Valve castings are one of the leading products in castings, and the annual market demand is very large. However, due to the rise in the price of raw materials, the production cost has increased significantly; Compared with other castings, the production efficiency is low and the scrap rate is high, which seriously affects the output of valve castings. A factory produced more than 2000 tons of valve castings in 2015, with a comprehensive waste of nearly 170 tons and a waste rate of more than 8%. Through the data analysis of the defects of valve castings in the factory, we found out the main causes of waste, and made a series of improvements, which achieved good results, ensured the qualified rate of valve castings and improved the product quality.
In 2015, a factory produced 2083.05t valve castings, with internal waste rate of 3.54% and external waste rate of 4.58%. See Table 1 and table 2 for the statistics of defect types causing the waste of valve castings.
| Casting defect | Sand holes | Slag hole | Stomata | Poor size | Poor performance | Falling sand | Deformation | Insufficient pouring | Other |
| Quantity | 29.58 | 25.85 | 5.36 | 2.30 | 2.18 | 1.32 | 1.43 | 1.24 | 4.48 |
| Proportion | 40.11 | 35.06 | 7,27 | 3.12 | 2.96 | 1.79 | 1.94 | 1.68 | 6.07 |
It can be seen from table 1 that the defects causing waste in valve castings are relatively concentrated. The main defects are sand holes, sand inclusions, slag holes and pores, resulting in a total of 60.79t of waste products, accounting for 82.44% of the total amount of waste.
| Casting defect | Shrinkage porosity | Sand hole | Slag hole | Stomata | Poor size | Poor performance | Deformation | Eccentric | Insufficient pouring | other |
| Quantity | 22.36 | 15.07 | 16.04 | 13.13 | 8.86 | 5.4 | 7.33 | 1.24 | 1.12 | 4.85 |
| Proportion | 23.44 | 15.80 | 16.81 | 13.76 | 9.29 | 5.66 | 7.68 | 1.30 | 1.18 | 5.08 |
It can be seen from table 2 that the defects causing the waste of valve castings are relatively scattered. The main defects are shrinkage cavity, shrinkage porosity, sand hole, sand inclusion, slag hole, air hole and poor size. A total of 75.46t of waste products are generated, accounting for 79.10% of the total amount of foreign waste.
From table 1 and table 2, the main reasons for the waste of valve castings are shrinkage cavity, porosity, slag hole, sand hole, sand inclusion and air hole, resulting in a total waste of 149.04t, accounting for 88.17% of the total waste.