Effects of the viscosity and thermal property of fluids on the residual wall thickness and concentricity of the hollow products in fluid-assisted injection molding

Abstract

Water-assisted injection molding (WAIM) is a method of producing plastic products with hollow internal sections. The flow instability and boiling effect of the water cause non-uniform residual wall thickness (RWT) and void in the wall in WAIM. Due to the high heat capacity and thermal conductivity of the water, the RWT is affected by the viscosity change of injected plastics when the hollow sections are formed. In this study, the effects of process parameters on the distribution of the RWT and the concentricity of the hollow sections in the products with amorphous ABS (Acrylonitrile Butadiene Styrene: amorphous polymer) and crystalline PP (Polypropylene: semi-crystalline polymer) were compared, and the water (0.86 mPa · s @ 25 °C) and high viscosity silicone oil (970 mPa · s @ 25 °C) were used as the secondary injection fluids in the experiments. The RWT was more stable, with the silicone oil having higher viscosity and boiling point compared to the water. The capillary number which could decide the shape of the flow front was also affected by the viscosity of the fluids. Due to the higher capillary number of the silicone oil, the flow front was formed with narrow and long elliptical shape. The higher capillary number also caused the stable flow of the flow front, resulting in stable RWT with the silicone oil. The concentricity of the hollow sections was higher with the silicone oil than with the water.