Why Do Your Coating Products Always Have Longitudinal Stripes?

Longitudinal stripes are one of the most common defects in coating processes — thin, uneven-thickness lines running along the substrate direction. Unlike transverse stripes, they’re irregular, often appearing and disappearing. Many engineers instinctively blame the material or equipment, but is that really the cause?

Let’s break it down.

1. Identify Your Stripe Type First

Before fixing anything, observe these details:

Width: Millimeter-scale thin lines, or centimeter-scale wide bands?

Location: Edges or center? Does it change with substrate width?

Pattern:

• Continuous, fixed position → Likely mechanical vibration or blade/die damage

• Intermittent, on-and-off → Coating solution issue or back roll oscillation

When do they appear?

• Dense, uniform lines → Check micro-vibration in the entire coating head system

• One or a few deep “grooves” → Blade notch or die lip damage

• Dense at edges, clean in center → Baffles or gaskets have dried buildup at edge seals

• Wavy, water-ripple pattern → Hot air drying too aggressive or uneven

Tip: If stripe spacing matches the coating roll’s circumference → roll issue. If only at edges → uneven tension or blade wear.

2. Root Causes & Solutions

A. Coating Head & Die Issues

(Most fundamental source)

B. Substrate Issues

• Thickness tolerance: Run substrate through without coating — check original flatness with a thickness gauge or backlight

• Static electricity: Excess static attracts dust; check static eliminator

• Back roll damage: Pits or contamination causes local substrate deflection

C. Fluid & Feed System

• Air bubbles in coating: Check degassing equipment and pipes for leaks

• Filter clogging: Causes pressure pulsation; check filter pressure drop, replace cartridges

• Unstable rheology: High thixotropy or fluctuating viscosity causes uneven flow at die lip

D. Drying System

• Hot air impact: If air velocity is too high at oven entrance, it blows wet film into longitudinal “wind marks”

• Solvent evaporation rate gradient: Fast surface evaporation → temperature drop → surface tension difference (Marangoni effect) → convection streaks

E. Mechanical Vibration

• Roll imbalance or worn bearings: Causes periodic vibration → leaves regular longitudinal light/dark stripes on wet film

3. Quick Diagnosis Workflow

Step 1 — Static Inspection (Must stop machine)
Use a magnifying glass or portable microscope. Inspect:

• Die lip and blade edge → look for physical damage, dried chunks, micro-scratches

• Use feeler gauges to check lip gap uniformity

• Forgravure rolls, feel for surface凹凸 (raised/low spots)

Tip: Often the culprit is a carbonized particle almost invisible to the naked eye.

Step 2 — Dynamic Tracking (Run slow)

• Do a low-speed thread-up test

• Observe substrate trajectory without coating

• Start coating at 30–50% normal speed

Key insight: If stripes disappear at low speed → fluid dynamics or drying rate issue. If still sharp and fixed → hardware damage or large particle.

Step 3 — Variable Isolation (The real test)

• Swap coating solution: Use finer-filtered same-batch solution. If stripes vanish → filter/dispersion problem

• Swap substrate: If stripes move with substrate or vanish → incoming material contamination

• Turn off hot air: Observe wet film. If stripes change → drying process issue

4. Common Q&A

Q: Stripes appear only after running for a while, not at startup?
A: Blade gradually wearing, or coating roll building up residue. Stop and inspect the coating head mid-run.

Q: Stripes only on one product, gone after switching?
A: Likely viscosity/solid content mismatch. Try adjusting coating speed or solution viscosity.

Q: Stripes only visible after drying, invisible on wet film?
A: Almost certainly drying — uneven coating shrinkage. Check oven temperature distribution or excessive air velocity.