Die casting mold push rod size and fit
The dimensional design and precision of the die-casting mold’s push rod are crucial factors in ensuring the proper functioning of the ejection mechanism and improving the quality of die-cast parts. They directly impact the smoothness of mold opening and closing, the release of die-cast parts, and the mold’s service life. Push rod dimensional parameters primarily include diameter, length, and head shape. These parameters must be determined based on the die-casting’s structural characteristics, the molding process requirements, and the ejection force. The selection of the push rod diameter is particularly important. A diameter that is too small can cause the push rod to bend or break due to excessive force during the ejection process. A diameter that is too large increases mold processing difficulty and may leave noticeable push rod marks on the die-casting surface, affecting the product’s appearance. Typically, the push rod diameter can be initially determined based on the ejection force calculation formula and then adjusted based on the die-casting’s wall thickness and material. For example, for thin-walled die-castings, the push rod diameter should be kept small to minimize surface damage. Larger, heavier die-castings, on the other hand, require a larger diameter to provide sufficient ejection force.
The push rod length must be designed to match the ejection stroke of the mold to ensure that the die-casting can be completely ejected from the cavity during the ejection process, while avoiding interference between the push rod and other parts of the mold during the return stroke. The calculation of the push rod length should include the distance from the push rod fixing plate to the cavity surface, plus the necessary margin to compensate for errors in the mold assembly process and wear of the push rod during use. In addition, the head shape of the push rod needs to be designed according to the demolding requirements of the die-casting. Common head shapes include flat top, spherical top, conical top, etc. Flat top push rods are suitable for die-castings with flat or shallow concave surfaces. Spherical top push rods can reduce contact stress with the casting surface to avoid crushing the casting. Conical top push rods are suitable for die-castings with deep cavities or complex structures and can better transmit the ejection force.
The precision of the fit between the push rod and related mold components directly impacts the motion accuracy and sealing of the ejection mechanism. The push rod, push rod retaining plate, and push plate guide sleeve typically utilize a clearance fit, but the size of this clearance must be strictly controlled. Excessively large clearances can easily cause the push rod to wobble during movement, affecting ejection accuracy and even causing deformation of the die-cast part. Excessively small clearances increase friction and wear between the push rod and mating components, leading to motion jamming and shortening the mold’s lifespan. Depending on the mold’s precision level and operating conditions, the clearance is generally controlled between 0.01-0.03mm. For high-precision die-casting molds, the clearance needs to be further reduced to 0.005-0.01mm to ensure smooth push rod movement. Furthermore, the surface roughness of the mating surfaces must also be strictly controlled, typically requiring an Ra value of no greater than 0.8μm to reduce friction and wear.
The fit between the push rod and the cavity is a key step in ensuring the dimensional accuracy and surface quality of die-cast parts. The fit between the push rod head and the cavity surface should ensure that the push rod head is flush with the cavity surface when the mold is closed, avoiding ridges or depressions on the die-cast part surface due to step differences. During design, the expansion of the push rod after heating must be considered. Especially in high-temperature die-casting processes, the die-casting temperature of materials such as aluminum alloys and magnesium alloys can reach 600-700℃. The push rod will elongate due to thermal expansion. If the fit clearance does not reserve sufficient compensation, it may cause interference between the push rod and the cavity, affecting the normal operation of the mold. Therefore, when determining the push rod length and fit clearance, thermal expansion calculations must be performed. Usually, reservations are made based on the material’s linear expansion coefficient (for example, the linear expansion coefficient of steel is approximately 12×10-6/℃) and the operating temperature difference to ensure that a reasonable fit clearance can still be maintained at the highest operating temperature.
The design of push rod dimensions and fit must also consider the mold’s processing and assembly processes. During processing, the push rod’s diameter tolerance is typically controlled within the h7-h8 range, and the length tolerance is controlled within ±0.02mm to ensure precise fit with other components. The push rod’s straightness error must be controlled within 0.01mm/m to prevent movement jamming due to bending. During assembly, push rods must be grouped and selected to ensure dimensional consistency within the same mold. Especially for multi-push rod ejection mechanisms, the push rod length error should be controlled within 0.01mm to ensure uniform force distribution across all push rods and avoid damage due to excessive force on individual push rods. Furthermore, after assembly, a trial mold verification is required. By observing the demolding of the die-cast part and the push rod’s movement, the push rod dimensions and fit clearances can be fine-tuned until optimal working conditions are achieved. With the development of precision machining technologies, such as CNC grinding and EDM, the dimensional accuracy and surface quality of push rods have been significantly improved, providing a strong guarantee for the high-precision matching of die-casting molds, and also laying a solid foundation for the high-quality production of die-casting parts.
