Experimental and Numerical Analysis of SMC Compression Molding in Confined Regions—A Comparison of Simulation Approaches

1. Introduction

The transition from conventional vehicles to battery electric vehicles creates new challenges for the structural design process. The new crash management system (CMS) has to overcome the missing substructure of the combustion engine with regard to crash safety and the increase in mass caused by the battery system by means of improved support structures.

One approach is the local reinforcement of metallic crash elements with fiber reinforced polymers (FRP), which offer high mass-specific stiffness and strength. The energy absorption capacity of the crash elements is an important factor here. For the specific energy absorption, honeycomb structures show a high potential. As crash elements, they are already being used by various car manufacturers and are mainly made of discontinuous fiber-reinforced polymers. An important advantage of discontinuous FRP are their low material and manufacturing costs, as well as the high freedom of design. The excellent mechanical properties of FRP are directly related to the fiber length, fiber volume fraction and the fiber orientation in the component. An increase in these properties up to a certain level while maintaining the complexity and design freedom of the manufacturing process would thus have a positive effect on the crashworthiness of the hybrid metal-FRP-CMS [ 1 2 ].

One possibility is the use of sheet molding compound (SMC), which has become a very attractive material in recent years, especially for exterior parts in the automotive industry. SMC consists of a thermosetting resin reinforced with chopped fibers and in most cases mineral fillers. SMC components are usually manufactured in a compression molding process, which allows the processing of long-fiber-reinforced polymers. Compared with injection-molded parts, compression-molded parts are superior in terms of increased strength and toughness due to the higher fiber length.

5,12,

During the compression molding of SMC, the initial fiber architecture is changed and is influenced by process parameters, e.g., the initial charge position or the manufacturing process conditions. At first, research on local changes in microstructure was mainly focused on fiber length and orientation rather than changes in FVC, which is commonly known as fiber matrix separation (FMS). It develops if fibers are not transported homogeneously with the matrix during mold filling. As a result, fiber content becomes non-uniform, even when averaged over macroscopic volumes. This leads to a lower than nominal FVC along the flow path and at the flow front in particular [ 3 ]. In this process, FMS results from a balance of forces between suspended fibers, fibers and mold surface, the elastic deformation of the fibers, and hydrodynamic forces acting as a result of fluid flow [ 4 6 ]. Basic experiments have shown that FMS is also formed in flat, round or rectangular compression-molded stacks [ 7 8 ] and depends, for instance, on the fiber-aspect ratio, the matrix viscosity [ 9 ] and the closing speed during production [ 10 ]. Usually, FMS is most pronounced in the top sections of ribs [ 11 13 ], which can be coupled with an increased fiber density in the rib base [ 14 ].

This fiber architecture has a direct influence on the mechanical behavior of the molded part. The effects of process parameters on the structural–mechanical component properties must therefore be mapped with a functional virtual process chain in order to create enhanced confidence in the class of discontinuous long-fiber-reinforced composites, as is already standard in metal processing with commercial software linking tools. Such a coupling has already been considered in short-fiber thermoplastic injection molding with a validation using a demonstrator and is also available for commercial use [ 15 ]. For the compression molding process of long-fiber-reinforced thermoplastics, Buck et al. [ 16 ] showed the effect of coupling process and structure simulation. Görthofer et al. [ 17 ] presented an approach for an integrated virtual process chain for SMC compression molding.

A here) to efficiently describe the orientation state in numerical models. Some experimental shortcomings, especially regarding the limitation of fiber length, have led to extended models by Wang et al. [

However, the base of any virtual process chain is a reliable and efficient process simulation, which is able to provide high-quality result data to map the material-specific characteristics of the specific material class. A brief overview of the state of research in fiber reorientation modeling is described below. Basically, a distinction is made between macroscopic models, which describe the fiber composite as a single-phase medium, and microscopic or mesoscopic approaches, which differentiate between fiber and matrix. Macroscopic models are used to predict the fiber orientation and pressure distribution in components and are already integrated in commercial software environments. Fiber orientation prediction started in 1922 with G. B. Jeffery’s approach to Einstein’s publication on the behavior of spherical particles in a liquid [ 18 ]. Jeffery extended Einstein’s work [ 19 ] for ellipsoidal particles and described their rotation in a dilute Newtonian flow. The velocity field of the fluid is not influenced by the particle and the contact between particle and fluid is assumed to be perfect. This approach was extended by Folgar and Tucker, who developed a phenomenological model to predict the orientation distribution of fibers in concentrated suspensions [ 20 ]. Their derived model is regarded as the initial model for the prediction of fiber orientation in process simulation. Advani and Tucker [ 21 ] proposed the use of statistical moments of the fiber orientation distribution function (called fiber orientation tensors and denotedhere) to efficiently describe the orientation state in numerical models. Some experimental shortcomings, especially regarding the limitation of fiber length, have led to extended models by Wang et al. [ 22 ], Phelps et al. [ 23 ] and Tseng et al. [ 24 ]. These tensor-based models provide good results for FRP with fiber lengths smaller than the characteristic length of the component [ 25 ], i.e., if scale separation applies. This term refers to the assumption that the fiber architecture is a local state of material without any significant extension at the part scale. It may be violated if the fiber length is in the order of magnitude of geometrical features.

All macroscopic modeling approaches published to date are phenomenological and cannot detail the specific mechanics during fiber–fiber interaction. In highly concentrated fiber suspensions, however, fiber interactions are the dominant factor for modeling fiber re-orientation and distribution. For this reason, such methods seem to be of limited suitability for use in a virtual process chain for long-fiber-reinforced plastics with confined regions.

26,27,28,29,

With simulation methods on the microscopic or mesoscopic level, fiber movements can be simulated for discretized fibers. In contrast to the phenomenological macroscopic fiber orientation models, modeling approaches on the microscopic scale allow a more precise approximation of the physical behavior [ 22 30 ]. At the micro level, different modeling approaches can be used. They can be roughly divided into particle-based methods, in which the polymer matrix and the fibers are treated as particles, or element-based methods, in which fibers are treated as particles and the matrix as a continuous medium. In contrast to particle-based methods, which are by nature two-way coupled, element-based simulations are often solved with one-way coupling. The feedback from the fiber motion on the fluid is computationally complex, but can be integrated in a general or refined way [ 31 32 ].

In this work, two direct element-based simulation methods with different coupling mechanisms between fiber and matrix and two macroscopic models are investigated for the process simulation of the SMC compression molding process. Furthermore, they are tested for their predictive accuracy using a real structure manufactured with different process conditions, based on fiber orientation, distribution, fiber matrix separation, compression forces and short-shot flow front.

An important factor for this comparison is the reproducibility of fiber orientation and distribution under real conditions, especially in complex three-dimensional structures. To the authors’ knowledge, previous works did not address this problem or only in geometrically simple structures, where the fiber length and component dimensions are at least one order of magnitude apart. Instead, this study examines the filling of a complex honeycomb structure with an SMC whose fiber length is in the same order of magnitude as the part’s geometric features. The process simulations are compared and validated by their prediction of the compression force, fiber volume content and fiber orientation inside a complex ribbed honeycomb structure.