Fatigue crack propagation of sheet molding compounds in various evironments

The effect of the absorption of water and isooctane on the rate of fatigue crack propagation of sheet molding compounds (SMC-R30 and SMC-R65) was investigated. A crack extension gage was used to measure the crack length. Results show that the absorption of water decreases the rate of fatigue crack propagation in the initial cycles but increases the rate of propagation in the final cycle. The absorption of isooctane into SMC-R65 tends to decrease the rate of fatigue crack propagation. Microscopic observation shows considerable swelling of the polyester matrix due to the absorption of water and no significant apparent effect due to the absorption of isooctane.     

Mesostructure development during molding of sheet molding compounds

Experiments have been carried out with sheet molding compounds in which the amount of shear developed during molding was varied by changing the charge size. The effects of pressure and temperature were also investigated. The moldings were characterized by their mesostructures and the influence of the mesostructures on the Izod toughness and flexural strength was examined. It was found that high degrees of shear resulted in fiber orientation and the spreading of the fiber bundles. However, this did not improve properties, and it was concluded that for optimum properties, small amounts of shear were desirable.     

Joining strength of sheet molding compounds

Bonding and bolting strengths of a sheet molding compound (SMC-R30) are investigated. Epoxy adhesives are found to give good bonding strength. Surface pretreatment does not affect bond strength. Artificial weathering has practically no effect upon bond strength. Bolting strength depends on the width of the specimen for a certain bolt size. Bolting gives stronger joints than bonding.     

Processing-property relationships in compression molding of sheet molding compounds

An experimental investigation was conducted into establishing relationships between the processing variables and the mechanical properties of compression-molded parts of sheet molding compounds (SMC). Emphasis was placed on investigating the effects on the tensile properties, impact strength, and dynamic mechanical properties of composite specimens, of low-profile additives, and of treating glass fibers (for reinforcement) with sizing chemicals. The processing variables investigated were cure time, mold temperature, and mold pressure. It was found that: (1) An optimum cure time and mold temperature exist for achieving molded SMC composites of the greatest tensile and impact strengths; (2) Of the four different types of low-profile thermoplastic additives employed, the poly(vinyl acetate) modified with acrylic acid gives rise to molded SMC composites having the greatest tensile and impact strengths; (3) An optimum cure time and mold temperature exist for achieving the highest glass-transition (T_g) of the low-profile additive; (4) The values of cure time and mold temperature that have yielded the greatest tensile and impact strengths also yield molded specimens having the highest T_g of the low-profile additive.     

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.     

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.     

Property modification of bulk molding compounds for use in injection molding

Although impact and flexural strength of injectionmolded bulk molding compounds increase initially with glass fiber content, these properties level out at a glass volume fraction between 0.1 and 0.2, limiting the achievable properties. Use of “special purpose” polyester resins gives no significant improvement in impact. The impact strength limitation is not worsened, however, by using the maximum processable level of filler, this being true for all fillers commonly used in polyester compounds. Replacement of a fraction of the glass by poly(ethylene terephthalate) fibers results in a substantial improvement in impact strength.     

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.     

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.     

Improving the crack resistance of zirconium dioxide

Translated from Steklo i Keramika, No. 6, pp. 17–18, June, 1991.     

多孔微珠用于制备低密度BMC材料

采用多孔微珠为填料制备了不饱和聚酯低密度团状模塑料(BMC)。选取多孔微珠的粒径及掺量,短切玻璃纤维的长度及掺量为4因素,设计L16(44)正交试验,并结合示差扫描量热法(DSC)和扫描电镜(SEM)对复合体系的增强机理进行了研究。结果表明,制备轻质BMC材料的最佳条件为:多孔微珠的粒径<0.710 mm,掺量4%,短切玻纤长度6 mm,掺量30%,此时制得BMC材料密度为1.314 g/cm3,弯曲强度为81.50 MPa,满足国标GB/T 23641—2009对BMC弯曲强度的要求(≥80 MPa)。多孔微珠的蜂窝壁对树脂的固化起到了阻碍作用,固化时间延长,放热不完全,同时多孔微珠的填充使得树脂基体的应力分散不均,样品的表观密度和弯曲强度降低。     

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.     

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.     

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.     

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.     

Notched strength of sheet molding compunds

The Tensile strength of sheet molding compounds containing circular coutouts is investigated. The results show that sheet molding compounds contain inherent voids making the material unaffected by notches smaller than a certain size, The results show a reasonable fit with a three parameter model.     

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.