Direct Sheet Molding Compound process (D-SMC)

The combination of highly crosslinked resin and long glass fiber reinforcement in direct sheet molding compound (D-SMC) provides the parts with high physical properties and low density suitable for large interior and exterior automotive applications. Different stages of D-SMC process including maturation zone and compression molding are studied using mathematical analysis, numerical simulation and experimental investigations. An analytical analysis is used to calculate the pressure and velocity in the first section of maturation zone and then the permeability of fiber bundle is calculated by simulation and the agreement between the available empirical equation and simulation was obtained. The flow is simulated in the next section of maturation zone using Ansys CFX and it was found the interlocking chain belt plays an important role in impregnation of bundles with paste. The microstructure of fiber glass bundles in compression is investigated in a flat plaque mold. Analyses of the bundle deformation, including bending and deformation of the tow shape, are done for the charge and specific sites within a square plaque part for 30 percent and 62 percent of initial charge mold area coverages. Microscopy and micro-computed tomography (micro-CT) of the samples clarify the microstructure of the bundles after flow. The bundles at the part edge and corner deform more than the bundles close to center of the mold in both initial charge cases. The bundles flatten at all positions and bundle bending is mainly observed at the corner. The tow width changes and tow deflections are higher in the samples of 30 percent mold area coverage. The micro-CT images show that the bundles keep their cohesion and stay straight within the middle of the flow path position, but bend at the edge of the mold. Mold filling simulation using MoldflowTM (Autodesk) predict fiber tow orientation through the thickness using the reduced-strain closure (RSC) models for fiber distribution for 30 and 62 percent initial mold area coverage. The measured value of orientation from micro-CT images confirms the random orientation through the thickness, consistent with the RSC model. The Carreau viscosity model explains the behaviour of the D-SMC paste. Another viscosity model for suspensions of high volume of planar randomly oriented glass bundles defines the flow behaviour in the process and is used to simulate a flow field. An open source CFD software package of OpenFOAM solves the equation of mass and momentum along with the introduced viscosity model by the volume of fluid technique in the three dimensional system. The simulation results show the plug flow profile at all positions and times during the process. To study bundle movement and deformation during the process, it is assumed that regions of very high viscosity with the same dimension of bundles represent bundles at different positions of the mold. The results reveal that the bundles close to the edges and corners of the mold moved more than bundles in the middle and diagonal positions. Moreover, the velocity profile shows higher velocity at these locations. Experimentally, the red bundles are placed at different locations of the charge and track during the process. The calculated movement distances of the bundles at different locations from the experiment and the OpenFOAM simulation follow the same trends and therefore agree. In D-SMC, there is a viscosity variation through thickness because of temperature gradient and glass bundle volume fraction variation. The charge is placed between two hot plates of mold which are then closed to squeeze the charge and filled the mold. The layers of charge close to the hot mold plates act as lubricant for layers of charge in center. Because of greater resistance of charge in the center to flow, the hotter lubrication layer may flow preferentially before core layers of charge. The simulation is run using Ansys Polyflow to study the preferential flow in D-SMC process. Several stacks of charges on top of each other are used for compression molding. The flow is simulated for two charges to investigate the evolution of interface during mold filling Images of cross sections from experimental samples are used to validate the simulation analysis