Tube End Forming Defects: Splitting vs Wrinkling

Tube end forming defects like splitting and wrinkling occur when the material’s mechanical limits are exceeded or when tool engagement is poorly controlled during expansion or reduction. Splitting indicates tensile failure where the material elongation limit is breached, whereas wrinkling indicates compressive instability due to improper clamping or excessive unsupported length during reduction. Resolving these issues requires a systematic analysis of material properties, tooling geometry, and machine parameters.

Why tube ends split during expansion

During expansion, the tube end is forced over a sizing punch, which increases the diameter of the tube. This process subjects the tube material to hoop stress and circumferential tensile strain. When the required expansion ratio exceeds the material’s maximum elongation capacity, the tube splits at the edge. This is particularly common in materials with high work-hardening rates or when using tubes with low-quality welded seams.

The weld seam is frequently the initiation point for splits. If the weld bead is not fully scarfed or if the heat-affected zone is brittle, the seam will fail under tension. To prevent splitting, the expansion ratio must be matched to the material’s annealed state properties. Additionally, the punch geometry plays a major role. A punch with a steep entry angle increases the rate of deformation, raising the risk of splitting. A shallower taper angle on the punch allows for a more gradual transition, distributing the tensile forces over a larger area during the stroke.

Why tube ends wrinkle during reduction

Reduction involves pushing the tube end into a die to decrease its outer diameter. This operation subjects the tube to axial and circumferential compressive stresses. If the tube wall is too thin relative to the diameter, or if the unsupported length of the tube between the clamp die and the reduction die is too great, the tube wall will buckle, resulting in wrinkles.

Wrinkling is a failure of structural stability under compression. If the clamping force is insufficient, the tube will slip backward into the clamp die during the reduction stroke, causing irregular folding at the transition zone. The clearance between the reduction die and the tube must be tightly controlled. Excess clearance allows the material to deflect outward or inward instead of flowing uniformly. To minimize wrinkling, the reduction should be performed in multiple progressive steps rather than a single aggressive stroke, especially when dealing with thin-walled materials.

Evaluating tooling alignment and lubrication

Concentricity between the clamping jaws and the forming punch or die is essential for preventing asymmetric defects. If the centerline of the machine ram is misaligned with the clamped tube, one side of the tube will experience higher localized stress. This misalignment causes premature splitting on one side during expansion, or localized wrinkling and flat spots during reduction.

Lubrication is another factor that directly influences material flow. Without adequate lubrication, high friction between the tooling and the tube surface restricts the material from sliding smoothly over the punch or into the die. This frictional resistance increases the tensile load during expansion, accelerating splitting, and increases the axial compressive load during reduction, promoting wrinkling. Applying a high-pressure drawing lubricant to both the inside and outside of the tube end reduces these frictional forces and ensures consistent material deformation. Tool wear must also be monitored; worn punches or dies with surface scoring will drag the material, leading to localized thinning and subsequent failure.

Selecting parameters on Vulcan end-forming machines

Optimizing the process on Boru Ucu Şişirme ve Büzme Makineleri (Vulcan) requires adjusting the stroke speed, clamping pressure, and sequencing. High-speed forming can cause rapid strain rates that exceed the material’s natural flow limit, leading to immediate cracking. Lowering the ram speed allows the material more time to yield plastically without fracturing.

For complex multi-step geometries, using a multi-station sequence is the most reliable method to prevent defects. For example, a severe expansion can be split into two separate stations with an intermediate annealing step if necessary. When integrating end-forming with bending operations performed on a CNC Boru Bükme Makinesi (Griffin), the sequence of operations must account for work hardening. Bending the tube first increases the hardness of the material near the bend, which can make subsequent end-forming more prone to splitting if the formed zone overlaps with the bend transition.

To obtain a precise tooling recommendation and process analysis, please provide a STEP or scaled drawing of the finished part, the material specification, the tube wall thickness, the desired end-forming geometry, and your estimated yearly volume.