Numerical simulation of non-isothermal SMC (sheet molding compound) molding

Design of molding tools and molding cycles for sheet molding compounds (SMC) is often expensive and time consuming. Computer simulation of the compression molding process is a desirable approach for reducing actual experimental runs. The focus of this work is to develop a computer model that can simulate the most important features of SMC compression molding, including material flow, heat transfer, and curing. A control volume/finite element approach was used to obtain the pressure and velocity fields and to compute the flow progression during compression mold filling. The energy equation and a kinetic model were solved simultaneously for the temperature and conversion profiles differential scanning calorimetry (DSC) was used to experimentally measure the polymer zation kinetics. A rheometrics dynamic analyzer (RDA) was used to measure the rheological changes of the compound. A series of molding experiments was conducted to record the flow front location and material temperature. The results were compared to simulated flow front and temperature profiles.     

Simulation of compression molding for sheet molding compound considering the anisotropic effect

An improved model of the anisotropic flow characteristics of SMC (sheet molding compound) during compression molding is developed. This study is intended to complement our previous paper, which was conducted to determine the anisotropic parameters for short fiber reinforced thermosets SMC (16). Our prior study measured flow viscosities and material anisotropy by means of axisymmetric and plane strain compression molding tests. The current study, in order to identify the superior flow model from the choices (1) isotropic, (2) constant anisotropic and (3) varying anisotropic, applies the finite element method to obtain numerical results, which are subsequently compared with experimental results to determine the flow model with the best fit. The anisotropic parameters of the shear directions are determined by use of normal and planar parameters because SMC is planar isotropic. Six varying anisotropic parameters and six viscosity values are estimated during molding experiments, which are conducted at room temperature so that the polymer does not cure. Two-dimensional molding numerical analyses are carried out to explain two experimental classes, axisymmetric and plane strain compression molding. The load-levels predicted by the isotropic model, anisotropic model (parameter values fixed) and anisotropic model (parameter values varying) are compared with the experimentally derived values, the results showing that the varying anisotropic model best fits SMC compression behavior.     

Chemical thickening of low profile polyester resin systems

Low profile polyesters and sheet molding compounds, SMC, are two of many resin-glass composites that have been commercialized in the past few years. They have evolved through application of earlier technologies and have opened up possibilities for fiber-reinforced plastics. Because of rapid growth, insufficient time has been spent documenting the technical aspects of making SMC. Techniques are thus described that we have used to characterize the rheological properties of the resin mix during its transition from liquid-like (viscous) behavior of the initial formulation through the solid-like (elastic) behavior of the resultant SMC.     

Yield stress and rheological characterization of the low shear zone of an epoxy molding compound for encapsulation of semiconductor devices

In encapsulation molding of IC packages, the melt flow inside the cavity is generally controlled in a low shear to prevent wire sweep, and other molding defects. Therefore, it is important to evaluate the rheological properties of epoxy molding compounds (EMC) in a low shear zone including determining the yield stress. In this study, a newly specialized Parallel‐Plate Plastometer for EMCs was built up. Using this plastometer, the yield stress and its temperature dependence were clarified, and the rheological properties in the low shear zone were evaluated. As a result, the rheological properties in a low shear zone of 0.1–10 s^?1 were characterized using the Herschel–Bulkley viscosity model which introduced the yield stress, the Castro–Macosko equation as a dependency model of cure, and the WLF equation as a dependency model for temperature. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Rheology of diallyl phthalate polymers below the gel point

The analysis of molding operations for thermosetting polymers requires information on the reaction rates and rheology of the materials. The purpose of this research was to measure kinetic and rheological data on diallyl phthalate resins and to develop models describing the flow behavior. The rheological data were measured with a mechanical spectrometer equipped with eccentric rotating discs. For the region below the gel point of the polymer, the elastic modulus and viscosity were correlated with molecular weight, temperature and shear rate through fairly simple models. By combining kinetic and rheological correlations, the viscosity of a reacting thermoset can be predicted as a function of time, temperature and shear rate.     

Processability study of in‐mold coating for sheet molding compound compression molded parts

In‐mold coating (IMC) is applied to compression molded sheet molding compound (SMC) exterior automotive or truck body panels as an environmentally friendly primer to make the part conductive for subsequent electrostatic painting operations. The coating is a thermosetting liquid that when injected onto the surface of the part cures and bonds to provide a smooth conductive surface. In order to identify the processability of IMC for SMC, it is essential to predict the time available for flow, that is the time before the viscosity starts to increase as well as the time when the coating has enough structural integrity so that the mold can be opened without damaging the part surface (mold opening time). In the present work, we study cure behavior of IMC based on differential scanning calorimetry and rheological experiments and show its relevance to both flow and mold opening time for the IMC process during SMC compression molding. POLYM. ENG. SCI., 59:1688–1694 2019. © 2019 Society of Plastics Engineers     

Squeezing flow of viscoplastic fluids subject to wall slip

Polymer processing operations such as compression molding, sheet forming and injection molding can be modeled by squeezing flows between two approaching parallel surfaces in relative motion. Squeezing flows also find applications in the modeling of lubrication systems, and in the determination of rheological properties. Here, analytical solutions are developed for the constant-speed squeezing flow of viscoplastic fluids. It is assumed that the fluid is purely viscous, and hence viscoelastic effects unimportant. The rheological behavior of the viscoplastic fluids is represented by the Herschel-Bulkley viscosity function. The deformation behavior of commonly encountered viscoplastic fluids is generally complicated by the presence of wall slip at solid walls, which is a function of the wall shear stress. The slip coefficient that relates the slip velocity to the shear stress is affected by the material of construction and also the roughness of the solid surfaces, leading to the possibility of different slip coefficients at various solid surfaces. The model developed in this study accommodates the use of different slip coefficients at different solid surfaces. The accuracy of the solutions is established, and the effects of various parameters such as slip coefficient and apparent yield stress are examined. The solutions provide useful design expressions that can be utilized for squeezing flows of viscoplastic fluids, with or without wall slip at the solid boundaries.     

Kinetics and flow of diallyl phthalate resins

The analysis of molding operations for thermosetting polymers requires knowledge of the rheology and reaction rates of the materials. The purpose of this research was to measure kinetic and rheological data on diallyl phthalate resins and to integrate these results into models describing the flow behavior. The chemical kinetics of the curing reactions were derived from calorimetric measurements taken with a differential scanning calorimeter. The rheological data were measured with a mechanical spectrometer equipped with eccentric rotating discs. A model based on the theory of ideal rubber elasticity was used to correlate the elastic storage modulus with reaction time and temperature. For the region below the gel point, the dynamic viscosity exhibited a power law dependence on angular frequency and an Arrhenius dependence on temperature.     

Optimization of compression molding of stand‐alone microlenses: Simulation and experimental results

Hot compression molding is a promising method to fabricate polymer stand‐alone microlenses. A reliable theoretical as well as statistical analysis is required for the optimization of the process to minimize the residual stresses and to predict the amount of springback to achieve a better replication of the mold profile. This article in this context focuses on the finite element simulation (FES), optimization as well as experimental validation of hot compression molding of polymer stand‐alone microlenses. Three steps such as molding, cooling, and demolding, under different molding parameters, were analyzed using ABAQUS/standard solver and the results were compared with experimental results. Compression test and compression relaxation test have been conducted at different temperatures and strain rates to characterize the rheological behavior of material. Two material models, linear viscoelastic and hyperelastic–viscoelastic models, were developed and used for compression test simulations. Hyperelastic–viscoelastic model is found to predict the material behavior in low strain rates better and, thus, is used for the simulation of actual lens compression molding. Good agreement is found between the FES‐predicted curve and the lens profile molded at different molding temperatures. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Predicting molding forces during sheet molding compound (SMC) compression molding. I: Model development

Because of its high strength‐to‐weight ratio, corrosion resistance, and low cost, Sheet Molding Compound (SMC) production offers great potential for growth in the automotive and trucking industry. Much attention is now being given to improving the economy of SMC compression molding by reducing the cycle time required to produce acceptable parts in steady production. One of the fastest‐growing applications of Sheet Molding Compound (SMC) compression molding panels is the manufacture of truck body panels. Owing to their large size, the molding forces developed are substantial and have a major influence in the molding cycle. The relevant process models for SMC flow are reviewed and a procedure is developed that can be used to obtain the closing force and calculate the needed material parameters. Experiments were done using commercially made SMC to verify the validity of this model and the compression force was predicted and compared to experimental values for commercially made automotive hoods.     

聚合物多相分层充模流动成型的粘性包围形成机理数值分析

基于聚合物多组分成型技术的工程背景，建立了全三维非稳态非等温多相分层充模流动的理论模型，提出了求解理论模型的稳定高效的数值算法。通过数值模拟，给出了不同流变性能参数、过程条件下多相分层充模流动成型时的粘性包围形成过程和其形貌的定量对比。在此基础上，通过理论分析，揭示了粘性包围的产生机理，并研究了流变性能参数和过程条件对分层界面形貌和粘性包围影响的规律性关系。模拟研究表明，模拟结果与Bamin Khomani等的实验研究结论相吻合。     

An approach towards the reduction of sink marks in sheet molding compound

Sink marks are shallow depressions normally observed above reinforcing ribs in molded Sheet Molding Compound (SMC) parts. In this paper, the effect of mold geometry, particularly the rib entrance shape, on the flow pattern of molding compound and the resulting sink marks in molded parts is presented. Flat plate specimens with a single reinforcing rib in the center were used in this work. Rib entrance shape was varied and its effect on both sink depth and fiber orientation measured. A reduction in sink depth from 0.0007 in. to less than 0.0001 in. was observed when comparing rounded and protruding rib entrances, respectively. The effect of inducing unequal flow rates from the two sides of the rib was also investigated and found to give a reduction in sink depth of about one-third. A computer simulation of the flow during molding was, used to compare observed flow patterns with simple theoretical predictions. The SMC was modeled as a highly viscous Newtonian fluid and finite difference methods were used to solve the Navier-Stokes equations. Extension of this modeling procedure to more complex geometries will aid in the design of nearly sink free molds.     

基于极差分析的SMC复合材料模压工艺参数优化

为了探讨模压成型参数对于片状模塑料（SMC）材料模压制品综合力学性能的影响,将模压成型过程分为3个阶段,以3个阶段的压力与时间共6个参数作为影响因子,对制品的拉伸强度、弯曲强度与冲击强度进行测试,设计了正交试验,以制品的力学性能作为评价指标,采用极差分析法,分析讨论了各阶段工艺参数对SMC复合材料制品力学性能的影响,并结合成型过程中材料状态变化分析造成实验结果的原因,最终得到优化后的工艺参数。     

Predicting molding forces during sheet molding compounds (SMC) compression molding. II: Effect of SMC composition

One of the fastest‐growing applications of SMC compression molding is the manufacture of truck body panels. Because of their large size, the molding forces required are substantial and have a major influence on the molding cycle. Also, as SMC moves towards parts requiring higher strength, the fiber length and percentage by weight of fibers must increase. This will also contribute to larger molding forces. In this paper, a procedure is presented to evaluate the SMC rheological parameters needed to predict molding forces. In addition, the effect of SMC composition on the molding forces is investigated. In particular, we evaluate the effect of reinforcement type (glass versus carbon) and level, filler level and thickener level. It was found that the factors most affecting molding forces are the reinforcement length and level; and the filler level. In addition, it was discovered that for SMC thickened with magnesium oxide, the level of thickener does not affect the molding force.     

Kinematics of flow in sheet molding compounds

Experiments utilizing charges constructed of black and white sheet molding compound (SMC) reveal the basic kinematic mechanisms controlling the flow of the fiber-filled compound in compression molding. The experimental results show that SMC deforms in uniform extension within individual charge layers, with slip occurring at the mold surface and, for slower closing speeds, also between the layers of SMC. When the mold closes rapidly, the charge extends uniformly through its thickness, with all slip concentrated at the mold surface.     

Correlation between sizing formulation and compressive behavior of concentrated glass bundle suspensions

The correlation between sizing formulation, bundle mechanical characteristics, and bundle–matrix static and dynamic interactions are investigated. Two glass‐fiber sizing formulations are considered, one containing polyvinyl acetate (PVAc) and the other polyester/PVAc, as conventionally used in sheet molding compounds (SMC). Axial compression tests are conducted on dry two‐dimensional (2D) random suspensions. The forced packing is governed by the bending of fiber bundle segments between bundle‐bundle contact points. Benchmarking of the experimental curves with a modified theoretical model provides an estimation of the fiber bundle bending rigidity under forced packing conditions. This value is found to depend on the bundle sizing as well as on the interaction with solvents present in the matrix as is the case for SMC. Free flow and molding experiments are performed on planar SMC sheets using the two different fiber bundles as reinforcements. The results confirm the dependence of the molding energy and the SMC rheology on the bundles chemical and mechanical characteristics. POLYM. COMPOS., 26:370–376, 2005. © 2005 Society of Plastics Engineers     

An evaluation of the curing characteristics of thermosetting epoxy carbon fiber sheet molding compounds and validation through computer simulations and molding trials

Sheet molding compounds (SMC) are ready-to-mold thermoset composite materials reinforced with discontinuous fibers, usually compression molded. Finite element (FE) based compression molding tools can be employed to optimize this process; FE tools require to define material models using raw material data measured through different characterization techniques. In this study, the cure kinetics of an epoxy-based carbon fiber SMC has been characterized by means of differential scanning calorimetry (DSC) and moving die rheometer (MDR) techniques. Based on these datasets, Claxton-Liska and Kamal-Souror models have been set and the compression molding of a validation plate was performed, both experimentally and virtually. The results indicate that, even if both characterization techniques are valid for SMC curing characterization, MDR technique enables the characterization of the material at real molding temperatures and the model based on MDR leads to more accurate results.     

Monitoring cure in sheet molding compound processing

During the sheet molding compound (SMC) compression molding process, a premeasured polymer charge is placed between the heated halves of a mold which are then brought together to squeeze the polymer and fill the mold, after which pressure is maintained while the part cures. The cure stage constitutes the larger part of the molding cycle and thus affords the largest potential for cycle time reduction. In general, cure times in SMC processing are set longer than necessary, since the inherent material and process variation make it difficult to predict cure times with more than 10 to 20% accuracy. Accurate methods to detect the end of cure would be very beneficial and would permit opening the mold as soon as the material has cured, avoiding unnecessary waste of time. In this paper, several techniques that show promise for monitoring the state of cure are reviewed and experimental results given. Their relative advantages and accuracies are compared. In particular, the use of linear variable displacement transducers, pressure transducers, and thermocouples is discussed. We also show how the measurements compare to theoretical predictions of the state of cure.