Design of the Spring-Pressure Unloading Device for Multi-Position Progressive Dies
The spring-loaded discharger of a multi-station progressive die is a key component for ensuring the smooth continuous stamping of strip material. Its primary function is to reliably remove the strip material from the punch after blanking is completed at each station. It also compresses the strip material during the stamping process to prevent it from warping or shifting. Compared to the discharger of a single-stage die, the spring-loaded discharger of a progressive die must adapt to the characteristics of continuous multi-station operation and possess features such as uniform discharge force, synchronized movement, and non-interference with feeding. Its design requires comprehensive consideration of factors such as strip thickness, feeding speed, and stamping process type.
The spring-loaded unloading mechanism consists of a stripper plate, unloading screws, springs (or rubber), guide pins and bushings, and other components. The stripper plate is the component that comes into direct contact with the strip material. Its shape must cover the entire working area of the punch, and its contact surface with the strip material must be flat and smooth, with a surface roughness of Ra1.6μm or less to avoid scratching the strip material. The thickness of the stripper plate is determined by the mold size and the unloading force, generally ranging from 15-30mm. The material used is 45 steel or Cr12, which reaches a hardness of HRC45-50 after quenching to ensure sufficient rigidity and wear resistance. For multi-station progressive dies, the stripper plate is typically designed as a single piece. However, for dies with multiple stations or complex shapes, a split structure can be used, connected together with screws for ease of processing and maintenance.
The selection and placement of springs are central to the design of a spring-loaded unloading device, directly impacting the magnitude and uniformity of the unloading force. The total unloading force of the springs must be calculated based on the blanking force and is generally 10%-20% of the blanking force. For thick sheet metal or high-strength materials, this can be increased to 25%. The spring force of a single spring, F = K × x (K is the spring stiffness, x is the compression), is determined based on the total unloading force and the number of springs. The springs should be evenly distributed across the width of the strip and placed as close to the punch as possible. For large or shaped punches, springs should be placed around them to ensure that the unloading force is concentrated in the blanking area. The spring pre-compression is generally 15%-20% of the free length to ensure sufficient spring reserve during the stamping process while avoiding fatigue failure caused by excessive compression. For example, a 16mm diameter cylindrical coil spring (free length 50mm, stiffness 5N/mm) with a pre-compression of 10mm can provide an initial spring force of 50N per spring, meeting the unloading requirements for thin sheet metal blanking.
The clearance between the stripper plate and the punch must be strictly controlled. Too large a clearance prevents reliable material removal, while too small a clearance increases friction and wear. The typical clearance is 0.05-0.1mm. For precision progressive dies (punching clearance ≤ 0.03mm), this clearance can be reduced to 0.01-0.03mm. Precision is ensured by grinding the stripper plate’s die hole. The stripper plate’s guiding accuracy is equally important. It is typically connected to the upper or lower die base via guide pins and bushings. The guide pin diameter is selected based on the stripper plate size (12-20mm), and the clearance is H7/H6, ensuring smooth, non-binding movement of the stripper plate. For high-speed progressive dies (punching speeds > 300 strokes/min), the stripper plate should be equipped with a cushioning device (such as a polyurethane pad) to reduce impact noise and vibration when in contact with the strip material.
Spring-loaded unloading devices for specialized workstations require targeted designs. In forming stations like bending and drawing, the unloading plate must also function as a material pressure. In these situations, the unloading force in this area needs to be increased. This can be achieved by using stiffer springs or increasing the number of springs to prevent wrinkling during the forming process. In flanging stations, the stripper plate’s die hole must provide clearance with the flanging punch, and clearance grooves should be provided in the flanging area to prevent interference. For workstations with small punches (diameter < 5mm), the unloading plate should be designed as a sheath-like structure, enveloping the punch. This not only serves as a material unloading function but also protects it from bending and breaking. Furthermore, the spring-loaded unloading device should be equipped with a limiter (such as a stopper post) to control the lowest position of the unloading plate. This ensures that the gap between the unloading plate and the die is equal to the strip thickness when the die is closed, preventing excessive pressure and deformation of the strip. Through refined design, the spring-loaded unloading device can achieve stable and efficient unloading and pressure functions in multi-station progressive dies, ensuring the quality of stamped parts.
