Sofie Sannen

The Influence of Melt and Process Parameters on the Quality and Occurrence of Part Defects in Water-assisted Injection Molded Tubes

Abstract This study investigates the influence of melt and process parameters on the quality of water-assisted injection molded tubes. The influence was determined by the aid of a design of experiments (DOE) for both a HDPE and a PP resin. This design was carried out on two different cavities, in which the residual wall thickness (RWT) and the part weight were set as response functions. This was combined with a visual inspection of the part surface to detect correlations in the occurrence of part defects such as fingering and double wall. It was found that the water injection delay time and the water volume flow rate have the largest influence on the RWT and part weight. Moreover, the shear viscosity of the applied material determines the width and shape of the water bubble and consequently the RWT and part weight. In addition, process parameters such as melt temperature, water hold time and water volume flow rate as well as melt parameters such as shear viscosity have an influence on the formation of part defects.

Water Penetration Behavior in Water-assisted Injection Molding (WAIM): A Study of Product Quality for Different Process and Material Parameters

Abstract This study investigates the relation between water penetration behavior and product quality in water-assisted injection molding (WAIM) for different process and material parameters. The principle mechanisms responsible for both building up the residual wall thickness (RWT) and formation of part defects as well as the influence of these process and material parameters on the RWT and occurrence of part defects could be revealed. This is done by relating the measurements of water pressure and water volume flow rate, visualizing in-mold variations during water injection, to both the part weight and a visual inspection of part cross-sections, for different process settings and materials. It was found that the RWT as well as the formation of part defects depend on the polymer resistance, since the variation of the latter leads to a change in both the water bubble width and velocity. On the one hand, the resistance is determined by the process parameters having an influence on the amount of melt ahead of the water bubble, depending on injector opening time, water volume flow rate and water injection delay time. On the other hand, the resistance also depends on the friction between the polymer chains, which is determined by the zero-shear viscosity and the shear thinning behavior of the polymer. In addition, the occurrence of part defects is directly related to the polymer crystallization behavior. The resulting product quality therefore depends on a combination of the process parameters and the applied material.