Precision in Motion: Advances in Chinese Roll Bending Machines

1. Overview

Plate Roll Bending Machine is one of the eight main forging machine products and falls into the category of bend correcting machines.

Roll bending machine is a metal forming equipment that uses the three-point circle principle to shape sheet metal. The movement and rotation of the work roll, whose axes are parallel to each other, result in continuous elastoplastic bending of the sheet, which leads to the creation of a part of predetermined shape and precision.

The work roller serves as the main working component and converts mechanical and hydraulic energy into kinetic energy to facilitate its change of position and rotation. This makes it easy to bend and roll sheet metal into single or multiple curvature curved or cylindrical parts in a cold, medium or hot state.

By adjusting the shape of the roll or adding a conical winding device, the material flow can be restricted and the movement speed of different parts in the same part can be changed. This allows you to create arc or cylinder parts with single or multiple curvature shapes such as ellipses, squares, and asymmetrical shapes.

Two methods, press bending and bending, can be used to bend sheet metal into simple or arc shapes with single or multiple curvatures. Press bending is carried out in a hydraulic press or press brake with the help of a die, mainly relying on transverse plastic bending. The bending process is a point-to-point or sectional discontinuous process.

Compared to press bending, roll bending has the following advantages:

• The bending process is continuous elastic-plastic bending with low springback and defined tensile force, resulting in precise forming, high-quality bends and high work efficiency.
• The absence of molding reduces operating costs.
• Roll bending requires lower bending force compared to press bending and the cost of a roll bending machine is lower than that of a hydraulic press.
• With the addition of auxiliary equipment, roll bending can also be used to create conical parts and bend tubes and profiles.

As a result, Sheet Metal Roll Bending Machine is widely used in various industries such as boiler manufacturing, shipbuilding, petroleum, chemical, hydraulic and metal structure.

In China, the development of roller bending machines has progressed from mechanical to hydraulic and now to CNC technology. The hydraulic and automatic control technology of roll bending machines has improved greatly in recent years.

In the 1990s, numerical control plate bending machines developed rapidly in China and underwent continuous improvements, becoming more and more mature. The transition from single-machine automation to a flexible production cell has also been completed.

Efficient, accurate, mass production is essential to modern industrial civilization. The development of high-efficiency, high-precision, safe, environmentally friendly, automated, flexible and intelligent bending correction equipment is crucial to improving the level of China's equipment manufacturing industry, meeting the needs of business competition and increasing benefits economic.

2. The basic situation of roller bending machines in China

2.1 Important role of roller bending machine in equipment manufacturing industry

The demand for sheet metal bending machines is increasing due to the development of various industries such as aerospace, military weapons, automobiles, locomotives and rolling stock, instruments and gauges, shipbuilding, chemical industry, pressure vessels, metallurgy, engineering and metal machinery components. The precision requirement of plate rolling is also increasing.

China's shipbuilding industry has greater demands for functionality, reliability, safety and automation of common and marine press brakes with winding compound functions due to the rapid growth of the manufacturing industry.

The electric power industry, which includes coal power, hydropower, nuclear power and wind power encouraged by the national clean energy policy, requires large sets of bending machines for pipelines and column towers.

The trend toward large-scale, high-parameter pressure vessels is becoming more apparent in industries such as offshore oil, natural gas, petrochemicals, and coal chemistry. Large-scale plate rolling machines, capable of forming thick, high-strength plates, are crucial for the production of thousand-ton hydrogenation reactors, two-thousand-ton coal liquefaction reactors, and 10,000-cubic-meter spherical natural gas tanks .

In the future, the requirements for energy efficiency, material savings and cutting reduction will become more and more intense. It is estimated that using sheet metal bending machines as process equipment increases efficiency by 10 to 40 times and saves more than 60% of materials.

Data shows that 70% of steel production needs to be deformed and 2/3 of steel in the metal forming industry needs processing. For every 10,000 tons of reprocessing of sheets, tubes and profiles, a tube bender, a roll bender and a profile roll bender are required.

2.2 Current status of plate bending machine

In recent years, the roller bending industry in China has grown significantly and its manufacturing capacity has improved. This has had a significant impact on improving the level of equipment manufacturing and promoting the industrialization process in China.

1) Main types and characteristics of sheet metal roll bending machine

Plate bending machines can be classified into several categories based on several factors:

• Number of work rolls: two rolls, three rolls, four rolls or multiple rolls
• Roll arrangement: symmetrical or asymmetrical
• Roll axis position: horizontal or vertical
• Roller adjustment mode: adjust up or adjust down
• Main driving mode: mechanical or hydraulic
• Roller drive mode: top drive, bottom drive or full drive
• Bearing body type: open or closed (marine)
• Function: common or multipurpose
• Lamination method: cold lamination, hot lamination or hot lamination
• Control mode: strong current control, NC control or CNC control

Table 1 shows the main types of sheet metal bending machines based on the number of work rolls, roll arrangement, position adjustment methods, etc. Common types of three-roll press brakes include:

• Symmetrical type
• Downward curve type
• Horizontal down type
• Cross type top roller (universal type top roller)

Other common types of sheet metal bending machines include:

• Four Roll Bending Machine (Common Type)
• Two Roll Bending Machine
• Three Roll Vertical Bending Machine
• Marine Bending Machine

2) Development history and current status of plate bending machine

Specialized production of sheet metal bending machines in China began in the early 1970s with the development of a three-roll symmetrical bending machine with mechanical transmission. Although it had a simple structure, it was unable to pre-bend the sheet ends, which resulted in low production efficiency and increased labor and material costs.

In the late 1970s, Changzhi Iron and Steel (Group) Forging Machinery successfully developed a four-roll sheet metal bending machine with mechanical transmission that could pre-bend both ends of the sheet metal. However, this machine had a large structure, high material consumption and a long manufacturing cycle.

In the mid-1980s, the factory introduced the four-roll fully hydraulic press brake and the three-roll press brake with hydraulic level adjustment. These machines not only pre-bend the sheet metal, but also completed the cylinder forming after welding, leading to a 2-3 times improvement in working efficiency compared with the original model.

In the late 1980s, three- or four-roll bending machines controlled by PC and NC entered the Chinese market. These machines had the functions of automatic work roll leveling and synchronous work roll lifting in inclined state, with synchronous precision between work rolls controlled within ±0.2mm.

In the early 1990s, Changzhi Forging developed the three-roll arc-down sheet metal bending machine and the four-roll arc-down sheet metal bending machine. These machines could pre-bend and roll the plate in a loading and unloading process and had reasonable bending strength, light weight and compact structure. The four-roll curved press brake was particularly suitable for rolling small diameter cylinders, with a minimum cylinder diameter of 1.1-1.2 times the top diameter of the roll.

In the mid-1990s, Changzhi Forging adopted the four-motor confluence main drive technology and applied it to the horizontal-down type regulating structure for large-scale three-roll bending machines. The first large-scale CDW11XNC -140×4000 horizontal down three-roll bending machine was developed for the Three Gorges Project, and in 2002, the factory produced the CDW11XNC -160/250×4000 horizontal down three-roll bending machine for the Factory of Nanjing Chemical Machinery.

Fig. 1 Horizontal Down Type Three Roll Bending Machine CDW11XNC-140 × 4000

At present, China is capable of producing the largest three-roll horizontal bending machine with negative regulation. This machine can roll cold plates with a thickness of up to 250 mm and hot plates with a thickness of up to 400 mm. The maximum plate width that can be processed is 3000mm to 4500mm, and the upper and lower roller pressure can reach 40,000kN to 60,000kN.

The main domestic manufacturers of large-scale three-roll horizontal press brakes are Changzhi Forging, Hubei Ezhong and Tai'an Hualu. Figure 2 shows the CDW11XNC -40×8000 horizontal-down type three-roll bending machine.

Fig. 2 Horizontal Down Type Three Roll Bending Machine CDW11XCNC-40 × 8000

The machine is capable of rolling plates with a maximum thickness of 40 mm, a maximum width of 8,000 mm, a yield strength of 240 MPa and a minimum coil diameter at full load of ø2500 mm, with a minimum coil diameter of ø900 mm.

The machine compensates for the deflection of the upper roll through the use of a wedge mechanism, driven by a motor, reducer and screw, which adjusts the deflection of the lower roll.

The NC system can automatically calculate and generate programs based on parameters such as plate thickness, width, drum diameter, yield limit and more, enabling automatic control.

To accommodate the rolling of thin and extra-long sheets (where the thickness/width ratio is small), Changzhi Forging has developed a new type of symmetrical or horizontal three-roll bending machine, with a support roller and cross beam on the upper roller (as per shown in Fig. 3).

Fig. 3 Horizontal Down Type Three Roll Bending Machine CDW11XCNC-8 × 9500

The machine is mainly driven by three rollers with a tilting bearing body, capable of rolling closed cylindrical parts. It also features computer control, making it particularly suitable for rolling long, thin-walled cylindrical parts with multiple curvatures.

The three-roll bending machine with cross-type top roller (universal type top roller) was first successfully manufactured by First Heavy Machinery Factory in the early 1980s. In the late 1980s, companies such as Nantong Hengli Heavy Industry started to produce it professionally.

The machine only requires adjustment of the upper roller to perform sheet lamination and pre-bending. The lower roller is fixed, providing ease of use for rolling materials and sheets. However, the machine's large frame can be challenging, making it more suitable for rolling plates with a thickness of less than 80mm to 100mm.

The marine plate bending machine was manufactured by First Heavy Machinery Factory and Changzhi Forging. Initially, it had a symmetrical structure with specifications such as W11TNC-20×8000 and W11TNC-25×9000. As China's shipbuilding production grew rapidly and the shipbuilding industry turned from weak to strong, the marine bending machine developed into a horizontal downward type in addition to its symmetrical structure.

This machine is capable of rolling and pre-bending curved and conical parts and can bend steel plates with the help of a bending die. It works as both a laminating machine and a bending machine. Deflection compensation technology for the upper and lower work rolls, two-way independent work roll drive technology and numerical control technology for bending and bending process parameters have reached a world-class level.

At present, the maximum downward pressure of the domestic marine sheet metal bending machine's upper roller is 22,000 kN. The width of rolled or bent sheets can reach more than 21,000 mm, meeting the needs of the shipping industry.

Figures 4 and 5 show the Changzhi Forging CDW11TNC-32×13500 marine three-roll bending machine and the CDW11TXNC-22000kN×16000mm horizontal three-roll marine bending machine, respectively. Major domestic manufacturers of marine bending machines include Changzhi Forging, Tai'an Hualu and Nantong Hengli.

Fig. 4 Marine three-roll bending machine CDW11TNC-32 × 13500

Fig. 5 CDW1ITXNC-2000kN×16000mm closed type (Marine) horizontal down type three roll bending machine

1. Main drive system 2. Frame 3. Main oil cylinder 4. Upper beam
2. Upper work roller 6. Upper support roller7. Lower the work roll 8. Lower the backup roll
3. Lower roller horizontal movement device 10. Lower beam 11. Connecting beam

The Vertical Roller is a bending machine with three or four rolls, where the axis is perpendicular to the horizontal plane. The most commonly used type is the three-roll symmetrical vertical press brake.

Changzhi Iron and Steel (Group) Forging has manufactured equipment such as CDW11TNC-36×1000 and 12×2000 for various industries, including the petroleum and chemical industries.

The First Heavy Machinery Factory introduced technology from the Hugh Smith Company of the United Kingdom and designed and produced a series of vertical press brakes with a single main roll. They manufactured a 30,000 kN x 3,600 mm vertical press brake.

Compared with horizontal press brakes, vertical press brakes have the advantages of being lightweight, taking up less space and having lower energy consumption, leading to reduced labor intensity, especially in hot rolling.

The two-roll bending machine has a rigid steel roller as the upper roller and an elastic roller with a steel core wrapped in elastic coating as the lower roller.

Rotary forming using elastic media (RSEM) is an advanced method of bending sheet metal parts with an elastic medium.

Currently, companies such as Changzhi Forging and others have the technology to design and manufacture these products, and the School of Mechanical and Electrical Engineering at Nanjing University of Aeronautics and Astronautics has also developed one.

Figure 6 shows the two-roll bending machine produced by Changzhi Forging. The upper roller is an elastic roller made of a steel core with an elastic coating, while the lower roller is a rigid steel roller. The machine has a bottom discharge structure.

Fig. 6 Drawing of a two-roll sheet metal bending machine

The two-roll bending machine was invented by Kauffman of the United States. Since then, countries such as Britain, Japan, France, Italy and Russia have also mastered the technology and produced highly automated CNC machine tools. These machines are used to manufacture various thin-walled parts in industries such as aviation, chemical and civil products. Parts include cylindrical and conical shells, tube sections, specially shaped thin-walled parts, multi-unit thin-walled parts, and reinforced or corrugated plate sections (see Figure 7).

Fig. 7 Thin-walled parts manufactured with RESM technology

Compared to three-roll and four-roll bending technologies, the RESM process has several notable features and benefits:

• High precision in forming parts: The RESM process results in a high degree of precision in the curvature and parallelism of the edges after bending, with the straight edge remaining smaller than the sheet thickness, making it ideal for forming porous sheets and ribbed sheets with protective layers.
• High curvature and bending yield: The roll bending process involves gradual deformation under low pressure, which reduces the risk of wrinkling and cracking during forming. The small force required for forming also makes it suitable for difficult-to-deform materials such as titanium alloys, stainless steel and composite materials.
• High production efficiency: The RESM process is easy to operate and has a high working speed, resulting in greater labor productivity.
• Good surface quality: The use of an elastic medium protects the surface against scratches during bending and forming, preserving surface quality, especially for sheets with protective layers and polished surfaces.

However, there are also some disadvantages to consider:

• Limited versatility: The need to use liners on the upper roller for cylinders with different curvatures makes RESM not ideal for products with many varieties and small batches.
• Processing thickness limitations: RESM is generally limited to processing sheets with a thickness of less than 10 mm.

In conclusion, RESM technology has a wide range of potential applications. Currently, as China's chemical industry and industries such as boilers and pressure vessels continue to develop, the demand for numerically controlled high-strength thick plate bending machines is growing. Meanwhile, the development of the tank truck and oil tank industries is driving the need for small bending machines with the ability to handle thin sheets, super-long types, multicurvatures and numerical control. The future of bending technology will also involve the development of flexible bending units, with CNC bending machines at the center.

3) Automatic control of plate rolling machine

The mechanical bending machine typically uses strong current control, while the hydraulic plate bending machine employs NC control (numerical control). The NC control system is mainly used to synchronize the position control of oil cylinders at both ends of the work roll in a static and dynamic manner.

In the rolling process, control is mainly carried out by the operator based on his experience. The numerical control rolling mill, however, allows automatic control and offers high rolling precision, efficiency and ease of operation. It is suitable for both batch rolling and single-piece small batch rolling.

At present, various bending machines such as horizontal down type, down curve type, top roll cross movable three roll bending machine, two roll bending machine, common and arc type four roll bending machine, marine bending, among others, are equipped with CNC functions.

The main component of these machines is an industrial computer processing PLC, which not only synchronizes the left and right oil cylinders of the work roll dynamically and statically, but also automatically calculates and optimizes process parameters such as rolling times , amount of reduction each time, horizontal movement. bottom roll, sheet metal feed rate, theoretical forming radius, load on each roll, and minimum residual straight edge during pre-bending. This optimization is based on parameters such as thickness, width, drum diameter and yield limit, among others.

However, the arrangement of the upper and lower rolls of the three-roll press brake is staggered, making it difficult to control the sheet metal feed rate, and therefore true numerical control is not possible. The four-roll press brake and two-roll press brake, on the other hand, can precisely control the sheet metal feed rate, thus allowing true CNC control.

Changzhi Forging has successfully developed a four-roll CNC press brake, filling the domestic gap and being comparable to the products of advanced plate bending machine manufacturers around the world.

The top roller of an NC (Numerical Control) four-roll bending machine is the main driving roller, powered by a DC motor. Speed ​​and position feedback is controlled through a closed-loop system, consisting of a DC speed regulator, DC motor and photoelectric encoder, to ensure precise control of sheet material feeding.

The bottom roller and rollers on both sides are driven by hydraulic cylinders for upward and downward movement. The PLC (Programmable Logic Controller) compares the input signal (Ur) with the feedback signal (Up) collected by the displacement sensor and adjusts the input signal (△U) accordingly. This input signal is then converted into digital/analog (D/A) format and used to drive the hydraulic system, ensuring precise control of the position of the lower roller and both rollers.

The control system of the four-roll CNC press brake is composed of the following hardware: a Eurotherm 590 series DC digital speed controller, an Omron C200HX PLC, a Mitsubishi A970GOT touch screen human-machine interface, a high-precision displacement sensor and a screen connected to the C200HX CPU via standard RS232 communication.

Based on parameters such as thickness, width, diameter, yield limit and springback correction coefficient, the numerically controlled four-roll press brake can automatically calculate and optimize process parameters including rolling times, the displacement of the lower roll and side roll, sheet feed rate, theoretical forming radius, load on each roll, top roll inclination and minimum residual straight edge during pre-bending. The machine also allows automatic control.

Precise control of the sheet metal feed rate and the position of the side roller and bottom roller enables true CNC rolling. The machine can store 100 programs and has the ability to edit and recall programs, as well as features such as error self-diagnosis, alarm, status monitoring and power-down memory.

4) Flexible coil processing unit

The Flexible Coil Processing Unit is typically equipped with a numerically controlled rolling mill and various other equipment, including a pre-treatment system for the front section of the sheet and a finished product delivery system for the rear section.

The unit is controlled by a control system that can be composed of one or several computers, forming an automated coil plate processing system.

The integration of information and material flow into the CNC press brake system allows for efficient and automated processing of small batches.

As a result, this unit offers high precision, efficiency and flexibility, making it an ideal manufacturing system.

Figures 8 and 9 show the layout of the Flexible Plate Bending Unit, which is composed of a CNC bending machine, a plate storage table, a loading manipulator, a loading table, a supporting device and an unloading device.

Fig. 8 Flexible coil processing unit layout I

1. Electrical panel 2. Charging handler 3. Support device

4. Loading table 5. Workpiece 6. Unloading device

7. Main engine of press brake machine 8. Plate stacking platform

Fig. 9 Flexible coil processing unit layout II

1. Bending machine main motor 2. Support device 3. Unloading device

4. Feed roller table 5. Loading handler 6. Plate stacking platform

During the rolling process, the loading manipulator takes the plate material from the plate storage table and places it on the loading work table. The manipulator then aligns the plate and sends it to the plate bender.

As the rolling process continues, the manipulator clamps the part based on its shape, thickness and diameter. It fixes the part from the inside or both sides and supports it with the curvature of the part until the desired shape is formed.

Once the rolling process is complete, the main machine releases the bearing body and the discharge manipulator pushes the part out. The unloading handler then picks up the part and transports it to the finished product table. Finally, the main machine and manipulators return to their original positions, ready for the next part to be rolled.

Some flexible coil processing units can also be equipped with additional features such as sheet metal centering, cylindrical inspection and welding.

3. The difference between China and foreign countries

3.1 B major introduction of bending machines abroad

Leading manufacturers of advanced rolling mills 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 Great Britain.

The four-roll sheet metal bending machine from HAUSLER in Switzerland is the most renowned, with its maximum specification reaching 78,000 kN, sheet thickness of 250 mm and sheet width of 4,000 mm, with only 1.28 times the sheet thickness remaining as straight edge.

In some companies, such as the German Schafer, the Italian Davi, the Italian MG, the Italian FAC-CIN, the Italian SERTOM and the Italian BODRINI, the down type three-roll horizontal press brake is common and is ideal for rolling thick plates and extra heavy.

In Japan, KURIMOTO's top roll cross motion type (top roll universal type) is more prevalent, and the large-scale marine plate rolling mills from Britain's HUGH SMITH and Italy's FACCIN have a high level of technology.

Three-roll and four-roll curved press brakes from Italian PROMAU DAVI and MG are more prevalent, with the maximum thickness of the three-roll curved press brake capable of reaching 140 mm.

DAVI's four-roll CNC arc bending machine has full hydraulic drive, and the rollers on both sides are in arc movement mode. The tangent point between the side roller and the upper roller is closest to the center line of the upper roller, and the smallest diameter of the roller can reach 1.1 times that of the upper roller.

The other straight sides are short, with almost zero frictional resistance between the mechanisms. The hydraulic planetary drive is directly coupled to the shaft ends of the upper roller and lower roller, providing high transmission efficiency and taking up little space. The bearings are maintenance-free and do not require lubrication. The machine features automatic linear speed compensation to ensure matching of the upper and lower speeds of the rollers during rolling.

CNC control is available, and rolling of parts with single and multiple bend radius can be automatically calculated and programmed. The program can be modified and stored, and network control is provided. It has a real-time 3D animation display function and is equipped with REAL AUTO-CAD (CAD/CAM).

3.2 T The main problems and the gap between China and foreign countries

It is estimated that China has the largest production of bending machines in the world, however, its global competitiveness in the industry is not strong. The main reasons are:

• In terms of product composition, a large proportion of domestic sheet metal bending machines are small to medium-sized and of low quality. Although there are some manufacturers that produce large-scale press brakes with a rolling force of 40,000 kN and a thickness of 160 mm or more, they are mostly imported. In addition, the added technical value and numerical control rate of domestic roll bending machines are low, with foreign products dominating the market of numerical control bending machines and flexible sheet metal bending units.
• Foreign companies have a strong ability to provide comprehensive solutions, complete equipment and technology to customers. For example, the company DAVI offers a complete set of forming equipment for wind energy towers, which well meets the demand of the national market. Currently, only single bending machines are available in China.
• Basic research on the forming mechanism, especially the formation of extra-thick and high-strength sheets, the simulation of the forming process and the establishment of mathematical models, need to be strengthened.
• Domestic low-speed, high-torque hydraulic motors and large-torque, high-speed ratio planetary reducers that can be directly coupled to the shaft end of the work roll are insufficient for the development of sheet metal bending machines.
• There remains a significant gap in the specialized development and suitability of CNC systems.
• Despite progress in the standardization of sheet metal bending machines in China, there are still considerable differences in machine types, configurations, technical parameters and safety, requiring further standardization efforts.
• The majority of national manufacturers of sheet metal bending machines are small and medium-sized private companies, without the capacity for technological development and general competitiveness.

4. Discussion on the development trend of plate bending machine in China

• The development of China's chemical industry, as well as the growth of the boiler and pressure vessel industries, has led to an increase in demand for large-scale three-roll horizontal bending machines. These machines are increasingly being used to process thick sheets, extra thick sheets, high-strength sheets and composite sheets. At the same time, the development of large-scale four-roll bending machines is being driven by the need for efficient and accurate forming, material savings and short remaining straight edges.
• The rapid growth of industries such as oil tankers and oil tankers has created a demand for small sheet metal bending machines that can handle thin sheets, super-long types, multicurvatures and numerical control.
• Using press brakes as the main processing equipment allows for a complete set of configurations, providing customers with a global solution. It is also worth considering the global solution for the formation of wind turbine towers.
• Future development of flexible sheet metal bending units will center on numerical control bending machines.

5. Conclusion

After decades of growth, China's bending machine industry has established a well-structured production cluster and efficient industrial chain. This made it possible to better meet the needs of the national equipment manufacturing industry and boost the industrialization process.

To further strengthen China's plate bending machine manufacturing industry, it is important for the industry to take advantage of the national industrial revitalization plan. This involves accelerating the adjustment of product structure, transforming and modernizing enterprises, incorporating advanced technology from abroad, increasing independent innovation capacity and expanding the reach of the industry.