Discussion on the Production of Large 1/4 Mill Head (End Cover)
Discussion on the Production of Large 1/4 Mill Head (End Cover)
Feb 26, 2025
Discussion on the Production of Large 1/4 Mill Head (End Cover)
1. Overview Mill heads(end covers) castings are one of the components of rotary kiln mills. Depending on the size of the rotary kiln, the structure of the mill head (end cover) is designed in various forms: full circle, half circle, and quarter circle. Many manufacturers, regardless of how many equal parts the mill head (end cover) is divided into, will produce it as a full circle as long as the hardware conditions allow. The advantage of full-circle production is that the mill head (end cover) undergoes uniform overall shrinkage and minimal deformation during the solidification process after pouring. If full-circle production is not feasible due to hardware limitations, casting manufacturers will adopt corresponding production processes based on their own conditions. This article discusses the single-piece production of a 1/4 circle mill head (end cover). The material of the mill head (end cover) is ZG20Mn, with high internal quality requirements. The joint surface and longitudinal flange must meet the GB7233.1 Level 2 non-destructive testing standards, while other parts must meet at least Level 3. The maximum diameter of the mill head (end cover) after machining reaches 7950mm, and the height reaches 800mm. The structure is shown in Figure 1. Through process discussion, relevant process data collection, and experience summarization, this method can be replicated and applied to other large circular structures with corresponding 1/4 products. Figure 1 1/4 mill head (end cover) 2. Process Plan For the 1/4 circle mill head (end cover), considering the specifications of the heat treatment furnace and the issue of the top box, it is not possible to produce the mill head (end cover) as a full circle. Therefore, the final process plan is to produce it as a single 1/4 circle piece, as shown in Figure 2. The overall approach is to use a sweep molding method for the bottom box and a false box for the top box to ensure a complete top box. The advantage of a complete top box is to avoid multiple core assemblies, which could lead to sand drop defects and gas holes in the casting. To ensure that the internal quality of the 1/4 mill head (end cover) meets the Level 2 non-destructive testing requirements, a CAE simulation was conducted for the process plan, and the results are shown in Figure 3. Figure 2 Process Plan of 1/4 mill head (end cover) Figure 3 CAE Simulation Results of 1/4 mill head (end cover)
3. Quality Control Since the 1/4 mill head (end cover) is not cast as a full circle, there is a higher possibility of deformation after pouring. The shrinkage and machining allowances must be reconsidered beyond conventional standards, and the process dimensions must account for the direction of solidification shrinkage. 3.1 Shrinkage and Machining Allowance Considering the single-piece casting of the 1/4 mill head (end cover), there is little obstruction to its solidification shrinkage, so a shrinkage rate of 2% is chosen. The machining allowance is intentionally set slightly larger. For the joint surface flange, the machining allowance is set at +60mm per side based on a 2% shrinkage rate of the chord length (5671mm after adding the machining allowance). The center is also radially reduced by 20mm. This means that even if the 1/4 mill head (end cover) shrinks completely by 2%, there will still be a 20mm machining allowance on the two joint surfaces in the 90-degree direction. In addition, the machining allowance for other surfaces is set at +35mm, and non-machining surfaces are given corresponding anti-deformation corrections according to standard practices. 3.2 Radial Shrinkage Direction Control The radial width of the 1/4 mill head (end cover) after adding the machining allowance is 2770mm. If the outer diameter is calculated using conventional methods, the shrinkage could reach 160mm, which might result in excessive machining allowance on the outer circle. If the inner diameter is designed using conventional enlargement methods, it might lead to insufficient machining allowance on the inner circle. Therefore, the radial shrinkage of the 1/4 mill head (end cover) is based on the effective width between the inner and outer diameters. To ensure that both the inner and outer circles have sufficient machining allowance, the center is radially reduced by 20mm as mentioned in section 3.1. 3.3 Quality Control To ensure that the internal quality of the 1/4 mill head (end cover) meets the non-destructive testing requirements, three rows of risers are set up. Since the entire surface of the mill head (end cover) is inclined, there is a risk of shrinkage porosity caused by water extraction between the risers. Therefore, dark chills are placed between each row of risers to partition the risers and ensure dense internal quality of the mill head (end cover). The quality control during the molding process strictly follows the process documents issued, especially the shrinkage dimension control, which is implemented with specialized process drawings to prevent operators from following normal process requirements. Additionally, since the entire cavity of the top box for the 1/4 mill head (end cover) is suspended at 2825mm × 5840mm, the strength of the top box is a concern, and the mesh reinforcement process is specifically strengthened. Figure 5 shows the molding of the lower cavity, and Figure 6 shows the top box molding. Figure 5 Molding of the Lower Cavity Figure 6 Top box Molding
After the first 1/4 mill head (end cover) was poured, insulated, demolded, and cleaned, the first marking was performed on the casting. The product dimensions were qualified, with sufficient machining allowance on all machined surfaces, and no deformation was observed. 4. Conclusion
The successful single-piece production of the 1/4 mill head (end cover) has accumulated practical experience that can be applied to the production of other similar 1/4 circle products, avoiding unnecessary detours.
Each casting manufacturer has its own advantages, and specific production methods and data need to be explored and summarized individually, taking into account the actual on-site conditions.
The trend toward large-scale cast steel parts is inevitable, and few manufacturers can achieve integrated casting. Therefore, based on the product's application and customer requirements, segmented casting is one of the production approaches.
