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Common Issues and Solutions for UV Coatings

Release time:

2014-09-22 15:29

Common Issues and Solutions for UV Coatings
Common Issues and Solutions for UV Coatings

UV varnish is a transparent coating, sometimes referred to as UV lacquer. After being sprayed or rolled onto the substrate surface and exposed to UV light, it transforms from a liquid state into a solid state, thereby hardening the surface. This coating offers excellent scratch and abrasion resistance, while also giving the surface a glossy, aesthetically pleasing appearance with a smooth, rounded texture. However, during use, certain issues frequently arise. Below are some common problems and their corresponding solutions:

1. Poor gloss and insufficient brightness (except for UV varnishes specifically designed to achieve certain matte or flat finishes, generally the higher the gloss of a UV varnish, the better):
Main reason: Solution:
A. The UV varnish itself has too low viscosity, resulting in an excessively thin printed coating.
B. Overdilution with non-reactive solvents such as ethanol and toluene
C. The glossiness of the light-curing resin itself is insufficient.
D. The paper is too absorbent.
E. The anilox roller is too fine, resulting in insufficient transfer of varnish.
F. Improper selection of squeegee and screen mesh count 		A. Select a high-gloss acrylic photopolymer resin.
B. Appropriately increase the viscosity of the varnish.
C. Strictly control the addition amounts of ethanol and toluene.
D. For papers with high permeability, you can first apply a primer coat or choose paper with good compactness.
E. Appropriately increase the diameter of the anilox roller.
F. Reduce the hardness of the scraper.
G. During printing, you can also choose a screen mesh with a lower mesh count to increase the coating thickness.

 

2. UV varnish cannot be applied to the surface of printed materials:  
Main reason: Solution:
A. UV varnish has low viscosity and the coating is too thin.
B. The surface of the printed material has oil stains.
C. The ink surface has become “glassy,” resulting in improper affinity with the varnish.
D. The ink surface contains too much anti-stick material (such as silicone oil or spray powder).
E. The coating anilox roller is too fine. 		A. Take necessary cleaning measures for printed materials to be coated with varnish.
B. Properly adjust the viscosity of the varnish.
C. Adjust the types and amounts of additives in the varnish; a small amount of wetting agent can be added.
D. The surface of the print can be wiped with a 2% alkaline solution.

 

3. After varnishing, the surface of the printed material feels sticky and doesn't dry properly:  
Main reason: Solution:
A. Insufficient UV light power
B. The UV lamp tube has aged and its intensity has weakened.
Improper selection of C.UV varnish initiator
D. Excessive addition of solvents such as ethanol and toluene
E. The machine’s paper feed speed is too fast.
F. The UV varnish itself has poor self-curing performance. 		A. Timely maintenance of equipment and replacement of light tubes
B. Add a certain amount of photoinitiator and photosensitizer, such as triethanolamine, to accelerate the photocuring speed.
C. Adjust the printing process

 

4. Uneven surface coating with streaks and orange-peel texture:  
Main reason: Solution:
A. The UV varnish has excessive viscosity.
B. Uneven pressure magnitude
C. Excessive coating amount
D. UV varnish has poor leveling properties.
E. The paper is uneven and too absorbent.
F. UV varnish has poor surface smoothness and flatness. 		A. Reduce the viscosity of UV varnish
B. Reduce the coating amount and ensure uniform pressure adjustment.
C. A small amount of leveling agent can be added.
D. Perform corona treatment on the surface of the printed material to enhance the affinity of the UV varnish for the surface.

 

5. The UV coating exhibits pinhole defects.  
Main reason: Solution:
A. The coating is too thin.
B. The anilox roller is too fine.
C. Excessive addition of non-reactive diluents, such as ethanol and toluene
D. The surface of the printed material has dust. 		A. Enhance the cleanliness of the work environment
B. A small amount of leveling aid and wetting aid can be added.
C. Use a dedicated UV diluent for dilution to prevent incompatibility among system components.
D. Try to increase the coating thickness during printing.

 

6. The surface of the printed material exhibits crater-like and fish-eye defects.  
Main reason: Solution:
A. The surface of the printed material is not clean and may have oil stains.
B. UV varnish has poor leveling properties.
C. The defoaming agent used for the gloss varnish is improperly selected, resulting in excessively strong defoaming performance. 		A. Clean the surface of the printed material
B. Properly select defoaming and leveling additives.

 

7. Poor adhesion of UV varnish  
Main reason: Solution:
A. The ink surface on printed materials undergoes “crystallization,” making it incompatible with and unable to be wetted by the varnish.
B. The selection of additives in printing ink is inappropriate.
The C.UV varnish itself lacks sufficient adhesion.
D. The photopolymerization conditions were not properly controlled, resulting in a curing time that was either too short or too long.
E. For non-polar or low-polarity substrates (such as PE, PP, PET, OPP, BOPP, etc.), apply UV varnish directly without prior treatment. 		A. The printing ink should be a low-wax variety.
B. Select light-curing resins and reactive monomers with good flexibility and excellent adhesion.
C. It is crucial to maintain a reasonable curing time. If the curing time is too short, the surface of the UV varnish will cure while the underlying layer remains incompletely cured, resulting in reduced adhesion. Conversely, if the curing time is too long, the UV varnish may receive excessive ultraviolet radiation, leading to post-curing polymerization that makes the coating film brittle and prone to cracking, thereby also causing poor adhesion.

 

8. The varnish coating has poor toughness.  
Main reason: Solution:
A. Improper selection of the resin and active monomers in the UV varnish itself.
B. The gloss coating is too thick.
C. Improper control of the curing process 		A. Select light-curing resins and active monomers with better toughness, control the amount of monomers used, and reduce crosslinking and density.
B. In printing, use a screen with a higher mesh count to reduce the coating thickness.
C. Master the appropriate curing time.

 

9. “Pitted” phenomenon  
Main reason: Solution:
A. The ink has undergone crystallization.
B. High surface tension results in poor wetting of the ink layer.
C. Insufficient post-spray dwell time, with residual solvents or slow-drying solvents remaining.
D. The curing rate is too fast, causing certain volatile substances within the film to fail to escape in time and instead coalesce into small, particle-like spots. 		A. Add 5% lactic acid to the UV oil to break down the crystallization film, remove the oil substance, or perform a roughening treatment.
B. Reduce the surface tension value by adding a surfactant or a solvent with a lower surface tension value.
C. After spraying, it should be left standing for a period of time to allow the solvent to evaporate.
D. Master the appropriate curing time.

 

10. Streaking and Wrinkling Phenomena  
Main reason: Solution:
The UV ink is too thick and the coating amount is excessive, which mainly occurs in roll coating. Reduce the viscosity of the UV oil by adding an appropriate amount of alcohol solvent for dilution.

 

11. Bubbling phenomenon  
Main reason: Solution:
The UV oil used is of poor quality, and the UV oil itself contains bubbles, which often occur during screen-printing glossing. Use high-quality UV oil or let it sit for a period of time before using it.

 

12. Strong residual odor  
Main reason: Solution:
A. Incomplete drying, such as insufficient light intensity or excessive non-reactive diluent.
B. Poor antioxidant interference resistance 		A. Ensure thorough curing and drying by selecting an appropriate light source power and machine speed, and minimize or eliminate the use of non-reactive diluents.
B. Strengthen the ventilation and exhaust system

 

13. The UV varnish has become thicker or exhibits gelation.  
Main reason: Solution:
A. Storage time too long
B. Failed to store completely away from light.
C. Storage temperature is too high 		A. Use according to the prescribed time, typically 6 months.
B. Store strictly away from light.
C. The storage temperature must be maintained between approximately 5℃ and 25℃.

 

14. Automatically bursts after UV curing.  
Main reason: Solution:
After the surface temperature of the illuminated area becomes too high, the polymerization reaction continues. If the surface being illuminated is too hot, increase the distance between the lamp and the surface of the object being illuminated, or use cold air or a chilled roller.

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