Water-assisted injection molding (WAIM): a study on the influence of melt, mold and process parameters on the product quality of water-assisted injection molded tubes

Water-assisted injection molding (WAIM) is one of the latest techniques to produce hollow or partially hollow parts. In comparison to gas-assisted injection molding (GAIM), WAIM offers advantages such as reduction of cycle time, flexibility in design, a lower residual wall thickness (RWT) and a better surface finish. The molding window and the process control, however, become more difficult so that the tooling and the mold process design will become more complicated. This study investigates the influence of melt, mold and process parameters on the product quality of water-assisted injection molded tubes, using a design of experiments (DOE). In these experiments the RWT and the part weight were set as response functions to determine the product quality, combined with a visual inspection of the part surface. To check the influence of melt parameters such as shear viscosity, both a HDPE and a PP resin were used. This test set-up was compared to the results of a previous study, which used a different part geometry. This way, the importance of the process parameters for both geometries were compared. The behavior in different curved sections was also deduced so that the influence of the part geometry on the RWT was marked. It was observed that a decrease of the water injection delay time and an increase of the flow rate lead to a minimization of both the RWT and the part weight. Furthermore, parts produced with the PP, which has a higher zero-shear viscosity and a steeper ‘shear thinning’ region in comparison to the HDPE, show a lower RWT and part weight. In addition, the behavior in the curved sections depends on both the applied material and the radius. The part geometry in this study as well as the part geometry in the former study show previously discussed patterns.

Publication year:2010

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