Integrative Simulation Method for the Prediction of Anisotropic and Time-Dependent Mechanical Behavior of Injection Molded Fiber-reinforced Fan Impellers – Creep Modelling
1.2 Fan Design & Materials
Injection molded fiber-reinforced plastics are widely used as structural or functional parts in fan applications. Due to the possibility of insert integration in injection molding and the short cycle times without the need of subsequent machining, injection molding is a cost-efficient process for the production of fans. Adding to this is the fact that the high degree of design freedom can be used to achieve lower noise emissions, higher efficiency factors and weight savings. For the latter option, a number of aspects need to be considered. Beneath the oper-ating loads and conditions, flow-induced static and dynamic loads on the surface of the fan can occur and the process-induced anisotropic, time-dependent mechanical behavior needs to be included for long-term use. In order to consider the different aspects, an integrative sim-ulation method is developed. At first a simulation of the flow conditions during the injection process and a flow simulation of the fan is carried out. The results, namely the fiber orienta-tion and pressure profile, are transferred to the structural mechanical model. The mechanical properties of fiber-reinforced plastic parts depend on the fiber orientations caused by the dif-ferent flow conditions during mold filling and the time- and load-dependent creep behavior of the polymer matrix.
Based on experimental approaches the anisotropic and time-dependent creep behavior were investigated and modelled. The Findley power law was used to represent nonlinear viscoelas-tic creep curves and compared to a time-stress superposition principle, which was applied to predict the creep behavior, based on short-term creep curves. The uniaxial tensile creep of a 40 percent by weight glass fiber reinforced injection molded polypropylene was measured into different directions for different specimen thicknesses (1 mm, 2 mm and 4 mm). Creep strength decreased and both creep strain and creep rate increased with loads perpendicular to the injection direction. This effect increases with reduced test specimen thickness and higher loads. Finally, the integrative method is outlined for a fan.