Abstract
A special mold provided with a glass window was used in order to directly evaluate the flow progression during the filling phase of the injection molding process in a thin-wall cavity and to validate the simulation of the process with particular focus on the hesitation effect. The flow of the polymer was recorded at 500 frames per second using a high-speed camera (HSC). Two unfilled thermoplastic polymers, acrylonitrile butadiene styrene (ABS), and polypropylene (PP), were used to fill two different 50 mm × 18 mm staircase geometry cavities, which were specifically designed to evaluate the hesitation effect with thicknesses of 1500, 1250, 1000, 750, 500 µm (cavity insert no. 1) and 1500, 1200, 900, 600, 300 µm (cavity insert no. 2). In addition to the video recordings, the simulations were validated using the timings and the data obtained by three pressure sensors and two thermocouples located in the cavity. For each injection cycle recorded on camera the machine data were collected to carefully implement the correct boundary conditions in the simulations. The analysis of the video recordings highlighted that flow progression and hesitation were mainly influenced not only by the thickness, but also by the velocity and the material type. The simulation results were in relatively good agreement with the experiments in terms of flow pattern and progression. Filling times were predicted with an average relative error deviation of 2.5% throughout all the section thicknesses of the cavity. Lower accuracies in terms of both filling times and injection pressure were observed at increasingly thinner sections.
Highlights
The hesitation effect in the filling phase of the injection molding process is a phenomenon that may occur wherever the melt encounters different thickness sections: the melt polymer tends to fill the thicker sections while hesitating in the thin sections
The flow front at position 3 obtained with a short shot experiment at high speed (SSF, 40 mm/s) will reach a position in the Y direction 1 mm farther than the corresponding position obtained with a full stroke filling (ComF)
The present study investigated the hesitation of unfilled thermoplastics through direct flow visualization and simulation
Summary
The hesitation effect in the filling phase of the injection molding process is a phenomenon that may occur wherever the melt encounters different thickness sections: the melt polymer tends to fill the thicker sections while hesitating in the thin sections. The effect can be reduced during the design phase of the part, i.e., avoiding large thickness variations or placing the gate in an opportune position [2]. In this case, modern simulation software is a powerful tool used to improve the design and reduce the lead time to successful production. The design choices may be limited by the complexity of the part, or be relevant only for a type of material. Current simulation software presents limits to its predicting capabilities, for thicknesses in the sub-millimeter down to the micrometer dimensional range, as in the case of thin-wall molding
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