Sheet Metal and Aluminum Extrusions Stretch Bending

Stretch Forming Process

Stretch forming/bending can be used to create various shapes, including gentle curves, compound curves, and even sharp angles. It makes it ideal for creating structural components with precise dimensions and contours that can be used with a variety of metals, including aluminum, steel, and titanium.

This article contains all the information you need to know about the Stretch Forming Process. Read further and learn more about:

Below you will learn:

• What is the Stretch Forming Process?
• Terminologies used in Stretch Forming
• Mechanics of Stretch Forming
• Types of Stretch Forming
• And much more…

What is the Stretch Forming Process?

The stretch forming process is where the profile material is pressed into the mold groove under the combined action of bending moment and longitudinal tension. 
When a material is stretch bent, the stress state within its deformation zone varies in different parts. The non-deforming part in the middle of the cross-section is called the neutral layer. The metal outside the neutral layer is subjected to tensile stress, resulting in elongation deformation. The metal inside the neutral layer is subjected to compressive stress, causing compression deformation. Due to the opposite directions of stress and strain in the metal on either side of the neutral layer, when the load is removed, the elastic deformation recovery direction of the metal on either side of the neutral layer is opposite, leading to varying degrees of springback. 
Although the stretch forming deformation is confined to a localized area of the material, the springback effect can influence the accuracy of the bent parts. There are many factors affecting springback, and these factors are difficult to control. The precision issues caused by springback in stretch bent parts have always been a critical aspect of stretch bending production.

Stretch forming of profile materials is a process where the material is pre-stretched to its yield limit while simultaneously subjected to bending with a certain axial tension. The stretch bending process applied to aluminum profiles offers rational allowances for shape correction in parts, enhancing manufacturing precision, and reducing production costs.

Top 4 Key Features of Profile Material Stretch Forming

• No Rebound or Residual Stress: The process ensures high precision and repeatability due to the absence of rebound and residual stress.
• Formation of Complex Shapes: It is capable of forming complex two-dimensional or three-dimensional shapes.
• Reduction in Assembly Operations: Stretch forming reduces the need for additional assembly operations.
• No Deformation after Welding or Machining: Parts remain undeformed after welding or machining processes.

Stretch forming of aluminum profiles plays a crucial role in industrial production, especially in the manufacturing of large-sized, high-precision, and relatively large-radius curved extrusions and sheet-bent profile material.

Wide Applications of Stretch Forming Process

• Aerospace Industry: Stretch forming is widely used in the manufacturing of profile components in the aerospace industry.
• Civil Applications: In civil applications such as aviation and high-speed rail, many skeletal components adopt profile frame structures.
• Home Appliance Manufacturing: In the manufacturing of home appliances, stretch forming is employed for producing components like door frames for refrigerators and drums for washing machines.
• Construction Industry: Stretch forming is used for producing various steel structural components in the construction industry, including steel beams and columns.

Stretch forming of profile materials provides an efficient and precise solution for manufacturing, applicable to various industries and applications. The forming process not only improves manufacturing efficiency but also achieves highly accurate control over the shape, offering reliable technological support for component manufacturing across diverse industries.

Top 7 Advantages of Stretch Forming

One of the key advantages of stretch forming is its ability to create complex, three-dimensional shapes with a high degree of accuracy and repeatability. It is particularly useful for producing curved and contoured parts such as aircraft fuselage sections, automobile body panels, and architectural components like curved facades.
Stretch forming offers several benefits, including reduced material waste, as it minimizes the need for cutting and welding, and it can produce parts with a smooth and consistent surface finish. Additionally, it can accommodate a wide range of materials, including aluminum, steel, and titanium.

The stretch forming process offers a set of advantages over roll bending, particularly when dealing with the forming of large hollow profiles, while ensuring the preservation of surface integrity. Stretch forming is well-suited for projects that demand tighter radii, special curves, and even 3D forming.

• Handling Large Hollow Profiles: Stretch forming excels in bending large, hollow profiles or sections. It is capable of accommodating the substantial dimensions of such materials, making it ideal for applications in construction, architecture, and other industries where large structures are involved.
• Preservation of Surface Integrity: Stretch forming minimizes the risk of surface damage or imperfections during the bending process. This is particularly important when working with materials that require a smooth and undamaged surface finish, such as aluminum or stainless steel used in architectural components.
• Tighter Radii: Stretch forming can achieve smaller bend radii compared to roll bending. This is crucial for projects that require precision in the curvature of materials, like those in the automotive and aerospace industries where the bending of tubes and components demands tight radii.
• Special Curves: The flexibility of stretch forming allows for the creation of special curves and custom geometries. It is well-suited for applications where standard bend radii do not meet the project’s specific requirements, such as in artistic or architectural designs.
• 3D Bending: Stretch forming is capable of producing three-dimensional bends, enabling the creation of complex and intricate shapes. This is particularly valuable in industries where unique and custom designs are common, such as in art installations, sculpture, or complex architectural elements.
• Capabilities: Stretch bending is particularly effective at forming complex, three-dimensional shapes with a high degree of accuracy. It can handle tight curves and shapes that have varying radii along the bend.
• Materials: It can work with various materials, including metals like aluminum, steel, and titanium.
• Precision: Stretch bending offers precision and can meet tight tolerances. It is ideal for applications where dimensional accuracy is crucial.

Versatility

Stretch forming can be used to create a wide variety of shapes and geometries, from simple curves to compound curves and sharp angles.

• Material Diversity: It can be applied to a wide range of materials, including metals like aluminum, steel, and stainless steel, as well as plastics, allowing flexibility in material selection based on specific requirements.
• Complex Shapes: Stretch forming can create intricate and complex shapes, making it suitable for producing a diverse array of components with varying geometries.
• Multiple Industries: Its adaptability extends across industries, including aerospace, automotive, architectural, and marine, showcasing its versatility in fulfilling the needs of different sectors.
• Variable Sizes: Stretch forming is applicable to both small and large components, accommodating a broad size spectrum, from small intricate parts to large structural elements.
• It can realize the stretch bending forming of profiles with multiple arc segments and variable curvature.
• The arc bending accuracy is high, the material rebound is stable, and the workpiece size is consistent.
• It can effectively eliminate the residual stress inside the material, and the product has good dimensional stability.
• Due to the cold work hardening of metal materials, the mechanical properties of the material can be improved after stretching and bending.

Precision

Stretch forming produces highly accurate and consistent shapes, making it ideal for high-tolerance applications.

• High Accuracy: Stretch forming achieves exceptional dimensional accuracy, ensuring that formed parts conform precisely to design specifications.
• Tight Tolerances: The process can consistently meet tight tolerances, crucial in industries like aerospace and automotive, where precision is paramount.
• Uniformity: Stretch forming produces parts with consistent shapes and dimensions across production runs, minimizing variations and ensuring part interchangeability. Uniform Wall Thickness: Stretch forming helps maintain a uniform wall thickness in the material, ensuring structural integrity and consistent product quality.
• Minimal Wrinkling and Distortion: The process minimizes wrinkling and distortion of the material, resulting in smooth and aesthetically pleasing finished products.
• Enhanced Material Strength: Stretch forming can improve the mechanical properties of certain materials, making them stronger and more durable.
• Minimal Material Distortion: Stretch forming minimizes material distortion, preserving the integrity of the formed parts and reducing the need for post-processing.
• High Surface Quality: The process yields parts with a high-quality surface finish, eliminating the need for additional surface treatments or refinishing.
• Reproducibility: Stretch forming’s repeatability ensures that identical parts can be produced consistently, maintaining quality and performance standards.

Efficiency

Stretch forming can produce complex shapes in a single step, reducing the need for multiple forming operations.

• Material Savings: The process minimizes material waste by stretching and shaping the material precisely where needed, reducing overall material consumption and cost.
• Quick Production: Stretch forming can produce parts efficiently, as it often requires fewer production steps, tool changes, and post-processing compared to alternative forming methods.
• High Production Rates: It is well-suited for high-volume production due to its efficiency and the potential for continuous, automated operations.
• Tooling Adaptability: Stretch forming can accommodate different tooling setups, allowing for rapid adaptation to varying part requirements without extensive tooling changes.
• Minimal Post-Processing: The precision achieved during stretch forming often eliminates the need for extensive post-processing, further streamlining production and reducing labor and time costs.
• Quality Assurance: The consistency of the process ensures that each part meets design specifications, reducing the need for inspection and rework.
• Cost-Effective: By optimizing material use, reducing waste, and enhancing production efficiency, stretch forming offers cost-effective solutions for industries requiring precision-formed components.

Strength

Stretch forming results in a material with higher strength and durability than other forming processes.

• Improved Material Properties: Stretch forming can enhance the mechanical properties of materials, such as increasing tensile strength and reducing material weaknesses.
• Structural Integrity: It creates components with excellent structural integrity, ensuring that formed parts can withstand mechanical stresses and maintain their shape under load.
• Uniform Thickness: Stretch forming helps maintain uniform material thickness across the part, contributing to consistent strength and durability.
• Suitable for Structural Components: In industries like aerospace and automotive, where strength is critical, stretch forming is favored for producing robust structural elements.

7 cons of the stretch forming process

1. Limitation on Cross-Sectional Size: The size and shape of parts that can be stretch-formed are limited by the tonnage of the equipment and the size of the jaws.
2. High Investment Costs for Tooling: Custom molds and tooling for stretch forming can be expensive, especially when compared to other metal forming processes.
3. Limited Mold Versatility: Stretch-forming molds are often designed for specific applications, limiting their adaptability.
4. Challenges with Asymmetric Profiles: Stretch forming can be problematic when working with asymmetric profile sections.
5. Material and Energy Costs: The stretching process requires significant force and energy, leading to higher operational costs. Material wastage, especially with expensive or rare materials, can be a concern.
6. Limitations in Forming Sharp Contours and Reentrant Angles: Stretch forming is not well-suited for parts with sharp contours or reentrant angles, making it less suitable for complex geometries.
7. Inability to Correct Surface Irregularities: Without pinching between mating dies, there’s no opportunity to correct slight surface irregularities in the material, potentially leading to imperfections in the final product.

Work Principle

Stretch forming is a very complicated process that involves high accuracy.

If a part were to be curved without first being stretched, two natural forces would occur during the process. First, one-half of the part would naturally be stretched. Secondly, the other half would go into compression, and the neutral axis would be the division between the two. Stretching the material compensates for the compression that would normally take place on the inner surface of the part, which, in turn, eliminates the wrinkling or twist. It could be said that a stretch-forming machine bends parts into curved shapes, but does so while pulling the wrinkles out of the part before they can occur.

In this combination, of bending and stretching, the outer fibers of the part being formed experience an elongation stretch sufficient enough to put this fiber into yield i.e. it has been stretched beyond its ability to return to its original length after the stretching force has been relaxed. Therefore, the parts remain close to the shape of the form block after it’s been removed from the stretch forming machine.

The stretching action taking place in the stretch-forming process not only eliminates wrinkling but also helps to eliminate the “spring-back”, of the part from the desired finished contour. Although it is a cold form of bending, heat can be introduced in the stretch forming process to allow for the bending of exotic alloys.

14 Stretch Forming Terminologies

• Die: A tool or mold used in stretch forming, which defines the final shape that the metal will take. Dies are typically custom-made to match the specific geometry of the part being formed.
• Stretching Force: The force applied to the metal sheet or part to stretch it over the die.
• Bend Radius: The radius of curvature that the metal sheet or part will achieve during stretch forming. It is an essential parameter for defining the desired shape.
• Tension: The state of stress in the metal where it is being stretched during the process. Tension is a critical factor in achieving the desired shape without material failure.
• Compression: The state of stress in the metal where it is compressed against the die during stretch forming. Compression occurs on the opposite side of the material from the tension.
• Wrinkles: Unwanted creases or folds that may form on the surface of the metal during stretch forming. Minimizing wrinkles is a key challenge in the process.
• Springback: The tendency of a material to partially return to its original shape after being stretched.
• Clamping: The method of securing the edges of the metal sheet or part to prevent movement during the stretch forming process.
• Material Thickness: The thickness of the metal sheet or part being used in stretch forming.
• Work Hardening: The process by which a metal becomes harder and less ductile as it is stretched or bent. Work-hardened materials may require different forming strategies.
• Form Block: A solid component that supports the metal sheet or part during stretching. The form block helps maintain the shape of the part and reduce the risk of wrinkles.
• Forming Lubricant: A lubricating material applied to the surface of the metal to reduce friction during stretching and prevent scratching or damage to the material.
• Hydraulic Stretch Forming: A stretch forming process that uses hydraulic pressure to achieve the required stretching force.
• Mechanical Stretch Forming: A stretch forming process that employs mechanical means, such as screws or ratchets, to apply the necessary stretching force.

Mechanics of Stretch Forming

The mechanics of stretch forming involve various physical changes per area that a tube experiences during bending. These changes depend on the bending technique used and the properties of the tubing material. Here are the key mechanical aspects:

• Outer Wall Thinning: During bending, the outer side of the tube undergoes tensile forces, leading to wall elongation and thinning.
• Inner Wall Wrinkling: The inner side of the tube experiences compressive forces, resulting in wall wrinkling and thickening.
• Ovality: Ovality refers to the distortion of the tube’s cross-section from its original round shape after bending. It occurs due to unbalanced forces, particularly in unsupported tube internals. Ovality may be acceptable in some applications, but industries with strict dimensional requirements must control it.
• Wall Factor: The wall factor represents the relative wall thickness, calculated as the ratio of the tube’s outer diameter to its wall thickness. It categorizes tubes as “thick-walled” or “thin-walled.” Tubes with lower wall factors are easier to bend because they require less material stretching.
• D of the Bend: “D of the bend” is a term used in tube fabrication to describe the ratio of the bend’s center line radius (CLR) to the tube’s outer diameter. A higher D of the bend indicates easier bending for tight radii. The ideal scenario for unsupported bends involves a low wall factor and a high D of the bend to prevent outer wall collapse.
• Elongation: Elongation measures a material’s ability to stretch before fracturing. The D of the bend and material properties, such as tensile strength and wall thickness, influence elongation. For example, stainless steel exhibits a higher elongation percentage compared to mild steel.
• Springback: Springback occurs when a bent tube attempts to return to its original flat shape, resulting in a slightly smaller bend angle. To compensate for this, operators may intentionally “overbend” to achieve the desired bend angle. Springback is influenced by material stiffness, tensile strength, wall thickness, type of tooling, and bending technique. Harder materials and smaller CLR values lead to more significant springback.

How does Stretch Forming Work?

Stretch forming is a process that uses a hydraulic or mechanical press to stretch a metal sheet or extrusion over a die. The die, which is made of a hard material such as steel or aluminum, has a shape that is the inverse of the desired finished product. The metal sheet is then clamped tightly around the die using grippers, and the press applies pressure to stretch the sheet, forming it into the shape of the die.

• Material Preparation: The process begins with a sheet of metal or other material, which is often pre-cut to the desired shape. The material’s thickness and type depend on the specific application and part requirements.
• Clamping and Fixturing: The sheet is securely clamped and held in place by a set of dies or molds. These dies are designed to shape the material into the desired form when pressure is applied.
• Tension and Compression: Stretch forming involves both tension and compression forces. A stretching force is applied to the sheet in the areas where elongation is required, while compressive forces are applied to areas that need to be compacted.
• Hydraulic or Mechanical Pressure: Hydraulic or mechanical systems apply controlled pressure to stretch and deform the material. This pressure is precisely adjusted to achieve the desired shape without causing excessive thinning or wrinkling.
• Gradual Forming: The forming process is typically gradual, with the material being incrementally stretched or compressed until it conforms to the shape of the dies.
• Quality Control: Throughout the process, quality control measures, such as monitoring material thickness and shape accuracy, are employed to ensure that the formed part meets the required specifications.

Host Of Three-Dimensional Stretch Forming Machine

The main frame of the three-dimensional bending equipment consists of a welded body frame structure installed on the foundation:

• The main frame is used to support the rotatable stretching rocker arm 2 and the hydraulic cylinder. A working platform for placing molds is installed on the top of the frame.
• The two stretching cylinder brackets are respectively installed on the upper part of the rocker’s arm and are electrically positioned through an electrically driven spiral guide rod to adapt to workpieces of different lengths. Each rocker’s arm is equipped with a tensioning cylinder.
• The twisting of the part is effected by a hydraulic motor with a gearbox mounted behind the stretching cylinder.
• The stretching cylinder is mounted on the bracket via a universal joint that allows the clamp jaws to rotate freely forward or backward.
• The clamp lifting cylinder realizes the gradual lifting of the stretching cylinder during the stretching process. The clamp-pitching cylinder can make the stretching cylinder pitch up and down. All the actions of the kinematic pair on the bracket (stretching-lifting-pitching-rotation) make the workpiece During the entire forming process, the three-dimensional mold surface is subjected to tangential stretching to form a three-dimensional three-dimensional workpiece.

Equipment structure and equipment working principle

The key technology of 3D bending is mainly mold design. The 3D bending machine will not give an ideal 3D bending program. Process designers need to conduct systematic analysis or CAE finite element analysis based on the material properties and bending forming, and continuous The trial production of workpieces requires process technicians to have high product research and development capabilities to achieve the best match between the three-dimensional bending mold and the three-dimensional bending program and meet the technical requirements of the workpiece.

Spatial three-dimensional workpieces have many applications in high-speed train front structural parts and aircraft manufacturing. The products have high technical content, high mold investment costs, long research and development cycles, and high added value of the products.

Materials Used in Stretch Forming

The stretch forming process is commonly used in the curving of aluminum sheets, extruded profiles, stainless sheets, bent or rolled profiles, structural shapes, various steel alloys, brass, bronze, copped, titanium, and even Inconel. It allows the curving of true radius contours, ellipses, compound bends, and helical bends and can include straight legs.

Stretch forming can be used with a variety of materials, including aluminum, steel, and titanium. These materials are often used in industries such as aerospace, automotive, and construction due to their high strength-to-weight ratios and durability.

Here are some of the materials that can be used in stretch forming:

1. Aluminum: Aluminum is a lightweight and corrosion-resistant material that is commonly used in aerospace and automotive applications.
2. Steel: Steel is a strong and durable material with high tensile strength. It is commonly used in the construction industry for applications such as bridge building and structural framing.
3. Titanium: Titanium is a lightweight and high-strength material that is commonly used in aerospace applications. It has excellent corrosion resistance and can withstand high temperatures.

What are the four methods of stretch forming?

1. Stretch draw forming: In this method, a flat sheet of metal is clamped at its edges and then pulled over a form or die, which stretches the metal into the desired shape.
2. Stretch wrapping, also called rotary stretch forming: In this method, a metal tube or extrusion is wrapped around a form or mandrel and then pulled or rolled over a series of rollers to stretch the material into the desired shape.
3. Compression forming: In this method, a metal sheet is clamped between two dies and then compressed to form the desired shape. This process can be done either hot or cold, depending on the material.
4. Radial draw forming: In this method, a flat sheet of metal is clamped at its edges and then drawn down into a form or die by a punch. The metal is stretched and formed into the desired shape as it is pulled into the die.

Curvature Design Principles for Stretch Forming Workpieces

What are the different types of stretch forming?

Longitudinal and transverse are the two basic types of stretch forming machines. Longitudinal stretch formers stretch the workpiece along its length, while transverse stretch formers stretch the workpiece along its width.

In both types of stretch forming machines, the jaws or grippers hold and stretch the metal sheet or extrusion while it is formed over the die table. The die table provides the shape for the metal to be stretched and formed over, and the hydraulic system provides the force needed to control the stretching process.

Two types of applications

Sheet Stretch Forming process

Sheet stretch forming is a manufacturing process used to shape sheet metal into complex three-dimensional forms. It involves stretching a sheet of metal over a die, causing it to deform and take on the shape of the die.

Sheet Stretch Forming is commonly used by aircraft builders to manufacture fuselage skin sections from special aerospace aluminum alloy sheets. During the metal stretch forming process (also known as ‘wrap forming’), in which a sheet of metal is wrapped around a mandrel or a form, using tension to create a three-dimensional shape. Stretch forming(wrap forming) typically involves wrapping the metal over the form and then pressing the metal onto the mold using a mechanical or hydraulic press. This process is commonly used in the aerospace industry to create complex curved or contoured parts for aircraft and spacecraft.

The process typically begins with a flat sheet of metal, which is clamped around its edges and held in place. The sheet is then forced to stretch and deform by applying controlled forces or pressure. This stretching causes the metal to thin out in some areas and stretch in others, allowing it to conform to the shape of the die.

Which metals can be used for the sheet stretch forming process

• Any ductile metal can be shaped by this method.
• The process is mainly applied to aluminum alloys for aircraft skins.
• Magnesium alloys are stretch-formed while hot.
• Stainless steel and titanium are stretch-formed on a commercial scale.
• Thinning and strain hardening are inherent in the processes. It is important to know the elongation values for the metal being used. Large elongations are best. Thickness reduction should not exceed 5% of the original thickness.

Extrusion stretch forming process

Extrusion stretch forming press designs for contouring aluminum aerospace alloys in the form of extrusion roll-formed profiles, or press braked shapes. These profile stretch presses are sometimes called stretch wraps or swing-arm presses. The basic press design has two arms or carriage beams that hold multiple-positioning gripping jaws. The jaws are attached to hydraulic tension cylinders that provide the stretch of the workpiece. The arms swing by rotating on large machined pins with bearings, thus allowing the workpiece to wrap around and against the forming die.

What is the difference between stretch forming and bending?

In stretch forming, the material is clamped at its edges and stretched over a form or die, which causes the material to thin and stretch in the direction of the stretching force. This can result in a smoother surface finish and tighter tolerances, particularly for complex shapes or parts with large radii.

In bending, a force is applied to the material to create a desired angle or shape. The material is not stretched, but instead undergoes compression on the inside radius of the bend and tension on the outside radius. The minimum bend radius for bending is typically smaller than for stretch forming, and the process can be used on a wider range of materials and geometries.

• Process: In the stretch forming process, a metal sheet or strip is clamped at the edges and then stretched over a die or form. This process can be done in a single pass or multiple passes. In contrast, bending involves applying a force to a metal sheet or tube to create a desired angle or shape.
• Material: Stretch forming is typically used for more ductile materials, such as aluminum, while bending can be used on a wider range of materials including steel, stainless steel, and aluminum.
• Complexity: Stretch forming is often used to produce more complex shapes and curves, while bending is better suited for simpler geometries and angles.
• Precision: Stretch forming is known for its high precision and accuracy, making it ideal for aerospace and automotive applications where tight tolerances are required. Bending, on the other hand, may have more variation in the final product due to factors such as springback and material thickness.

Stretch forming applications

Almost any shape that can be produced by other sheet-forming methods can be produced by stretch forming.
Stretch forming is used to form aerospace parts from steel, nickel, aluminum, and titanium alloys and other heat-resistant and refractory metals. Some of these parts are difficult or impossible to form by other methods, for example, the titanium alloy gas-turbine ring.
Stretch forming is also used to shape automotive body panels, both inner and outer, and frame members that could be formed by other processes but at a higher cost.

What is stretch forming used for?

Typical stretch-formed parts are large curved panels such as door panels in cars or wing panels on aircraft. The variety of shapes and cross-sections that can be stretched and formed is almost unlimited. Window systems, skylights, storefronts, signs, flashings, curtain walls, walkway enclosures, and hand railings can be accurately and precisely formed to the desired profiles.. Other stretch-formed parts can be found in window frames and enclosures. Close and consistent tolerances, no surface marring, no distortion or ripples, and no surface misalignment of complex profiles are important benefits inherent in stretch forming. A smooth and even surface results from the stretch forming process. This process is ideally suited for the manufacture of large parts made from aluminum but does just as well with stainless steel and commercially pure titanium. It is quick, efficient, and has a high degree of repeatability.

Read More:

• 4 Steps for Stretch Forming Process
• Measuring Arched Openings for Stretch-Formed Metal

Summary

Stretch forming offers several advantages over other forming methods. It allows for the production of large, curved components with high precision and repeatability. It also minimizes material thinning and maintains the structural integrity of the metal, resulting in components with excellent strength-to-weight ratios. Additionally, stretch forming eliminates the need for multiple forming operations, reducing production time and costs.

Overall, stretch forming is a versatile and efficient process for shaping metal sheets and extrusions into complex, curved components, making it a valuable technique in various industries.