in the FPTS has the same behaviour as in the unidirectional case. Both spaces reduce the dimensionality of the problem to 2D or 1D allowing a simpler set out of the LCM optimization and control of problems. The concept of configuration spaces is a powerful tool to solve complex problems for LCM processes in a simple manner. To the authors’ knowledge, this is the first time that this concept has been applied to LCM processes. Some examples of applications are presented at the end of this paper.

Induction technique in manufacturing preforms

The prepreg technology is a state-of-the-art method to produce high-performance CFRP parts. Due to the high material prices, the restricted process rate, and limitations to the component complexity, in future, more and more parts will be assembled by using liquid composite moulding. Especially in the case of series larger than 100 parts per year, the LCM technology offers the best cost-effectiveness. This technology is based on resin injection into dry multilayer fibre textiles (preforms). The Institute of Joining and Welding (TU, Braunschweig), together with the Institute of Composite Structures and Adaptive Systems (DLR), has elaborated a new technology to speed up the preform process, which is the most labour-intensive step within the LCM process chain. A novel concept to consolidate binder-coated fabrics is under development. By applying the high energy transfer rate of induction technology, it is possible to heat up a preform with rates up to 50 K/s to melt the binder and consolidate the preform.

Flow numerical computation through Bezier shape deformation for LCM process simulation methods

The flow front advance computation is commonly used in Liquid Composite Molding simulation for design optimization algorithms, quality process performance, etc. It can be used also to take corrective decisions in on-line control systems for the flow redirection during filling. In these cases, the flow front can be composed by a large number of nodes. In particular, it can be computed by thousands of nodes using a CCD as Finite Element sensor with high resolution [1]. Therefore, a methodology to reduce it in a few significant points is required to improve the computational costs of whatever algorithm that uses the flow front behavior information. In other previous research results, a new mathematical formulation of the flow front using CAGD techniques was used [1], [2]. Therefore, the flow front is formulae in a Bezier curve permitting to represent whatever flow front shape in a few representative points. In particular the common flow front shapes can be represented in just 10 points. The flow front nodes were approximated in a Bezier curve using a least square method to estimate the control points of the Bezier curve and a projection of this into itself. In CAGD fields, there are another interesting techniques that can be used for this propose. In particular, there is an interesting topic known as “Bezier shape deformation” [3]. This topic treats the deformation of a predefined Bezier curve through vectors. Hence, whatever point of the Bezier curve can be moved using the modulus and the direction of this vector. Therefore, the velocity vector field obtained solving the flow kinematics with Finite Element Method (FEM) or Natural Element Method (NEM) can be used to deform a predefined Bezier curve. This flow front parameterization permits a continuous numerical formulation of the flow front avoiding the approximation techniques used in [1], [2]. This computation was also extended to study particle tracking during filling.

Regulation of apoptosis by peroxisome proliferators

Peroxisome proliferators (PPs) constitute a large and chemically diverse family of non-genotoxic rodent hepatocarcinogens that activate the PP-activated receptor α (PPARα). In order to investigate the hypothesis that PPs elicit their carcinogenic effects through the suppression of apoptosis, we established an in vitro assay for apoptosis using both primary rat hepatocytes and the FaO rat hepatoma cell line. Apoptosis was induced by transforming growth factor β1 (TGFβ1), the physiological negative regulator of liver growth. In this system, PPs could suppress both spontaneous and TGFβ1-induced apoptosis. In order to understand the mechanisms of this regulation of apoptosis, we conducted microarray analysis followed by pathway-specific gene clustering in TGFβ1-treated cells. After treatment, 76 genes were up-regulated and 185 were down-regulated more than 1.5-fold. Cluster analysis of up-regulated genes revealed three clusters, A–C. Cluster A (4 h) was associated with 12% apoptosis and consisted of genes mainly of the cytoskeleton and extracellular matrix such as troponin and the proteoglycan SDC4. In cluster B (8 h; 25% apoptosis), there were many pro- and anti-apoptotic genes such as XIAP, BAK1 and BAD, whereas at 16 h (40% apoptosis) the regulated genes were mainly those of the cellular stress pathways such as the genes implicated in the activation of the transcription factor NFκb. Genes found down-regulated in response to TGFβ1 were mainly those associated with oxidative stress and several genes implicated in glutathione production and maintenance. Thus, TGFβ1 may induce apoptosis via a down regulation of oxidant defence leading to the generation of reactive oxygen species. The ability of PPs to impact on these apoptosis pathways remains to be determined. To approach this question, we have developed a technique using laser capture microdissection of livers treated with the PP, clofibric acid coupled with gene expression array analysis. Results show that some of the key steps of the LCM process had an impact on the gene profiles generated. However, this did not preclude accurate determination of a PP-specific molecular signature. Thus, the choice of appropriate controls will ensure that meaningful gene expression analyses can be performed on tissue microdissected from the foci generated in clofibric acid treated livers. These data will allow the identification of specific genes that are regulated by PPs leading to changes in apoptosis and ultimately to tumours.