How to avoid burrs on the parting surface affecting assembly of zinc alloy die-cast bag accessories through mold optimization?
Release Time : 2025-08-28
Optimizing the mold parting surface for zinc alloy die-cast bag accessories primarily focuses on improving its fit and flatness, which are fundamental to preventing burrs. The parting surface is the interface between the movable and fixed molds. If there are any flatness errors or gaps, the molten zinc alloy will seep into the gap under the pressure of the die casting process, forming burrs upon cooling, which directly affects the assembly of the accessory. During design, the parting surface must be finely machined using high-precision machining equipment (such as CNC milling machines and surface grinders) to ensure minimal flatness errors. Furthermore, a "grinding" process is used to ensure perfect fit between the movable and fixed molds. During this grinding process, red lead powder is applied to the parting surface, the molds are rotated to rub the two together, and high points are polished along the red lead contact marks until the contact area of the parting surface reaches over 95%. This completely eliminates any local gaps and reduces the risk of melt overflow.
Optimizing the mold's clamping guide and positioning system further ensures precise alignment of the parting surface during the die casting process, preventing burrs caused by misalignment. Bag accessories often incorporate delicate assembly features such as clips and holes. If the movable and fixed molds are misaligned during mold closing, gaps can easily form along the parting surface. This can lead to burrs in critical assembly areas due to molten metal infiltration. High-precision guide posts and bushings should be placed at the mold corners or symmetrically. The guide posts should be hardened for wear resistance, while the inner surfaces of the guide bushings should be polished to reduce friction, ensuring smooth, non-binding guidance during mold closing. Positioning pins (such as tapered locating pins) should also be added, with the clearance between the pins and the pin holes carefully controlled. This ensures precise positioning of the movable and fixed molds during mold closing, preventing parting surface deviation caused by uneven clamping force. This ensures a tight, gap-free parting surface throughout the die-casting process.
The sealing structure of the parting surface should be designed in accordance with the shape of the bag accessories. Appropriate overflow and venting slots should be used to direct excess melt away from critical assembly areas. Zinc alloy die-casting generates gas. Poor venting can lead to uneven pressure within the mold cavity, potentially pushing the melt out of the parting surface and forming burrs. Furthermore, small amounts of excess melt, if left with nowhere to drain, can also form burrs along the parting edge. During design, overflow chutes should be designed in non-assembly areas of the parting surface (such as the non-functional area around the edge of a component). The overflow chute's volume should match the excess melt volume during die-casting, ensuring that excess melt flows smoothly into the chute rather than overflowing onto the assembly surface. Vents should also be located in locations within the mold cavity where gas is likely to accumulate (such as corners and thicker wall areas). These vents should be connected to the overflow chute, allowing gas to drain into the chute along with a small amount of melt. This ensures smooth venting and prevents accumulation of melt at critical locations on the parting surface, forming burrs.
Optimizing the junction between the mold cavity and the parting surface can prevent burr concentration caused by structural abrupt changes. If the assembly areas of bag accessories (such as the mating surfaces of clips or the edges of holes) are close to the parting surface, and the junction between the cavity and the parting surface is sharp or right-angled, the molten metal can easily form eddies when flowing into this area, resulting in burrs at the junction after cooling, which can affect subsequent assembly. During design, the junction between the cavity and the parting surface should be machined into a smooth, rounded transition. The radius of the fillet should be determined based on the wall thickness of the accessory. This ensures that the transition does not affect the appearance and function of the accessory while guiding the molten metal to flow smoothly and avoid localized accumulation. Furthermore, the contact width of the parting surface should be appropriately reduced in areas corresponding to critical assembly points (such as the snap-fitting surface). This makes it more difficult for the molten metal to penetrate the gap, further reducing the likelihood of burrs in the assembly area.
To meet the assembly requirements of different bag accessories, a "differentiated sealing" design for the parting surface can achieve precise burr prevention. Some bag accessories have one side for assembly and the other for decoration. Applying a uniform sealing standard to the parting surface may result in fine burrs on the assembly surface. During design, the parting surface area corresponding to the assembly surface requires enhanced sealing. For example, a stepped parting surface is employed in this area. This creates a slight step difference between the movable and fixed molds (the step height must be less than the minimum flow clearance for the molten metal). This prevents the molten metal from penetrating the assembly surface. Meanwhile, the decorative parting surface area can be equipped with overflow chutes to allow a small amount of excess molten metal to escape. This ensures a burr-free assembly surface without compromising the aesthetic quality of the decorative surface, achieving a balance between function and appearance.
Wear compensation design for the mold parting surface ensures a consistent fit during long-term production, preventing burrs from developing due to wear. During the zinc alloy die-casting process, the movable and fixed mold parting surface wears due to repeated opening and closing, resulting in friction. Over time, this gap increases, increasing the likelihood of burrs. During the mold design phase, a small allowance for wear compensation should be reserved in areas of the parting surface prone to wear (such as near guide pins and areas where clamping force is concentrated). The amount of compensation should be determined based on the expected mold life and wear rate. Furthermore, removable inserts should be designed in key sealing areas of the parting surface. When these inserts wear to a certain extent, they can be replaced without replacing the entire mold. This reduces maintenance costs, ensures long-term sealing of the parting surface, and prevents burrs caused by wear.
Iterative optimization of the parting surface during the mold trial phase is a key step in eliminating burrs for specific bag accessories. After the new mold is manufactured, the parting surface design should be verified through trial molds. During trial molds, the location, size, and shape of burrs should be observed. If burrs appear in the assembly area, the cause should be analyzed. If the cause is a gap problem, the parting surface should be re-calibrated; if the cause is poor venting, the location and size of the venting grooves should be adjusted; if the cause is mold misalignment, the accuracy of the guide and positioning system should be checked. After the mold trial, the mold is adjusted according to the problems and the mold is tested again until there are no visible burrs on the accessory assembly area. At the same time, the optimized mold parameters (such as parting surface processing accuracy and overflow groove size) are recorded to provide a reference for the subsequent mold design of similar accessories. This forms a closed loop of "design-mold trial-optimization" to ensure that the final mass-produced bag accessories have no parting surface burrs that affect assembly.