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Publication

Prediction of interphase strength in 2K injectionmolding of thermoset rubbers and thermoplastics

Book - Dissertation

The project targets at modeling the overmolding process and predicting the mechanical properties of the interphase between a semi-crystalline HDPE and an EPDM thermoset rubber. The first challenge is to accurately predict process-related parameters such as local temperatures, and local curing degree as a function of time. For this specific 2K-production process, no process-specific simulation tools are available. Nonetheless, these tools are indispensable for the determination of the final material and product properties. Within this project, a validated method has been developed using Autodesk Moldflow to predict all process-related parameters by using accurately determined characteristics for both materials. The flow behavior and the vulcanization behavior of the rubber are investigated via capillary rheometry and MDR, respectively. The relevant mechanical properties of the EPDM material are measured as a function of the degree of vulcanization. Different physical models were used to describe the relevant material characteristics. To model the viscosity behavior of the EPDM, the Cross-model is combined with the Castro-Macoscko model. The vulcanization behavior is modeled with the Kamal-Saurour model. The stress-strain behavior of the cured rubbers is included via hyper-elastic material models. Secondly, the process simulations are used to determine the product-specific properties. The most important product quality parameter is the interphase strength between the rubber and the thermoplastic. Moreover, for thermoset rubbers, the product properties depend both on the interphase properties and the local vulcanization degree. To develop a model to predict the interphase strength, test samples are produced in a tailor-made 2K-plate mold. A combined numerical and experimental study investigates the influence of local mold and product temperatures and vulcanization on the final interphase properties. The local properties of the rubber are determined and compared to the expected adhesion strength. The latter depends on the interdiffusion of the rubber material over the interface. This interdiffusion on its turn depends on the local temperature and the melting of the thermoplastic at the interface. For both parameters, data is generated via samples produced with accurately defined process parameters. With these data, the main goal of this project has been achieved, namely defining a model to predict the interphase strength. The proposed model is validated on two types of 2K products: a plate product and the 2K wheel product for which a mold was designed and optimized during this project. The interphase strength could be accurately predicted for both products. To visualize the indicated interphase strength and its important parameters, a numerical simulation strategy is developed in which a self-developed script is included to visualize the simulated properties. In this way, it is possible to identify opportunities for interphase improvement. Finally, a procedure is developed to map the local strength and stiffness properties of both rubber and interphase on a structural simulation model to perform an accurate simulation. A mapping method is created for two implicit structural solvers, Adina and Abaqus/standard. Both simulation methods contain different challenges in solver stability while predicting failure behavior. The Abaqus solver proved to be the most robust, combining the reduced polynomial hyper-elastic material model for the thermoset rubber and cohesive elements for interphase failure. The results of the numerical simulations including the local mechanical properties proved to be in a good correlation between numerical simulation results and experimental data. This was the case for both tested product geometries.
Publication year:2020
Accessibility:Open