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.     

Silane treated alumina trihydrate: A new formulating tool for flame retardant polyester fiber reinforced plastics

The high strength to weight ratio of glass reinforced plastics (FRF) combined with their fabrication flexibility has led to increasing use in new applications that no other materials could satisfy. FRF often replaces metals in traditional applications. Increasingly stringent flammability requirements have led to the use of alumina trihydrate filler, usually in combination with halogenated rosins. However, difficulties in compounding and fabrication often limit, the alumina trihydrate loading. This requires substantial increments of halogenated resin to meet specific flammability restrictions. In such composites, the price of the compound may be too high for a given application and one must often compromise substantially on both processing and physical property performance. Alumina trihydrate filled systems commonly suffer from poor glass fiber wet out and mold flow, with reduced strength and uniformity in fabricated parts. This paper describes how a unique silane pretreatment for alumina trihydrate permits bulk molding compound (BMC)/sheet molding compound (SMC) formulations to be developed with increased loadings of the filler, reduced filler wet out time, and improvements in glass dispersion, mold flow out, strength and uniformity of the composite. The paper also shows how the halogenated resin content of flame retarded systems may be minimized by the use of this silane treated alumina trihydrate.     

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     

Double‐vacuum‐bag technology for volatile management in composite fabrication

A nonautoclave vacuum bag process using atmospheric pressure alone that eliminates the need for external pressure supplied normally by an autoclave or a press is an attractive method for composite fabrication. This type of process does not require large capital expenditures for tooling and processing equipment. The traditional single‐vacuum‐bag (SVB) process is best suited for molding epoxy matrix‐based composites because of their superior flow and the absence of reaction byproducts or other volatiles. This is not the case for other classes of materials such as polyimides and phenolics. Polyimides and phenolics are cured by condensation reactions which generate water as a reaction byproduct. In addition, these materials are commonly synthesized as oligomers using solvents to facilitate processability. Volatiles (solvents and reaction byproducts) management therefore becomes a critical issue. SVB molding, without additional pressure, normally fails to yield void‐free quality composites for these classes of resin systems. A double‐vacuum‐bag (DVB) process for volatile management in composite fabrication using common molding equipment was designed and built at the NASA Langley Research Center. This experimental DVB process affords superior volatiles management compared with the traditional SVB process. Void‐free composites are consistently fabricated as measured by C‐scan and optical photomicroscopy for high‐performance polyimide and phenolic resins. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

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.     

Interrelationship of strength and flow characteristics of polystyrene

This study investigated the interrelationship between strength and flow characteristics of general-purpose polystyrene (GPPS) used in injection molding applications. The ease of flow was chosen as a measure of processability and was evaluated using the melt flow rate and capillary rheometer techniques. Of the different strength tests that were examined, flexural and notched tensile strength tests were most effective in differentiating between commercial grades of high and low molecular weight GPPS. While characterizing strength of injection molded specimens, the degree of molecular orientation was taken into consideration. For unplasticized resins, increasing the weight average molecular weight by about 100,000 enhanced the flexural strength by 10%, but also increased the viscosity at low shear rates (10 to 100 s^?1). The increase in molecular weight had virtually no effect on viscosity at the highest shear rates (up to 10,000 s^?1). Plasticized resins displayed a 6% loss in flexural strength as well as a significant reduction in viscosity (throughout the shear rate range) as compared with the unplasticized resins. As expected, the improvement in strength achieved by increasing molecular weight leads to a simultaneous increase in the viscosity, i.e., a deterioration of processability. In addition, our study indicates that for samples without preferential molecular orientation, narrowing the molecular weight distribution significantly improves the balance of strength and melt flow rate properties.     

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     

Vinyl molding compounds: Formulation and performance evaluation

Today's vinyl molding compounds are successfully meeting the combined challenges of physical properties, appearance, processability, and cost requirements in a variety of specialty injection molding applications such as appliance parts, business equipment, and electrical enclosures. One of the major reasons why vinyl materials are so versatile is that the poly(vinyl chloride) resins on which they are based can be easily modified with a wide variety of additives to tailor the particular performance features of the compounds to their intended applications. Determination of an appropriate combination of PVC resin and additives to produce an effective and cost-competitive compound, however, is not a simple process. Important considerations in formulating a vinyl molding compound and evaluating its performance are discussed here.     

Processing and physical properties of native grass-reinforced biocomposites

Big blue stem grass fiber-reinforced high density polyethylene powder biocomposites were fabricated using two separate processing schemes: (1) by compounding biofiber with the thermoplastic powder in an extruder and subsequently injection molding the extrusion pellets and (2) by combining biofiber and the powder thermoplastic powder using a modified sheet molding compounding (SMC) line and subsequently compression molding the sheet material. The physical properties including storage modulus, heat deflection temperature (HDT), notched Izod impact strength, and morphology were evaluated with dynamic mechanical analysis, Izod impact strength measurement, and microscopy observation. It was found that compression-molded specimens achieved similar modulus values to injection molded specimens for grass-reinforced high density polyethylene (HDPE) composites. The stiffness of the compression-molded specimens is related to the consolidation state of the samples, which depends on compression molding conditions such as temperature, pressure, and mold type. Compression-molded specimens exhibited a higher HDT and notched Izod impact strength compared to injection-molded samples. Grass fiber-reinforced cellulose acetate butyrate (CAB) biocomposites made with SMC processing had similar physical properties with grass fiber-reinforced HDPE composites, which indicates that natural fiber-reinforced CAB biocomposites have the potential to replace polyolefin-based composites for automotive applications. POLYM. ENG. SCI. 47:969–976, 2007. © 2007 Society of Plastics Engineers.     

SMC模塑料在汽车中的应用

轻量化、环保和舒适安全性将成为我国汽车用材料未来发展方向。介绍了SMC材料生产工艺流程和特性;描述了SMC材料在汽车零部件中的应用。SMC模塑料具有质量轻、性能优、耐腐蚀、使用寿命长、可靠性高和模压工艺简便等优点,将在汽车上得到广泛应用。     

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.     

Advanced injection molding mold and molding process for improvement of weld line strengths and isotropy of glass fiber filled aromatic polyester LCP

An advanced injection molding tool for measurement of mechanical strength and anisotropy of liquid crystal polymers (LCP)/mineral filler composites was developed. The mold produces thin‐walled LCP specimens that can be used by water cutting technique for production of an injection molded flow direction test bar, a transverse‐to‐injection molded flow direction test bar, a test bar for knit line strength measurement, and a test bar for butt weld line strength measurement. This tool and its use for molding experiments were optimized by experimental research and by computational calculations based on experimental parameters obtained by molding of several LCP test materials. Different pressure profiles and different injection speeds were tested as well as application of mold overflow phenomenon in production of test specimens. It was observed that a pressure controlled X‐melt technique and on the other hand fast injection speeds with overflow in conventional molding methods gave the best strength and isotropy properties for the test specimens. Results indicate that the mold developed is useful for determination of anisotropic and weld line strength properties of LCP composites. When developing “isotropic LCP” by different possibilities of nanotechnology this tool significantly reduces time of LCP material and process development. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

The balance between durability,reliability, and affordability in structural composites manufacturing

Fiber reinforced structural composites will play a key role in the development of the next generation of transportation vehicles (passenger cars, vans, light trucks, and heavy trucks) because of their high strength‐to‐weight and stiffness‐to‐weight ratio compared with metals. An integrated assessment of the durability, reliability, and affordability of these materials is critical to facilitate the inclusion of these materials into new designs. The result of this assessment should provide information to find the balance between the three performance measures. This paper describes a method to develop this assessment in the fabrication of sheet molding compound (SMC) parts, and discusses the concept of Preform Insert Assembly (PIA) for improved affordability in the manufacturing of composite parts. POLYM. COMPOS., 28:233–240, 2007. © 2007 Society of Plastics Engineers     

基于阀体制造的先进树脂基复合材料性能研究

研究了不同改性方法对酚醛团状膜塑料（BMC）、酚醛片状膜塑料（SMC）、乙烯基SMC材料力学性能的影响,分析了树脂基与纤维相的改性作用机制,得到了三者中具有最佳力学性能的复合材料,并通过仿真分析验证了不同复合材料用于制造阀体的可行性。结果表明,3种材料的拉伸性能为乙烯基SMC>酚醛BMC>酚醛SMC,弯曲性能为酚醛BMC>乙烯基SMC>酚醛SMC,乙烯基SMC的综合力学性能最佳;成型温度为160 ℃、模压压力为9 MPa、保温时间为30 min时,乙烯基SMC的拉伸强度、拉伸模量、弯曲强度、弯曲模量分别达到148.26 MPa、4.50 GPa、92.33 MPa、2.39 GPa;阀体静力学分析结果表明,乙烯基SMC与酚醛BMC均满足阀体制造要求。     

不饱和聚酯片状模塑料的研究

采用聚氨酯预聚物作增稠剂对带端羟基的不饱和聚酯进行增稠,制备出一种新型的不饱和聚酯片状模塑料(SMC),并研究了其增稠性能、增韧性能及贮存稳定性。结果表明,增稠剂PU400的用量要控制在8％～10％,否则树脂糊无法正常浸渍玻璃纤维;不饱和聚酯SMC的体积收缩率随PU400加入量的增加而明显变小;当PU400含量为36％,时,不饱和聚酯SMC固化物的拉伸强度、断裂伸长率、冲击强度都维持在一个较高的水平;该不饱和聚酯SMC有良好的贮存稳定性。     

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。     

高光表面SMC的研究

本文主要对高光表面SMC进行了研究.采用正交实验设计法对影响SMC表面光泽度的主要影响因素LPA、MgO、CaCO3和模压温度进行了系统的探讨与分析,以SMC制品的表面光泽度和弯曲强度为参考值,得出了4因素对SMC制品表面光泽度的影响趋势曲线.综合表面光泽度和弯曲性能得到一组进一步优化的SMC配方,根据该配方所压制的SMC制品的表面光泽度可达到90,并具有较好的力学性能.     

碱处理竹纤维对聚酰胺6结构和性能的影响

采用10 ％的（质量分数,下同）NaOH碱溶液对竹纤维进行了处理,通过双螺杆挤出和注射成型制备了聚酰胺6（PA6）/竹纤维复合材料。用扫描电子显微镜﹑差示扫描量热仪、X射线衍射仪和热失重分析仪等表征了材料的形貌、结构和热性能,测试了熔体流动速率和力学性能。研究表明,碱处理可清除竹纤维中的胶质物,增大其比表面积,有利于改善PA6和竹纤维间的界面结合。PA6与竹纤维之间具有较好界面结合,相界面间无明显间隙。竹纤维使复合材料中PA6晶相的完整程度降低,使复合材料刚性增加,冲击强度、熔体流动性和热稳定性下降。在挤出和注射过程中,PA6/竹纤维复合材料较好的熔体流动性和一定的热稳定性使其具有熔体加工性能。     

高流动性无碱玻璃纤维SMC粗纱

通过对络纱工艺及张力调节装置的改进,在无碱玻璃纤维络纱过程中加入5.5%～7.0%的热塑性PBT（聚对苯二甲酸乙二醇酯）纤维。为使两种材料混合均匀,络纱时将每2股403,tex的玻纤原丝夹入1股60,tex的PBT纤维束。生产的产品（代号PSR）抗静电性好,毛丝少,具有更好的分散性,与SMC（片状模塑料）其它原料相适性好。用其生产的SMC片材在压制时流动性好,纤维分布均匀,制品收缩纹小,抗冲击强度高。     

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.