The compression molding of sheet molding compounds (SMCs) is typically thought of as a fluid mechanics problem. Recently however, the use of carbon fiber based SMC materials (CF-SMCs) with high fiber volume content (up to 50%) and very long carbon fibers (up to 50 mm long) has challenged this point of view. These materials allow to manufacture parts with much better structural properties than ordinary glass fiber based SMC materials, and furthermore, the rheological behavior of these types of materials cannot be characterized using standard rheometry equipment. At the IVW, the material characterization of high fiber volume fraction CF-SMC materials has been carried out. The characterization is conducted using two variations of the “press rheometer” experiment which measures the force response and flow front progression of pressed SMC charges. The goal of the material characterization is to provide the input and validation data for a solid mechanics material model to allow the compression molding simulation of CF-SMC materials by using general purpose FE analysis codes such as ABAQUS and LS-DYNA.
To characterize the compaction and flow behavior of the CF-SMC material, an open circular plate press rheometer experiment is carried out. In this experiment, several layers of SMC material are stacked and positioned centrally between two circular tooling plates, and then the layers are compressed, where the lateral outflow is unhindered during the process. The upper plate moves at a defined speed or force. Two different configurations allow the pressing to be performed at either a constant cross-sectional area or at constant volume. The test specimens themselves are either circular or rectangular as shown in the scheme of the two tests in Figure 1. The large circular specimens can be used for constant area tests to produce cleaner compression force curves free of edge effects while rectangular specimens can be used for the constant volume tests allowing the clearest visualization of the flow front progression. Figure 2 (left) shows the characterization results from an open circular plate constant volume press rheometry experiment for a structural CF-SMC material (6 layers) at a press closing speed of 3 mm/s and for decreasing plate closing distances (short shots). The dashed square in the center shows the initial stack size and position. The markers represent the outlines of the average of 3 flow front experiments for each short shot. Together with the press force curves of the constant area press rheometry experiments, Figure 2 (right), the necessary material model data can be extracted and also validated.
In the past, a limited number of software tools were developed to simulate the compression molding of sheet molding compounds (SMC). First attempts were based on 2D or 2.5D modeling approaches without considering the flow in the thickness direction. Based upon the simulation of injection molding, there are still only a few specialized software tools commercially available which can simulate compression molding in a 3D format. The material models used in these software solutions, however, are based purely on fluid shear (viscosity) and are not capable of describing the complex behavior of a SMC material consisting of high fiber volume content and long fiber reinforcement. In many current solutions the model for the fiber orientation is based on the Folgar-Tucker equation. Another possibility to describe the fiber orientation in a SMC material is through the explicit modeling of the fibers as beam elements. Here the fibers can be represented with customized lengths and effects like fiber breakage and interaction can also be taken into account. However, this mesoscopic modeling approach demands very high computing resources and computational times. As the specialized software tools are restricted with respect to the available material models and methods, general FEM-Software such as ABAQUS® or LS-DYNA® provide a great advantage for the development of customized user defined material models for CF-SMC materials. With multi-physics solvers and the necessary tools for creating user-defined material models, most of the necessary factors and effects can be taken into account. Figure 3 shows the conceptualization of this model for a user-defined material created in LS-DYNA®. Most of the mechanical and thermal effects influence directly the velocity gradient which plays an important role in the Folgar-Tucker equation. In most recent software solutions, SMC compression molding is typically thought of as a viscosity based fluid mechanics problem. However, with the increasing use of high fiber volume content CF-SMCs, it is better described as a solid mechanics problem. The flow of the material is therefore based on an elastoplastic deformation instead of a purely viscosity based formulation. In addition, some CF-SMC materials show an anisotropic behavior which is based on the local fiber orientation. Therefore, the elastoplastic formulation should be orthotropic with directional scale parameters described by the fiber orientation tensor of the subsequent time-step. When all influencing factors are taken into account, one can compare the fiber orientations resulting from the simulated and experimental part. It is expected that the Folgar-Tucker formulation is not adequate enough to accurately describe the fiber orientation in all CF-SMC materials; therefore, a step back will be taken towards developments in the mathematical model in order to change the formulation from a short fiber to a high fiber volume fraction long fiber based description.
With the correct material characterization and setup of the simulation model, the prediction of the compression molding behavior of full 3D parts with different SMC materials is possible. Filling behavior, approximate fiber orientation, along with a prediction of the required press forces for part processing at different pressing conditions are all predictable. In addition, the resulting data from the simulation can then be used for warpage prediction and determination of stiffness and strength for the final part.
M.Sc. Dominic Schommer
Institut für Verbundwerkstoffe GmbH
Telephone: +49 (0) 631/2017 151