Design of the progressive die for punching and bending of upper and lower covers
The progressive die design for punching and bending upper and lower covers utilizes a multi-station continuous forming technology developed for cover parts of household appliances (such as air conditioners and washing machines). This technology enables punching (mounting holes, buckle holes), multi-directional bending ( 90° hemming, arc bending), and shaping of both covers within a single die. Production efficiency reaches 80-120 pieces per minute, with a dimensional accuracy of ±0.05mm. It is suitable for cold-rolled steel sheets (SPCC) with thicknesses of 0.8-2mm. The core challenge of the progressive die lies in the structural differences between the upper and lower covers. Flexible production is achieved by sharing a common base mold and replacing some station components. DFMEA analysis is required during the initial design phase to identify potential failure modes in key processes (such as buckle hole punching and arc bending) to ensure forming stability.
The mold structure adopts a replaceable modular progressive layout, with 12-15 stations: stations 1-3 are common processes (punching guide holes, pre-punching of the shape), stations 4-8 are special processes for the upper cover (punching φ5mm mounting holes, 90° folding), stations 9-13 are special processes for the lower cover (punching long round holes, R5mm arc bending), station 14 is for shaping, and station 15 is for separation and blanking. The key modules are made of Cr12MoV steel with a quenching hardness of HRC58-62. The wearing parts (such as punching punches) adopt a quick-change structure, and the replacement time is less than 5 minutes. The bending die adopts an insert design, and arc bends of different radii can be achieved by replacing inserts (R3-R10mm), and the insert positioning accuracy is ±0.005mm.
Coordinated control of multi-directional bending is a technical difficulty. For the 90° folding of the upper cover, a wedge-driven swing block mechanism is used, and the bending angle error is less than 0.5°; the R5mm arc bend of the lower cover adopts roller bending, which is gradually formed by 3 sets of rollers (the curvature radius is R10mm→R7mm→R5mm in turn) to avoid excessive stretching of the material (thinning rate <8%). Each bending station is set with springback compensation, the punch angle of the 90° folding is set to 92°, and the mold radius of the arc bend is 0.1mm smaller than the part requirement to ensure that it meets the size requirements after springback. The order of punching and bending is based on the principle of "punching first and then bending", and the distance between the hole edge and the bending line is ≥3mm (2mm thick material) to prevent cracking of the hole edge.
The feeding and positioning system adopts the combination of “servo feeding + multiple sets of guiding”. After the coil is leveled by the leveling machine (flatness <0.1mm/m), it is pushed by the high-precision feeder (positioning accuracy ±0.01mm). A set of precision guiding pins (φ4mm, H7/g6 matching) is set for every 3 stations, and side guides are added to the 8th and 13th stations to control the cumulative positioning error within 0.03mm. For the buckle holes (size φ2mm×5mm) of the upper and lower covers, special-shaped punches are used for punching, and the matching clearance between the punch and the die is 0.01-0.015mm to ensure that the hole shape is complete and burr-free.
Commissioning and changeover management must be standardized. During mold trials, a three-dimensional coordinate measuring machine (CMM) is used to check key dimensions (e.g., the folding edge height of the upper cover is 10±0.1mm). Five pieces are sampled per batch, and the CPK value must be ≥1.33. During changeovers, the mold management system calls up process parameters for the upper and lower covers (e.g., feed pitch and pressing force). After replacing dedicated workstation parts, three first-piece inspections are conducted. Only qualified pieces can be used for mass production. During routine maintenance, iron filings are cleaned from the surface of the bending insert every 2,000 cycles, and the lubrication of the cam mechanism is checked weekly (with special grease) to ensure smooth movement. The mold is equipped with a fault diagnosis system that uses sensors to monitor the blanking force of each workstation. In the event of an abnormality, the machine automatically shuts down and displays the fault location.
