Achieving a perfect bend is rarely the initial outcome. Several defects can arise during the tube bending process:

• Wrinkling: These are folds or ripples that typically appear on the inner radius of the bend where the material is compressed. They indicate insufficient support or improper pressure settings.
• Flattening: This refers to the distortion of the tube's cross-sectional shape, where it becomes oval or even collapses. It often occurs when the outer radius stretches excessively without adequate internal support.
• Buckling: A more severe form of deformation characterized by a localized collapse or bulging of the tube wall, usually occurring on the outer radius due to excessive compressive forces.
• Wall Thinning: While some thinning on the outer radius is unavoidable due to material stretching, excessive thinning can compromise the structural integrity of the tube.
• Cracking: This is a fracture or tear in the tube material, usually occurring on the outer radius where tensile stresses are highest. It can be caused by exceeding the material's ductility or using incorrect tooling.
• Excessive Springback: As mentioned earlier, while some springback is expected, excessive amounts can lead to out-of-tolerance bends.
• Over-bending: This occurs when the bend angle exceeds the intended specification, often due to miscalculations in springback compensation or incorrect machine settings.

While the fundamental principles are similar, pipe bending often presents challenges related to material stress, ovality (a form of flattening), and achieving consistent bend angles. Pipes, often having thicker walls and larger diameters than tubes, require more force to bend. This increased force can lead to higher residual stresses within the bent pipe, potentially weakening its structural integrity over time. Maintaining a circular cross-section (minimizing ovality) and achieving the desired bend angle consistently are crucial for ensuring proper fit and functionality in piping systems.

A fundamental guideline in tube bending is the 3D bend radius rule. This suggests that the minimum recommended bend radius should be at least three times the outer diameter of the tube (3D). Adhering to this rule generally helps to prevent common defects like wrinkling and flattening, especially for less ductile materials or tighter bends. However, this is a general guideline, and specific applications and materials may allow for tighter bends with appropriate tooling and techniques.

To reiterate, common defects of bending across various materials include:

• Cracking: Fractures in the material, often on the tension side.
• Thinning: Reduction in material thickness, particularly on the outer radius.
• Wrinkles: Compressive folds on the inner radius.
• Excessive Springback: Greater than anticipated recovery of the original shape.
• Over-bending: Bending beyond the specified angle.
• Flattening/Ovality: Distortion of the circular cross-section.

These defects often arise from a combination of factors, including incorrect tooling selection, improper machine settings (like pressure and speed), inadequate lubrication, and the inherent properties of the material being bent.

Preventing wrinkles is crucial for achieving high-quality bends. Several strategies can be employed:

• Use a Mandrel: A mandrel is an internal support tool inserted into the tube during bending. It provides internal support, preventing the inner radius from collapsing and forming wrinkles. Different types of mandrels exist, such as plug mandrels, ball mandrels, and link mandrels, each suitable for different bending requirements.
• Adjust Pressure Die Settings: The pressure die, located opposite the bending die, applies counter-pressure to the tube. Optimizing the pressure and position of the pressure die is essential for controlling material flow and preventing wrinkling.
• Ensure Proper Lubrication: Applying the correct type and amount of lubricant between the tube and the tooling reduces friction, allowing the material to flow more smoothly and minimizing the risk of wrinkling.
• Use Suitable Material Thickness: For tighter bends or less ductile materials, using a tube with a thicker wall can provide more resistance to wrinkling.
• Optimize Clamp Force: The clamp secures the tube to the bending die. Insufficient clamp force can allow the tube to slip, leading to wrinkles. However, excessive force can also cause damage.

The minimum bend radius achievable without causing significant defects is highly dependent on several factors, including the material's ductility, the tube's diameter and wall thickness, and the bending method and tooling used. Generally, the minimum bend radius is often expressed as a multiple of the tube's outer diameter (D). While the 3D rule is common, tighter bends down to 1.5D to 2D are often possible with appropriate techniques, such as using mandrels and advanced bending machines. However, attempting bends tighter than the material's capacity will likely result in defects like flattening or cracking.

Aging lines, also known as Lüders bands or stretcher strain marks, are visible surface deformations that can appear in some ductile materials like low-carbon steel after forming processes, including bending. These lines are caused by discontinuous yielding of the material. To minimize or prevent aging lines:

• Controlled Cooling: After hot forming processes, controlled cooling can help to prevent the formation of these lines.
• Uniform Heat Application: In processes involving heat, ensuring uniform heat distribution can reduce the likelihood of localized yielding.
• Material Conditioning (Temper Rolling or Skin Passing): Before bending, subjecting the material to a slight amount of plastic deformation through temper rolling or skin passing can eliminate the yield point elongation responsible for aging lines.
• Using Stabilized Steel: Certain grades of steel are specifically produced to be non-aging.

Line bending is a specific technique used to create sharp, angled bends in thermoplastic sheets by applying localized heat along a line. Common plastics suitable for this process due to their thermoforming properties include:

• Acrylic (PMMA - Polymethyl methacrylate): Known for its clarity and ease of bending.
• Polycarbonate (PC): Offers high impact resistance and good bendability.
• Polyvinyl Chloride (PVC): A versatile and cost-effective option for various applications.

The choice of plastic depends on the desired properties of the final product, such as clarity, strength, and flexibility.

Calculating the required length of the tube before bending to achieve the desired final dimensions involves considering several factors, most notably the bend deduction. The bend deduction is the amount of material that is effectively shortened during the bending process. A simplified formula for bend deduction is often used:

Bend Deduction (BD) = (π/180) * Bend Angle (α) * (Bend Radius (R) + K * Material Thickness (T))

Where:

• α = Bend angle in degrees
• R = Inside bend radius
• T = Material thickness
• K = K-factor (a material-dependent factor, typically ranging from 0.3 to 0.5 for common metals)

More complex calculations may involve considering the neutral axis shift and other factors for high-precision applications. Specialized tube bending software often automates these calculations.