The BYD3313 gearbox is a newly developed product by Shantui Construction Machinery Co., Ltd. specifically designed for a 20-ton road roller. During the initial stages of trial production and small-scale manufacturing, casting defects significantly impacted the yield of the cast parts. In response, the company established a dedicated QC (Quality Control) team to investigate and resolve these issues. Through multiple process improvements, the casting yield was eventually brought up to the required quality standards. This QC project was recognized with the "First Prize of National Machinery Industry Excellent Quality Management Team Activities" at the 30th Double Generation Conference of the China Machine Quality Association, held in Beijing in 2011.
**Current Situation Analysis**
As our company produces a wide variety of products in small batches, we use the furan resin sand molding and core making method. The box blank weighs approximately 150 kg, made from HT250 material, and its main structure is illustrated in Figure 1. There is an oil passage located on the upper surface of the housing, positioned above the pouring area, with the corresponding sand core shown in Figure 2.
In the early stages of trial production, the rejection rate of the box reached as high as 15%. The primary defects were concentrated in porosity and broken cores within the oil passage, as seen in Figures 3 and 4. Among these, porosity accounted for 50% of the defects, while broken cores made up 33%.
**First Process Improvement and Verification**
Given the tight timeline for the trial, a QC team was formed to conduct technical research. Using the QC methodology, we identified the key factors among the five elements: people, machines, materials, methods, and environment. It was determined that the core-making process and gas venting mode were the two main causes of the casting defects.
To address this, the following measures were taken:
1. Furan resin sand generates a significant amount of gas, and if the venting channels are too small or absent, it can lead to porosity. To mitigate this, ceramic tubes were embedded into the main core, one of which was placed beneath the oil passage core to direct the gas away from it. Additionally, a riser was added near the oil passage plane.
2. The oil passage core had a thickness of only 10 mm, and using furan resin sand made it difficult to ensure sufficient strength. Therefore, the method of producing the oil passage core was changed from handmade furan resin sand to coated sand hot core boxes, improving the core's strength. To prevent the core from lifting during casting, adhesive was applied to secure it on top of the sand core, as shown in Figure 5.
After the first round of process improvements, the scrap statistics showed a significant reduction in the defect rate, dropping from 15% to 8%. While porosity issues were largely resolved, the problem of broken cores remained severe, accounting for over 50% of the defects.
**Second Process Improvement and Verification**
Further testing of the coated sand revealed that its tensile strength at room temperature was ≥ 3.5 MPa, and its bending strength was ≥ 7.5 MPa. The gas generation was ≤ 14 mL, meeting all necessary physical and chemical requirements.
Upon analyzing the core fixing method, it was found that improper operation of the nails could cause cracks, leading to core breakage during casting. Additionally, if the core was not securely fixed, it might be lifted by the buoyancy of the molten metal, resulting in thin walls and defects. Some castings exhibited thin oil passage walls that could not be tapped during drilling, as shown in Figure 7.
Based on this analysis, further process improvements were necessary. After studying the part’s structure, we modified the M26 hole from the cooler outlet, changing it from a through-hole to a casting hole to better hold the oil core. A similar core was also added at the other end, as shown in Figure 8. This change allowed us to eliminate the nail fixing method entirely.
After implementing the second round of process improvements, a new trial production run was conducted. The results showed that the scrap rate dropped to around 4.5%, and the quality remained stable during mass production.
**Conclusion**
Through the improvement of the oil core process for the BYD3313 gear box, the oil passage opening was increased, which created better conditions for shot blasting and significantly improved the internal cleanliness of the oil passage. The overall scrap rate was reduced from 15% before the improvements to approximately 4.5%, lowering production costs and providing valuable experience for quality enhancement in similar products.
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