As more companies turn to engineering thermoplastics to access high-performance material properties at lower cost, finite element analysis tools increasingly are called upon to assist in efficient part design. Importantly, the use of a coupled engineering process between mold-filling simulation, non-linear structural analysis and advanced optimization tools is providing answers for discrete manufacturers in search of solutions.
With many of today’s engineering thermoplastics, simplified isotropic representations of material properties have limited accuracy in the prediction of structural performance. The use of more advanced material models, as well as a product development workflow that accurately captures the “as-molded” condition of a structural injection molded part, are keys to high-performance product design. The “as-molded” condition can and often does significantly change injection molded part behavior; capturing this behavior is key to robust analytical predictions.
This paper outlines and discusses a computer-assisted engineering (CAE) centered product design workflow that successfully integrates material testing at the coupon level and injection molding simulation, and subsequently imports these as-molded simulation results directly into a finite element solver for high-fidelity performance predictions and, ultimately, into optimization software that can help determine optimal topological feature construction for the best performance at the lowest cost.
Material properties for mold-filling simulation
CAE-centered product design workflow described in this paper is based on a close interaction of the mold filling (MF) and finite element analysis (FEA) simulations. Results of MF simulations such as shrinkage, warpage and residual stresses act as inputs to the FEA simulations. Therefore, the accuracy of the FEA simulation is a function of the MF simulation results, which are governed by the rheological, pressure-specific volume-temperature (PVT) and thermal properties of the thermoplastic resin. As such, the properties of the simulated material have a direct effect on final part performance as predicted by FEA simulations. Hence, they are a crucial element of the CAE workflow.
Figure 1 shows the set of properties for a typical thermoplastic material that are needed to perform an MF simulation. Complex flow behavior of thermoplastics is characterized by the viscosity vs shear rate curves as obtained by means of a capillary rheometer. PVT curves are the backbone of the MF simulation, as they govern the state of the thermoplastic for a given temperature and pressure combination. Thermal properties such as thermal conductivity and specific heat govern the overall heat transfer between the melt front and the mold base.
These properties must be available for multiple temperatures, which are relevant to the processing conditions of the subject thermoplastic material. For common materials, these properties are available in the simulation software (Moldex 3D, for example) database. However, for customized proprietary materials these properties can be easily tested for a relatively low cost to gain maximum accuracy in MF simulation results.
|Figure 1: Material properties needed to run a mold filling simulation.|
In addition to the aforementioned properties, standard datasheet properties such as coefficient of linear thermal expansion (CLTE) and Young’s modulus are needed to perform the MF simulation. If unavailable, these properties can be tested as well, along with the other properties. As such, obtaining the desired material