How To Troubleshoot Scratch Issues in Coaters
To troubleshoot scratch issues in coaters effectively, it is essential to identify different scratch types, understand how particles are generated, and analyze defect mechanisms throughout the coating process. A systematic troubleshooting approach helps improve coating quality, reduce defects, and ensure stable production performance.
I. Classification of Scratches
Particle scratches: Formed at the wet film stage. Hard particles in the slurry or along the coating path plow grooves into the undried coating via the coating head or drive rollers. They feature smooth interior surfaces and regular distribution along the coating direction.
Mechanical scratches: Formed after the electrode sheet is dried or even wound, caused by direct scraping from hard objects. The edges of such scratches are often accompanied by curling of the foil and coating, and they are classified as external impact damage.
II. Formation of Particles
Slurry: Slurry is a dispersion of solid particles including active materials, conductive agents and binders in a solvent. Nano conductive agents have high surface energy; insufficient stirring shear force or improper process parameters can easily lead to agglomeration.
Inadequate dissolution of PVDF binder or contact with trace moisture will precipitate elastic gel particles. Both agglomerates and gel particles can cause scratches under the doctor blade.
Raw materials: The cathode material has a wide particle size distribution with oversized particles; environmental dust gets mixed in during transportation and feeding processes.
Equipment and pipeline system: Residual slurry lumps on insufficiently cleaned inner walls of mixing tanks, delivery pipelines and slurry pumps turn into hard particles; metal shavings are generated from equipment wear; filter screens with too fine a mesh fail to intercept particles, while damaged screens lose their interception function entirely, allowing particles to pass straight through to the coating head.
Environment and operation: Despite high cleanliness of the coating head, poor environmental control allows airborne dust to fall into the slurry tank or onto the undried wet film surface, directly forming scratches.
Guide roller adhesion: If solidified slurry particles adhere to the surface of guide rollers at the oven inlet and outlet, they will press scratches into the passing electrode sheet.
III. Evolution of Particles into Scratches
Position: Particles embedded in the coating only form protrusions. If they get stuck in the coating gap or adhere to the doctor blade edge, they will form scratches as they are dragged along with the moving web.
Pressure: Excessive doctor blade pressure or an unreasonable gap between the back-up roll and coating roll will press particles into the substrate, increasing the depth and width of scratches.
Motion state: When the coater is running, a fixed particle produces a continuous straight scratch, while an intermittently falling particle produces segmented scratches.
IV. Particle Defect Mechanisms Vary by Coating Method
Comma doctor blade coating: Highly sensitive to particles. The doctor blade is in direct contact with the wet film, and particles caught on the blade edge will drag out straight, continuous scratches with uniform width.
Micro-gravure coating: Particles trapped in gravure roll cells cause pinhole-shaped coating deficiencies, while particles adhering to the roll surface press out pits.
Slot die coating: Particles clogging the die lip gap create scratches of varying depths, and damage to the die lip itself will amplify such defects.
V. Standardized Troubleshooting Process
Identify the root cause Analysis by morphology: First confirm three core characteristics: Are the scratches continuous or intermittent? Is their spacing fixed? Do they occur only in specific batches or across all batches?
Slurry screening: Check mixing records to verify whether the feeding sequence, mixing duration and dispersion shear parameters comply with specifications.
Equipment screening: Check maintenance records to confirm the filter replacement cycle, pipeline cleaning records and guide roller cleaning frequency.
Environment screening: Check cleanliness records to confirm whether the dust particle count in the coating head area meets the standard.
Verify hypotheses through single-variable tests: Replace with a new batch of slurry - if scratches disappear, the problem lies in the slurry. If defects are eliminated after cleaning the guide rollers, the issue is guide roller adhesion. If scratches change after adjusting the doctor blade pressure, the pressure parameters are unreasonable.
