The main factors affecting the core pulling force of die casting
The magnitude of die-casting core-pulling force is directly related to the selection of the core-pulling mechanism and the quality of die-casting demolding. It is influenced by a combination of factors, including the physical interaction between the die-casting and the core, material properties, mold structure, and process parameters. A deeper understanding of the mechanisms of these influencing factors can be used to reduce core-pulling force through targeted optimization, thereby minimizing mechanism load and the risk of die-casting damage. The core factor affecting core-pulling force is the clamping force generated by the solidification shrinkage of the molten metal, while other factors indirectly influence the magnitude of core-pulling force by altering the clamping force or friction.
The shrinkage characteristics of die-casting materials are intrinsic factors determining core-pulling force. The shrinkage rates of different alloys vary significantly. The greater the shrinkage rate, the stronger the core-holding force and the greater the core-pulling force. For example, a copper alloy with a linear shrinkage rate of 1.2% to 1.5% has a core-pulling force that is 1 to 2 times greater than a zinc alloy with a linear shrinkage rate of 0.5% to 0.8%. The core-pulling force of aluminum alloys lies between these two, decreasing with increasing silicon content, as silicon refines the grain size and reduces shrinkage. Furthermore, the alloy’s elastic modulus also influences core-pulling force. Materials with a high elastic modulus (such as magnesium alloys) experience less elastic recovery during core pulling, exerting less additional pressure on the core and resulting in a relatively low core-pulling force. Conversely, materials with a low elastic modulus (such as some aluminum alloys) experience increased elastic recovery, which increases core-pulling resistance.
The structural parameters of the core have a significant impact on the core-pulling force, with the draft angle being a key parameter. The smaller the draft angle, the larger the contact area between the die-casting and the core, the stronger the clamping force, and the greater the core-pulling force. Increasing the draft angle from 0.5° to 2° can reduce the core-pulling force by 30% to 50%. However, excessive draft (over 5°) can affect the dimensional accuracy of the die-casting, particularly the flatness of the mating surfaces. The surface roughness of the core is equally important. The smoother the surface (the smaller the Ra value), the lower the friction and the smaller the core-pulling force. Reducing the Ra from 6.3μm to 0.8μm can reduce the core-pulling force by 25% to 40%. Therefore, the core surface is typically polished or chrome-plated. The core’s molding area and shape also affect the core-pulling force. The larger the molding area and the more complex the shape (such as with bosses or grooves), the greater the core-pulling force. For example, the core-pulling force of a core with a molding area of 200 cm² is 1.8 to 2.2 times that of a core with a molding area of 100 cm².
Die-casting process parameters affect core-pulling force by altering the solidification state of the molten metal. Excessively high pouring temperatures increase molten metal shrinkage, increasing holding force and core-pulling force. For example, increasing the pouring temperature of aluminum alloy from 650°C to 700°C can increase core-pulling force by 15%-20%. However, excessively low temperatures can lead to poor molten metal fluidity and inadequate filling, potentially resulting in uneven holding force distribution between the die-casting and the core, and increased core-pulling force in certain areas. Uneven mold temperature can cause uneven cooling and shrinkage of the die-casting, generating additional stress and increasing core-pulling resistance. When the mold temperature difference exceeds 30°C, core-pulling force can fluctuate by more than 25%. Holding pressure and holding time also have an impact. Excessive holding pressure (exceeding 80% of the die-casting machine’s rated pressure) can cause overfilling of the molten metal, increasing pressure on the core and leading to increased core-pulling force. Excessive holding time prolongs solidification time, causing a continuous increase in holding force. Therefore, holding parameters should be appropriately set based on the thickness of the die-casting.
The structure and surface condition of die-castings also indirectly affect core-pulling force. Uneven wall thickness in die-castings can lead to uneven shrinkage. Thicker areas shrink more, creating a stronger clamping force on the core, which in turn increases local core-pulling force. For example, the core-pulling force in a 5mm wall thickness is 1.5-1.8 times greater than that in a 2mm wall thickness. The surface roughness and grain direction of die-castings alter friction. Rougher surfaces increase friction between the die-casting and the core, leading to greater core-pulling force. Graining along the core-pulling direction reduces frictional resistance and core-pulling force. Furthermore, the depth and number of side holes or undercuts in a die-casting also affect core-pulling force. The deeper the hole and the greater the number, the greater the total core-pulling force. For example, a 30mm deep undercut has a core-pulling force 2.5-3 times greater than a 10mm deep one, while the combined core-pulling force of two symmetrical undercuts is approximately 1.9-2.1 times greater than that of a single one (taking into account synergistic effects).
The impact of mold usage and maintenance on core-pulling force cannot be ignored. Core wear increases surface roughness and friction, and core-pulling force gradually increases with mold usage. After a mold has been used for more than 100,000 cycles, core-pulling force may increase by 20%-30%, necessitating regular core polishing and repair. The amount of release agent used and the uniformity of its application are also crucial. The right amount of release agent forms a lubricating film on the core surface, reducing friction. However, excessive application can carbonize at high temperatures, increasing friction and, in turn, increasing core-pulling force. Mold cleanliness is equally important. Residual metal debris or oxide slag on the core surface can exacerbate friction and lead to an abnormal increase in core-pulling force. Therefore, the core surface should be cleaned before each production run to ensure it is free of debris. Through effective mold maintenance, fluctuations in core-pulling force can be kept below 10%, ensuring stable production.
