Mold fixing plate and pad design
The die holder and backing plate are crucial structural components of stamping dies. The holder secures the punch, die, and other working parts, ensuring their precise positioning and operational stability. The backing plate, installed between the holder and die base, withstands the impact of the stamping process and protects the die base from damage. The design quality of both directly impacts the die’s precision, rigidity, and service life, requiring targeted design based on the die type, stamping force, and layout of the working parts.
The design of the retaining plate must meet positioning accuracy and connection strength requirements. The material typically used is 45 steel or Q235 steel. The thickness is determined by the mold size and punch height, generally ranging from 15-50mm. For small and medium-sized molds, the retaining plate thickness should be 1/3-1/2 the punch height; for large molds, the thickness should be increased to ensure sufficient rigidity. The punch mounting holes on the retaining plate must have a transition fit (H7/m6) with the punch. The hole positioning accuracy must be controlled within ±0.01-±0.02mm, with a perpendicularity error of no more than 0.01mm/100mm to ensure that the punch does not tilt during operation. For special-shaped or large punches, the retaining plate should be provided with pin positioning holes. Pins (fitting H7/r6) are used to further secure the punch and prevent rotation or displacement during stamping. The retaining plate is connected to the die base with hexagon socket head cap screws. The screw diameter should be selected based on the thickness of the retaining plate (M6-M16), and the spacing should be 80-150mm to ensure a secure connection.
The core design of the backing plate is to withstand and distribute the punching force, preventing deformation or damage to the die base due to excessive localized stress. The backing plate material must possess high strength and hardness. 45 hardened steel (HRC40-45) or T8 hardened steel (HRC50-55) are commonly used. For large dies or thick material , Cr12MoV steel (HRC55-60) is used for improved wear and impact resistance. The thickness of the backing plate is determined by the punching force, generally ranging from 10-30mm. The greater the punching force (over 500kN), the thicker the backing plate. The backing plate’s surface area must cover the projected area of the entire working part to ensure even transfer of punching force to the die base. The contact surfaces of the backing plate with the fixed plate and die base must be ground to a flatness tolerance of no more than 0.02mm/m and a surface roughness of Ra 1.6μm or less to reduce contact stress. For multi-station progressive dies, avoidance holes need to be set on the pad to avoid the installation positions of parts such as punches and guide pillars. The gap between the avoidance holes and the parts is 2-5mm to prevent mutual interference.
The design of the fixing plate and the pad must ensure positioning accuracy and force balance between the two. Pins are required to locate the fixing plate and the pad. The diameter of the positioning pins should be 8-16mm, and there should be 4-6 of them, distributed symmetrically to ensure that the position deviation between the two does not exceed 0.01mm. For large molds, the connecting screws of the fixing plate and the pad need to be thickened (M12-M20) and arranged more densely around the working part, with a spacing of 50-80mm, to prevent relative displacement during stamping. When the stamping force is large (over 1000kN), a key connection should be installed between the fixing plate and the pad. The cross-sectional dimensions of the key are 10mm×8mm-20mm×12mm and the length is 50-100mm to enhance shear resistance. In addition, the edges of the fixing plate and the pad need to be rounded (R2-R5mm) to avoid sharp angles and reduce stress concentration.
The design of the retaining plate and backing plate for special operating conditions requires targeted optimization. For high-speed stamping dies (>500 strokes/min), the retaining plate and backing plate should be made of lightweight, high-strength materials (such as cast steel ZG310-570) to reduce inertial forces. Furthermore, the connection between the two should use anti-loosening structures (such as anti-loosening nuts and spot welding) to prevent the screws from loosening due to long-term vibration. For precision progressive dies, the material of the retaining plate and backing plate requires aging treatment to eliminate internal stress and ensure dimensional stability. The aging treatment temperature is 180-220°C for 4-6 hours. For dies used in corrosive environments (such as stainless steel stamping), the retaining plate and backing plate should be made of stainless steel (such as 304, 316) or chrome-plated (coating thickness 0.01-0.03mm) to improve corrosion resistance.
The machining process for the retaining plate and backing plate requires strict control to ensure design accuracy. The mounting holes for the retaining plate are machined using a jig boring machine or machining center, achieving a dimensional accuracy of IT7 and a surface roughness of Ra0.8μm. The avoidance holes for the backing plate are machined using wire cutting or milling machines, achieving a contour accuracy of ±0.02mm. After heat treatment, the parts are ground to eliminate deformation and ensure that flatness and perpendicularity meet requirements. Before assembly, the retaining plate and backing plate must be cleaned and inspected to remove burrs and oil stains, and the hole positions and dimensions must be checked for compliance with the design drawings. Deformation of the retaining plate and backing plate must be monitored during mold trials. This can be measured using strain gauges or a dial indicator. If deformation exceeds 0.03mm, the thickness must be increased or the material replaced. Through scientific design and precision machining, the retaining plate and backing plate provide reliable support for the stable operation of the mold.
