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

The compression molding process of sheet molding compound (SMC) is an economical manufacturing process for lightweight parts. However, molding defects, such as fiber matrix separation, and fiber re-orientation, may develop during the molding process in confined regions, such as ribs and bosses. Hence, the mechanical properties of the composite depend on the local fiber architecture. Consequently, this work compares the predictive capabilities of tensor-based and directly modeled process simulation approaches regarding compression force, fiber volume content and fiber orientation on the example of honeycomb structures molded from SMC. The results are validated by micro-computed tomography and thermal gravimetric analysis. The fiber orientation in the honeycomb varies between individual samples because a sheet molding compound is macroscopically heterogeneous and thus the fiber architecture is strongly influenced by random events. Tensor-based fiber orientation models can not reliably predict fiber volume content and fiber orientation in the part’s thickness direction if there is a lack of scale separation. Therefore, directly modeled process simulations should be preferred in cases in which fiber length and mold dimensions prohibit scale separation. The prediction of fiber volume content is a difficult task and no simulation can predict the severity of fiber matrix separation precisely in all cases.