Selection of die casting parting surface
The selection of parting surfaces is a critical step in die-casting mold design, directly impacting casting quality, mold structural complexity, and production efficiency. Choosing the right parting surface requires adherence to certain principles, taking into account factors such as the casting’s structural characteristics, dimensional accuracy, surface quality, demolding methods, and mold processing and operating costs. A reasonable parting surface can simplify the mold structure, improve casting qualification rates, and reduce production costs. Conversely, an inappropriate parting surface can lead to complex mold structures, increased casting defects, and impacted production schedules.
First of all, the selection of the parting surface should ensure that the casting can be demoulded smoothly, which is the first principle of parting surface selection. The parting surface should be set at the largest cross-section of the casting as much as possible, so that the casting can remain on the side of the movable mold after the mold is opened, which is convenient for the ejection mechanism to eject it. For example, for cylindrical castings, the parting surface should be set at the maximum diameter of the cylinder to ensure that the casting moves with the movable mold when the mold is opened, to avoid the casting being stuck in the fixed mold and unable to be removed due to improper position of the parting surface. At the same time, the position of the parting surface should make the demoulding angle of the casting reasonable. For the inner and outer walls, sufficient demoulding angles should be set respectively to reduce demoulding resistance. For castings with undercuts or bosses, if the parting surface cannot avoid these structures, the use of a core pulling mechanism or an inclined ejector mechanism to assist in demoulding should be considered to ensure that the casting can be removed smoothly.
Secondly, the selection of parting surfaces should be conducive to ensuring the dimensional accuracy and surface quality of the casting. Parting surfaces should be placed away from important functional areas and high-precision surfaces of the casting, such as bearing holes, sealing surfaces, and mating surfaces, to prevent these areas from generating flash or misalignment due to the presence of the parting surface, which would affect dimensional accuracy and assembly performance. For example, for castings with precision bearing holes, the parting surface should be set away from the bearing holes to ensure that the machining accuracy of the bearing holes is not affected by the parting surface. At the same time, the parting surface should be selected so that the molding surfaces of the casting are located in the same mold cavity as much as possible, reducing surface joints caused by the parting surface and improving the surface quality of the casting. For castings with high appearance requirements, such as automotive decorative parts, the parting surface should be set on the non-appearance surface or hidden part of the casting to avoid parting marks affecting the appearance.
The selection of parting surfaces should also facilitate the processing and manufacturing of the mold, simplify the mold structure as much as possible, and reduce the difficulty and cost of processing. Parting surfaces should be flat or regularly curved as much as possible, and avoid complex irregular curved surfaces to reduce the processing steps and time of the mold. For example, for castings with complex shapes, if the parting surface can be made flat by adjusting its position, the processing cost and cycle of the mold can be greatly reduced. At the same time, the selection of parting surfaces should make the mold cavity and core structure simple, easy to process and assemble. For example, for box-shaped castings, setting the parting surface at the edge of the box body can make the cavity structure of the fixed mold and the movable mold simple and easy to process; if the parting surface is set in the middle of the box body, it may lead to a complex cavity and core structure, and increase the difficulty of processing.
In addition, the selection of the parting surface should be conducive to exhaust and overflow, ensuring that the gas and cold material in the cavity can be discharged smoothly, reducing defects such as pores and cold shuts in the casting. The parting surface should be set as far as possible at the last filling part of the casting, so that it is convenient to open an exhaust groove at the parting surface to guide the gas and cold material into the overflow groove. For example, for cup-shaped castings, the parting surface can be set at the edge of the cup mouth, and an exhaust groove can be opened at the edge of the cup mouth to allow the gas to be discharged in time during the molten metal filling process. At the same time, the selection of the parting surface should make the arrangement of the exhaust groove simple and effective, avoiding the difficulty of setting the exhaust groove or poor exhaust effect due to improper position of the parting surface.
Finally, the selection of parting surfaces should also consider production volume and mold life. For high-volume castings, the parting surface should facilitate rapid mold replacement and repair, minimizing mold downtime. For example, a symmetrical parting surface design allows for interchangeability of mold components, facilitating replacement of worn parts. Furthermore, the parting surface should be selected to minimize excessive impact and wear on the mold cavity and core, extending mold life. For example, when die-casting high-melting-point alloys, the parting surface should minimize direct impact with the molten metal to reduce thermal fatigue and mechanical wear on the mold.
When selecting a parting surface, it’s often necessary to compare and optimize multiple options, weighing the impact of various factors before selecting the optimal one. For example, a company producing a complex automotive engine hood die-casting initially considered two parting surface options: a flat parting surface located in the center of the hood, and a stepped parting surface located along the hood’s contour. After analysis and comparison, it was determined that the flat parting surface option would simplify mold processing but struggled to ensure the dimensional accuracy and surface quality of the casting. The stepped parting surface option, however, was complex to manufacture but maintained quality. Ultimately, given the high quality requirements of the casting, the company chose the stepped parting surface option. By optimizing the machining process, they reduced mold processing costs while ensuring a high casting yield. This demonstrates that the selection of a parting surface requires comprehensive consideration of various factors, with a rational decision based on the actual situation.
