Wrinkling (also known as orange peel, crinkling, or cratering) in UV varnish during the curing process is a common technical issue, typically caused by uneven stress within the film or on the surface during curing. The following are the main causes and a detailed analysis:

I. Core Cause: Imbalance in Curing Shrinkage Stress

UV varnish transforms instantly from liquid to solid under ultraviolet irradiation, accompanied by significant volume shrinkage. When this shrinkage occurs unevenly between the surface and bottom layers, or across different areas, internal stress develops, pulling and causing surface wrinkling.

II. Detailed Cause Analysis

1. Excessive Coating Thickness

a. Mechanism: An overly thick coating (especially >10μm) causes the surface layer to absorb UV energy and cure/shrink rapidly, while the bottom layer remains liquid or semi-solid. The shrinkage of the surface layer pulls and deforms the incompletely cured bottom layer, leading to wrinkles.

b. Typical Manifestation: Wrinkles are often large and irregular, commonly found in recessed areas or where the coating has built up at edges.

c. Solutions:Reduce coating weight (adjust anilox roller line screen/volume, doctor blade pressure).

i. Apply and cure two thin coats (cure a thin layer before applying the second).

2. Insufficient UV Curing Energy

a. Mechanism:Aged/Insufficient Power Lamps: Decreased output intensity of UV lamps fails to penetrate thick coatings, resulting in incomplete bottom layer cure.

i. Conveyor Speed Too Fast: Insufficient exposure time under UV lamps.

ii. Dirty Reflectors/Incorrect Focal Length: Reduces effective radiation intensity.

b. Typical Manifestation: Wrinkling accompanied by surface tackiness and poor rub resistance; wrinkles often appear in thicker coating areas.

c. Solutions:Check UV lamp power (measure mJ/cm² with a radiometer), replace aged lamps.

i. Reduce conveyor speed or add more UV lamp units.

ii. Clean reflectors, adjust the distance between lamps and substrate (typically 5-10cm).

3. Underlying Ink Not Fully Dry or Incompatible with UV Varnish

a. Mechanism:Residual Solvent in Solvent-based Inks: Solvent volatilizes and expands during UV curing, pushing up the surface varnish causing bubble-like wrinkles.

i. Inks Containing Silicone/Wax Additives: Reduce surface tension, causing local uneven shrinkage of the varnish.

ii. Insufficient Ink Curing: Weak support layer unable to resist varnish shrinkage stress.

b. Typical Manifestation: Wrinkles concentrated over undried ink areas, possibly accompanied by the ink being “lifted.”

c. Solutions:Ensure the underlying ink is completely dry (extend drying time/increase air flow).

i. Choose inks compatible with the UV varnish (avoid those containing silicone oil, high wax content).

ii. Apply a UV primer/link coat over the ink to improve adhesion.

4. Substrate Issues

a. Mechanism:Excessively Absorbent Substrate (e.g., coarse fiber paper): Rapidly absorbs monomers from the varnish, causing local viscosity increase, poor leveling, and uneven shrinkage during cure.

i. Low Surface Energy Substrate (e.g., PE/PP plastic): Poor varnish wetting, causing it to shrink into bead-like wrinkles.

ii. Poor Adhesion to Substrate Coating: The curing shrinkage of the varnish pulls up the substrate coating.

b. Typical Manifestation: Wrinkling accompanied by craters, especially over exposed substrate fibers or low surface energy areas.

c. Solutions:Treat low surface energy substrates (e.g., corona treat to ≥38 dyne/cm).

i. Apply a dedicated UV primer to enhance adhesion.

ii. Use a UV varnish with better wetting agents (leveling additives).

5. UV Varnish Formulation or Storage Issues

a. Mechanism:Overly Reactive Monomer/Resin: Surface cures instantly, bottom layer lacks time to react fully.

i. Excessive Diluent Added: Evaporation of non-reactive solvents (e.g., ethanol) exacerbates shrinkage.

ii. Expired or Improperly Stored Varnish: Can lead to partial pre-polymerization, causing abnormal stress during curing.

b. Typical Manifestation: Uniform wrinkling across the sheet, possibly accompanied by grit or gel particles.

c. Solutions:Contact supplier to adjust formulation (reduce proportion of highly reactive monomers, add flexible resins).

i. Avoid dilution with non-UV solvents! If dilution is necessary, use dedicated reactive diluents.

ii. Check varnish expiration date; store in a cool, dark place.

6. Uncontrolled Environmental or Process Parameters

a. Mechanism:Temperature Too Low: High varnish viscosity leads to poor leveling and uneven coating thickness.

i. Humidity Too High: Moisture incorporation into the varnish can cause bubbling and wrinkles as water vapor escapes during curing.

ii. Excessive Delay between Coating and Curing: Solvent evaporation leads to uneven surface viscosity.

b. Solutions:Control ambient temperature (20-25°C) and humidity (50-60% RH).

i. Cure immediately after coating (delay < 3 minutes).

III. Quick Diagnostic Process

1. Observe Wrinkle Pattern:

a. → Large wrinkles → Coating too thick / Insufficient curing

b. → Dense small wrinkles (orange peel) → Poor leveling / Unsuitable environment

c. → Wrinkles with bubbles → Solvent residue / High humidity

2. Check Cure Degree:

a. → Wipe surface with acetone → If it dissolves → Insufficient cure

b. → Perform tape peel test → If it lifts → Poor adhesion

3. Check Equipment Parameters:

a. → Check radiometer reading → Compare against varnish requirements

b. → Measure coating weight → Check if it exceeds specification

IV. Ultimate Solutions

Key Tip: Wrinkling is often the result of multiple overlapping factors! A systematic investigation is needed. Prioritize adjusting these three core parameters: coating weight, UV curing energy, and underlying layer dryness. If the problem persists, collaborate with your varnish supplier’s technical staff for formulation optimization.