Die casting mold structure
The die-casting mold’s structural form determines the quality of die-cast parts, production efficiency, and mold cost. Depending on the number of parting surfaces, core-pulling method, and number of cavities, it can be divided into various structural types, each with its own application scenarios and advantages and disadvantages. Choosing the right structural form requires considering the casting’s shape, size, material, and production batch size to achieve a balance between functionality and cost-effectiveness. Common structural forms include single-parting surface molds, double-parting surface molds, inclined core-pulling molds, and multi-cavity molds.
A single-parting surface mold (two-plate mold) is the most basic structural form, consisting of a fixed mold and a movable mold with a single parting surface. This simple structure, ease of manufacturing, and low cost make it suitable for castings with simple shapes and no or minimal core pulling, such as covers and end caps. The operating process is as follows: After the mold is closed, molten metal is injected into the mold cavity. After solidification, the fixed and movable molds separate, and the casting moves with the movable mold and is ejected by an ejector mechanism. The advantages of a single-parting surface mold include fewer parts, simple assembly, and high production efficiency (shorter mold opening and closing times). However, its disadvantages are that it cannot form complex undercut structures and that the core pulling mechanism is limited in its configuration. For an aluminum alloy cover plate, a single-parting surface mold is used, resulting in a 40% reduction in mold cost compared to a double-parting surface mold and a 15% reduction in cycle time, making it suitable for high-volume production (annual output >100,000 pieces).
Double-parting surface molds (three-plate molds) add a parting plate to the fixed mold, forming two parting surfaces. They are primarily used for castings requiring point-gating feed, particularly where cavities are distributed across multiple locations or center-feed, such as small, complex parts and multi-cavity castings. When the first parting surface separates, the sprue solidifies from the casting, while when the second parting surface separates, the casting is ejected. While their advantages include minimal gate marks and uniform feed, their disadvantages include a complex structure, high cost, and long mold opening and closing strokes. A multi-cavity connector using a double-parting surface mold achieves uniform feed across all eight cavities, leaving only 0.5mm of gate residue and requiring no subsequent machining. While this increases mold costs by 30%, it eliminates the gate removal process, reducing overall costs by 15%.
Oblique core-pulling molds are used to form castings with inclined holes, bosses, or undercuts. The core-pulling mechanism is driven by inclined guide pins and hydraulic cylinders. Depending on the direction of the core pull, these systems can be categorized as inclined guide pin core-pulling, bent pin core-pulling, or hydraulic core-pulling. The inclined guide pin core-pulling system is simple and low-cost, making it suitable for applications with short core-pulling distances (<50mm) and small angles (<25°). A housing with a 15° inclined hole utilizes an inclined guide pin core-pulling system, demonstrating its reliability and low failure rate. Hydraulic core-pulling is suitable for complex applications with long core-pulling distances (>50mm) and large angles (>25°). A 40° deep hole in an automotive engine bracket utilizes hydraulic core-pulling. With a core-pulling force of up to 100kN, it meets molding requirements, but costs 50% more than mechanical core-pulling. The key to inclined core-pulling molds lies in the coordination between the core-pulling and ejection mechanisms, requiring an anti-interference device to prevent conflicting movements.
Multi-cavity molds have multiple identical cavities in a mold. They are suitable for small, simple, and mass-produced castings (such as home appliance accessories and toy parts). They can significantly improve production efficiency and reduce unit costs. The number of cavities is determined by the size of the casting and the tonnage of the die-casting machine. Common cavities are 2, 4, 8, and 16 cavities. It is necessary to ensure that each cavity is fed evenly and the force is balanced. The advantages of multi-cavity molds are high production efficiency (high output per mold) and low unit energy consumption. The disadvantages are large mold size, high manufacturing precision requirements, and damage to a cavity that will affect the overall use. A zinc alloy toy part uses an 8-cavity mold, which has a production efficiency 7 times that of a single-cavity mold and a 60% reduction in unit cost. However, the mold manufacturing precision must reach IT7 level to ensure that the dimensions of each cavity are consistent (error ≤ 0.02mm).
Combination molds are made up of multiple modules and are suitable for large, complex, or multi-variety castings produced in small batches. By replacing different modules, different castings can be molded, improving the mold’s versatility. Modules include cavity modules, core pulling modules, and ejection modules, which can be manufactured and replaced individually. Their advantages are high flexibility, high mold utilization, and easy maintenance. Their disadvantages are that the joints between modules are prone to flash and require high manufacturing precision. A large machine tool base uses a combination mold. By replacing different cavity modules, it can produce three different base specifications, reducing mold investment by 40%. It is suitable for small and medium-sized batch production of 5,000-10,000 pieces per year. The joint surfaces of the combination mold require precision machining (flatness ≤ 0.01mm), and locating pins are provided to ensure accuracy and avoid flash.
Specialty molds, including stacked molds, vacuum die-casting molds, and extrusion die-casting molds, are suitable for castings with specialized requirements. Stacked molds feature multiple cavity layers between the movable and fixed molds, effectively stacking multiple molds. This significantly increases production output and is suitable for thin, small parts (such as mobile phone holders). One stacked mold boasts a production output twice that of a standard mold, while consuming only 30% more energy. Vacuum die-casting molds incorporate a vacuum system and are used for castings requiring high airtightness (such as hydraulic valve bodies). Vacuuming (≤50 mbar) reduces porosity. For one valve body, the porosity was reduced from 5% to 0.5% after vacuum die-casting. Extrusion die-casting molds combine the advantages of die-casting and forging, increasing the density of castings and making them suitable for high-strength parts. For example, one aluminum alloy wheel hub, extrusion die-cast, achieved a 20% increase in tensile strength. Specialty molds are expensive and require high technical expertise, making them suitable for the production of high-value-added products.
