Uncertainty in the manufacturing of fibrous thermosetting composites: A review

Composites manufacturing involves many sources of uncertainty associated with material properties variation and boundary conditions variability. In this study, experimental and numerical results concerning the statistical characterization and the influence of inputs variability on the main steps of composites manufacturing including process-induced defects are presented and analysed. Each of the steps of composite manufacturing introduces variability to the subsequent processes, creating strong interdependencies between the process parameters and properties of the final part. The development and implementation of stochastic simulation tools is imperative to quantify process output variabilities and develop optimal process designs in composites manufacturing

Direct Sheet Molding Compound process (D-SMC)

The combination of highly crosslinked resin and long glass fiber reinforcement in direct sheet molding compound (D-SMC) provides the parts with high physical properties and low density suitable for large interior and exterior automotive applications. Different stages of D-SMC process including maturation zone and compression molding are studied using mathematical analysis, numerical simulation and experimental investigations. An analytical analysis is used to calculate the pressure and velocity in the first section of maturation zone and then the permeability of fiber bundle is calculated by simulation and the agreement between the available empirical equation and simulation was obtained. The flow is simulated in the next section of maturation zone using Ansys CFX and it was found the interlocking chain belt plays an important role in impregnation of bundles with paste. The microstructure of fiber glass bundles in compression is investigated in a flat plaque mold. Analyses of the bundle deformation, including bending and deformation of the tow shape, are done for the charge and specific sites within a square plaque part for 30 percent and 62 percent of initial charge mold area coverages. Microscopy and micro-computed tomography (micro-CT) of the samples clarify the microstructure of the bundles after flow. The bundles at the part edge and corner deform more than the bundles close to center of the mold in both initial charge cases. The bundles flatten at all positions and bundle bending is mainly observed at the corner. The tow width changes and tow deflections are higher in the samples of 30 percent mold area coverage. The micro-CT images show that the bundles keep their cohesion and stay straight within the middle of the flow path position, but bend at the edge of the mold. Mold filling simulation using MoldflowTM (Autodesk) predict fiber tow orientation through the thickness using the reduced-strain closure (RSC) models for fiber distribution for 30 and 62 percent initial mold area coverage. The measured value of orientation from micro-CT images confirms the random orientation through the thickness, consistent with the RSC model. The Carreau viscosity model explains the behaviour of the D-SMC paste. Another viscosity model for suspensions of high volume of planar randomly oriented glass bundles defines the flow behaviour in the process and is used to simulate a flow field. An open source CFD software package of OpenFOAM solves the equation of mass and momentum along with the introduced viscosity model by the volume of fluid technique in the three dimensional system. The simulation results show the plug flow profile at all positions and times during the process. To study bundle movement and deformation during the process, it is assumed that regions of very high viscosity with the same dimension of bundles represent bundles at different positions of the mold. The results reveal that the bundles close to the edges and corners of the mold moved more than bundles in the middle and diagonal positions. Moreover, the velocity profile shows higher velocity at these locations. Experimentally, the red bundles are placed at different locations of the charge and track during the process. The calculated movement distances of the bundles at different locations from the experiment and the OpenFOAM simulation follow the same trends and therefore agree. In D-SMC, there is a viscosity variation through thickness because of temperature gradient and glass bundle volume fraction variation. The charge is placed between two hot plates of mold which are then closed to squeeze the charge and filled the mold. The layers of charge close to the hot mold plates act as lubricant for layers of charge in center. Because of greater resistance of charge in the center to flow, the hotter lubrication layer may flow preferentially before core layers of charge. The simulation is run using Ansys Polyflow to study the preferential flow in D-SMC process. Several stacks of charges on top of each other are used for compression molding. The flow is simulated for two charges to investigate the evolution of interface during mold filling Images of cross sections from experimental samples are used to validate the simulation analysis

Analysis of Reactive Injection Compression Molding by Numerical Simulations and Experiments

Injection compression molding is an injection molding process with the addition of a compression stage after the injection. This process is useful for the injection molding of precision parts. A stable and controlled manufacturing process is needed to guarantee reliability of complex products, and usually process optimization is achieved by experimental and time consuming approaches. However, for being competitive a minimal market time is a very important requirement and computer simulations can help to optimize the process at the only expense of computational time. This paper reports and discusses for the first time the results of a 3D finite element simulation of reactive injection compression molding (RICM) by commercial software for the production of rubber diaphragms. In particular, the stages of mold filling dynamics and material curing are analyzed and the results verified with experimental tests. To get an accurate representation of the process, the rheological behavior, thermal properties, and kinetic behavior during curing of the real rubber compound were described by mathematical models. A differential scanning calorimeter (DSC) and a capillary rheometer are employed to characterize the rubber material in order to achieve an appropriate curing reaction and viscosity models, respectively. The computations are found to be in good agreement with the experimental results, indicating that reliable information on material viscosity and curing kinetics can play a key role in making well-founded predictions and avoiding trial and error methods

Compounding of short fiber reinforced phenolic resin by using specific mechanical energy input as a process control parameter

For a newly developed thermoset injection molding process, glass fiber-reinforced phenolic molding compounds with fiber contents between 0 wt% and 60 wt% were compounded. To achieve a comparable remaining heat of the reaction in all compound formulations, the specific mechanical energy input (SME) during the twin-screw extruder compounding process was used as a control parameter. By adjusting the extruder screw speed and the material throughput, a constant SME into the resin was targeted. Validation measurements using differential scanning calorimetry showed that the remaining heat of the reaction was higher for the molding compounds with low glass fiber contents. It was concluded that the SME was not the only influencing factor on the resin crosslinking progress during the compounding. The material temperature and the residence time changed with the screw speed and throughput, and most likely influenced the curing. However, the SME was one of the major influence factors, and can serve as an at-line control parameter for reactive compounding processes. The mechanical characterization of the test specimens revealed a linear improvement in tensile strength up to a fiber content of 40–50 wt%. The unnotched Charpy impact strength at a 0° orientation reached a plateau at fiber fractions of approximately 45 wt%

Simulación del proceso de inyección del elastómero EPDM, mediante modelado fenomenológico

In chemical injection processes, the chemical kinetics is determinant, that is, the velocities of the reactions; and the mechanisms through which they are produced. The validity and applicability of mathematical models of kinetics in semi-salid state is considered a controversial issue. Due to the complexity of the curing kinetics, models of order n are used according to the shape of the reaction trajcetories. To describe the curing fraction and the curing speed of the EPDM polymer from a phenomenological approach, the Kamal-Sourour and Isayev-Deng models were used in this investigation. The EPDM polymer was tested in a mobile chamber rheometer (MDR) under isothermal conditions at six different ternperatures. Using non-linear adjustments, the kinetic parameters were determined as a function of time and temperature in the Kamal-Sourour rnodel; the reaction arder n was proposed as an Arrhenius type expression. In order to validate the proposed model, a comparative study was carried out between the Isayev-Deng model and the experimental results. Finally, using the adjusted models, a graphical and analytical description of the fraction of curing α (T, t) and curing speed (dα / dt) of the material was made. The mobile camera rheornetry (MDR) technique is used to measure the elastic and viscous components of rubber. The analysis of rheometry and kinetic modeling are used to obtain mathematical models of viseosity as a function of time, temperature and degree of curing. With the prediction of kinetic and viscous behavior, it is possible to control, optimize and design the process according to the properties of the material. The phenomenological model of Kamal-Sourour is used to describe the curing kinetics, while the Carreau-Macosko model is used to model the viscous behavior. In this work, non-linear regression tools are used to find the parameters of the models and a mathernatical model is proposed to describe the apparent viscosity according to the degree of vulcanization. Said rnudel is contrasted with experimental results made to the industrial ethylene-propylene-diene EPDM elastorner that includes chemical agents as additives for processing. The mathematical model is consistent for the material tested, validating the methodology developed to be applied to any thermosetting or elastomeric resin .En los procesos de inyección de hule es determinante la cinética química, es decir, las velocidades de las reacciones y los mecanismos a través de los cuales estas se producen. La validez y aplicabilidad de los modelos matemáticos de la cinética en estado semisólido es considerado un tema controvertido. Debido a lo complejo de la cinética de curado se emplean modelos de orden n de acuerdo con la forma de las trayectorias de reacción. Para describir la fracción de curado y la velocidad de curado del polímero EPDM desde un enfoque fenomenológico, en esta investigación se emplearon los modelos de Kamal-Sourour e Isayev-Deng respectivamente. El polímero EPDM fue ensayado en un reómetro de cámara móvil (MDR) en condiciones isotérmicas a seis diferentes temperaturas. Mediante ajustes no lineales se determinaron los parámetros cinéticos en función del tiempo y la temperatura en el modelo de Kamal-Sourour; se propuso el orden de reacción n como una expresión tipo Arrhenius. Con el fin de validar el modelo propuesto, se realizó un estudio comparativo entre el modelo de Isayev-Deng y los resultados experimentales. Finalmente, utilizando los modelos ajustados, se hizo una descripción gráfica. y analítica de la fracción de curado α(T, t) y la velocidad de curado (dα/dt) del material. La técnica de reometría de cámara móvil (MDR) se utiliza para medir las componentes elástica y viscosa del caucho. El análisis de las reometría y el modelado cinético se utilizan para obtener modelos matemáticos de viscosidad en función del tiempo, la temperatura y el grado de curado. Con la predicción del comportamiento cinético y viscoso es posible el control, la optimización y el diseño del proceso en función de las propiedades del material. El modelo fenomenológico de Kamal-Sourour se utiliza para describir la cinética de curado, mientras que el modelo Carreau-Macosko sirve para modelar el comportamiento viscoso. En este trabajo se utilizan herramientas de regresión no lineal para encontrar los parámetros de los modelos y se propone un modelo matemático para describir la viscosidad aparente en función del grado de vulcanización. Dicho modelo se contrasta con resultados experimentales efectuados al elastómero de etileno -propileno -dieno EPDM de tipo industrial que incluye agentes químicos como aditivos para el procesamiento. El modelo matemático resulta consistente para el material ensayado. validando la metodología desarrollada para aplicarse a cualquier resina termoestable o elastomérica

Warpage issues in large area mould embedding technologies

The need for higher communications speed, heterogeneous integration and further miniaturisation have increased demand in developing new 3D integrated packaging technologies which include wafer-level moulding and chip-to-wafer interconnections . Wafer-level moulding refers to the embedding of multiple chips or heterogeneous systems on the wafer scale. This can be achieved through a relatively new technology consisting of thermal compression moulding of granular or liquid epoxy moulding compounds. Experimental measurements from compression moulding on 8” blank wafers have shown an unexpected tendency to warp into a cylindrical-shape following cooling from the moulding temperature to room temperature. Wafer warpage occurs primarily as a result of a mismatch between the coefficient of thermal expansion of the resin compound and the Si wafer. This paper will delve into possible causes of such asymmetric warpage related to mould, dimensional and material characteristics using finite element (FE) software (ANSYS Mechanical). The FE model of the resin on wafer deposition will be validated against the measurement results and will be used to deduce appropriate guidelines for low warpage wafer encapsulation.peer-reviewe

Plastic Ball Grid Array Encapsulation Process Simulation on Rheology Effect

The integrated circuit should be encapsulated for protection from their intended environment. This paper presents the flow visualization of the plastic ball grid array (PBGA) chip encapsulation process considering of the rheology effect. In the molding process, encapsulant flow behavior is modeled by Castro-Macosko viscosity model with considering curing effect and volume of fluid technique is applied for melt front tracking. The viscosity model is written into C language and compiled using User-Defined Functions into the FLUENT analysis. Three types of Epoxy Molding Compound namely case 1, 2, and 3 were utilized for the study of fluid flow inside the mold cavity. The melt front profiles and viscosity versus shear rate for all cases are analyzed and presented. The numerical results are compared with the previous experimental results and found in good conformity. In the present study, case 1 with greater viscosity shows the higher air trap and higher pressure distributions.

Solid rocket motor internal insulation

Internal insulation in a solid rocket motor is defined as a layer of heat barrier material placed between the internal surface of the case propellant. The primary purpose is to prevent the case from reaching temperatures that endanger its structural integrity. Secondary functions of the insulation are listed and guidelines for avoiding critical problems in the development of internal insulation for rocket motors are presented

Hybrid Single Shot Manufacturing of Multi-Materials Structure for Automotive Applications

Multi-material design is one of the most attractive methods for automakers to reduce production cost while achieving lightweighting to meet stringent regulations and fuel efficiency concerns. Lightweighting, parts consolidation, reduction in assembly time and cost, and diverse functionalities are some advantages to the use of multi-material design in the automotive industry. However, the current technology of multi-material manufacturing faces some drawbacks, such as high cycle time, the necessity of various tooling and machinery systems, tight tolerance requirements, and extended planning effort on the production line. In this study, a technique named the Hybrid Single Shot (HSS), which is similar to Polymer Injection Forming (PIF), is used to manufacture CF/Epoxy-Thermoplastic components in a single operation. Unlike the PIF method, a carbon fiber /epoxy prepreg sheet is used as an insert material instead of sheet metal. In this technique, an injected polymer melt behaves like a forming medium to form the inserted thermoset sheet, in a single operation. Molten polymer not only forms but also bonds with the thermoset sheet using the high temperature of the polymer, in one process. CF/Epoxy sheet with injected thermoplastic is a hybrid structure that combines high mechanical properties of thermoset composite with the toughness and complex geometries of injected thermoplastic into a single component. A feasibility study was conducted for developing an integrated technology for the manufacturing of thermoset CF/Epoxy prepreg sheet with an injection of polypropylene to overcome the high cycle time and production cost associated with the manufacturing of such hybrids. Several sample parts were manufactured to demonstrate the effect of the process parameters on the process performance and the appearance of the final hybrid component. Although the results were promising, it showed some practical challenges such as excessive penetration, inadequate deformation, and warpage. Various process and design parameters are applied to the hybrid single shot process to circumvent these challenges. For example, a lower injection speed rate and the injection temperature are applied to increase the viscosity to prevent the penetration of polymeric melt through the thermoset sheet. Also, to evaluate the impact of polymer injection on the degree of cure of the prepreg sheet, Differential Scanning Calorimetry (DSC) analysis is conducted at a different pre-heat time before and after injection. The results showed that an increase in pre-heating time and injection temperature significantly enhanced the curing of the prepreg sheet after injection. Further, the mechanical properties of the hybrid part will be examined to identify the effect of individual properties of CF/ Epoxy and PP on the final component. Another contribution of this study is that it avoids many difficulties that conventional TS/TP joining techniques face. Specifically, these traditional joining methods, namely mechanical fastening, adhesive bonding, and welding, are time-consuming and labor-intensive. Also, mechanical fastening causes delamination and possible galvanic corrosion while adhesive bonding requires extensive surface preparation. Despite the time and weight advantages, welding techniques tend to create local delamination due to high local temperature. The hybrid single shot method is a promising alternative to overcome all the challenges that conventional methods face. A lap shear test is conducted to address the bonding conditions between polypropylene and CF/Epoxy prepreg. The experimental results presented in the previous chapters have revealed that the final geometry of the hybrid part is highly dependent on the preheating conditions and pressure field applied on the prepreg sheet during the injection phase. The pressure distribution is then a function of selected polymer, process settings, and most importantly of the geometry of the flow channel. To model the forming of the prepreg sheet due to this non-uniform pressure field, it is essential to couple all the physical events occurring inside the cavity. Therefore, the last contribution of this study is to have a better understanding on the effect of interaction injection, forming and curing on the final geometry of prepreg sheet, a quick yet accurate simulation of the HSS process. This simulation includes the consideration of the non-uniform pressure distribution of the melt flow and the prepreg sheet deformation behavior based on a new experimentally calibrated numerical approach

Influence of fiber breakage on flow behavior in fiber length- and orientation-dependent injection molding simulations

Injection molding is one of the most important processes for manufacturing discontinuous fiber reinforced polymers (FRPs). The matrix of FRPs shows a transient chemo-thermomechanical behavior and the fibers create anisotropy influencing physical properties. Hence, FRPs are complex materials, but also likely used in volume production. In this work, the fiber-induced anisotropic behavior during mold filling is modelled with an anisotropic fourth order viscosity tensor. The viscosity tensor takes second and fourth order fiber orientation tensor, fiber length and non-Newtonian matrix viscosity into account. In this way, the macroscopic simulation captures the influence of the flow field on the fiber re-orientation and vice versa. The fiber orientation tensor is used to determine reference fibers in every element for calculation of hydrodynamic forces. This information is used in a novel fiber breakage model, based on buckling of fibers in Jeffery’s orbit. The result is a macroscopic molding simulation with not only transient fiber orientation distribution, but also fiber length distribution. Due to the anisotropic viscosity tensor, the predicted fiber breakage influences the material’s viscosity and flow behavior, which is also visible in the simulated cavity pressure. The results are validated with injection molding experiments, performed with a glass fiber reinforced phenolic compound, showing good agreement