What causes wrinkles on the surface of a pipe after it has been processed by a pipe bending machine?

Wavy wrinkles on the inside of a pipe bend are one of the most common quality defects in pipe bending. Wrinkles not only affect the appearance but also reduce the effective wall thickness of the pipe and weaken its structural strength. The following analysis examines the main causes and solutions from three dimensions: Process, mold, and materials.

1. Improper adjustment of anti-wrinkle die

Cause: The gap between the anti-wrinkle die and the bending die is too large, causing the inner side of the pipe to lose effective support, resulting in material instability and wrinkling under compressive stress.

Solution: Adjust the anti-wrinkle die so that it fits snugly against the groove of the bending die. The gap should be such that an a4 sheet of paper can slide along it without loosening. Also, ensure that the leading edge of the anti-wrinkle die is aligned with the tangent point of the bending die.

2. Mandrel not used or incorrectly positioned

Cause: For thin-walled pipes or small-radius bends (r/d≤3), a mandrel is not used, or the mandrel extension is insufficient, or the ball head is positioned too far back, causing the inner side of the bend to lose internal support.

Solution: Select a ball head mandrel or a rigid mandrel according to the pipe diameter and wall thickness, and adjust the mandrel extension position to 0.5-1.5mm before the tangent point of the bending die.

3. Insufficient bending radius

Cause: The ratio of the bending center radius (r) to the pipe's outer diameter (d), r/d, is too small (usually below 2.5-3 times), exceeding the material's compressive stability limit.

Solution: Increase the bending radius or use a thicker-walled pipe. If a small-radius bending is necessary, a combined process using a mandrel, anti-wrinkle die, and filling medium (such as a bending spring) should be employed.

4. Thin pipe wall or soft material

Cause: Thinner walls result in poorer resistance to instability. Soft materials (such as annealed copper or aluminum) are prone to wrinkling on the compression side.

Solution: Replace with a thicker-walled pipe or use a harder material. For soft materials, appropriately reduce the bending speed to minimize material flow unevenness.

5. Excessive bending speed

Cause: Excessive bending angular velocity prevents the material inside the pipe from stabilizing and flowing, leading to rapid localized wrinkling.

Solution: Reduce the bending speed, especially at the beginning of the bending process. For cnc pipe bending machines, a speed-changing curve can be set—low speed for initial bending, constant speed in the middle, and low speed for final bending.

6. Die wear or poor surface finish

Causes: Wear or accumulation of chips and rust on the working surfaces of the bending die and anti-wrinkle die leads to uneven friction, and local resistance differences induce wrinkles.

Solutions: Inspect the die surface, repair or replace worn dies. Clean the die and apply special pipe bending oil to maintain adequate lubrication.

7. Insufficient clamping force or eccentricity

Causes: Insufficient clamping die pressure, or misalignment between the clamping die and the bending die, causing the pipe to slide or shift during bending, resulting in uneven force on the inner side.

Solutions: Check the clamping cylinder pressure to ensure it reaches the set value. Calibrate the alignment accuracy of the clamping die and the bending die.

Quick troubleshooting approach

When wrinkles appear, check in the following order:

1. Check the anti-wrinkle die clearance → 2. Confirm the mandrel position → 3. Calculate whether the R/D is too small → 4. Check the wall thickness and material condition → 5. Test at a reduced bending speed → 6. Check die wear and lubrication

Most wrinkling problems stem from two core causes: insufficient support and compression instability. Identifying the root cause will effectively resolve the issue.