Selection of injection molding guide pin diameter
Injection molding guide pins serve as guiding components within the mold. Their diameter directly impacts mold opening and closing precision, motion smoothness, and service life. Guide pins primarily ensure accurate positioning of the movable and fixed molds during mold closing, prevent collision between the core and cavity, and withstand lateral forces generated during mold opening and closing. A diameter that is too small results in insufficient guide pin rigidity and can easily bend and deform. A diameter that is too large increases mold weight and processing costs. Therefore, a guide pin must be selected based on mold size, clamping force, and part precision requirements.
The basic principle for selecting guide pin diameter is the mold’s plate size. Generally, guide pin diameter is proportional to plate length. For example, for small molds with a plate length of less than 300mm, the guide pin diameter can be 16-20mm; for medium-sized molds with a plate length of 300-500mm, the diameter is 22-28mm; and for large molds with a plate length of over 500mm, the diameter should be 30-40mm. This matching ensures sufficient support span for the guide pins, preventing bending due to excessive cantilever when the plate warps under clamping forces. Furthermore, the number of guide pins must be coordinated with the diameter. Small molds typically use two guide pins of the same diameter, arranged symmetrically; medium-to-large molds use four guide pins, arranged diagonally. Two of the guide pins can be appropriately larger in diameter (2-4mm larger than the other two) to improve guiding stability.
The clamping force is a key factor in determining the guide pin diameter. During the clamping process, the guide pin withstands a certain lateral force component, approximately 5%-10% of the clamping force. Therefore, the guide pin diameter must meet strength requirements to avoid plastic deformation during long-term use. The calculation formula is: d ≥ √(4 × F side / (π × [σ])), where d is the guide pin diameter (mm), F side is the lateral force (N), and [σ] is the allowable stress of the guide pin material (for 45 steel, [σ] is approximately 160 MPa). For example, when the clamping force is 500 kN, the lateral force is approximately 25-50 kN. Substituting this into the formula, the guide pin diameter must be ≥ 20-25 mm. In practice, the larger value should be used to allow for margin. For large molds with a clamping force exceeding 1000kN, the guide pins need to be made of alloy structural steel (such as 40Cr) and tempered. The allowable stress can be increased to 200MPa. Under the same lateral force, the diameter can be appropriately reduced to reduce the weight of the mold.
The precision requirements of plastic parts significantly influence the selection of guide pin diameter. Precision plastic parts (dimensional tolerances IT5-IT7) require mold guide accuracy of 0.01-0.02mm. In these cases, larger diameter guide pins are used in conjunction with high-precision guide bushings (clearance of 0.005-0.01mm) to minimize the impact of guide clearance on positioning accuracy. For example, in molds for precision plastic parts like mobile phone cases, the guide pin diameter is typically 2-4mm larger than that of standard molds of the same size. A combination of sliding and rolling fits (sliding guide bushings at both ends and a ball guide bushing in the middle) is employed to ensure guiding accuracy while reducing motion resistance. For lower-precision plastic parts (IT8-IT10), the guide pin diameter can be selected according to conventional standards, with clearances as wide as 0.02-0.05mm to reduce processing costs.
The guide pin’s installation method and location also influence the diameter selection. When the guide pin is fixed in a stepped manner (one end is fixed to the fixed platen, the other extends into the guide sleeve of the movable platen), the diameter of the fixed end should be 5-10mm larger than the guide section to enhance the root strength and prevent the guide pin from loosening due to repeated stress. When the guide pin extends through both the movable and fixed platens (fixed at both ends), the diameter can remain consistent, but locknuts are required at both ends. The guide pin should be positioned as close to the mold edge as possible and away from the cavity area. This allows for a longer lever arm and greater lateral bending resistance for the same diameter, so the diameter can be appropriately reduced. For example, when the guide pin is installed in the corner of the platen, the diameter can be 2-3mm smaller than when it is installed in the middle, and still meet the guiding requirements.
The final determination of the guide pin diameter requires careful consideration of the mold material and processing technology. Guide pins are commonly made of 20 steel (carburized and quenched) or 45 steel (quenched and tempered), with a surface hardness of HRC50-55. If high-strength materials (such as SKD61) are used, the diameter can be reduced by approximately 10% while maintaining strength. Regarding machining accuracy, the guide pin’s cylindricity error must be controlled within 0.005 mm/m, with a surface roughness Ra ≤ 0.4 μm. The actual diameter should be ground to the guide sleeve dimensions to ensure the clearance meets the design requirements. During the mold trial phase, the guide pin’s stress response should be inspected. If scratches or excessive wear are observed on the guide pin surface, the diameter may be too small and should be increased by 0.5-1 mm. Excessive resistance during mold opening and closing may indicate an oversized diameter or an overly tight fit, requiring appropriate adjustment. By comprehensively considering mold specifications, stress response, and precision requirements, the appropriate guide pin diameter can be selected to ensure an optimal balance between guiding performance and mold life.
