Determination of core pulling distance of die casting mold
Determining the core-pulling distance in a die-casting mold is a critical parameter for ensuring smooth demolding of castings. It directly affects whether the core can completely separate from the casting, preventing strain, deformation, or damage during the demolding process. The core-pulling distance refers to the minimum displacement required for the core to be pulled from the molding position to a position that does not interfere with demolding. Its determination requires comprehensive consideration of the casting’s structural dimensions, the core’s shape, and the overall mold layout. A reasonable core-pulling distance not only ensures smooth production but also reduces mold space and eases mechanical design complexity. Therefore, precise calculation and verification are essential during the mold design phase.
The core pulling distance is calculated based on the maximum dimension of the core in the core pulling direction, typically the sum of the length of the core protruding from the casting surface and a necessary safety margin. For example, for a casting with a side hole, if the side hole is 20mm deep, the maximum length of the core in the core pulling direction is 20mm. In this case, the core pulling distance must be greater than 20mm. A safety margin of 3-5mm is generally added to this, resulting in a maximum of 23-25mm. For cores with steps or concave-convex structures, the dimensions of the deepest step or most prominent area are used as the basis for calculation to ensure that all protrusions are completely clear of the casting. For example, if a core has a 15mm deep step and a 25mm protrusion, the core pulling distance should be based on 25mm, with a safety margin of 28-30mm.
The elastic deformation characteristics of a casting are crucial factors when determining the core-pulling distance. Metal castings experience a certain amount of elastic deformation during the demolding process due to core extraction, especially in thin-walled parts and highly plastic materials (such as aluminum and copper alloys). This deformation can cause the casting to rub against the core before it is fully withdrawn. Therefore, for such castings, the core-pulling distance should be appropriately increased, typically adding a 5%-10% margin to the basic calculated value. For example, for a thin-walled aluminum alloy part with a basic calculated core-pulling distance of 30mm, the actual core-pulling distance should be 32-33mm to compensate for the risk of interference caused by the casting’s potential elastic recovery. For brittle materials (such as zinc alloys) or thick-walled castings, the safety margin can be reduced due to the smaller deformation, typically adding 2-3mm.
The structural complexity of the core will affect the determination of the core-pulling distance. For cores with slopes, curves, or complex partings, the core-pulling process must be simulated through 3D modeling to ensure that all molding surfaces can be completely separated from the casting within the core-pulling distance. For example, the core-pulling trajectory of a curved core may have nonlinear characteristics. In this case, the relative position of the core and the casting must be measured point by point along the core-pulling direction to find the maximum separation distance as the benchmark for the core-pulling distance. For modular cores, that is, molding structures composed of multiple split cores, the core-pulling distance of each core must be calculated separately, and it must be ensured that the core-pulling actions of each core do not interfere with each other. In this case, the core-pulling distance must be based on the displacement of the farthest core, and the core-pulling sequence of each core must be coordinated.
The overall mold layout and the movement space of other mechanisms can also limit the design of the core-pulling distance. The core-pulling mechanism must be coordinated with the ejection mechanism, guide mechanism, cooling system, and other components to avoid interference with other components due to excessive core-pulling distance. For example, when the core-pulling mechanism is close to the edge of the mold, the core-pulling distance is limited by the mold installation space and must be shortened as much as possible while ensuring that the casting can be ejected. In this case, the core structure can be optimized (such as using a modular core) to reduce the dimensions in the core-pulling direction, thereby reducing the required core-pulling distance. In addition, the stroke of the core-pulling mechanism must match the mold opening stroke of the die-casting machine. For mechanical core-pulling mechanisms (such as inclined guide pin core pulling), the core-pulling distance is limited by the mold opening stroke and the inclined guide pin angle. It must be verified using the formula L=S/sinα (where L is the mold opening stroke, S is the core-pulling distance, and α is the inclined guide pin angle) to ensure that the mold opening stroke meets the core-pulling distance requirements.
The final determination of the core-pulling distance requires adjustment through mold trials. The theoretically calculated core-pulling distance may deviate from actual requirements due to factors such as the actual shrinkage of the casting and mold assembly errors. During mold trials, observe the casting’s release from the mold. If slight scratches are observed, the core-pulling distance is insufficient and should be increased appropriately. If the core-pulling mechanism operates smoothly but excessive stroke leads to mold space constraints, the distance can be reduced while ensuring safety. For example, if scratches are observed on the edge of the casting at a core-pulling distance of 30mm during mold trials, the distance can be adjusted to 32mm and further verified until the optimal core-pulling distance is found. Only by combining theoretical calculation with practical verification can a safe and economical core-pulling distance parameter be determined.
