Flow Front Profiles Research Articles

Effect of fins on melting of phase change material in a closed vertical cylinder under microgravity

In this study, the melting dynamics of a phase change material (PCM) filled in a closed vertical cylinder under microgravity are investigated numerically, and the results are compared with the melting dynamics of PCM under gravity. In addition, the effect of inserting fins inside the PCM unit under microgravity on melting dynamics is evaluated. A three-dimensional symmetric numerical model is developed to analyze temperature field, fluid flow, and melting front profile, as well as heat flux, melting time, power, stored energy and stored energy per mass during the melting of PCM RT27 under microgravity field. The results show that the melting performance of PCM in the microgravity field is lower than in the gravity field. The melting process is 64.7% slower in the case of the PCM unit under microgravity compared to the PCM unit under gravity. However, when the fins are inserted into the PCM unit, the melting is accelerated. The reduction of the total melting time of the PCM under microgravity is 78.8%, 58.1%, and 30.7% for the non-dimensional fin widths of 0.8, 0.6, and 0.4, respectively, compared to the no-fin configuration. When the fins are inserted, the amount of energy per mass is decreased, but the power increases dramatically. For the non-dimensional fin width of 0.8, the decline in the energy per mass is the highest, which is 17.6%, and the increase in the power is the highest, which is 349.4%.

Void content reduction of composites with sensor-aided injection strategy in liquid composite molding process

Void is one of the critical issues affecting the mechanical performance of fiber-reinforced polymer composites. Aiming at the composite parts formed by Liquid Composite Molding, this develops a sensor-aided injection strategy, including a real-time online monitoring module and a sensor-aided injection system, which realizes continuous detection of resin flow and the automatic injection flow rate adjustment to minimize void content. Flow front profile continuously recorded by the monitoring module in resin flow direction is reconstructed in real-time. It provides a digital representation of the saturation state of the fibrous preform, which is referred to Digital Process Twin (DPT) in present work. The local flow front velocity is then extracted from the DPT, thus the sensor-aided injection system can maintain the flow front velocity at a optimal level, regardless of the cross-sectional geometry. Experimental investigation confirms the representative of DPT to actual flow. Characterization of void content by micro-computed tomography shows that the proposed sensor-aided injection strategy not only effectively decreases the void content from 0.32%, 0.34%, and 0.79% to 0.18%, 0.18%, and 0.19%, but also significantly improves the part quality consistency. Furthermore, to the best knowledge of the authors, This work is the first to use micro CT to investigate the effect of velocity on void content from the mesoscopic and microscopic scales, although the topic has been widely investigated theoretically.

A METHODOLOGY TO CHARACTERIZE FIBER PREFORM PERMEABILITY BY USING KARDAR–PARISI–ZHANG EQUATION

Permeability tensor describes the resistance to fluid flow through the fibrous porous media, which may not be spatially uniform. This nonuniformity in fiber architecture causes variation in the permeability value of the fibrous domain. The time evolution and geometry of the rough interfaces of the fluid flow in fibrous porous medium are analyzed using the concepts of dynamic scaling and self-affine fractal geometry, and they are shown to belong to the Kardar–Parisi– Zhang (KPZ) universality class. The resulting growth exponent, β is found to match the 1 + 1 KPZ values, and the roughness exponent, α, describes the standard deviation of the variation in fiber preform permeability. Additionally, this characterization is used to develop a tool to quantify the percentage and strength of defects within the fibrous porous media from flow front profile analysis.

Flow pattern control in resin transfer molding using a model predictive control strategy

Resin transfer molding (RTM) is an efficient manufacturing process for fabricating polymer composites, in which liquid thermosetting resin is injected into a closed mold to saturate a fiber preform. In RTM, effective flow control is necessary to direct the resin to flow in the desired manner and to prevent the formation of defects. Most existing methods are based on numerical flow simulations, whose accuracy is directly tied to the fidelity of the physics and material models used in the codes. The control performance of these methods largely depends on the quality of the models. The traditional proportional–integral–differential controllers are unsuitable as well, because of the nonlinear and time‐varying characteristics of the RTM system. In this research, a model predictive control strategy is proposed for adjusting the flow behavior of the resin inside the mold, and it does not rely on process simulators. Recursive least squares with an adaptive directional forgetting factor is adopted as a method to identify the input–output relationship of the process under control. Based on the identification results, both the flow velocity and the flow front profile can be controlled simultaneously. The feasibility of the proposed strategy are illustrated with experimental results. POLYM. ENG. SCI., 58:1659–1665, 2018. © 2017 Society of Plastics Engineers

An experimental method for characterizing unsaturated 1D flow in tubular braided preforms during bladder‐assisted resin transfer molding

The work presented in this paper addresses the development of an efficient approach to investigate the unsaturated in‐plane impregnation behavior of tubular braided reinforcement materials in a bladder‐assisted resin transfer molding process. The basic idea is to record the macroscopic flow front advancement during a pressure‐driven injection of a test fluid by using a transparent monolithic mold and optical measurement methods. The experimental setup further comprises sophisticated process control to accomplish a fully automated and highly repeatable test procedure. As the filling behavior of the braided fabrics predominantly shows unidirectional characteristics, the so‐called unsaturated apparent permeability of these textile reinforcements can be determined, which can be used as an input parameter in one‐dimensional flow simulations. A verification of the test setup is accomplished by means of saturation measurements on different braided preforms, which are compared with predicted flow front profiles. POLYM. COMPOS., 39:4666–4677, 2018. © 2017 Society of Plastics Engineers

CUF scaling effect on contact angle and threshold pressure

PurposeThis paper aims to present experimental and finite volume method (FVM)-based simulation studies on the scaling effect on the capillary contact angle and entrant pressure for a three-dimensional encapsulation process of ball-grid array (BGA).Design/methodology/approachWith the development of various sizes of BGA packages, the scaling effect of BGA model on capillary underfill (CUF) process is investigated together with the influences of different industrial standard solder bump arrangements and dispensing methods used as case study.FindingsThe experimental results agree well to the simulation findings with minimal deviation in filling time and similar flow front profiles for all setups. The results revealed that the capillary contact angle of flow front decreases in scale-up model with larger gap height observed and lengthens the encapsulation process. Statistical correlation studies are conducted and accurate regression equations are obtained to relate the gap height to the completion filling time and contact angle. CUF threshold capillary pressures were computed based on Leverett-J function and found to be increasing with the scale size of the package.Practical implicationsThese statistical data provide accurate insights into the impact of BGA’s scale sizes to the CUF process that will be benefiting the future design of BGA package. This study provided electronic designers with profound understanding on the scaling effect in CUF process of BGA, which may be extended to the future development of miniature-sized BGA and multi-stack device.Originality/valueThis study relates the flow behaviour of encapsulant to its capillary contact angle and Leverett-J pressure threshold, in the CUF process of different BGA and dispensing conditions. To date, no research has been found to predict the threshold pressure on the gap between the chip and substrate.

Comparative Study of the Scaling Effect on Pressure Profiles in Capillary Underfill Process

Optimization of the capillary underfill (CUF) encapsulation process is vital to enhance the package’s reliability. Therefore, the design and sizing of the newly developed ball grid array (BGA) device must be considered so that it is compatible with the CUF process. The scaling effect of BGA on CUF flow and its dynamic properties is thoroughly investigated by means of fluid-structure interaction (FSI) numerical simulation. This paper generally highlighted the differences in CUF flow behaviours, together with the pressure distributions between the actual industrial size BGA and the scaled up models for large BGA setup. While flow front profiles appeared to be similar across BGA of various sizes at relative error less than 10%, the CUF filling time gradually increases as the BGA become larger. The scaling limit is found to be at 20, based on the analysis of dimensionless number. The entrant pressure however decreases when the BGA device being scaled up. These findings will assist in the future BGA designs for various sizes used in the CUF encapsulation process.

Numerical modeling and experimental validation of nonisothermal resin infusion and cure processes in large composites

This paper presents an experimentally validated model to simulate the nonisothermal resin infusion and cure processes in a relatively large and thick composite laminate fabricated using vacuum-assisted resin transfer molding. Compaction effects were accounted for by using a step-wise scheme wherein the panel was divided into three discrete zones along the flow direction, with each zone being assigned different porosities and other material properties. The experimentally observed dynamic evolution of the resin flow front profile due to permeability difference between the high-permeable infusion medium and the glass-fiber preform as well as the heat transfer in the preheated system was captured accurately. Both model predictions and experimental measurements indicate subtle variation in the spatiotemporal distributions of temperature and degrees of resin cure from the infusion stage can result in large differences in the resin cure profile across the laminate, which may cause undesirable residual stresses in large composites.

Fast method to monitor the flow front and control injection parameters in resin transfer molding using pressure sensors

The quality of composite parts manufactured by resin transfer molding is sensitive to material and process variations during the preform impregnation. In order to improve the process robustness in two-dimensional injection cases, this work proposes a fast method for tracking and controlling the resin flow through the preform, using only a small number of pressure sensors embedded in the mold. The approach combines pressure signals and flow modeling in a quick algorithm that returns on-line estimations of the flow front profiles. Virtual and real injection tests demonstrated the accuracy of the methodology and provided information for designing the sensing system. Moreover, the investigation proved the feasibility of a simple and effective procedure of flow rate control. Based on a computer-controlled pressure regulator acting on the inlet gate, a feedback mechanism was implemented in the system and allowed keeping the flow front velocity within a target range of values. Such a control procedure may be used to limit the formation of voids and enhance the final part quality.

Lattice Boltzmann method study of effect three dimensional stacking-chip package layout on micro-void formation during encapsulation process

The current study applied the lattice Boltzmann method to examine the effects of stacking chips layout to the micro-void formation in three-dimensional (3D) packaging. Three-dimensional 19-velocities commonly known as D3Q19 scheme is utilized in this study. Three different cases, which are different in layout design, are examined. For code verification purpose, an experimental work is also presented to compare the flow front results between numerical and experimental at different filling percentage. The numerical predictions compared well with the experimental results. Minor differences are observed in their flow front profile. The numerical findings identified the predicted locations of micro-void formation during the encapsulation process. The entrapment of micro-void was visualized clearly in the simulation because of the unbalanced molecular force at the interface during encapsulation. Knit lines were also identified at the interface between the flows that occurred in the encapsulation. Different layout of stacking flip-chips package have influence the micro-void in the package, which tended to form at the stacking chips region. The results show that the lattice Boltzmann method has a good performance in the IC encapsulation simulation.

Numerical Analysis of the Resin Transfer Molding Process via PAM-RTM Software

This work aims to investigate the infiltration of a CaCO3filled resin in fibrous porous media (resin transfer molding process) using the PAM-RTM software. A preform of glass fiber mat (fraction 30%), with dimensions 320 x 150 x 3.6 mm, has been used in rectilinear injection experiments conducted at room temperature and injection pressure 0.25, 0.50 and 0.75 bar. The polyester resin contain 0% and 40% CaCO3. The numerical results were evaluated by direct comparison with experimental data. The flat flow-front profile of the rectilinear flow was reached approximately half length of the mold. It was observed, that the both velocity infiltration and permeability have decreased with increasing the CaCO3content, thus, increasing the time to processing of the composite material.

Resin Transfer Molding Process: A Numerical Analysis

This work aims to investigate the infiltration of a CaCO3filled resin using experiments and the PAM-RTM software. A preform of glass fiber mat, with dimensions 320 x 150 x 3.6 mm, has been used for experiments conducted at room temperature, with injection pressure of 0.25bar. The resin contained 10 and 40% CaCO3content with particle size 38μm. The numerical results were evaluated by direct comparison with experimental data. The flat flow-front profile of the rectilinear flow was reached approximately halfway the length of the mold. It was observed, that the speed of the filling decreases with increasing CaCO3content and,the higher the amount of CaCO3in the resin, the lower the permeability of the reinforcement that is found. The reduction in permeability is due to the presence of calcium carbonate particles between the fibers, hindering the resin flow in the fibrous media. The computational fluid flow analysis with the PAM-RTM proved to be an accurate tool study for the processing of composite materials.

A Distribution Medium Considered Resin Flow Simulation Model for Vacuum Infusion Molding Process

Based on the flow characteristics of resin in fiber perform, a simulation model considering distribution medium was developed, and impregnation of fiberglass reinforced resin matrix composites was numerically simulated. The fiberglass layer thickness on VIMP microscopic impregnation was analyzed in simulation. The results show that increasing fiberglass layer thickness can reduce the flow velocity of the resin and the resin flow front profile approximates a straight line type, so the fluctuation is small, and the final product has few dry spots; reducing the glass fiber layer thickness can improve wetting speed but resin flow front profile approximates a parabolic type, so the fluctuation is large, and the final product has more dry spots, the resin flow front profile can provide guidance for prediction and optimization of the infusion process.

A methodology for flow-front estimation in LCM processes based on pressure sensors

Liquid Composite Molding (LCM) processes offer nowadays considerable advantages for composite manufacturers; nevertheless, their robustness is still an open issue, since part quality is sensitive to slight material and process variations, mainly arising during preform impregnation. An accurate monitoring system is thus required for early identification of filling troubles and potentially adoption of control actions. This work proposes an approach for resin flow monitoring, based on a combination of a sensing system and numerical modeling, which can be easily implemented into a generic LCM process. Using pressure data provided by few sensors placed in strategic positions inside the mold cavity, the developed methodology enables reconstruction of flow-front patterns at any impregnation time, without simulation of cavity filling. The effectiveness of the methodology is demonstrated by two test cases, which validate the approach through comparison between real and estimated flow-front profiles. Potential and limitations of the method in presence of flow disturbances have been studied through several virtual experiments, defining the range of applicability and key issues for future development.

Effects of outlet vent arrangement on air traps in stacked-chip scale package encapsulation

The current paper analyzes the effect of outlet vent arrangements on the air traps and pressure distribution of a stacked-chip scale package (S-CSP) during encapsulation. A three-dimensional model of S-CSP is created using GAMBIT and analyzed using FLUENT code. In the molding process, the epoxy molding compound flow behavior calculated using the Castro–Macosko viscosity model by considering the curing effect, and the volume of fluid technique are applied for flow front tracking. The viscosity model is written in C language and compiled using User-Defined Function in FLUENT code. Three different types of outlet vent arrangement are considered in the analysis, namely, Type A, Type B, and Type C. Type A, which has the minimum outlet vent area, shows the minimum air trap and the highest average pressure distribution. The simulation results of the flow front profiles agree well with previous experimental results.

Numerical analysis on the effects of different inlet gates and gap heights in TQFP encapsulation process

Finite volume method (FVM) based simulation of encapsulation process in thin quad flat pack (TQFP) packages is presented in this paper. The 3D model of TQFP package is built and meshed with tetrahedral elements using GAMBIT, and simulated by FLUENT software. Castro–Macosko viscosity model and volume of fluid (VOF) technique are applied for flow front tracking of the encapsulant. Curing kinetics is taken into consideration in the simulation using Kamal’s equation. To solve the Castro–Macosko and Kamal models, suitable user defined functions (UDFs) are developed using MS VISUAL STUDIO.NET software and incorporated into the FLUENT. The parameters such as mold filling time, flow front profiles, pressure distribution in the package and void formation, for three different inlet gate arrangements and gap heights, are analyzed. The degree of conversion of the molding compound during the encapsulation process is also studied for different number of inlet gates. It is found that the filling time and void occurrence could be reduced by increasing the number of inlet gates, and the variation of gap height within the cavity is crucial in controlling the peak pressure. Moreover, the combined effect to two competing events, such as, reduction of viscosity with shear rate due to non-Newtonian behavior of the polymer fluid and increase in viscosity during the curing reaction, are effectively demonstrated. The simulation results are compared with previous experimental results and found in good conformity.

Effect of vertical stacking dies on flow behavior of epoxy molding compound during encapsulation of stacked-chip scale packages

This paper presents three dimensional (3D) simulation of flow visualization in the encapsulation of stacked-chip scales packages (S-CSP), using finite volume method. The S-CSP model is constructed using GAMBIT and simulated using FLUENT CFD software. The epoxy molding compound is Hitachi CEL-9200 XU (LF) and its flow is assumed laminar and incompressible. Cross viscosity model and volume of fluid technique are applied for flow front tracking of the encapsulant. The meshing is performed using tetrahedral elements and the discretization is done by first order upwind scheme. SIMPLE algorithm is selected for solving the governing equations. The top view and 3D view of simulation flow front profiles in the encapsulation process are presented. The percentage of filled volume versus filling time, viscosity versus shear rate and number of voids versus rows of stacked die are plotted. The temperature and pressure distributions within the mold cavity during the encapsulation process are also observed. Further, the possibility and cause of void formation during the encapsulation process are analyzed and discussed in detail. The number of vertical stacking dies and horizontal rows of packages are found to be crucial in the void formation. The numerical results are compared with previous experimental results and found in good conformity.

Visualization of the surface tension and gravitational effects on flow injection in center-gated disks

Flow injection in center-gated disks is numerically studied in this paper for possible applications in the manufacturing of composite materials in microgravity environment. The numerical method, which combines the finite element method with a predictor/corrector scheme, is used to determine the transient flow field. The effects of gravitation and surface tension on the development of flow front profile and velocity field are examined for a wide range of the governing parameters (namely, the capillary and Bonds numbers). It has been found that surface tension tends to hold the flow front in symmetric shape while gravitation is to distort it. The balance of these two forces has significant effects on the front shape, front tip traveling speed and required injection pressure. Good agreement is found between the numerical prediction and the experimental results concerning the advancement of the flow front and the front shape. The present results provide useful information in the design of resin transfer molding process in space.

Analysis of Mold Filling in Lost Foam Casting of Aluminum: Part II-Example Applications

This paper is the second in a two-part series on mold filling analysis for lost foam casting of aluminum. Part I (published in the IJMC Volume 2, Issue 3, Summer 2008) describes the analysis method, assumptions, and governing equations, and this paper (Part II) discusses applications of the analysis to a variety of pattern shapes that exhibit different types of mold filling behavior, many of which are unique to lost foam casting. The first example compares mold filling in a horizontal foam strip filled from the side with filling in the same strip oriented vertically and filled from the top or bottom. The results illustrate the unusual effect that pattern orientation and thickness have on metal fluidity in lost foam casting. The second example, a rectangular plate filled through a single inlet located on a top, bottom or side edge, illustrates how different modes of foam decomposition can interact during the filling of a single pattern. The mold filling sequence turns out to be dramatically different depending on inlet location and pattern thickness. The third example is a generic box pattern that has long been used at GM and elsewhere to study the lost foam process. This pattern is filled from the back through three separate inlets, creating two merging flow fronts inside the cavity. The likelihood of these metal fronts enclosing foam between them when they come together is explored by examining the concavity of their flow front profiles. For several cases considered in this paper, the results of the analysis are compared with X-ray and neutron radiography images taken during actual mold filling.

FLOW ANALYSIS OF MICRO-CHANNEL IN INJECTION MOLDING

Many micro-devices have been successfully injection molded. Efforts need to be made to identify the significant factors that affect micro-filling behaviors. Polymeric flows in micro-channels are found to differ significantly from those in macro geometries. To have a cavity with micro-channels, an electrical-discharge wire cut insert was employed. Ten micro-channels were machined with different geometries to investigate the geometric effects on the flow behaviors. Both the numerical simulations and experiments were conducted to study the filling pattern through the micro-channels. Numerical simulation was executed based on the two-dimensional software C-MOLD and three-dimensional software Moldflow. The simulation and experimental data were compared and discrepancies were found in the flow front profiles. However, a three-dimensional simulation brought better results than a two-dimensional simulation result in this study. The effects of the process conditions, injection speed and mold temperature, on the flow behaviors were also investigated.