Why Do Longitudinal Streaks Always Appear on Your Coated Products?

Introduction

Longitudinal streaks are one of the most common defects in the coating process. They are defined as thin, elongated lines with uneven coating thickness that form along the running direction of the coated substrate.

Unlike transverse streaks, which follow a regular pattern, longitudinal streaks often appear and disappear intermittently. Many engineers habitually attribute this defect to poor material quality or equipment failure, but the root cause is usually more specific and traceable.

This document breaks down the identification methods, root causes, corrective solutions, and rapid diagnostic workflow for longitudinal streaks in coating production.

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1. Identify the Type of Longitudinal Streak

Different streak characteristics correspond to specific root causes. Record the following details during observation:

• Streak width: millimeter-scale fine streaks, or centimeter-scale wide bands?
• Occurrence position: concentrated on the substrate edges or middle area? Does it change with substrate width?
• Morphology change: Does the streak change before and after drying? Does it disappear after shutdown and restart?
• Continuity: Is the streak continuous or intermittent?

Correlation Between Streak Features and Root Causes

表格

Streak Feature	Most Likely Root Cause
Continuous, full-length unbroken lines	Mechanical vibration, or physical damage to the doctor blade or coating die lip
Intermittent, on-and-off streaks	Abnormal coating slurry properties, or periodic runout of the back-up roll
Dense, uniform fine lines	Micro-vibration in the coating head system (e.g., motor pulsation, feed pump fluctuation)
One or more deep “grooves”	Notches on the doctor blade edge, damage to the die lip, or foreign matter trapped on the shim
Streaks dense at edges, clean in the middle area	Slurry accumulation and caking at the edge seal of the baffle or shim
Water ripple-like longitudinal streaks	Drying process defects: excessive or uneven hot air velocity distorting the wet film

Field Reference: If the interval between streaks exactly matches the circumference of the coating roll, the defect is most likely roll-related; if streaks only appear at the substrate edges, it may be caused by uneven edge tension or edge wear of the doctor blade.

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2. Root Cause Analysis and Corrective Solutions

2.1 Coating Head and Die Defects

This is the primary source of longitudinal streaks, as it determines the initial state of the wet coating layer.

2.1.1 Contamination or Damage to the Die Lip Gap

• Root Cause: Dried coating particles, dust, or tiny foreign matter on the die lip edge. As the coating fluid exits the die, these particles disturb the flow field, forming a persistent liquid-starved line or edge thickening downstream.
• Inspection & Correction: Clean and inspect the die lip for scratches. Even a scratch as small as 1/10 the diameter of a human hair can cause permanent longitudinal streaks.

2.1.2 Thermal Expansion Deformation of the Die

• Root Cause: Uneven temperature distribution on both sides of the slot coating die causes differential thermal expansion of the metal, leading to micro-bending of the die lip gap.
• Inspection & Correction: Verify the stability of the die heating/cooling system, and check if ambient air flow causes local overheating or overcooling of the die.

2.1.3 Shim Issues

• Root Cause: Improper shim installation, creases, or uneven thickness will result in uneven coating fluid distribution.
• Inspection & Correction: For multi-layer shims, confirm correct assembly and neat, undamaged edges.

2.2 Substrate-Related Issues

2.2.1 Substrate Thickness Tolerance Deviation

• Root Cause: If the substrate itself has inherent longitudinal thickness unevenness (e.g., thickness variation from film casting), these defects will be amplified after coating due to surface tension or coating fluid flow.
• Inspection & Correction: Without coating, inspect the original flatness of the substrate with a thickness gauge or visual backlight inspection.

2.2.2 Static Electricity and Adsorption

• Root Cause: Excessive static electricity on the substrate attracts airborne dust, which scratches the wet coating layer when passing through the coating gap, forming streaks.
• Inspection & Correction: Check if the static eliminator is functioning properly.

2.2.3 Back-Up Roll Condition

• Root Cause: Pits or foreign matter on the surface of the substrate-supporting back-up roll cause local depression or protrusion of the substrate at the coating point, altering the coating gap.
• Inspection & Correction: Clean and inspect the radial runout and surface finish of the back-up roll.

2.3 Fluid and Feed System Issues

2.3.1 Air Bubbles in the Coating Fluid

• Root Cause: Rupture or flow of micro-bubbles inside the die leaves thin, elongated streaks on the coating layer.
• Inspection & Correction: Check the deaeration equipment, air leakage points in the feed pipeline, and whether air bubbles are generated by return flow.

2.3.2 Filter Screen Clogging or Rupture

• Root Cause: A clogged filter causes feed pressure pulsation; a ruptured filter screen allows large particles to enter the die and get stuck at the die lip.
• Inspection & Correction: Monitor the filter differential pressure and replace the filter element regularly.

2.3.3 Unstable Rheology of the Coating Fluid

• Root Cause: Excessively strong thixotropy or unstable viscosity of the coating fluid leads to inconsistent flow expansion at the die lip exit, forming longitudinal flow marks.
• Inspection & Correction: Optimize the fluid formulation to stabilize viscosity and rheological properties.

2.4 Drying and Hot Air System Issues

2.4.1 Defects Caused by Hot Air Impact

• Root Cause: When the wet film enters the oven, excessive or concentrated hot air velocity blows longitudinal “air marks” on the uncured coating. Such streaks usually originate at the oven inlet.
• Inspection & Correction: Check the air pressure and nozzle design at the oven inlet, and optimize for convective balance if necessary.

2.4.2 Solvent Evaporation Rate Gradient

• Root Cause: Excessively fast solvent evaporation on the coating surface causes local temperature drop, leading to surface tension differences (Marangoni effect), which forms longitudinal micro-convection streaks.
• Inspection & Correction: Adjust the oven temperature gradient and air volume to control the solvent evaporation rate.

2.5 Mechanical Vibration and Transmission Issues

2.5.1 Roll Dynamic Imbalance or Bearing Wear

• Root Cause: Damaged bearings or poor dynamic balance of a guide roll generate periodic mechanical vibration. When the vibration frequency is superimposed with the web running speed, regular longitudinal light and dark streaks form on the wet film.
• Inspection & Correction: Inspect the runout and vibration of all rolls related to the coating point (especially the coating back-up roll and traction roll).

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3. Diagnostic Process: Rapid Root Cause Location

Step 1: Static Inspection

Mandatory prerequisite: Shut down the equipment completely.

Use a high-power magnifying glass or portable microscope to directly inspect the coating die lip and doctor blade edge.

• Key Inspection Points: Look for physical damage, dried slurry agglomerates, and micro-scratches.
• Operation: Use a feeler gauge to check the consistency of the die lip gap. For gravure rolls, manually touch the surface to check for bumps or pits.

Field experience shows that in most cases, the problem stems from a single carbide particle barely visible to the naked eye.

Step 2: Dynamic Tracking

If no abnormalities are found in the static inspection, run the equipment at reduced speed for a low-speed web guiding test:

1. Without coating, observe the substrate running track to rule out periodic vibration caused by guide roll runout.
2. Start the coating process, but reduce the speed to 30%-50 of the normal operating speed.

Key Judgment Criteria:

• If streaks disappear or fade at low speed: The problem is strongly related to fluid dynamics (e.g., Reynolds number change) or drying rate.
• If streaks remain clear and in a fixed position: The root cause is hardware damage or large foreign particles.

Step 3: Variable Isolation

This core step requires designing a minimal controlled test to isolate variables:

• Slurry replacement: Use the same batch of slurry filtered through a finer mesh. If streaks disappear, the root cause is a filtration/dispersion problem.
• Substrate replacement: Switch to another roll of substrate. If streaks move or disappear with the substrate, the root cause is incoming material contamination or defects.
• Hot air shutdown: Briefly turn off part of the oven heating and observe the wet film state. If streaks change, the root cause is the drying process.