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MATERIALS IN DESIGN & PROTOTYPING


FEATURE MATERIALS AT A STRETCH


Using ‘stretch forming’ to stretch materials such as steel and stainless steel beyond their yield point can create stronger parts, increase production and reduce processing steps...


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nitially developed as an efficient means to produce components with complex curved profiles in the aircraft industry, stretch


forming is now widely used for similar components in automotive, aerospace, construction, rail and rocketry. But what is it? Stretch forming is a process where material such as steel, stainless steel, aluminium or even titanium is stretched beyond its yield point and simultaneously wrapped around net shaped dies. Forming sheet or extruded material using this method essentially shifts the neutral axes of a part to the perimeter of the die, resulting in a smooth, wrinkle-free, contour that closely retains the shape of the die, improves the mechanical properties, and eliminates many post process steps used to attain dimensional accuracy. “As part producers seek to expand their


capabilities, many have approached us with new stretch forming applications,” said Dave Gardner, senior mechanical engineer at Erie Press Systems. Park Ohio acquired Erie Press Systems in 2020 and is now a part of Ajax-CECO-Erie Press (ACE). “Combining modern CNC technology with our precise digital die mapping technology and historically robust machine structure provides for the finely tuned automated motion control necessary to maintain constant strain in the part throughout the cycle, all resulting in industry leading dimensional stability,” he added.


PRODUCING PARTS According to Gardner, stretch forming machines may be classified into three primary design types. Sheet stretch forming machines are used to produce complex curved sheet metal parts such as exterior panels and leading edges on aircraft, high-speed trains, RVs, buses and commercial rockets. Extrusion stretch forming machines are used to produce extruded structural components with complex cross sections and curved profiles such as stringers and support beams for aircraft, skeletal components for locomotives, buses and transportation structures, and curved shapes used in the elevator and architectural industry. High-speed, high-volume stretch forming machines are generally reserved for forming extrusions in automotive or other high production applications. The company works with its clients to


develop the technical details of a machine that will fully meet their unique requirements. Stretch forming offers numerous advantages, including the ability to accurately form complex shapes while maintaining the part’s total volume. Once the component is stretched into the yield state, less force is required to wrap the part around the die than with other forming methods. Part cost is substantially reduced by the ability to form accurate and repeatable components with little wasted material. Additionally, the process


induces work hardening in many materials, which increases strength while reducing internal residual stress and spring back as compared to conventional bending operations. Gardner advises that clients consider the


purchase of stretch forming machines with these qualities: Structural integrity and longevity: Stretch


forming is not a high impact process, but forces involved can be over 3,000 tons in some aerospace applications. High-quality stretch forming machines with very precise motion control and the ability to digitally map the contour of the die with respect to the forming axes will form the part with nearly constant strain throughout the process and create very precise, consistent parts, for many decades. Selecting a stretch forming machine with a


robust, heavy, frame and properly designed connection points to the die table is essential to ensure its accuracy and reliability. For sheet stretch forming equipment, it is particularly important to maintain a rigid structural frame to ensure accurate, consistent part creation. ACE’s robust structure is complemented by stiff connection points between moving machine members, which provide a system that reduces the total deflection throughout the forming cycle. The result is a process that is more precise and accurate, with predictable part quality. Superior control: Stretch forming operates within the material conditions between the yield point and its ultimate strength. Commonly, high temper materials have a narrow operating band to form above the yield point and do not exceed its ultimate strength. Precise motion and force control of the process is a vital consideration in machine selection if you expect to consistently achieve repeatable part production. According to Gardner: “A perfectly envisioned


stretch forming cycle would include the ability for the machine to monitor the strain rate within the part as force increases along its axis, capture the strain when the part transitions slightly above the yield point of the material, and form the part with nearly constant strain throughout the forming cycle.” Control system upgrades: Most of the


sophistication in today’s stretch forming machines involves control system improvements for axes positioning systems, hydraulic force control, part programming, and ease of use for operators. In ACE’s stretch forming presses, for instance, the OEM simplifies operation by incorporating touchscreen, menu-based Human Machine Interfaces (HMIs) to automate the forming cycle. Utilisation of the HMI approach facilitates


automated production, reliability and consistency while greatly reducing broken or out-of-tolerance


parts. Modern, high quality control systems for stretch forming machines include automatic yield point detection systems, the ability to automate the programming process, monitor the forming process, contain machine diagnostic protocols, and have advanced safety systems for operator protection. Spring back can be a problem when forming


tempered parts, however the company’s software predicts these reactions based on actual material properties at the time of forming. Spring back values are used to make virtual die geometry adjustments and the feasibility study is duplicated until simulations indicate accurate parts are produced. “Certain types of alloyed material, particularly some grades of extruded aluminium, will age harden at room temperature in a relatively short amount of time. Processing these parts directly from the annealing furnace, before age hardening occurs, will improve the consistency of the formed profile. Age hardening after they have been formed usually will not affect the dimensional stability of a properly racked part,” said Gardner. Forming aluminium closer to the ‘0’ temper


condition (or annealed condition) results in a consistent process that will more closely adhere to the net shape of the die, according to Gardner. “A lot of our clients will form tempered aluminium extrusions, say up to a T4 temper condition, but tempered material retains some amount of residual stress during the forming process and the die must be designed to accommodate some degree of spring back. So, you will have some spring back when forming tempered material, but certainly a lot less than witnessed in normal bending operations,” Gardner explained. To facilitate a faster, higher rate production,


ACE offers options for automated part loading/unloading and improved tool change systems in their stretch forming machines. In addition, to speed production for both short


and long production run manufacturers, the company can accommodate complete or partial automation for tooling or part changing systems.


Ajax/CECO/Erie www.AjaxErie.com


JULY/AUGUST 2023 DESIGN SOLUTIONS 59


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