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.     

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.     

A boundary element analysis of flow in sheet molding compound

A new boundary element method has been developed for analyzing the flow of sheet molding compound (SMC) during compression molding. The boundary element equations can be used to determine the velocities on the perimeter of the charge. Successive flow front configurations are then generated by a simple explicit updating procedure. This approach was used to predict the flow front progression for elliptical, rectangular, and L-shaped charges. Comparisons with experimental data for elliptical and rectangular charges were encouraging. The fact that it was possible to obtain reasonable agreement for charges with different shapes and thicknesses lends support to the underlying flow model. Furthermore, valuable insight regarding knit line formation was acquired by analyzing the L-shaped charge. Results from the boundary element analysis showed that the initial thickness of the charge has a pronounced effect on knit line development. Even though there is considerable industrial experience in making SMC parts, the important role of charge thickness on knit line formation appears to have been largely overlooked. Prior analyses gave no indication of this effect because they were based on lubrication models that were independent of charge thickness.     

Rheological characterization of unsaturated polyester resin sheet molding compound

This paper presents a theoretical and experimental analysis of the rheological behavior of sheet molding compound (SMC). The work analyses the squeeze flow in a parallel plate plastometer of SMC discs which contain 25 percent of fiber glass by weight. This method of flow characterization gives a good insight into the basic rheological behavior of SMC for the compression molding process when producing flat parts. The theoretical analysis applies to thickened and matured SMC at room temperature. The analysis treats SMC as a viscoelastic material having an equation of state with viscous, elastic and yield elements. The time variation of compressive force when squeezing SMC discs between two parallel plates (one fixed and one mobile) has been derived from the equation of state. The values of the viscous, elastic and yield parameters were determined by using a least squares method of curve fitting to the experimental results. There are two aspects to the reported experimental work. One aspect is concerned with showing that the three element model for the equation of state provides a realistic mathematical basis for characterizing the rheological behavior of SMC at room temperature. The other shows how the parallel plate plastometer can be used to give data which characterize SMC flow behavior under conditions similar to those of the actual compression molding process.     

Simulation-based design of sheet molding compound (SMC) compression molding

The design of moding tool and molding cycle for sheet molding compounds (SMC) is often expensive and time consuming. Computer simulation of the compression molding proces is a desirable approach to reduce experimental prototypes. The focus of this work is to develop an automatic optimization scheme utilizing an earlier developed SMC plrocess simulation program which is capable of simulating material flow, heat trensfer, and curing. The proposed scheme reduces computing time by using approximate responses, instead of actual simulated responses, to perform the optimization. The automated optimization package minimizes user intervention during optimal design by creating an automatic link between the optimization and simulation routines. A 2-level factorial design combined with regression analysis is adopted to gather and analyze computed information, and to serve as the approximation formula. Two examples are presented to test the applicability of the optimization scheme.     

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.     

Experimental and analytical procedures for flow dynamics of sheet molding compound (SMC) in compression molding

A package of procedures have been developed to collect and analyze the response of dynamic variables such as pressure, temperature, and mold separation during the compression molding of Sheet Molding Compound (SMC). From the dynamic responses, the molding process was found to consist of two regions: the flow and the subsequent curing reaction region. With an R-25 formulation and a mold closing rate of 30 mm/s, these two regions are well separated and the average flow time is not significantly affected by the maturation time for the material up to 30 days. Several mechanical parameters were estimated based on relatively simple flow models. The relationship between the press force, mold separation, and mold closing rate is found to be sensitive to the restrictions of the flow.     

Effect of two stage pressure molding on surface quality of sheet molding compounds

The goal of this work was to investigate the effect of two stage pressure molding on the compression molding of a sheet molding compound (SMC). It has been shown in previous studies that a rapid drop in pressure during SMC curing significantly reduced severity of sink marks. This study concentrated on a method of predicting the optimum time during curing to release pressure by examining material behavior through process data from in-mold sersors. A simple control scheme was them applied for automatic pressure release at the optimum time corresponding with the peak of the material expansion and the onset of the reaction exotherm.     

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。     

Compression molding of sheet molding compounds in plate-rib type geometry

The flow of fiber-reinforced composite materials in a plate-rib type mold geometry during compression molding was investigated using a series of sheet molding compounds (SMC). Material anisotropy in relation to the amount and the length of reinforcing fibers was analyzed. The influence of the interfacial friction between SMC charge and the mold surface on the flow and sink mark formation was also examined. The results were explained qualitatively by computer simulation.     

Compression molding of glass reinforced thermoplastics: Modeling and experiments

The shearing and extensional behavior of glass mat‐thermoplastic (GMT) material under compression molding was investigated with a special model being developed for the case of non‐lubricated mold‐plate surfaces. Mathematical expressions for the radial and through‐thickness flow velocities were derived that enabled the derivation of extensional and shear strain rates. The GMT non‐lubricated (no‐slip wall conditions) compression molding was modeled as a combination of extensional and shearing flow and the two extensional and shear viscosities were determined. Scott's approach was used in this work to determine the radial velocit in the r‐direction, which depends on the shear power‐law expression. The velocity component in the z‐direction was then calculated using the continuity equation. The velocity profiles were used to calculate the shear and extensional strain rates. Scott's shear viscosity did not satisfy the constitutive equation for the extensional part, but a power‐law expression with new parameters depending on the deformation tensors was successfully used to calculate an independent extensional viscosity using the same non‐lubricated squeezing experiment. Lubricated squeezing flow was carried out for the same material to achieve a pure extensional flow, and the extensional viscosity calculated using this approach agreed with the extensional viscosity determined using the non‐lubricated experiment. GMT material used in this study is confirmed to have two layers of continuous long fibers orientated randomly inplane, separated by short chopped fibers in the middle, which suggests that the material can be treated as an isotropic material, and the fiber‐matrix separation is seen to be high at the extremities of the flow.     

Modeling fountain flow and filling front shape in reaction injection molding

A numerical model of the reaction injection molding process was developed to test front shape and flow approximations employed in previous models. The model was two-dimensional and simulated the flow, reaction, and heat transfer in the typically long axial dimension and the typically small thickness dimension of a mold. The filling front shape and the velocity profiles in the filling fluid were determined by numerical solution of the momentum equation with the appropriate stress boundary conditions using the method of Patankar (1980). The predicted temperature and conversion results agreed with calculations assuming that the front was flat perpendicular to the flow and that a parabolic velocity profile existed behind the fountain flow region at the front. Thus, simple assumptions about front shape and velocity in the thin dimension of a reaction injection mold can be employed without significant loss of accuracy in modeling reaction injection molding.     

Optimization of polyester sheet molding compound. Part II: Theoretical modeling

A mechanistic kinetic and heat transfer model is used to describe the cure of sheet molding compounds (SMC). Kinetic parameters such as rate constant of initiator decomposition and rate constant of propagation are estimated from the induction time and the time to reach the peak exotherm of isothermal reaction curves measured by the differential scanning calorimetry (DSC). Temperature and conversion profiles inside plate sections of SMC parts during molding are measured. The predicted results compare well with the experimental data except the limiting conversion. A set of predictive parameters are proposed from this model as guidelines for the optimal molding of SMC. Several moldability diagrams are also constructed which can be easily used to design the optimum SMC recipe for a given processing condition.     

Reaction and thermal analysis for SMC (sheet molding compound) molding in complicated geometries

A finite element technique has been developed for coupled reaction and heat transfer analysis in which mass diffusion is negligible. The temperature unknowns are located at nodal points, while the reaction variables (species concentrations, reaction rates) are at the Gauss points in each element. With a mechanistic kinetic model, the SMC (sheet molding compound) cure in 2-D and 3-D geometries was analyzed. The results for plate-and-rib configurations show the progression of cure and heat transfer and the influence of geometry on the progression. The analysis for a flat sheet of SMC in a mold with localized heating using bubblers indicates the thermal interaction between the mold and the curing SMC. Temperature and reaction profiles are given for each case.     

The effects of molding conditions on the surface composition of sheet molding compound

An improved internal reflection infrared spectroscopy (ATR) technique (1) has been found to be effective for measuring the relative concentrations of polyester and polystyrene (resin) and calcium carbonate filler on sheet molding compound (SMC) surfaces. The technique has been used to determine the effect of molding conditions on the surface compositions of three commercial SMC materials. The surface compositions of two of the materials, of the same formulation but obtained from different sources, were the same and were unaffected by molding conditions. The surface of the third material (or a different formulation) was found to have substantially less resin than the first two materials. The surface composition of the third material varied with molding conditions, the greatest uniformity being obtained with high molding temperatures and pressures. This study has shown that the ATR technique is suitable for determining the relative surface compositions of SMC formulations. This method will be used to correlate the SMC surface composition to SMC properties, such as surface appearance, paintability, and adhesive bond durability.     

Decreasing the cycle time for in‐mold coating (IMC) of sheet molding compound (SMC) compression molding

In‐mold coating (IMC) is a thermosetting liquid applied to compression molded sheet molding compound (SMC) exterior automotive or truck body panels as an environmentally friendly primer to improve surface quality and make the part conductive for subsequent electrostatic painting. The IMC is injected onto the surface of the SMC then cures and bonds to provide a smooth conductive and protective surface. In IMC as in many other reactive polymer processes, to have short cycle time while maintaining adequate flow time and pot life is required. This allows enough time to fill the mold before solidification. In this study, the effect of inhibitor (p‐benzoquinone), initiator (t‐butyl peroxybenzoate), and mold temperature on the flow and cure time of IMC materials has been experimentally investigated using differential scanning calorimeter. A cure model is developed based on experiments to predict inhibition and cure time. A multiple criteria optimization method was employed to identify the setting parameters of the controllable process variables that provide the best compromise (Pareto frontier [PF]) between flow and cure time. The analysis shows that simultaneous addition of initiator and inhibitor allows the molding to be performed at a higher temperature, which moves the PF toward the ideal location. Hence, minimizes the cure time and maximizes the flow time simultaneously. POLYM. ENG. SCI., 59:1158–1166 2019. © 2019 Society of Plastics Engineers