Figure 1 shows a motorcycle buffer casting with an average wall thickness of approximately 2.5 mm. After machining and fitting a suitable sprocket, it becomes the buffer assembly. To ensure smooth rear wheel movement, the four sprocket mounting holes of the buffer and the central bearing hole require precise positioning during assembly.
Figure 1: Buffer Casting
1. Defects and Improvements of the Original Casting
As shown in Figure 1, due to the limitations of the casting structure, the clamping force during demolding is relatively large. To facilitate demolding, the ejector pin design of early die-casting molds is shown in Figure 2. To accommodate the ejector pin, the bottom holes for the four sprocket mounting holes on the casting cannot be made on the mold and must be completed through subsequent machining. However, during subsequent machining, due to the thick wall at the mounting holes, severe internal shrinkage cavities occur in the casting, seriously affecting product quality. Meanwhile, because the threaded mounting holes lack pilot holes, the machining positioning requirements are very high. Even slight negligence can lead to positional errors in the machined casting, failing to meet usage requirements and resulting in extremely low production efficiency, unable to meet the demands of mass production.
Figure 2: Schematic diagram of the original casting
To fundamentally solve this problem, the die-casting mold structure must be improved and enhanced. It is necessary to create pilot holes for the threaded mounting holes on the blank. To do this, the position of the ejector pins must be changed.
After analysis and discussion, it was decided to change the ejector position to the location shown in Figure 3, and increase the number of ejector pins from the original 4 to 8.
Figure 3: Schematic diagram of the improved casting
During production, it was found that because the casting wall thickness was relatively thin at the location of the ejector pins, and the clamping force of the casting was large, the casting could not be ejected smoothly. Often, the bottom surface of the casting was pierced during ejection, resulting in scrapped castings.
2. Improvements to the New Mold Design
To solve this new problem, it would be necessary to either increase the number of ejector pins or reduce the clamping force of the casting. Due to space constraints, increasing the number of ejector pins is impossible; the only option is to reduce the clamping force of the casting.
Based on the mold structure, we decided to reduce the ejection force required for the first ejection by allowing the casting to be ejected in two stages. The specific solution is as follows:
The tail step of the moving mold core is made 6 mm thick, and the depth of the mounting holes on the moving mold core is made 10 mm (as shown in Figure 4). During mold opening, the moving mold core moves forward 4 mm along with the die casting, completing the first demolding. Afterward, the ejector plate continues to eject, and the die casting is ejected from the moving mold core again, completing the second demolding. By performing two demolding operations, the force required for each demolding is reduced, allowing for smooth ejection of the die casting.
Figure 4: Buffer Body Moving Mold Core
Having solved the demolding problem of the die casting, it is still necessary to accurately reset the fixed mold core in the next cycle; otherwise, the dimensions of the casting will change, and the quality will not be guaranteed. The repositioning of the fixed mold core can be achieved using the mold's own structure. The moving mold core and the fixed mold core fit together; during mold closing, the fixed mold core pushes back the moving mold core that followed the ejection, ensuring accurate repositioning.
This improvement in mold structure fundamentally solves the problem of the lack of a sprocket mounting hole in the buffer die-casting, which previously made subsequent machining difficult. It reduces the scrap rate and significantly improves the production efficiency of subsequent machining.
