Application and Control of Sheet Metal Bending Process in the Processing of New Energy Vehicle Power Battery Boxes

Introduction

Amid the rapid development of the new energy vehicle industry, the power battery is regarded as the "heart" of new energy vehicles, and the processing quality of its supporting structural components directly determines the safety, sealing performance and service life of the battery pack. The power battery box is a core sheet metal component that carries the power battery pack, protects the internal cells, and adapts to the entire vehicle assembly. Mostly made of cold-rolled steel sheets, galvanized sheets or aluminum alloy sheets with a thickness of 1.5-6mm, this type of box is a typical complex sheet metal bending part, which not only requires extremely high dimensional accuracy and precise bending angles, but also needs sufficient structural rigidity, impact resistance and sealing adaptability, imposing stringent standards on the sheet metal bending process.

Sheet metal bending is a cold working process that applies external force to metal sheets through bending equipment to produce plastic deformation and form a predetermined angle and radian. As the core process of forming a power battery box, it directly determines the forming quality, assembly accuracy and structural performance of the box. Compared with ordinary sheet metal bending parts, the bending processing of new energy vehicle power battery boxes involves multiple technical difficulties such as multi-station bending, large-size bending, high-precision angle control, springback compensation, and crack and deformation prevention, and needs to meet the industry requirements of mass production and standardized control. This paper takes the new energy vehicle power battery box as the core processing product, deeply analyzes the application points, process control, difficulty overcoming and quality optimization schemes of the sheet metal bending process, providing practical references and technical support for the processing and production of high-precision complex sheet metal bending parts in the industry. The full text focuses on a single product and all-dimensional details of the bending process to ensure professionalism and pertinence.

I. Product Characteristics and Bending Processing Requirements of New Energy Vehicle Power Battery Boxes

1.1 Basic Product Structure and Material Selection

A new energy vehicle power battery box is not a single component, but mainly composed of a box bottom plate, side coamings, reinforcement rib plates, flanges, mounting supports and other parts, presenting a closed box-type structure as a whole. Some models are equipped with special-shaped curved boxes, which need to take into account space utilization and adaptability to the vehicle chassis. In terms of material selection, mainstream power battery boxes are divided into two categories: one is cold-rolled galvanized steel sheet, with a thickness of 2.0-4.0mm, featuring moderate cost, excellent weldability and bending formability, and good anti-corrosion performance of the galvanized layer, suitable for mid-to-low-end new energy passenger vehicles; the other is 5052/6061 aluminum alloy sheet, with a thickness of 1.5-3.0mm, remarkable lightweight advantages, high specific strength and strong corrosion resistance, which can effectively reduce the vehicle's dead weight and increase the cruising range, mostly used in high-end new energy passenger vehicles and commercial vehicles.

The structural characteristics of this type of box determine the complexity of bending processing: the side coamings of the box require 90° right-angle bending and arc transition bending, some reinforced parts need multi-angle composite bending, the flange bending must ensure flatness and parallelism, the bending around mounting holes should avoid deformation and offset, the overall structure has no sharp edges to prevent bumping and damaging internal cells, and the bending joints must meet the requirements of welding and sealing assembly without excessive gaps and deformation defects.

1.2 Core Bending Processing Technical Requirements

Combined with the industry standards of new energy vehicles and the application scenarios of power battery boxes, the sheet metal bending processing must meet the following core requirements, which are also the core indicators of process control:

First, dimensional accuracy requirements: the overall overall dimensional tolerance of the box must be controlled within ±0.5mm, the key assembly dimensional tolerance ≤±0.3mm, and the bending side length error ≤0.2mm, to avoid problems such as failure to assemble the box with the vehicle chassis and misalignment of battery cell installation due to dimensional deviation; second, angle accuracy requirements: bending angle error ≤±0.3°, 90° right-angle bending without collapse and obtuse angles, and uniform radian of arc bending to ensure the closed sealing performance and structural regularity of the box; third, surface quality requirements: no cracks, scratches, wrinkles, strains, zinc layer shedding (galvanized sheets) and other defects at the bending parts, and no surface cracking or coarse grain problems in aluminum alloy sheets after bending to ensure the protection performance and appearance quality of the box; fourth, structural performance requirements: no residual stress concentration on the sheet after bending, qualified overall rigidity of the box, impact and vibration resistance meeting the vehicle road test standards, and no problems of excessive bending springback and structural deformation; fifth, batch consistency requirements: in mass production, the bending size, angle and forming state of boxes in each batch remain consistent, adapting to automatic assembly line assembly and standardized quality inspection.

II. Analysis of the Whole Process of Sheet Metal Bending for Power Battery Boxes

The sheet metal bending processing of new energy vehicle power battery boxes is not a single process, but a complete process integrating early cutting, mold preparation, bending programming, practical processing, post-process inspection and other links. Each step must be precisely controlled to ensure the final bending quality. For the bending processing of this product, the whole process can be divided into five core links, which are closely connected and interlocked.

2.1 Pre-Bending Pretreatment: Blank Preparation and Quality Inspection

The premise of bending processing is to obtain qualified sheet metal blanks. Most power battery box blanks are prepared by laser cutting process. Compared with traditional shearing and punching blanking, laser cutting features higher precision and smooth, burr-free cuts, adapting to the blanking of complex contours and special-shaped structures of the box, and avoiding uneven bending stress and angle deviation caused by burrs and dimensional deviations on the edge of the blank. After blanking, a full-dimensional quality inspection of the blank is required: first, check whether the sheet thickness and material meet the design requirements, and prevent unqualified sheets from flowing into the bending process; second, inspect the overall dimension and cut flatness of the blank, remove edge burrs and flashes to avoid scratching the mold and straining the sheet surface during bending; third, for special materials such as aluminum alloy and galvanized sheets, check the integrity of the surface coating and oxide layer to avoid bending cracks at the damaged part of the coating; fourth, level the blank to eliminate slight deformation generated during blanking, ensuring the blank is flat and warpage-free before bending.

2.2 Bending Process Planning: Parameter Setting and Mold Selection

Process planning is the core link of power battery box bending processing. A dedicated bending plan needs to be formulated based on the product drawings, sheet material, thickness and bending accuracy requirements, mainly including two contents: bending parameter setting and mold selection.

In terms of bending parameters, the focus is on determining the bending radius, bending force, bending sequence and springback compensation value. The bending radius must follow the principle of "material adaptation, crack prevention first": the inner bending radius of cold-rolled steel sheets R≥1.2t (t is the sheet thickness), and that of aluminum alloy sheets R≥1.5t, to avoid stress concentration and cracking caused by an excessively small radius; the bending force is accurately calculated through formulas to ensure sufficient and non-overloaded pressure of the bending machine, preventing incomplete bending due to insufficient pressure; the bending sequence follows the logic of "inner first, outer later; small first, large later; complex first, simple later": first process the bending of internal reinforcement ribs and small-angle bending of the box, then conduct large-size 90° bending of the side coamings, and finally process the flange bending to prevent the formed parts from interfering with subsequent bending operations; springback compensation is the key to high-precision bending. Metal sheets produce elastic springback after bending, with a springback angle of about 1-3° for steel and 2-5° for aluminum alloy. The springback value needs to be calculated through early test bending, and the bending angle is adjusted during programming to offset the springback error.

In terms of mold selection, power battery box bending needs to adapt to special precision bending molds: standard right-angle upper and lower molds for conventional 90° right-angle bending, arc forming molds for arc transition bending, forming pressing molds for reinforcement rib bending, and narrow groove molds for flange bending. The mold material is high-hardness chromium vanadium alloy steel with surface quenching treatment, meeting the hardness standard and featuring strong wear resistance, avoiding mold wear and deformation in long-term mass production that affects bending accuracy; meanwhile, the mold gap is adjusted according to the sheet thickness, generally 1.05-1.1 times the sheet thickness. An excessively small gap is likely to strain the sheet and increase the bending force, while an excessively large gap will lead to a larger bending angle and dimensional deviation.

2.3 CNC Programming and Equipment Debugging

Modern power battery box bending processing adopts CNC bending machines, requiring professional programming software to input product bending parameters, dimensions, angles, sequences and other information to generate CNC machining programs and realize automatic bending. During programming, the bending line and back gauge position need to be accurately positioned, the whole bending process simulated, and problems such as bending interference and deformation predicted in advance to optimize the program. After program input, equipment debugging is carried out: first, calibrate the levelness of the bending machine worktable and the perpendicularity of the slider to ensure the equipment operation accuracy; second, debug the back gauge positioning device to ensure accurate and non-offset positioning; finally, run the machine empty to check whether the program operation, mold opening and closing, and equipment action are normal, and eliminate potential equipment failures.

2.4 Practical Bending Processing: Refined Operation and Process Control

Practical bending is the core link of forming, requiring strict compliance with CNC programs and process specifications, and refined operation to control quality. First, place the pre-treated qualified blank on the bending machine worktable, fit the back gauge for positioning, and ensure the blank is placed flat and non-offset; second, start the equipment to conduct first-piece test bending. After the test bending is completed, use equipment such as angle rulers, calipers, and three-coordinate measuring instruments to detect the bending angle, side length dimension and surface quality, and check whether they meet the design requirements. If there is a deviation, timely adjust the springback compensation value, back gauge position or bending force; mass bending processing can only be carried out after the first-piece inspection is qualified.

In the batch processing process, good process control is required: first, conduct regular sampling inspection of bending part accuracy, once every 20-30 pieces, to prevent batch defective products caused by equipment wear and parameter drift; second, pay attention to the sheet surface state, and timely clean up debris and oil stains on the mold surface to avoid scratching the box surface; third, for large-size box bending, use auxiliary support devices to prevent bending deviation caused by the self-weight sag of the blank; fourth, standardize the operation process, and prohibit forced bending and illegal parameter adjustment to ensure processing safety and product quality.

2.5 Post-Bending Inspection and Shaping

After bending processing, a full-item inspection is carried out on the semi-finished power battery box, including overall dimension, bending angle, surface quality, structural flatness, crack-free deformation and other items, in strict accordance with industry standards and product drawings to eliminate unqualified products. For semi-finished products with slight excessive springback and angle deviation, secondary fine-tuning shaping can be carried out through special shaping molds, and violent knocking shaping is strictly prohibited to avoid sheet cracking and surface damage; defective products with cracks, severe deformation and oversize dimensions are uniformly recycled and disposed of, and are prohibited from flowing into the subsequent welding and assembly processes. Qualified bent semi-finished products after inspection are stored with proper protection to avoid deformation caused by stacking and extrusion, waiting for subsequent welding and surface treatment processes.

III. Common Difficulties and Solutions in Sheet Metal Bending of Power Battery Boxes

3.1 Excessive Bending Springback: Core Pain Point of Out-of-Control Accuracy

Bending springback is the most common technical difficulty in power battery box bending processing, especially for aluminum alloy sheets and thick steel plates, where the springback phenomenon is more significant, directly leading to unqualified bending angles and poor box sealing. The reason is that when the metal sheet is bent, the outer layer is stretched and the inner layer is compressed, resulting in elastic recovery after the external force is removed. In addition, factors such as sheet material, thickness, bending radius and bending force make it difficult to control the springback amount.

Core solutions to this problem: first, accurate springback compensation, establish a springback database for sheets of different materials and thicknesses through a large number of early test bends, directly call the data during programming, and preset the over-bending angle to offset the springback error; second, optimize the bending process, reduce the bending radius and increase the bending force to make the sheet plastic deformation more sufficient and reduce elastic springback; third, improve the mold structure, adopt a special bending mold with springback compensation function, and accurately control the bending forming angle through fine-tuning the mold angle; fourth, secondary shaping, conduct fine-tuning on workpieces with slight excessive springback by cold shaping process to ensure angle accuracy.

3.2 Cracking at Bending Parts: Dual Problems of Material and Process

Cracking of power battery boxes during bending mostly occurs at small-radius bending parts of aluminum alloy sheets and thick galvanized sheets. Cracks not only affect the structural strength of the box, but also damage the sealing performance, leading to moisture and corrosion of the battery cells and causing potential safety hazards. The main causes of cracking include excessively small bending radius, poor sheet plasticity, burrs on the blank edge, excessive bending force, coarse sheet grains, etc.

Solutions: first, optimize the bending radius, strictly set the minimum bending radius in accordance with material characteristics, and prohibit forced bending with small radius; second, improve blank quality, thoroughly remove edge burrs after blanking, and anneal aluminum alloy sheets to enhance sheet plasticity; third, adjust the bending process, reduce the bending speed and adopt progressive bending to avoid cracking due to excessive instantaneous stress; fourth, strictly control sheet quality, select high-quality sheets with fine grains and qualified plasticity, and prohibit processing of unqualified sheets; fifth, lubrication treatment, apply special bending lubricant to the contact part between the mold and the sheet during bending to reduce friction resistance and lower the risk of cracking.

3.3 Bending Deformation of Large-Size Boxes: Out-of-Control Flatness

Most new energy vehicle power battery boxes are large-size components, with a length of up to 1.5-2.5m. During bending of large-size blanks, self-weight sag and uneven bending stress are prone to occur, resulting in warpage, distortion and uneven sides of the box, affecting subsequent welding and vehicle assembly. This problem is more prominent in thin sheet bending processing, and is a key control difficulty in mass production.

Solutions: first, install auxiliary support devices, install adjustable pneumatic support arms on the bending machine worktable to support the suspended parts of large-size blanks and prevent self-weight sag; second, optimize the bending sequence, adopt segmented bending and symmetrical bending processes to make the sheet stress uniform and reduce deformation; third, improve equipment rigidity, select high-rigidity, large-tonnage CNC bending machines to avoid uneven bending caused by deformation of the equipment slider; fourth, leveling treatment, level slightly deformed boxes through a leveling machine after bending to ensure overall flatness; fifth, standardize storage, store bent boxes vertically with special material racks to avoid secondary deformation caused by horizontal stacking and extrusion.

3.4 Surface Strain and Coating Shedding: Appearance and Protection Defects

Zinc layer shedding is prone to occur when galvanized sheet boxes are bent, and surface strains and scratches are easy to appear on aluminum alloy boxes, which not only affect the product appearance, but also reduce the anti-corrosion performance of the box and shorten the service life. The main reasons are rough mold surface with debris, unreasonable mold gap, and excessive friction during bending, resulting in damage to the surface coating and base material.

Solutions: first,refined mold maintenance, regularly polish and polish the mold surface to remove debris and scratches, and keep the mold smooth; second, reasonably adjust the mold gap, accurately set the gap according to the sheet thickness to avoid surface damage caused by excessive extrusion; third, use bending protective film, paste a special protective film on the contact part between the sheet and the mold to reduce direct friction; fourth, clean the equipment in a timely manner, regularly clean up metal debris and oil stains on the worktable and mold during processing to prevent scratching the sheet; fifth, optimize lubrication, select suitable bending lubricants to reduce the friction coefficient and surface damage.

IV. Quality Control System for Sheet Metal Bending of Power Battery Boxes

The power battery box of new energy vehicles is related to driving safety, so its bending processing quality must be subject to the whole-process and standardized control, and a complete quality control system must be established to control all dimensions from raw materials, equipment, process, operation to inspection, and eliminate potential quality hazards.

4.1 Raw Material Control: Guarantee Quality from the Source

Establish a strict raw material warehousing inspection system. For each batch of cold-rolled steel sheets, galvanized sheets and aluminum alloy sheets entering the factory, inspect the material certificate, thickness tolerance, surface quality and plastic performance. Raw materials that do not meet the design requirements will be rejected and strictly prohibited from flowing into the production link. Meanwhile, standardize raw material storage, provide moisture, rust and scratch protection, and avoid sheet deterioration and deformation affecting bending processing.

4.2 Equipment Control: Ensure Stable Processing Accuracy

CNC bending machines, laser cutting machines and testing equipment are the core equipment for bending processing, and a regular maintenance system needs to be established: check the equipment operation status and mold wear before starting up every day; conduct equipment lubrication and accuracy calibration every week; carry out comprehensive maintenance every month to troubleshoot equipment faults; conduct accuracy retest and overhaul every year to ensure that the equipment accuracy and operation stability meet the standards. Meanwhile, keep equipment operation records, monitor equipment parameters in real time, and handle abnormal conditions in a timely manner to avoid product quality fluctuations caused by equipment failures.

4.3 Process Control: Implementation of Standardized Operations

Formulate a dedicated Standard Operating Procedure (SOP) for power battery box bending processing, clarify bending parameters, mold selection, operation process and testing standards, and prohibit operators from arbitrarily changing process parameters. Implement a process disclosure system, conduct process training for operators before production to ensure that all personnel are familiar with process requirements; meanwhile, establish a process optimization mechanism, timely analyze the causes of quality problems occurring in production, optimize process schemes, and continuously improve bending processing quality.

4.4 Inspection Control: Whole-Process Closed-Loop Quality Inspection

Construct a four-level quality inspection system of "raw material inspection - first-piece inspection - in-process patrol inspection - finished product final inspection": raw material inspection controls the source quality, first-piece inspection locks process parameters, in-process patrol inspection prevents batch defective products, and finished product final inspection ensures factory eligibility. Select high-precision three-coordinate measuring instruments, digital display angle rulers, flatness detectors, hardness testers and other testing equipment to ensure accurate and reliable testing data; quality inspectors strictly implement inspections in accordance with standards, keep inspection records, and implement closed-loop management of identification, isolation, rework and scrapping for unqualified products to prevent the circulation of unqualified products.

4.5 Personnel Control: Improve Professionalism of Operation

Bending operators and quality inspectors must take professional training and pass the assessment before taking up their posts. Regular skill improvement training and safety training are carried out to strengthen quality awareness and operational standardization. Establish a personnel performance appraisal mechanism, linking product quality and pass rate with performance, to encourage operators to carry out refined operations and reduce quality problems caused by human errors.

V. Development Trend of Sheet Metal Bending Process for Power Battery Boxes

With the upgrading of the new energy vehicle industry towards high-end, intelligent and lightweight, the structural design of power battery boxes is becoming more complex, and the requirements for sheet metal bending process are continuously improving. The bending process will develop in three major directions in the future:

The first is intelligence and automation. Fully automatic CNC bending units and robot loading and unloading bending production lines are gradually popularized, realizing the whole-process automation of blank loading, bending, unloading and transfer, reducing manual intervention, and improving production efficiency and batch consistency. Meanwhile, bending equipment equipped with AI intelligent algorithms can automatically identify sheet material and thickness, accurately calculate springback compensation values, adaptively adjust bending parameters, and realize intelligent high-precision bending.

The second is high precision and micro-forming. In response to the processing needs of ultra-thin aluminum alloy boxes and special-shaped curved boxes, precision micro-bending processes are gradually applied, with bending accuracy improved to ±0.1mm and angle error controlled within ±0.1°, meeting the processing requirements of lightweight, high-precision structural components for high-end new energy vehicles.

The third is greening and high efficiency. The application of environment-friendly bending lubricants and scratch-free bending molds reduces environmental pollution and product surface damage during processing; meanwhile, the optimization of composite bending and one-step forming processes reduces bending processes, shortens processing cycles, reduces production costs, and adapts to the development demand of cost reduction and efficiency increase in the new energy vehicle industry.

Conclusion

The sheet metal bending process is the core lifeline of new energy vehicle power battery box processing, and its process level and control accuracy directly determine the quality performance of the box and the driving safety of new energy vehicles. For high-precision and complex sheet metal bending parts such as power battery boxes, it is necessary to base on product characteristics, optimize bending process parameters, standardize whole-process operations, overcome technical difficulties, and improve quality control to ensure the accuracy, quality and efficiency of bending processing.

Against the background of continuous iteration of the new energy vehicle industry, the sheet metal bending process also needs continuous innovation and upgrading, integrating intelligent, automated and precise technologies to break through the limitations of traditional processing and adapt to the high-end processing needs of power battery boxes. At the same time, industry practitioners need to deeply cultivate process details, strengthen quality control awareness, continuously optimize processing schemes, and improve the level of sheet metal bending processing, providing solid structural component processing support for the high-quality development of the new energy vehicle industry. In the future, with the continuous progress of process technology, sheet metal bending will play a core role in the processing of more high-end sheet metal components, promoting the steady progress of the manufacturing industry towards high precision and intelligence.