Defect reduction in alloy casting industry

The objective of this research is to reduce the defect rate in aluminum alloy casting process. The selected product is chair's leg of a garden set, which has the most production volume in the case study company. There are three defect types that mostly occur in the casting process, which are Metallic Projection, Cavities, and Defective Surface. These defect types cause rework costs for the case study company. The research was started from defining potential factors that might be the causes of three defect types by using brainstorming and research. There are six potential factors that found. The six potential factors are pouring height, number of gates, snow powder quantity, solidification time, aluminum combination, and gate size. These six potential factors were tested using DOE techniques, where the three types of defects were used as three responses. Then, ANOVA was used to test for significant factors. The result shows that the effects of some factors are not consistent between some responses. Thus, the total cost of rework was then used as the decision criteria for factor level selection. The chosen levels are pouring height at 10 centimeters, 1 number of gate, 10 grams of snow powder, 10 minutes for solidification time, aluminum combination of 18 bars of solid type and 2 bars of resilient type, and gate at size 2x7 centimeters. The findings are confirmed by follow-up runs. After improving the process, the production time for a garden set has reduced from 6 hours and 1 minute to 5 hours and 40 minutes, or it reduces by 6% as a result of the reduction of rework time. If calculate it in term of products that the company can gain after implementing this new setting. The company can actually produce the products 26.43 extra garden sets from the original setting and gain the income by about 330,313 baht. However, the new setting only applies to the chair's legs. If this experiment expands to the rest of the chair parts, the income will likely increase.

本研究以降低铝合金铸造工序的次品率为目标。本次研究选取案例企业产量最高的产品——户外休闲套装椅腿铸件作为研究对象。该铸造工序中频发三类缺陷：金属突起（Metallic Projection）、孔洞（Cavities）与表面缺陷（Defective Surface），上述缺陷会给案例企业带来额外的返工成本。研究首先通过头脑风暴与文献调研，明确了可能导致这三类缺陷的6项潜在影响因素，分别为浇注高度、浇口数量、雪粉（snow powder）用量、凝固时间、铝合金配比以及浇口尺寸。随后采用试验设计（DOE, Design of Experiments）技术对这6项因素开展测试，将三类缺陷分别作为三项响应指标，并通过方差分析（ANOVA, Analysis of Variance）检验各因素的显著性。研究结果显示，部分因素对不同响应指标的影响并不统一，遂将总返工成本作为因素水平选取的决策准则。最终确定的最优工艺参数为：浇注高度10厘米、浇口数量1个、雪粉用量10克、凝固时间10分钟、铝合金配比为18根铸造型铝合金锭与2根弹性铝合金锭的组合，以及浇口尺寸2×7厘米。上述优化结果通过后续试生产得到验证。工序优化后，单套户外休闲套装的生产时长从6小时1分钟缩短至5小时40分钟，因返工耗时减少，整体生产效率提升6%。以该企业原有生产参数为基准测算，采用该优化工艺后，企业可额外生产26.43套户外休闲套装，新增营收约330313泰铢。但该优化工艺仅适用于椅腿铸件；若将该试验拓展至其余椅类零部件，企业营收有望进一步提升。