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.

What is the Stretch Forming Process?

Stretch forming(also called stretch bending) is a versatile method used to shape materials into complex curves and shapes. The stretch forming process is a method of forming metal parts such as steel shapes, aluminum extrusions, and sheet metal, into permanently curved shapes of predetermined contour, the curved shapes are formed from sheet metal pressed, stretched, and bent over a die. Stretch forming can be used to create various shapes, including gentle curves, compound curves, and even sharp angles.

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.

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.

The 4 Pros of Stretch Forming

Stretch forming offers several advantages over other forming processes, such as stamping or roll forming. Here are some of the pros of stretch forming:

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.

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.
• 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.

4 cons of the stretch forming process

• The stretch-forming process is seldom suited to progressive or transfer operations. It is limited in its ability to form sharp contours and reentrant angles. The stretch forming process is at its best in forming shallow or nearly flat contours.
• If the piece is not pinched between mating dies, there is no opportunity to coin out or iron out slight irregularities in the surface of the metal.
• In some applications, especially in stretch wrapping, The stretch forming process is slower than competitive processes, and it is not suited to high-volume production. However, stretch draw forming with mating dies can be done as rapidly and automatically as conventional press operations. In fact, punch presses are used with dies incorporating draw beads or other means of gripping the blank in order to perform some stretch-forming operations.
• Metals with yield strength and tensile strength very nearly the same, such as titanium, necessitate the use of automatic equipment for determining the amount of strain for uniform results.

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.

Stretch forming calculations and formulas

Stretch forming involves the controlled stretching of a material to achieve a desired shape. While there isn’t a single universal formula for stretch forming due to the complexity of the process and the variability in materials and shapes, there are several key parameters and equations that can be used to calculate specific aspects of the process.

• Stretch Ratio (λ): The stretch ratio is the ratio of the final length (L_f) to the initial length (L_i) of the material. It is calculated as follows:
  λ = L_f / L_i
• Strain (ε): Strain measures the deformation of the material during stretching and is defined as the change in length (ΔL) divided by the original length (L_i):
  ε = ΔL / L_i
• Forming Force (F): The forming force required for stretch forming depends on factors like material properties, stretch ratio, and the shape being formed. The specific equation for calculating forming force can be complex and often requires experimental data or finite element analysis (FEA).
• Blank Size (B): The initial blank size can be calculated based on the desired final dimensions and the stretch ratio. The relationship between the initial blank size, final size, and stretch ratio is not a simple linear equation and may require iterative calculations.
• Die and Tooling Design: The design of the die and tooling involves considerations of the part’s geometry, material properties, and desired deformation. Calculations for die radius, die opening, and clamping force can be derived based on specific design requirements.
• Material Selection: Material selection involves assessing material properties such as ductility, yield strength, and elongation to determine its suitability for stretch forming. The choice of material can significantly impact the stretching process.

Euler-Bernoulli theory calculations and formulas

In general, the Euler-Bernoulli theory can be used to calculate the tensile and bending stress of the profile.

The tensile bending stress σ can be calculated by the following formula: σ = M * y / I

Among them, M is the bending moment, y is the distance from a certain point on the cross section of the profile to the neutral axis, and I is the cross-sectional moment of inertia of the profile.

In addition, the section moment of inertia I of the profile can be calculated by the following formula:

I = (b * h^3) / 12

Among them, b is the width of the profile, and h is the height of the profile.

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.