Common Issues and Solutions in UV Coating: Particles and Pinholes

During the UV coating process, particles and pinholes are common defects that not only affect the appearance quality of the product but may also lead to a decline in coating performance. This article provides an in-depth analysis of the causes of particles and pinholes and offers feasible solutions.

I. Causes of Particles and Pinholes

1. External Factors

(1) Excessive dust in the environment
UV coating requires a high level of environmental cleanliness. If the air cleanliness in the spray booth is insufficient, dust, fibers, and other pollutants in the air can easily adhere to the substrate or coating surface, forming particle defects.

(2) Inadequate cleaning of the substrate surface
If the substrate surface contains dust, oil, static electricity, or other contaminants and is not thoroughly cleaned before coating, particles or pinholes may appear on the coating surface.

(3) Defects in the primer coating process

• Insufficient fineness or re-coarsening: If the primer is not ground sufficiently, the pigment or filler particles may be too large, or the coating may re-coarsen due to solvent evaporation, leading to particle formation.
• Special issues with black primer: Due to the high pigment concentration in black primer, uneven dispersion can easily cause local agglomeration, resulting in pinholes.

2. Coating Material Factors

(1) Unreasonable formulation system

• Excessive volume shrinkage: If the coating undergoes excessive volume shrinkage during curing, it may lead to surface depressions or pinholes.
• Poor wetting and leveling: If the coating has poor wetting properties, the pigments or fillers may distribute unevenly in the coating, causing local agglomeration and pinholes.

(2) Improper control of primer fineness

• Color paint fineness >12μm: If the color paint fineness is too high, the pigment particles may be too large, leading to surface particles in the coating.
• Improper use of metallic paint powders: If the dispersion of aluminum powder or pearlescent powder in metallic paint is poor or the amount used is excessive, it may cause pinholes or uneven gloss on the coating surface.

(3) Influence of thinner

• Insufficient dissolving power: If the thinner cannot fully dissolve the resin, the coating may have poor fluidity, leading to surface particles.
• Imbalanced evaporation gradient: If the thinner evaporates too quickly, the coating surface may dry too fast, trapping solvents inside and forming pinholes or particles; if it evaporates too slowly, it may cause sagging or particle accumulation.

II. Feasible Solutions

1. Improve the cleanliness of the spray booth

• Use a cleanroom: Ensure the air cleanliness in the spray booth meets the required standards to reduce the impact of dust and fibers.
• Regularly clean equipment: Clean spray guns, paint delivery pipelines, and air supply systems regularly to prevent residual contamination of the coating.

2. Strengthen substrate surface treatment

• Degrease and dewax: Use white spirit, IPA (isopropyl alcohol), or ethanol to clean the substrate, removing oil, wax, and other contaminants.
• Electrostatic dust removal: Use an electrostatic gun to remove dust from the substrate surface, improving coating adhesion and surface smoothness.

3. Optimize the primer coating process

• Ensure the primer surface is clean: Before UV coating, ensure the primer surface is free of contamination to avoid dust or oil affecting coating quality.
• Control the dispersion of matte powder: For matte primers, optimize the dispersion process of matte powder to prevent agglomeration and pinholes.
• Balance primer film thickness, smoothness, and hiding power: Avoid overly thin film thickness causing exposure of the substrate or overly thick film causing sagging or particle accumulation.

4. Control coating fineness and dispersion

• Keep color paint fineness below 12μm: Ensure sufficient grinding of pigments to avoid oversized particles affecting coating smoothness.
• Optimize the use of metallic paint powders: Adjust the dispersion process of aluminum or pearlescent powder and avoid excessive use to prevent pinholes or uneven gloss.

5. Optimize thinner formulation

• Balance dissolving power, evaporation rate, and bite resistance: Select an appropriate thinner that can fully dissolve the resin and evaporate evenly during coating to avoid surface defects.

III. Conclusion

Particles and pinholes are common defects in UV coating, with causes involving the environment, substrate, coating materials, and process. By improving workshop cleanliness, strengthening substrate treatment, optimizing coating formulations, and refining processes, the occurrence of particles and pinholes can be effectively reduced, enhancing coating quality. Coating manufacturers should adjust process parameters based on their production conditions to achieve more stable coating results.