By Iris Xu, Quality Engineer at ZTRFM | Last Updated: September 15, 2026
A distributor in Kenya sent us photos of corrugated sheets with wavy edges and asked if our machine was defective. It wasn't. The real problem was that they'd switched from 0.4mm SGCC to 0.3mm SPCC without adjusting the roller gaps. The thinner, softer material was being over-formed at the edges.
1. Wavy Edges (Edge Buckling)
What it looks like: The edges of the profile ripple or wave instead of running straight. Most common on wide, thin material (0.3-0.5mm).
Root cause: The edges are being compressed more than the center. This happens when roller gaps are too tight for the material thickness, or when the flower pattern (the progressive bending sequence) puts too much edge strain in early stands.
Fix: Open the roller gaps at stands 1-4 by 0.05-0.1mm. The material needs room to flow. If that doesn't work, check whether you're running material below the machine's minimum thickness spec -- some machines simply can't handle 0.3mm without edge waves.
2. Twist (Spiral Deformation)
What it looks like: The finished profile twists like a corkscrew along its length. Lay it flat on the floor and one end lifts off the ground.
Root cause: Uneven forming. One side of the profile is being bent more than the other. Usually caused by misaligned roller stands, worn bearings on one side, or a flower pattern that's asymmetrical.
Fix: First, check stand alignment with a straightedge across all stands. If stands are aligned, check bearing play on each stand by grabbing the roller shaft and pushing side to side -- any movement over 0.05mm means the bearing needs replacing. Finally, if the machine is mechanically sound, the flower pattern may need adjustment (this requires the original tooling engineer).
3. Springback (Profile Opens Up After Forming)
What it looks like: The angle or radius of the finished profile is wider than the roller geometry. A 90-degree bend comes out at 92-95 degrees.
Root cause: The metal "remembers" its flat shape and partially springs back after leaving the rollers. High-strength steels (SGC400, Q345) spring back more than mild steel (SGCC, Q235). Thicker material springs back more than thin.
Fix: Overbend. If the profile springs back 3 degrees, set the rollers to 3 degrees past the target angle. For high-strength steel, you may need 4-5 degrees of overbend. The last 2-3 stands should include a "finishing pass" that holds the final angle firmly. If springback suddenly increases on a machine that was running fine, check material certification -- the supplier may have substituted a higher-strength grade.
4. Surface Scratches and Marks
What it looks like: Linear scratches along the length of the product, or repetitive marks at regular intervals.
Root cause: Scratches come from damaged roller surfaces (nicks, rust, or built-up material). Repetitive marks at fixed intervals usually come from a damaged spot on one specific roller -- the distance between marks equals the circumference of that roller.
Fix: Identify which roller is causing the marks by measuring the mark spacing and dividing by pi to get the roller diameter. Inspect that roller under good light. Small nicks can be polished out with 600-grit paper (never use coarser). Deep grooves mean the roller needs regrinding. For PPGI (pre-painted) material, consider switching to chrome-plated rollers to prevent scratching the paint.
5. Dimension Drift (Profile Changes Over Time)
What it looks like: The first 500 meters of a run are perfect, then dimensions start creeping out of tolerance.
Root cause: Roller gaps open up as the machine warms up (thermal expansion), or bearings settle under load. Also common when switching from thick to thin material without recalibrating -- the rollers were pushed apart by the thick material and don't spring back fully.
Fix: Run 50-100 meters of scrap material at the start of each production batch to let the machine settle. Then check dimensions and fine-tune. If drift is severe (more than 0.3mm over a shift), the bearing housings may be worn and need replacement.
6. Oil Can (Oil-Can Effect / Center Buckle)
What it looks like: The center of a wide panel buckles up or down when pressed. Like pressing an oil can.
Root cause: The center of the strip is being stretched more than the edges during forming. Opposite of wavy edges. Common on wide panels (over 800mm) with stiff center profiles.
Fix: Add a slight crown (0.1-0.2mm higher in center) to the leveling rollers. Reduce the forming angle per stand in the middle of the flower pattern. In severe cases, a strip tension control system can help.
Quick Reference Table
| Defect | Most Likely Cause | First Fix to Try |
|---|---|---|
| Wavy edge buckling | Roller gaps too tight for material | Open gaps at stands 1-4 by 0.05mm |
| Twist / spiral | Misaligned stands or worn bearings | Check alignment with straightedge |
| Angle springback | Insufficient overbend for material | Increase overbend by 2-3 degrees |
| Surface scratches | Damaged roller surface or buildup | Polish roller with 600-grit paper |
| Dimension drift | Thermal expansion or bearing wear | Run 50m warmup scrap, recalibrate |
| Oil can buckle | Center stretched more than edges | Add crown to the leveling rollers |
When to Call the Manufacturer
If you've tried the fixes above and the defect persists, the issue may be in the tooling design itself (the flower pattern). This is not something you can fix on the floor -- it requires the original tooling engineer to review and modify the roller geometry. Contact your machine supplier with photos, material specs, and a sample of the defective product. A good supplier can diagnose the problem remotely.
On ZTRFM, you can find verified roll forming machine suppliers who provide after-sales technical support.
Frequently Asked Questions (FAQ)
How can wavy edges be corrected?
Open the roller gaps at stands 1-4 by 0.05-0.1 mm. If the problem continues, verify that the material is not below the machine's minimum thickness specification.
What causes a roll-formed profile to twist?
Common causes are misaligned roller stands, worn bearings on one side, or an asymmetrical flower pattern. Check stand alignment first, then bearing play.
How is springback corrected?
Overbend the profile by the amount it springs back. High-strength steel may require 4-5 degrees of overbend, and the final stands should hold the target angle in a finishing pass.
How can the roller causing repetitive surface marks be identified?
Measure the distance between marks and divide it by pi to estimate the responsible roller's diameter, then inspect that roller for nicks, rust, or buildup.
What should be done when dimensions drift during a run?
Run 50-100 meters of scrap at startup so the machine can settle, then measure and fine-tune. Severe drift over a shift may indicate worn bearing housings.
