Determination of mold component dimensions
Determining mold component dimensions is a core step in mold design, directly impacting mold assembly accuracy, service life, and product quality. The dimensions of each component must be comprehensively calculated based on product requirements, process performance, and material properties, while also taking into account manufacturing tolerances, assembly clearances, and wear allowances to ensure stable forming accuracy over long-term mold use. The dimensions of mold components must be standardized and serialized, prioritizing standard sizes to reduce custom processing and improve interchangeability.
The dimensions of the working parts (punch, die, and core) must be determined based on the product dimensions and adjusted according to the characteristics of the forming process. For the punch and die of a stamping die, the die dimensions are calculated based on the maximum limit dimensions of the product. The punch dimensions are calculated by subtracting the clearance between the two sides of the die. Specifically, Dconcave = (Dmax – xΔ), dconvex = Dconcave – Z, where Dmax is the maximum limit dimensions of the product, Δ is the product tolerance, x is the correction factor (0.5-0.75), and Z is the clearance between the two sides of the die. For example, for a stamping part with a maximum dimension of 50mm and a tolerance of 0.3mm, x = 0.6, and Z = 0.2mm, Dconcave = 50 – 0.6 × 0.3 = 49.82mm, and dconvex = 49.82 – 0.2 = 49.62mm. For the core and cavity of the plastic mold, the cavity size needs to take into account the shrinkage rate of the plastic. The formula is Dtype = (Dsystem + Dsystem × S) + xΔ, and the core size dtype = (dsystem + dsystem × S) – xΔ, where S is the shrinkage rate, such as S = 1.5%-3% for polyethylene.
The dimensions of guide components (guide pins and guide bushings) must meet guiding accuracy and strength requirements. The guide pin diameter is selected based on the mold size: 16-30mm for small and medium-sized molds and 30-60mm for large molds. The length should be 3-5 times the diameter to ensure guiding stability. The guide pin’s cylindricity tolerance should be ≤0.005mm/100mm, with a surface roughness of Ra 0.4μm. The clearance between the guide pin and the guide bushing should be 0.005-0.01mm (for precision molds) or 0.01-0.02mm (for standard molds). The guide bushing should be 1-2 times longer than the guide pin’s diameter and have a wall thickness of ≥5mm to prevent deformation. The material is typically 20 steel, carburized and quenched (hardness 58-62 HRC), and the inner bore should be ground and polished. For example, for a small or medium-sized mold measuring 200×200mm, a guide pin with a diameter of 20mm and a length of 100mm is selected. The guide bushing should have an inner diameter of 20mm (H7), an outer diameter of 32mm (M6), and a clearance of 0.008mm.
The dimensions of the template components (upper die base, lower die base, and fixing plate) must ensure sufficient strength and rigidity. Thickness is determined based on the mold’s closing height and the applied forces. Template thickness for small and medium-sized molds ranges from 20-50mm, while for large molds it ranges from 50-150mm. The template’s flatness tolerance should be ≤0.05mm/300mm, and the parallelism of the upper and lower surfaces should be ≤0.1mm/300mm to ensure uniform force distribution after the workpiece is installed. The template’s overall dimensions should be 50-100mm larger than the workpiece, and the perimeter should be equipped with lifting holes and locating pin holes. The locating pin holes have a diameter of 10-16mm and a fit of H7/m6 with the locating pins. For example, a template for a φ100mm die should have overall dimensions of at least 200×200mm, a thickness of 40mm, and lifting holes with a diameter of 16mm, located diagonally.
The dimensions of fasteners (bolts, screws, and pins) must meet the required connection strength. The bolt diameter is selected based on the formwork thickness, typically 1/4-1/3 of the thickness . For example, a 40mm thick formwork requires M10-M12 bolts with a tightening torque of 20-30 N · m to ensure a secure connection. Pins are 1-2mm smaller in diameter than the bolts and are used for positioning. For example, an M10 bolt is paired with an 8mm φ pin. The fit between the pin and the hole is H7/m6 , with a positioning accuracy of ≤ 0.01mm . The number of fasteners should be evenly distributed according to part size. Each part requires at least two bolts and two pins. Large parts require an additional bolt for every 100mm of length . For example, a 300mm long die plate requires four M12 bolts and four 10mm φ pins, symmetrically distributed on both sides of the part.
The dimensions of auxiliary components (ejector pins, springs, and stop pins) must be determined based on functional requirements. The ejector pin diameter is calculated based on the ejection force (F = K × A × p, where K is the safety factor, A is the ejection area, and p is the specific ejection force). The diameter is typically 5-20 mm, and the length is 10-20 mm greater than the ejection stroke. The spring size is selected based on the required spring force, with compression reaching 30%-50% of the free length. For example, if a spring force of 50 N is required, a cylindrical coil spring with a diameter of 10 mm and a free length of 50 mm is used. When compressed to 35 mm, the spring force is approximately 55 N. The stop pin controls the mold’s closing height. It has a diameter of 20-50 mm and a height 0.1-0.2 mm less than the mold’s minimum closing height. This ensures that the stop pin contacts the mold first during operation, protecting the working parts. Accurately calculating the dimensions of each component ensures mold assembly accuracy and performance, preventing mold damage or product failure due to improper sizing.
