Overview of Digitally Controlled Incremental Forming Methods for Sheet Metal and Future Market Trends

In recent years, due to changes in international regulations and consumption habits, the product life cycle is short, resulting in a small amount of diversification in the market. If the manufacturing cost remains the same, but the demand decreases, it will be difficult for the manufacturer to recover the cost. Rapid trial production technology can meet the needs of consumers and producers, and make molds or products with low-cost manufacturing methods to achieve the purpose of rapid sample delivery. It has become a popular research project in the world to win orders and reduce development risks. In response to market trends, the current plastic and metal product industries mostly use rapid prototype technology (3D printing or CNC processing) to directly print or process products. Usually, samples can be delivered quickly within three weeks. In the case of a large number of samples, direct manufacturing of products will result in high product development costs. In the case of a large amount of trial production, Rapid tolling will be used to make trial samples. Generally, die casting and sheet metal industry will require 2~2.5 months. Due to the increasing demand for special curved surfaces in recent years, At the same time, high-precision, high-strength requirements are required, which complicates the mold and requires repeated mold repairs to meet the quality requirements. The application of trial molds cannot effectively shorten the development time and provide samples, and the opportunity to grab orders is lost. RP technology is the technology to quickly complete the prototype of the product. Different manufacturing methods can be divided into removal (CNC processing), addition multilayer manufacturing, and forming (injection, stamping, casting).

Market Research on the Application of Incremental Forming Technology to the Trial Production of Sheet Metal Parts:

Sheet metal incremental forming technology belongs to the forming category, and its purpose is to produce highly customized products at the most reasonable manufacturing cost. ISF technology has become one of the more and more important technologies in recent years. At present, it has become the focus of international and many research and development units.

In the traditional processing technology of plastic forming materials, it is usually necessary to heat the material or apply high pressure to shape the material into the target shape. Considering the microstructure change of the material and the flow mode of the material, material heating plays an important role, but the heating system, whether it is the construction of the heating furnace or the electric power consumed by heating, etc., will increase the manufacturing cost. On the other hand, stamping is used in sheet metal forming, which requires a die and punch. At the same time, the mechanical properties of die steel are high, including high hardness, temperature resistance, or low friction, resulting in a high cost of die materials.

To deal with these problems, the development of new technology is urgent, with a shorter life cycle and shorter production time to manufacture products in small batches. Using ISF technology, with simple molds and room temperature, digital path control technology, and simple equipment, the forming purpose of sheet metal can be completed. The application markets of ISF technology are automobiles, ships, and aerospace, and there are researchers in the unit applies this technology to the medical field, such as fake human bones, fake joints, etc.

In recent years, to cope with global production, the product development cycle has been shortened, and the industry must have the ability to respond quickly to obtain international orders, so rapid trial production technology has emerged. The incremental forming technology that can produce parts in a short time through digital path control and reduce the use of special molds is the current international development trend in related forming technologies.

The Development Trend of Incremental Forming Technology:

In response to global production, the product development cycle is shortened. To meet this demand, it is necessary to have the ability to respond quickly to obtain international orders, resulting in a shortened mold manufacturing cycle. Domestic manufacturers need to be able to provide customers with high-quality trial parts with small batches and low-cost manufacturing processes, to be able to compete for orders. In addition, in response to a small number of diverse markets, brand factories need to develop new products and new parts every year, and the cost of product trial production increases year by year. At present, various industries including plastic products, metal products, sheet metal, block materials, and other different forms of products mostly use rapid prototype technology to directly print or machine-process the products. The required product shape can be completed within a week, but in the case of a large number of submitted samples, the cost of direct production or 3D printing for product development is too high. At present, there is rapid tooling, which can be quickly processed by using resin molds or free-cutting steel. Small batch trial proofing of the mold structure, or 3D printing of sand molds combined with recyclable zinc alloy casting molds to reduce the cost of trial molds. Compared with direct proofing, although the cost of small batch trial production can be reduced, the production schedule will be increased.

The elastic multi-point mold forming technology discretizes the overall surface of the traditional mold into a series of highly controllable and regularly arranged punch matrixes. Each discrete point is an independent mobile unit. The shape of the forming surface can be changed by controlling the relative vertical height of each point. The envelope surface formed by the punch matrix is the required forming surface. It can be controlled without changing the mold. Point position rapid prototyping, various curved surface products with different complex shapes, have been applied in many fields internationally, including high-speed train head covers, automobile covers, ship outer plates, and other large sheet metal products. In traditional spinning, the material is rapidly rotated axially, and the forming roller head is similar to a lathe to approach the rotating workpiece, causing the workpiece to deform axisymmetric ally. The elastic spinning forming is traditional spinning forming plus multi-axis control to increase the degree of freedom of form, and numerical control can be used to form non-axisymmetric rotating thin-walled parts, and the accuracy of the product obtained by spinning is relatively high. Higher and better quality, it has been widely used in the fields of metal precision processing such as aerospace, military, and nuclear energy the world.

Progressive forming technology also uses numerical control methods to control the roller head mold according to the path by hardware facilities such as CNC or robotic arm and plastically deforms the plates clamped on the fixture in sequence, gradually forming the final product shape. Different types of incremental forming techniques, including single-point, double-point, and multi-point modes, and the forming direction is positive or negative. Most of the above-mentioned forming processes are carried out in the form of numerical control because it can reduce the number of special molds used in the traditional manufacturing process. Reduce the cost of the mold and the processing time, and can effectively provide rapid response to design changes during the product trial development period, or respond to industries that require small batch production that does not need to amortize the cost of large molds.

Incremental Forming Process Passes and Parameter Design:

The formability of incremental forming technology is affected by many parameters, including roll head diameter, feed rate, roll head speed, friction between sheet and roll head, and forming wall angle, that is, the angle between the forming surface and the horizontal. Among them, the forming wall angle is limited by the shape of the product, and it is difficult to change it by adjusting the forming parameters at will. The thickness of the metal blanking plate is greatly affected by the angle of the formed product in the progressive forming. When the forming angle exceeds the maximum forming angle of the material, cracks will occur. Many scholars in the world have found through experiments that the forming limit and material thickness are consistent with each other. The forming angle is closely related. The forming limit is a measure used to judge whether the forming is broken during the sheet metal forming process. Since there is no criterion for judging the failure mode corresponding to the gradual forming, the forming state is predicted through the evaluation of the thickness and the forming angle. During incremental forming, the thickness of the material follows a sinusoidal law. The deformed sheet thickness is a function of the forming angle Φ and the initial sheet thickness. The thickness of the sheet material gradually becomes thinner under the shear force applied by the forming roller head. Since the area of the roller head relative to the product is quite small, the deformation of each point has little or no influence on the undeformed area.

The single maximum forming angle is studied for different materials. The maximum forming angle of most materials is between 60° and 70°. Taking mild steel as an example, the maximum forming angle is about 65°. When the product feature exceeds the maximum forming angle, it must be planned through multi-pass paths. For example, for 90° extended products, the intermediate pass can be used to make the forming angle 40°~50°, and then the intermediate pass is designed to make the material The strain is more evenly distributed in each position to avoid deformation and thinning of local positions. With the extension method, the material outside the forming area can be used to make up for the thinning of the material in the forming area during the forming process. It can be realized by clamping.

A six-axis robotic arm was used to carry out incremental forming tests. The incremental forming CAE analysis technology was initially established. The biggest difference from the stamping forming analysis is that the actuation mode of the roller head path needs to be considered in the incremental forming process. Since the forming method of the roller head is to move in various directions over time, its operating conditions need to be achieved by discretization of the path. Software such as Mastercam or visual mesh can be used to discretize the path. The discretized path is converted into the coordinate system of each point, and then the X, Y, and Z moving distances between the points are obtained from the coordinate point data, so that after the roller head is moved to the initial position, it will follow the X, Y, Z moving distance in three directions for moving on the path.

Incremental Forming Technology and its Applications:

There are many different types of incremental forming technology. Since ISF does not require special dies and punches, it is suitable for low-volume production in the aerospace and automotive industries. This technique utilizes localized deformation to increase formability, so it can be effectively used to form materials with low strength. Complex path settings can be made using digital control, so complex geometric parts can be formed relatively easily compared to other traditional forming methods.

Single Point Incremental Forming (SPIF) can be easily performed on a CNC milling machine. The following are the basic components of SPIF:

• CAM software for tool path planning.
• Hemispherical-shaped roller head with a small diameter.
• CNC milling machines for automated tool movement.
• Sheet metal clamping tools.
• Sheet metal blanks.

Sometimes CAD software is required in complex geometries to generate the roll head path from the CAD model. The material of the roller head is cold die steel, high-speed steel, hard alloy, plastic, etc. And the blank material is copper, high-temperature steel, low-carbon steel, brass, aluminum alloy, stainless steel, gold, silver, and platinum.

Market Future of the Incremental Forming Method:

In the face of the increasing demand for customization from consumers in the future, special forming methods such as flexible multi-point plate metal forming technology. Flexible spinning forming technology has been developed internationally to meet the demand for a small number of thin plate parts. And it has been applied to the incremental sheet metal forming technology for trial production of various sheet metal parts.