The modern automotive industry is characterized by large-scale production, small batch sizes, rapid product variety changes, and the production of multiple models on the same line. Consequently, body panels are becoming larger and more integrated. These trends necessitate that stamping technology and equipment continuously evolve towards greater flexibility and automation. Highly flexible and efficient automated stamping equipment is gradually replacing traditional rigid production lines, becoming the mainstream of stamping production worldwide. See the main text for details.
In modern automotive stamping production, various new stamping equipment, advanced mechanized systems, new processes and dies, and intelligent control technologies are being gradually adopted, which greatly improves stamping production efficiency and finished product quality, and reduces the cost of mass production.
The modern automotive industry is characterized by large-scale production, small batch sizes of different models, rapid product variety changes, and the production of multiple models on the same production line. As a result, body panels are becoming larger and more integrated. These trends require stamping technology and equipment to continuously develop towards flexibility and automation. Highly flexible and efficient automated stamping equipment is gradually replacing traditional rigid production lines and becoming the mainstream of stamping production worldwide.
The following section will introduce the technological advancements and applications in stamping equipment, mechanized systems, stamping processes and dies, and intelligent control.
stamping equipment
For a long time, traditional stamping equipment has ensured the quality of stamped parts due to its stable production conditions, and the molds have a relatively long service life. However, the lack of flexibility is a significant drawback of this production method. The application of integrated control technology and servo drive technology in presses has brought more flexibility to stamping production. Large multi-station presses, flexible automatic stamping lines, and other equipment are rewriting the history of stamping production.
1. Integrated control
To adapt to the requirements of flexible production, all control functions of the press should be integrated to achieve menu-based management of the entire mold system. This mainly includes parameter settings for various aspects such as slide stroke adjustment, balancer air pressure adjustment, air cushion stroke adjustment, and automated control system settings. Fieldbus technology, featuring field communication networks, interconnection of field devices, interactive operability, distributed functional modules, and open functionality, represents the development direction of press control technology and has a significant driving effect on achieving automation.
The integration of the press control system enables all control functions of the press and mold to be realized through a single operating interface, including fault diagnosis, mold menu configuration, programmable limit switches, and mold monitoring and adjustment, making equipment maintenance more convenient and significantly increasing the effective working time of the press.
Currently, the large stamping lines produced domestically for manufacturing large automotive body panels have reached the advanced international level (see Figure 1). They adopt multi-link precision optimization compensation technology, networked fully automatic mold quick change technology, vector AC frequency speed regulation main drive technology, and multi-point hydraulic overload protection unloading control technology. Using Ethernet network technology, they can be connected to the production line to monitor the equipment status in real time. They have remote communication and remote fault diagnosis functions. The entire line supports a rapid feeding system and can produce 10 to 14 large automotive body panels per minute.
Figure 1. 3200t large stamping line produced by Jinan No.2 Machine Tool Group
2. Servo driver
The new servo press adopts highly adaptable servo drive technology, organically combining the flexibility of hydraulic presses with the high efficiency of mechanical presses. Enterprises can provide their users with various stamped parts in both large and small batches, significantly increasing production capacity. Komatsu of Japan and Schuler of Germany have successively developed and produced this type of servo press (see Figure 2). Currently, in my country, FAW Toyota Motor Corporation and Guangzhou Toyota Motor Corporation have adopted this equipment.
Figure 2 shows a new servo press developed by Schuler AG in Germany in 2007.
The most prominent feature of a servo drive system is that the motor is directly connected to the eccentric crank gear. Since the speed of the motor can be adjusted arbitrarily, the movement speed of the crank mechanism can be adjusted arbitrarily, and the stamping speed can be accelerated or decelerated at will.
Another major feature of servo presses is their high adaptability. Because the motor speed of a servo press is infinitely adjustable, it's possible to achieve stepless adjustment of the stamping motion and blanking force during the stamping process, allowing the press's operating curve to match various application requirements. To ensure precise crank movement, users can achieve or maintain a constant stamping speed through deceleration drawing processes. This not only improves the quality of sheet metal processing but also extends the lifespan of the dies.
The speed variation and control of the servo press have been rationally optimized and designed, and it adopts an integrated module and is manufactured according to a modular structure. When its servo function is not needed, the machine can work like a traditional press, maintaining constant power and stroke.
3. Large multi-station press
Large multi-station presses capable of flexible stamping production represent the highest level of international stamping technology today and are the development direction for stamping forming equipment for car body panels. These presses integrate mechanical, electronic, control, and testing technologies, achieving full automation and intelligence, safe operation, high productivity, high part quality, and low overall cost, meeting the needs of mass production in the automotive industry. Of course, the press's technology and performance have been continuously improved and developed over the past decade, with advancements such as the use of variable-speed multi-link mechanisms in the stretching station, the replacement of double-action presses with CNC hydraulic stretching pads, and the application of fieldbus control technology.
Currently, renowned European and American automakers use large multi-station presses and crossbar automatic presses to mass-produce body panels (see Figure 3), meeting the highest precision and surface quality standards for contemporary automotive stamping parts. No Chinese automakers currently use large multi-station presses, primarily due to two reasons: firstly, the initial investment in such equipment is substantial; secondly, domestic automakers started later, and their existing stamping equipment is still in service, which to some extent restricts the improvement of my country's automotive stamping technology.
Figure 3. A multi-station press used by a foreign automobile manufacturer.
4. Flexible automatic stamping line
Flexible stamping automated lines typically consist of a single double-action stretching press or a multi-link single-action stretching press and 4-5 single-action presses, forming a production line for large body panels. This line offers high safety and excellent stamping quality. The production line is equipped with an automated loading and unloading system, comprising a destacking machine, a cleaning and oiling machine, a centering and loading machine, loading and unloading robots, and a shuttle (flipping) automatic conveyor. The entire process, from loading the blank to the stamped part leaving the line, is automated. The maximum production cycle is 6-9 times/min. The entire stamping line is approximately 60-80m long and requires about 6-7 operators.
Some companies use internationally advanced high-speed compact stamping lines, which can rapidly improve the production efficiency of stamping machines. Sheet metal alignment is achieved using state-of-the-art optical camera systems. The stretching press uses CNC hydraulic stretching pads, and each advanced automated unit is equipped with a robotic arm, eliminating the need for shuttle conveyors, shortening conveying distances, reducing press spacing, and achieving a production cycle of up to 10 pieces/min.
With the increasing competition in my country's automobile market, it is an indisputable fact that the appearance quality and internal quality of automobiles have become prerequisites for automobile manufacturers to compete in the market. Therefore, adopting mechanized and automated production methods for stamping production is the most effective way to improve the appearance quality of automobiles.
5. Adjusting the press
Large-scale automotive stamping companies should equip each large-scale automated stamping line with a commissioning press to shorten the trial molding time of metal forming dies and increase the uptime of large-scale stamping lines.
For commissioning testing, hydraulic high-speed testing presses and tensile mechanical presses are the primary choices. Especially on production-type mechanical presses, mold testing time can be reduced by 80%, demonstrating significant application value. The development trend of mold testing mechanical presses is towards multi-link tensile presses equipped with CNC hydraulic tensile pads and featuring parameter setting and status memory functions.
Mechanized systems
The development of automated stamping production lines, the mechanization of stamping production lines, and the manufacturing of large vacuum feeding stamping presses for automobile side panel production have promoted the rapid development of the automobile industry.
For domestic automotive stamping parts manufacturers, the most valuable technological improvements are concentrated in the automated transportation and transfer devices for stamping parts, as well as the automation of material loading on stamping equipment and the product-oriented stamping production process.
Since the limits of hydraulic technology are difficult to overcome in deformation processing, we can only seek benefits from mechanical automation. Moreover, we cannot only seek benefits from mechanical automation in the stamping process, but also from areas outside of stamping technology, from new stamping part delivery methods, from stamping part off-line methods, and from all areas where economic benefits can be obtained.
A new integrated high-speed transfer system (see Figure 4) enables direct transfer between presses, achieving a production cycle of 12-15 times/min. The robotic arm's end effector can be automatically programmed for angle adjustment to adapt to the die slot position. In the production of large and extra-large stamped parts, this robust and powerful stamping part transfer mechanism is widely used not only in the stamping of large workpieces and double-piece stamping (stamping two finished products at once), but also in the production of automotive parts. The swing-arm stamping part transfer system will greatly support and assist stamping equipment in rapid die changes, enabling simultaneous output of stamping dies and workpieces, thus laying the foundation for a multi-functional stamping production line.
Figure 4 Conveying System
Stamping process and molds
In an era of diverse automotive products worldwide, the diversification of molds and the short mold replacement cycle have become particularly prominent, while also placing new demands on stamping technology and bringing about a transformation in the production and processing technology of automotive stamping parts.
1. Number of stamping processes
Automotive stamping parts possess excellent manufacturability and economic efficiency. Key indicators of their quality include the number of stamping processes, the number and size of body assembly segments, and the structure of the stamped parts. Reducing the number of stamping processes means reducing the number of stamped parts, saving on tooling, simplifying the stamping process's conveyor system, and reducing the number of operators and equipment footprint. This is an excellent measure to save investment and energy consumption. Therefore, stamping manufacturers consider designing the number of stamping processes as an important way to reduce automobile manufacturing costs, even at the cost of improving product design to meet manufacturing process requirements. At the same time, they also use reasonably sized body assembly segments, such as one-piece left and right side panels and roof panels, which not only make the car's appearance more aesthetically pleasing and practical, reduce air resistance, but also reduce the number of stamped parts and welding points, effectively reducing costs.
When designing a stamping process for a workpiece, handling the distribution and concentration of processes is a complex issue. It depends on the workpiece's batch size, structural shape, quality requirements, and process characteristics. For automotive stamping parts, which are typically produced in large batches, a process concentration approach should be adopted as much as possible, using compound or progressive dies for stamping. This improves productivity while ensuring safe production. Practical experience shows that for compound dies, the number of processes concentrated on a single die should not be too high, generally 2-3 processes, with a maximum of 4. For progressive dies, the number of concentrated processes can be higher because their die structure layout offers greater flexibility.
2. Multi-piece stamping die process
With the promotion of the overall side panel stamping technology for mid-to-high-end sedans and the increase in the size of the press table, the stamping process has also developed rapidly. Stamped parts such as doors and fenders have evolved from single-piece production using a single die to double-groove die production, which has increased production efficiency many times over. The latest development has been to use four-groove dies to produce the inner and outer panels of doors, which has doubled production efficiency again.
When production volume is large, using multiple parts to stamp simultaneously can reduce mold costs, material costs, and processing costs. For the forming process, it is also beneficial for the material stress and strain to be symmetrical and uniform, and it also improves production efficiency and reduces energy consumption.
The latest trend in dual-piece production is the simultaneous production of inner and outer panels. Its advantages are: matching production quantities, simultaneous delivery to welding lines for timely pressing, and smooth and convenient logistics.
3. Progressive Module
Progressive combination dies are widely used in the automotive industry of developed countries. Their advantages include high productivity, low die cost, and no need for sheet metal shearing. Compared with stepped dies used on multi-station presses, progressive dies can save 30% of the cost. However, their application is limited by the drawing depth, the surface hardening of the strip edge material for guiding and transmission, and they are mainly used for simple parts with relatively shallow drawing depths.
Intelligent control technology
In recent years, intelligent control technology for stamping production has developed rapidly. Based on the integration of materials and processes, it achieves online or intelligent control (also known as "adaptive control") of the stamping process using existing material and process databases. First, an online detection system is established for material or process parameters. When material properties or process parameters change or fluctuate, the automatic detection system (sensors and signal conversion system) determines the instantaneous values of the relevant parameters online. Then, computer simulation analysis and optimization software (such as artificial neural network methods and expert systems) determine the optimal combination of process parameters after the parameter changes. After the automatic control system adjusts the process parameters, adaptive control of the stamping process can be achieved. The gradual accumulation of new production data can further serve as the basis for process optimization in subsequent processing steps.
Conclusion
Stamping technologies and equipment with greater flexibility will adapt to the development trend of small-batch, multi-variety, mixed-flow production modes and diversified and personalized market demands, and enhance enterprises' ability to respond quickly to market changes.
