Identification of the Root Causes for Blowhole Defect in Castings Using Quantitative Risk Ishikawa Diagrams

Abstract

The primary objective of this work is to reduce blowhole defect occurring in a cast iron transfer box. The transfer box moulds were produced using green sand moulding process, and cores were made using CO2 process. It was found that the transfer box had a rejection rate as high as 60%, and it was mainly due to a blowhole on the top surface of the casting. The Ishikawa and risk Ishikawa diagrams were used to analyze this major defect. The risk Ishikawa diagram was constructed by assigning weights to the major and sub-causes due to which the defect occurs. Further, the probability of risk and its impact values were employed to compute risk scores for the main causes and also global risk. From the work carried out, it was found that that the moulds and the sands along with melting and pouring parameters were the major causes for this defect while damp chill, pouring delays and high moisture content in sand were identified as the root causes for the blowhole defect. The main root cause for the blowhole defect was pouring delay that eventually causes dampness in the chill pieces kept inside the moulds. The remedial measures of keeping the moisture content of the sand within 3.5% and pouring the moulds in the shift II within half an hour after closing them were implemented to eliminate it.