Pinholes / missed coating

Discontinuous holes or exposed substrate missed-coating spots appear in the coating, appearing as bright spots under light transmission inspection, and as regular or irregular tiny holes under magnification. Pinholes and missed coating make the coating completely lose function at those points — zero barrier property, broken conductivity, white spots in decoration. A pinhole density of more than 10 per square meter can make food packaging hygiene safety unqualified.

1. Mold/tooling factors: Bubbles in the coating liquid burst at the coating head. 2. Process parameter factors: Local low surface tension contamination spots on substrate surface cause the coating liquid to fail to wet. 3. Mold/tooling factors: Local micro-blockage of the coating head. 4. Material/formulation factors: Coating amount too low makes the liquid film unable to cover microscopic high and low points on substrate surface. 5. Curing process factors: Curing temperature and time determine the completion degree of the coating crosslinking reaction — insufficient temperature or time causes incomplete curing (tacky, low hardness, poor solvent resistance), excessive temperature or time causes over-curing (embrittlement, yellowing, cracking); the curing window of the coating liquid system must be precisely matched.

1. Process parameter optimization and adjustment: Perform defoaming treatment on coating liquid (vacuum defoaming or standing). 2. Mold/tooling improvement and maintenance: Improve substrate surface treatment uniformity. 3. Equipment inspection and system maintenance: Regularly clean the coating head. 4. Material/formulation optimization: Appropriately increase coating amount. 6. Preventive management: Establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluations. For recurring problems, use fishbone diagrams or 5Why analysis to trace the root cause, and solidify countermeasures as PFMEA control points. Establish a regular equipment maintenance system and implement life management for key tooling such as dies/spinnerets/calender rolls/coating heads.

The essence of pinholes and missed coating is that the coating liquid fails to form a continuous and complete covering film. The solution approach is "defoam + thicken + clean surface": defoam the coating liquid to eliminate bubble sources, appropriately increase coating amount to ensure the liquid film thickness is sufficient to cover substrate microscopic unevenness, and improve substrate surface treatment uniformity to eliminate low surface tension spots.