Die casting guide pins and guide bushings
Guide pins and guide bushings in die-casting molds are critical components for ensuring precise opening and closing of the mold. Their design quality directly impacts the mold’s service life and the dimensional accuracy of the die-casting. They must meet the requirements for precise guidance, smooth movement, and wear resistance and durability. The precise fit between the guide pin and bushing is crucial to guiding performance. Typically, a clearance of H7/H6 is used, with a clearance of 0.01-0.03mm. This ensures flexible movement of the guide pin within the bushing while avoiding the loss of guiding accuracy caused by excessive clearance. The diameter of the guide pin is determined by the mold size: small molds use φ16-φ25mm, medium-sized molds use φ30-φ50mm, and large molds require φ60-φ100mm. The guide pin’s length should cover the mold’s maximum opening stroke to ensure consistent guidance throughout the entire mold opening and closing process. For example, for a medium-sized mold with a 300mm opening stroke, the guide pin should be 350-400mm long, φ35mm in diameter, and have a clearance of 0.02mm with the guide bushing to ensure precise guidance.
The structural design of a guide pin must balance its guiding function with its mounting requirements. It typically consists of a working portion, a mounting portion, and a transition portion. The working portion is cylindrical, ground and polished to a surface roughness of Ra 0.4μm or less and a straightness error of no more than 0.01mm/m, ensuring a perfect fit with the guide sleeve. The mounting portion, 2-5mm larger in diameter than the working portion, is press-fitted into a template using an interference fit (H7/r6). The fit length is 1.5-2 times the guide pin diameter, ensuring a secure fit. The transition portion features fillets or tapers with a radius of 2-5mm to avoid stress concentration. Guide pins are made of 20CrMnTi or SUJ2 (high-carbon chromium bearing steel), carburized and hardened, or through-hardened. The working portion reaches a hardness of HRC 58-62, while the mounting portion reaches HRC 30-40, providing both high wear resistance and ease of press-fit installation. For example, a φ35mm guide column has a working part length of 300mm, a mounting part diameter of φ38mm, a length of 60mm, and is made of SUJ2. After overall quenching, the working part is ground to ensure dimensional accuracy.
The guide sleeve’s structural design must match the guide post. It is typically categorized into straight and stepped styles. Straight sleeves are simple and suitable for small molds. They feature a through-hole inner bore, an interference fit (H7/r6) on the outer diameter of the mold plate, and a length 1.5-2 times the guide post diameter. Stepped sleeves feature a flange at one end for axial positioning and are suitable for medium-sized and larger molds. The flange is 5-10mm thick and 10-20mm larger in diameter than the guide sleeve’s outer diameter. It is secured to the mold plate with screws for enhanced mounting stability. The guide sleeve’s inner bore surface roughness should be below Ra0.4μm, with a roundness tolerance of no more than 0.005mm. The mating surface with the guide post should be honed to enhance fit accuracy. Guide sleeves are made of tin bronze (ZCuSn10Pb1) or wear-resistant cast iron (HT300). For high-temperature die-casting molds, powder metallurgy materials (such as oil-impregnated bearings) can be used, offering self-lubricating properties and reducing lubrication requirements. For example, the stepped guide sleeve that matches the φ35mm guide column has an inner hole diameter of φ35mm (H7), an outer diameter of φ50mm, a length of 70mm, a flange diameter of φ65mm, a material of ZCuSn10Pb1, and an inner hole honed.
The layout of guide pins and guide bushings must ensure balanced force distribution and stable guiding in the mold. Common layouts include corner, diagonal, and center-assisted arrangements. The corner arrangement is the most common, with a guide pin placed at each corner of the mold. This arrangement is suitable for rectangular or square molds and ensures uniform guiding. The diagonal arrangement is suitable for small or irregularly shaped molds, with guide pins placed at opposite corners and aligned with locating pins to ensure precision. The center-assisted arrangement is used for large molds. In addition to the corner arrangement, one or two guide pins are added to the center of the mold to enhance guiding stability and prevent wobbling during mold opening and closing. Guide pins should be placed away from the cavity and gate to avoid direct contact with the molten metal. They should also maintain a sufficient distance (at least 50 mm) from the mold plate edge to ensure mold strength. For example, a medium-sized mold measuring 600 mm x 500 mm would use φ35 mm guide pins at the four corners, spaced 500 mm x 400 mm apart, with the center of the guide pin 70 mm from the mold plate edge. This ensures stable guiding without affecting the cavity layout.
Lubrication and maintenance of guide pins and guide bushings are crucial to ensuring long-term, stable operation. Oil grooves should be provided on the guide pins’ surfaces. These grooves should be spiral or annular, 0.5-1mm deep and 2-3mm wide, to facilitate lubrication. Before using the mold, high-temperature grease (such as lithium-based grease) should be added between the guide pins and bushings. Relubrication is required every 500-1000 die-cast parts to ensure smooth movement. For molds used in continuous production, an automatic lubrication system can be installed to provide a constant and quantitative lubrication supply, reducing manual maintenance workload. During maintenance, guide pins and bushings should be regularly inspected for wear. If the clearance exceeds 0.05mm or scratches or strains appear on the surface, they should be replaced promptly to prevent compromising guiding accuracy. Furthermore, the mounting surfaces of the guide pins and bushings must be kept clean to prevent iron filings, dust, and other debris from entering the clearance and exacerbating wear. With proper design, material selection, and maintenance, guide pins and bushings can achieve a service life of over 100,000 cycles, providing reliable assurance for stable mold operation.
