Abstract
In water assisted injection moulded parts, structural ribs are usually designed to reinforce the parts as well as to serve as water channels. Water penetration in the channels results in improved mouldability and minimises sinkmarks. However, water leaves a rib core void, which reduces part rigidity. The present study examines the core out shapes of channels in water assisted injection moulded parts. The effects of channel geometry as well as processing conditions such as water injection delay time, water temperature, water pressure, etc., on the core out shapes were investigated. For the parameters selected, melt temperature, short shot size and water injection delay time were found to be the principal factors affecting the core out shapes of moulded parts. The core out geometries in water assisted injection moulded parts were compared with those in gas assisted injection moulded products. Water penetrated the parts with higher core out ratios, which are the ratios of the cross-sectional area of the water (gas) channel to the area of the water (gas) bubble in moulded parts, than gas. The core out geometry of water assisted injection moulded parts mainly resembles that of the water channel, while the void shape of gas assisted injection moulded parts tends to be more round shaped. Channels with ribs on top exhibited the highest bending strengths. Furthermore, a numerical simulation of the temperature distribution in the channels was completed to better interpret the water core out behaviour in water assisted injection moulded products.