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.     

Effect of reinforcement type and length on physical properties,surface quality,and cycle time for sheet molding compound (SMC) compression molded parts

One of the fastest growing applications of sheet molding compound (SMC) compression molding is the manufacture of truck body panels. The trucking industry requires parts with high strength and stiffness, but the surface quality is also important. In this study, the effect of reinforcement type and length on physical properties, surface quality, and cycle time are evaluated. In particular, the effect of different lengths of carbon fibers and glass fibers with different sizing are studied. It was found that for the same volume percent, carbon fibers greatly improve the stiffness of the SMC at the sacrifice of strength and surface quality and also require larger fill times for the same molding force, as compared to glass fibers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2557–2571, 2003     

Variability in static strengths of sheet molding compounds (SMC)

In this paper, the variability in static strengths of six glass fiber-reinforced sheet molding compounds (SMC) is reported. This variability was shown not to be caused by macroscopic variations in fiber, resin, filler, or void content nor gross fiber anisotropy. Weibull statistics were used to accurately characterize this variability, Random flaw models with the flaws being distributed in the volume, surface, or edge were shown not to predict the observed static strength variability in SMC.     

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.     

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.     

Air entrapment in sheet molding compounds (SMC)

Entrapped air can commonly lead to large delaminations in thick walled sheet molding compound (SMC) products. In this work different sources of air entrapment in SMC are investigated. The critical process is shown to be the impregnation of the fibers. If no surface active additives are used, large volumes of air may be entrapped in this process, unless the viscosity of the compound is very low. In this situation of poor wetting, the viscosity of the compound during fiber impregnation will critically determine the interlaminar, tensile strength of the product. However, if surface active additives are used, the air escapes entrapment even at relatively high viscosities. The lowering of the viscosity, which is a side effect of the additives, has practically no importance under these conditions of good wetting. Large numbers of small air bubbles are also entrapped during the mixing of the components, but it is shown that these bubbles have very little effect on the mechanical properties of the finished part.     

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.     

Powder coating of sheet molding compound (SMC) body panels

Sheet Molding Compound (SMC) compression molded parts are prone to porosity. During top coat baking, trapped air in the surface porosity expands and often blows through the paint leaving unacceptable craters in the final finish. The accepted solution to this problem in the SMC industry is to use a coating compound on the SMC part. The coating compound (called in‐mold coating (IMC)) is injected and cured on the SMC molding after its cure is complete, but before removing it from the mold. Another potential solution is to powder coat the parts once they have been de‐molded. While powder coating adds time to the process, it is performed outside of the mold and frees the mold for the next molding cycle earlier than if the IMC process is used. In the present paper, we develop a simplified model for the powder coating of plastic parts. We show how the model can be combined with chemo‐rheological measurements to guide the optimization of the process and material parameters. Although with the powders currently available, the surface appearance is inferior to the one obtained with IMC, this process shows potential.     

Practical guidelines for predicting steady state cure time during sheet molding compound (SMC) compression molding

The longest part of the molding cycle during SMC compression molding is the curing stage. Thus it is extremely important to be able to predict its duration to estimate the cost of manufacturing a new part. During an SMC molding cycle, the mold surface temperature drops suddenly when it contacts the cold charge. The surface temperature then gradually recovers as heat is conducted from the interior of the mold and the resin releases heat during curing. In general, this exchange of heat remains locally unbalanced, causing a gradual decrease in the local surface temperature. To avoid blistering, the cure time must be increased with consecutive moldings until a steady state value is achieved (t_css). In this paper, we present a series of charts that can be used to estimate the steady state cure time for new parts. These values can then be used to estimate the manufacturing cost.     

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.     

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     

TDI增稠改性SMC研究

分别以甲苯二异氰酸酯(TDI)和氧化镁(MgO)/TDI体系作为增稠剂改性不饱和聚酯(UP)基片状模塑料(SMC),考察了单一增稠剂TDI和复配增稠剂MgO/TDI体系用量对SMC物理性能的影响。结果表明,TDI质量分数为7%,MgO/TDI质量比为1.5/6时,SMC能得到最小的收缩率和最好的力学性能。     

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.     

Sheet molding compound characterization using spiral flow

Sheet molding compound (SMC) is a fiber‐reinforced polymeric composite. It is often used in automotive, marine, and industrial applications over other materials because of its high strength to density ratio, resistance to corrosion, and low cost. There is a demand in the SMC industry to be able to characterize SMC processability. This is particularly true for heavy truck body panels, one of the fastest growing applications of SMC. Because of their large size and high strength requirement, the molding forces have a major influence in the molding cycle. Also because of the long flow paths involved, the ability of the paste to carry glass needs to be properly characterized when developing new SMC materials. In this article, we demonstrate the benefits of using spiral flow as a processability tester. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Application of artificial neural networks for the optimal design of sheet molding compound (SMC) compression molding

A sequential design optimization scheme based on artificial neural networks (ANN) is proposed. It is a combination of an ANN model and a nonlinear programming algorithm. The proposed scheme is implemented with network training, optimization, and sheet molding compound (SMC) process simulation in a closed loop. A “cyclic coordinate search” technique is employed to initiate the optimization process, to collect training data for the neural network model, and to perform a preliminary design sensitivity analysis. Emphasis is placed on the development of an integrated, automatic optimization-simulation design tool that does not rely on the designer's experience and interpretation. Testing results based on the design of heating channels in an SMC compression molding tool show that the optimal design can be achieved with fewer data points than other methods, such as factorial design. The efficiency of the ANN method would be greater as the number of design variables grows.     

SMC轻量化初步探究

为实现SMC轻量化,通过模压工艺,从原材料选型、配方设计以及工艺过程控制三个方面对轻质SMC(片状模塑料)进行了探究。首先,通过研究不同类型中空玻璃微珠(HGS)对制品比重、光亮度以及弯曲强度的影响发现,VS5500和H40适合作为轻量化SMC轻质填料,制品的设计密度和真实密度比较接近,且力学性能损失较小。其次,通过配方设计,研究了中空微珠用量、增稠剂类型以及增稠剂用量对制品比重的影响。同时研究了玻纤含量和树脂类型对制品力学性能的影响。研究结果表明,轻质SMC的设计密度不能过低,否则制品中HGS的破损比例将会增加。研究发现EK100作为增稠剂,树脂糊前期粘度可以有效控制,后期粘度快速上升,可以有效防止中空微珠相分离的发生。此外,随着玻纤含量从25%增加到30%,制品力学性能呈现增加趋势,弯曲强度从148 MPa增加到172 MPa,但随着玻纤进一步提高,弯曲强度反而出现大幅度衰减,降到140 MPa。通过研究三种不同类型树脂对制品外观和力学性能的影响,使用P18-03树脂压制的制品外观最好,其弯曲强度为172 MPa,满足汽车外饰件力学性能要求。最后,通过工艺过程控制,研究了微珠处理工艺对制品比重的影响。结果表明,烘干处理的HGS可以有效降低树脂糊的水含量,从而保证树脂糊后期粘度可以达到适合模压的窗口。     

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.     

An economical way of using carbon fibers in sheet molding compound compression molding for automotive applications

The automotive industry is extremely cost sensitive. This is one of the main reasons why sheet molding compound (SMC) compression molding is the most popular fiber‐reinforced polymeric composites manufacturing method in this industry. SMC compression molding economics are better suited for the automotive industry than processes such as resin transfer molding or any of its variations. For automotive SMC molders to take advantage of the added stiffness provided by carbon fibers, as an alternative to the widely used glass fibers, the manufacturing process needs to be simplified as much as possible. In actual manufacturing of SMC it is not easy to combine glass fibers with carbon fibers; it will be a lot more convenient to combine SMC plies only with carbon fibers together with SMC plies with only glass fibers. This will also allow molders that will normally use only glass fibers‐based SMC to secure carbon fiber SMC from a separate supplier and use it only where it makes economic sense. The main purpose of this study is to investigate the relative improvement in physical properties that can be achieved by substituting glass fibers by carbon fibers in a per ply basis. POLYM. COMPOS. 27:718–722, 2006. © 2006 Society of Plastics Engineers