The plate roll bending machine belongs to the bending correction machine among the eight major products of forging machinery.
The roll bending machine is a metal forming equipment that makes the metal sheet according to the three-point circle principle, which uses the relative position change and rotation of the work roll to produce continuous elastoplastic bending of the sheet to obtain a predetermined shape and precision workpiece.
It takes the work roll whose axes are parallel to each other as the main working part, and converts the mechanical and hydraulic energy into kinetic energy to realize the position change and rotation movement of the work roll.
Therefore, it is very convenient to bend and roll various metal sheets into curved or cylindrical parts with single curvature or multi curvature with straight generatrix under cold, medium or hot state.
By changing the shape of the roller or adding a cone winding device, the material flow can be limited and the movement speed of different parts in the same workpiece can be changed.
The arc or cylinder parts with single curvature or multi curvature, such as ellipse, square and asymmetric shape, can be rolled by changing the shape of roller or adding cone winding device, etc.
In order to bend sheet metal into a simple or arc with single or multiple curvature, two methods of bending and bending can be used.
Press bending is carried out on hydraulic press or press brake with the help of die, mainly relying on transverse plastic bending. The bending process is discontinuous point by point or section bending.
Compared with press bending, roll bending has the following characteristics:
1) The bending process is a continuous elastic-plastic bending with a certain tensile force and small springback, so the forming is accurate, the bending quality is high and the work efficiency is high;
2) Without die forming, operation cost is low;
3) The bending force is usually smaller than that of press bending, and the cost of roll bending machine is lower than that of hydraulic press;
4) Equipped with auxiliary device, it can roll conical parts and realize the bending of pipe and profile.
Therefore, the plate roll bending machine is widely used in boiler, shipbuilding, petroleum, chemical, hydraulic, metal structure and other machinery manufacturing industries.
China’s roll bending machine has experienced the development process from mechanical to hydraulic, and then to CNC.
At present, its hydraulic and automatic control technology has made great progress.
In the 1990s, the numerical control plate bending machine developed in succession in China is becoming more and more mature after years of continuous improvement.
The process from single machine automation to flexible manufacturing cell has been completed.
Rapid, precise and mass production is the foundation of modern industrial civilization.
It is very important to develop high efficiency, high precision, safety, composite, environmental protection, automation, flexibility and intelligence bending correction equipment to improve the level of China’s equipment manufacturing industry, meet the needs of enterprise competition and improve economic benefits.
2. The basic situation of roll bending machines in China
2.1 Important role of roll bending machine in the equipment manufacturing industry
With the development of aerospace, military weapons, automobiles, locomotives and rolling stock, instruments and meters, shipbuilding, chemical industry, pressure vessel, metallurgy, engineering machinery, metal components and other industries, the demand for plate bending machine is increasing day by day, and the requirements of rolling plate precision are higher and higher.
With the rapid development of China’s manufacturing industry, the shipbuilding industry has put forward higher requirements for the functions, reliability, safety and automation of the ordinary bending machine and the marine bending machine with winding compound function;
With the rapid development of the electric power industry, coal power, hydropower, nuclear power and wind power encouraged by the national clean energy policy, large complete sets of bending machines are needed for pipelines and column towers etc;
In the offshore oil, natural gas, petrochemical, coal chemical and other industries, the trend of large-scale and high-parameter pressure vessel products are becoming increasingly obvious.
Thousand-ton hydrogenation reactors, two-thousand-ton coal liquefaction reactors, and 10,000 cubic meters of natural gas spherical tanks are widely used in China, making large-scale plate rolling machines for forming thick and high-strength plates become key equipment.
With the development of industry in the future, the requirements for energy saving, material saving and less cutting will be more and more intense,
It is estimated that the efficiency can be increased by 10 ~ 40 times and the material can be saved by more than 60% by using the plate bending machine as the process equipment.
Some data show that 70% of the steel output needs to be deformed, and 2/3 of the steel in the metal forming industry needs to be processed.
It is estimated that for every 10000 tons of plate, pipe and profile reprocessing, one pipe bender, one roll bending machine and one profile roll bending machine are needed.
2.2 Current situation of plate roll bending machine
In recent years, the development of the roll bending machine in China has formed a considerable scale, and its manufacturing level is also improving. It has a profound impact on improving the level of the equipment manufacturing industry and promoting the process of industrialization in China.
1) Main types and features of plate roll bending machine
According to the number of rolls, the machine can be divided into two rolls, three rolls, four rolls and multiple rolls;
According to the roller arrangement, it can be divided into symmetrical type and asymmetric type;
According to the position of roller axis, it can be divided into horizontal and vertical type;
According to the roller adjustment mode, it can be divided into up adjustment type and down adjustment type;
According to the main driving mode, it is divided into mechanical and hydraulic;
According to the transmission mode of the roller, it can be divided into upper drive (upper roll driving, other roller driven), lower driving (lower roll driving and other roller driven), and full drive (working roll is all main drive);
The bearing body can be divided into open type and closed type (marine type) according to whether the upper roller is tilted or not;
According to the function, it can be divided into common type and multi-purpose type;
According to the rolling method, it can be divided into cold rolling, hot rolling and warm rolling;
According to the control mode, it can be divided into strong current control, NC control and CNC control.
Plate bending machines are usually classified according to the number of work rolls, their arrangement, and position adjustment methods, and the main types are shown in Table 1.
In the table, the following types of three-roll bending machine are common:
- symmetrical type
- curve down type
- horizontal down type
- upper roller cross type (upper roller universal type) ;
Four-roll bending machine (common type);
Two-roller bending machine;
Vertical three-roll bending machine; marine bending machine.
2) Development history and current situation of plate bending machine
The specialized production of plate bending machine in China began in the early 1970s.
The first one is the symmetrical three-roll bending machine with mechanical transmission.
This kind of bending machine has a simple structure, but can not pre-bend the plate end.
When rolling the cylinder workpiece, it is necessary to pre-bend both ends of the sheet metal or cut the remaining straight edge before welding, which costs both labor and material, and the production efficiency is low.
In the late 1970s, Changzhi Iron and steel (Group) forging machinery successfully developed a four-roll plate bending machine with mechanical transmission,
This kind of bending machine can pre-bend both ends of the plate.
Compared with the three-roll bending machine, the performance is greatly improved.
However, the machine has large structure, high material consumption and long manufacturing cycle.
In the mid-1980s, the full hydraulic four-roll bending machine and the three-roll bending machine with hydraulic level adjustment were put into the market by the factory.
The two models can not only pre-bend the sheet metal, but also complete the cylinder forming after welding.
The working efficiency of the two models is 2~3 times higher than that of the original model.
At the end of 1980s, three or four roll bending machines controlled by PC and NC entered the Chinese market.
The machine has the functions of automatic leveling of work roll (upper roll or lower roll) and synchronous lifting of work roll in inclined state.
The synchronous precision between working rolls is controlled within±0.2 mm.
In the early 1990s, the arc down type three roll plate bending machine and the arc four roller plate bending machine was first developed in China by Changzhi Forging.
The end pre-bending and rolling can be completed by loading and unloading the plate at one time.
The arc swing of the rotary arm is less than the linear guide rail friction loss, the bending force is reasonable, the machine weight is light, and the overall structure is compact and reasonable.
The curved four roller plate bending machine is more suitable for rolling small-diameter cylinder than other plate bending machines, and the minimum cylinder diameter can reach 1.1~1.2 times of the upper roll diameter.
In the mid-1990s, four motor confluence main drive technology was adopted for Changzhi Forging, and the horizontal down type regulating structure was applied to large-scale three-roll bending machine for the first time.
The first large-scale CDW11XNC -140×4000 horizontal down type three-roll bending machine (Figure 1) was developed for the Three Gorges Project.
In 2002, the plant produced CDW11XNC -160/250×4000 horizontal down type three-roll bending machine for Nanjing Chemical Machinery Factory.
Fig. 1 CDW11XNC-140×4000 horizontal down type three-roll bending machine
At present, the largest horizontal down-regulation three roll plate bending machine in China can be produced.
The thickness of the cold rolling plate can reach 250 mm, the thickness of the hot coil plate can reach 400 mm, the maximum width of the plate is 3000 mm ~ 4500 mm, and the pressure of the upper roller and lower roller can reach 40000 kN ~ 60000 kN.
Domestic large-scale horizontal down type three-roll bending machine professional manufacturers are mainly Changzhi Forging, Hubei Ezhong, Tai’an Hualu, etc.
Figure 2 shows CDW11XNC -40×8000 horizontal down type three-roll bending machine.
Fig. 2 CDW11XCNC-40×8000 horizontal down type three-roll bending machine
The machine can roll a maximum plate thickness of 40 mm, a maximum plate width of 8000 mm, a yield limit of 240 MPa, a full-load minimum reel diameter of ø2500 mm, and a minimum reel diameter of ø900 mm.
The upper roll deflection of the machine is compensated by the wedge mechanism driven by motor, reducer and screw by adjusting the deflection of the lower roll.
According to the parameters of plate thickness, plate width, drum diameter, yield limit and so on, the NC system can automatically calculate and generate programs and realize automatic control.
In order to adapt to the rolling of extra-long thin plate (the ratio of plate thickness to plate width is very small), a new type of symmetrical or horizontal down type three-roll bending machine with the back-up roll and cross beam on the upper roll has been developed by Changzhi Forging (Fig. 3).
Fig. 3 CDW11XCNC-8×9500 horizontal down type three-roll bending machine
The machine is mainly driven by three rollers with a tilting bearing body, which can roll closed cylindrical parts.
At the same time, it adopts computer control, which is especially suitable for the rolling of thin-walled long cylindrical parts with multiple curvatures.
The upper roller cross-type (upper roller universal type) three-roll bending machine was first successfully manufactured by the First Heavy Machinery Factory in the early 1980s.
In the late 1980s, Nantong Hengli heavy industry and other enterprises began to produce professionally.
The machine only needs to adjust the upper roll to realize plate rolling and pre-bending.
The lower roll is relatively fixed, which is convenient for material and plate rolling.
However, it is not easy to deal with the structure of large machine, so it is more suitable for rolling plates with thickness less than 80 mm~100 mm.
The marine plate bending machine was first manufactured by the First Heavy Machinery Factory and Changzhi Forging.
In the early stage, it was of symmetrical structure, with specifications of W11TNC-20×8000 and W11TNC-25×9000 etc.
With the rapid increase of China’s shipbuilding output and the development requirements of the shipbuilding industry from the weak to the strong, in addition to the symmetrical structure, China’s marine bending machine also develops the horizontal down type.
This kind of machine can roll and pre bend curved and conical workpieces.
At the same time, with the help of bending die, the steel plate can be bent.
It can be used as both rolling machine and bending machine.
The deflection compensation technology of the upper and lower work rolls, the bi-directional independent driving technology of the work roll, and the numerical control technology of the bending and bending process parameters of the machine have approached the world’s advanced level.
At present, the maximum downward pressure of the upper roll of the domestic marine plate bending machine has reached 22000 kN.
The width of rolled or bent plates can reach more than 21000 mm, which can meet the development needs of the shipbuilding industry.
Figure 4 shows the Changzhi Forging CDW11TNC-32×13500 marine three-roll bending machine.
Figure 5 shows the CDW11TXNC-22000kN×16000 mm marine horizontal down type three-roll bending machine.
Domestic large-scale marine bending machine professional manufacturers are mainly Changzhi Forging, Tai’an Hualu, Nantong Hengli, etc.
Fig. 4 CDW11TNC-32×13500 marine three-roll bending machine
Fig. 5 CDW1ITXNC-2000kN×16000mm closed type (Marine) horizontal down type three-roll bending machine
- Main drive system 2. Frame 3. Main oil cylinder 4. Upper beam
- Upper work roll 6. Upper backup roll7. Lower work roll 8. Lower backup roll
- Lower roll horizontal moving device 10. Lower crossbeam 11. Connecting beam
The vertical roller is the symmetrical three-roll and the normal four-roll roll bending machine whose axis is perpendicular to the horizontal plane, among which the vertical symmetrical three-roll bending machine is the most commonly used.
Changzhi Iron and steel (Group) Forging has produced CDW11TNC-36×1000, 12×2000 and other kinds of equipment for the petroleum, chemical industry and other industries.
The first heavy machine factory introduced the UK Hugh Smith Company’s proprietary technology, designed and manufactured a series of vertical bending machines with only one main roll, and has produced 30000kN×3600mm vertical bending machine.
Compared with the horizontal bending machine, it has the advantages of light weight, less floor area, low energy consumption, and can reduce labor intensity, especially in hot rolling.
The upper roll of the two-roll bending machine is steel rigid roll, and the lower roll is an elastic roll with steel roll core wrapped with elastic coating.
It is an advanced sheet metal manufacturing process to roll bend sheet metal parts with elastic medium (RSEM, rotary shaping with the use of elastic mediums).
At present, Changzhi Forging and other companies have the design and manufacturing technology of such products and have produced several sets, and the school of mechanical and electrical engineering of Nanjing University of Aeronautics and Astronautics has also successfully developed one.
Figure 6 shows the two-roll bending machine produced by Changzhi Forging.
The upper roll of the machine is the elastic roll of the elastic roll core of the steel roll core, and the lower roll is the steel rigid roll.
The structure type of discharging from the bottom is adopted.
Fig. 6 Outline drawing of two-roll plate bending machine
The two roll bending machine was first developed by Kauffman of the United States. Britain, Japan, France, Italy, Russia and other countries have mastered the technology, and produce highly automatic CNC machine tools, which are used to manufacture various thin-wall parts in aircraft, chemical industry and civil products, such as cylindrical and conical shells, pipe parts, thin-walled special-shaped parts, thin-walled parts with different units, and stiffened or corrugated plate parts, etc. (Fig. 7)
Fig. 7 Thin wall parts manufactured by RESM technology
Compared with three roll and four roll bending technology, RESM has the following characteristics and advantages:
- The precision of forming parts is high.
After bending, more accurate curvature and higher edge parallelism can be obtained, and the remaining straight edge at the end can be less than one time of plate thickness, which is conducive to the accurate forming of porous sheet and ribbed plate coated with protective layer.
- Higher bending curvature and higher yield can be obtained.
The principle of roll bending is that under small pressure, the plate will gradually deform locally.
Therefore, it is not easy to wrinkle and crack during forming.
The required forming force is small, so the bending radius is small, which is suitable for forming difficult-to-deform materials, such as titanium alloys, stainless steel, composite materials, etc.
- High production efficiency
Because of the convenience of operation and the high speed of work roll, labor productivity is greatly improved.
- The surface quality of the parts is good.
Due to the protection of elastic medium, the sheet with protective layer and the polished surface can be bent and formed without scratching the surface
The main disadvantages are:
The cylinder with different curvature needs to be equipped with the liner on the upper roller, which is not suitable for the products of many varieties and small batches;
The processing plate thickness is limited, generally can only process the sheet metal below 10 mm.
In a word, the application prospect of RESM technology is quite broad.
At present, with the rapid development of China’s chemical industry, boiler, pressure vessel and other industries, the bending machine is developing in the direction of thick plate, high strength plate and numerical control.
With the rapid development of oil tank truck and the oil tank industry, small bending machine is required to develop in the direction of thin plate, super long type, multi curvature and numerical control.
Taking CNC bending machine as the center, the realization of flexible bending unit is also an important development direction in the future.
3) Automatic control of plate rolling machine
Generally, the mechanical bending machine adopts strong current control, and hydraulic plate bending machine adopts NC control.
Its NC control system is mainly used for static and dynamic synchronization and position control of oil cylinders at both ends of the work roll.
The rolling process is mainly controlled by the operator according to experience.
Numerical control rolling machine can realize automatic control, high rolling precision, high efficiency and easy operation.
It is an ideal equipment for both batch rolling and single piece small batch rolling.
At present, horizontal down type, curve down type, upper roll cross movable three-roll bending machine, two-roll bending machine, ordinary and arc type four-roll bending machine, marine bending machine and other various bending machines allegedly have CNC functions.
Its core component is industrial computer processing PLC can not only realize the static and dynamic synchronization of the left and right oil cylinders of the work roll, but also automatically calculate and optimize the process parameters such as the rolling times, the reduction amount each time, the horizontal movement of the lower roll, the feed rate of the sheet metal, the theoretical forming radius and the load of each roll, and the minimum residual straight edge during pre-bending according to the parameters of the thickness, width, diameter of the drum and yield limit etc., and realize automatic control.
In fact, due to the staggered arrangement of the upper and lower rollers of the three-roll bending machine, the sheet metal is easy to slip, so it is impossible to realize the real numerical control.
Only the four-roll bending machine and the two-roll bending machine can accurately control the feed rate of sheet metal, which can realize the real CNC control.
At present, Changzhi Forging has successfully developed CNC four roll bending machine, which fills in the domestic lack and is comparable with the products of the world’s advanced plate bending machine manufacturers.
Generally, the upper roll of NC four-roller bending machine is the main driving roll with DC motor.
Speed feedback and position feedback closed-loop control are composed of DC speed governor, DC motor, photoelectric encoder, etc., to achieve precise control of sheet material feed.
The lower roller and the rollers on both sides are driven by hydraulic cylinders to move up and down.
The PLC compares the input given signal Ur with the feedback signal Up collected by the displacement sensor according to the program requirements and obtains the input signal △U.
The AU is converted into D/A (digital/analog) and then the converted analog signal is used to drive the hydraulic system to realize the precise control of the position of the lower roller and the two rollers.
The hardware composition of the control system of the CNC four-roll bending machine mainly includes:
Eurotherm 590 series DC digital speed controller, OMRON C200HX PLC (programmable controller), MITSUBISHI A970GOT touch screen human-machine interface, high-precision displacement sensor, the display screen is connected to the C200HX CPU through RS232 standard communication.
According to the thickness, width, diameter, yield limit and springback correction coefficient, the numerical control four-roll bending machine can automatically calculate and optimize the process parameters, such as the rolling times, the up and down displacement of the lower roll and the side roll, the feed rate of the sheet, the theoretical forming radius and the load of each roll, the inclination of the upper roll and the minimum residual straight edge during pre-bending.
It can also realize automatic control.
Because the feed rate of sheet metal and the position of side roll and lower roll can be accurately controlled, the real CNC rolling can be realized.
At the same time, the machine can store 100 programs, edit and call programs, and has the functions of error self-diagnosis and alarm, state monitoring and power-off memory etc.
4) Coil flexible processing unit
Coil flexible processing unit is generally equipped with a numerical control rolling machine with the equipment such as the pretreatment of the front section of the plate and the delivery of the finished products of the rear section.
It is controlled by a control system composed of one or several computers to form the automatic processing unit of the coil plate.
The unit integrates information flow and material flow into CNC bending machine system, and can realize small batch processing automation.
It is an ideal manufacturing system with high precision, high efficiency and high flexibility.
Fig. 8 and Fig. 9 are the layout of the flexible plate bending unit.
The unit is mainly composed of CNC bending machine, plate storage table, loading manipulator, loading table, bracket device, unloading device, etc.
Fig. 8 Layout I of coil flexible processing unit
1. Electrical cabinet 2. Loading manipulator 3. Bracket device
4. Loading table 5. Workpiece 6. Unloading device
7. Main engineof bending machine 8. Sheet stacking platform
Fig. 9 Layout II of coil flexible processing unit
1. Main engine of bending machine 2. Bracket device 3. Unloading device
4. Feeding roller table 5. Loading manipulator 6. Plate stacking platform
When the plate is rolled, the loading manipulator takes the material from the plate storage table and puts it on the loading work table.
The manipulator aligns the plate and sends it to the plate bending machine.
During the rolling process, according to the shape, thickness and diameter of the rolled workpiece, the manipulator always attaches the workpiece from the inside or both sides of the workpiece (automatic setting), and always attaches and supports the workpiece with the curvature of the workpiece until the workpiece is formed.
Then the main machine dumps the bearing body and falls down, the unloading manipulator pushes out the workpiece, the discharging manipulator grabs the workpiece to the finished product table, the main machine and the manipulators at various positions return to their original positions and prepare for the next workpiece to be rolled.
In addition to the above configuration, some coil flexible processing units can also be equipped with auxiliary facilities such as sheet metal centering, cylindrical inspection and welding etc.
3. The gap between China and abroad countries
3.1 Brief introduction of abroad roll bending machines
The world’s advanced rolling machine manufacturers mainly include HAEUSLER in Switzerland, SCHAFER in Germany, PROMAU DAVI MG, SER-TOM and BODRINI in Italy, ROUNDO in Sweden, KURI-MOTO in Japan and HUGH SMITH in Britain.
Switzerland HAEUSLER’s four-roll plate bending machine is the most famous, its maximum specification has reached 78000 kN, plate thickness 250 mm, plate width 4000 mm, the remaining straight edge is only 1.28 times of the plate thickness.
Horizontal down type three-roll bending machine is common in some companies like German Schafer, Italy Davi, Italy MG, Italy FAC-CIN, Italy SERTOM, Italy BODRINI etc, which is one of the ideal models for rolling thick plate and extra heavy plate.
Japan KURIMOTO’s upper roller cross-moving type (upper roller universal type) is more common, and the large-scale marine plate rolling machines of British HUGH SMITH and Italian FACCIN have a high technical level.
The curved three-roller and four-roller bending machine of Italian PROMAU DAVI, MG is more common, and the maximum thickness of the curved three-roller bending machine can reach 140mm.
DAVI’s CNC arc four-roll bending machine adopts full hydraulic drive;
The rollers on both sides are in an arc motion mode.
The tangent point between the side roller and the upper roller is closer to the centerline of the upper roller.
The smallest roll diameter can reach 1.1 times that of the upper roller.
The remaining straight sides are short, and the friction resistance between the mechanisms is almost zero;
The hydraulic planetary drive is directly coupled to the shaft ends of the upper roller and the lower roller, with high transmission efficiency and small footprint;
The bearings are maintenance-free and do not need lubrication;
Automatic linear speed compensation to ensure the matching of the upper and lower roller speeds when rolling;
CNC control, rolling single curvature and multiple curvature radius workpieces can automatically calculate and generate programs.
The program can be modified and stored, and network control can be provided;
It has 3D animation real-time display function;
It equipped with REAL AUTO-CAD (CAD/CAM).
3.2 The main problems and the gap between China and foreign countries
It is estimated that China is the country with the largest output of bending machines in the world, but the overall competitiveness is not strong.
The main manifestations are as follows:
1) In terms of product composition, small and medium-sized and low-grade domestic plate bending machines account for a large proportion.
Although there are very few manufacturers that can produce large-scale bending machines with a rolling force of 40000 kN and a thickness of 160 mm or more, they are mainly imported at present.
At the same time, the technical added value and numerical control rate of domestic roll bending machine are low, and the main market of numerical control bending machine and flexible plate bending unit is still occupied by foreign products.
2) Foreign enterprises have strong ability to provide customers with overall solutions, complete sets of equipment and technology.
For example, DAVI company is a complete set of forming equipment for wind power tower, which well meets the domestic market demand.
At present, only single bending machine can be provided in China.
3) The basic research on the forming mechanism, especially the forming mechanism of extra thick and high strength sheet, the simulation of forming process and the establishment of mathematical model needs to be strengthened.
4) Domestic low-speed high torque hydraulic motor, large torque and high-speed ratio planetary reducer which can be directly coupled to the work roll shaft end can not meet the needs of the development of plate bending machine.
5) There is still a big gap in the specialized development and matching of CNC system.
6) Although the standardization work of plate bending machine in China has made great progress, there are still great differences in the aspects of machine type, configuration, technical parameters and safety, so the standardization work needs to be further strengthened.
7) At present, most of the domestic plate bending machine manufacturers are small and medium-sized private enterprises, which lack technological development ability and overall competitiveness.
4. Discussion on the development trend of the plate bending machine in China
1) With the development of China’s chemical industry, boiler, pressure vessel and other industries to large-scale, large-scale horizontal three-roll bending machine is developing in the direction of processing heavy plate, extra thick plate, high-strength plate and composite plate.
At the same time, due to the short remaining straight edge, material saving, precise forming and high efficiency, the large-scale four-roll bending machine will also be developed.
2) With the rapid development of oil tanker, oil tank and other industries, small plate bending machine is required to develop in the direction of thin plate, super long type, multi curvature and numerical control.
3) It takes the bending machine as the main processing equipment to realize the complete set of configuration, which provides the overall solution for customers.
As mentioned above, the overall solution of wind turbine tower forming is worthy of attention.
4) It takes the numerical control bending machine as the center, forming the flexible plate bending unit is an important development direction in the future.
After decades of development, China’s bending machine has formed a relatively perfect manufacturing cluster and reasonable industrial chain, which can better meet the needs of the domestic equipment manufacturing industry and promote the process of industrialization.
The bending machine industry should make use of the national industrial revitalization plan, speed up the adjustment of product structure and the transformation and upgrading of enterprises, digest and absorb foreign advanced technology, improve the ability of independent innovation, and strengthen and expand China’s plate bending machine manufacturing industry.
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