This is perhaps the most frustrating problem. You pull a printed sheet, and the ink smudges or scrapes off too easily. Several culprits could be at play.
- Low UV Intensity: The UV lamps might not be emitting enough energy. This can happen if the lamps are nearing the end of their lifespan or if there’s an issue with the power supply. Check the lamp’s output regularly with a radiometer.
- Incorrect Wavelength: Different UV inks are formulated for specific UV wavelengths. Ensure your lamps are emitting the correct spectrum. Mercury vapor lamps offer a broad spectrum, while LED UV systems are designed for narrow, specific wavelengths.
- Ink/Coating Formulation: The ink or coating itself might be the issue. Some formulations are less reactive to UV light. Always confirm compatibility with your UV system and consult the ink manufacturer if problems persist. Pigments, especially heavy ones like titanium dioxide (white), can block UV light from reaching the photoinitiators deeper in the ink layer.
- Speed Too High: If the printing press is running too fast, the ink doesn’t have enough exposure time under the UV lamps to cure fully. Slow down the press and re-test.
- Distance Too Far: The distance between the UV lamp and the substrate is critical. If the lamp is too far away, the energy intensity drops significantly. Ensure the correct gap is maintained.
- Contamination: Opaque contaminants on the substrate or in the ink can block UV light. Maintain a clean printing environment and substrate.
Pinholing and Blistering: Small Holes, Big Problems
These defects appear as tiny holes or raised blisters on the printed surface. They often indicate trapped air or solvents.
- Trapped Air: During the printing process, air can get trapped between the ink layers or between the ink and the substrate. This is more common in thick ink laydowns.
- Outgassing: If the substrate contains volatile organic compounds (VOCs) or if the ink/coating has residual solvents, these can be released as gases during the UV curing process, creating bubbles. This is especially a concern with certain plastics and varnishes.
- Speed Mismatch: Sometimes, a mismatch in curing speed and press speed can exacerbate outgassing issues.
- Excessive Heat: While UV curing is a “cool” cure compared to thermal drying, excessive UV energy in a confined space can still generate heat. This heat can cause trapped air or solvents to expand.
Yellowing or Discoloration: Losing the True Colors
The printed image or coating turns yellow over time or immediately after curing.
- Over-Curing: Excessive UV exposure, especially with certain types of mercury vapor lamps, can lead to photo-oxidation and yellowing of the ink or coating binder.
- Lamp Aging: As mercury vapor lamps age, their spectral output can shift, potentially producing more heat or shorter wavelengths that cause yellowing.
- Ink Chemistry: Some UV inks and coatings, particularly those with certain pigments or binders, are more prone to yellowing when exposed to UV light.
- Substrate Interaction: The substrate itself can sometimes react with the UV-cured ink or coating, leading to discoloration.
Poor Adhesion: The Ink Won’t Stick
The cured ink or coating peels off easily or has weak adhesion.
- Surface Contamination: The substrate might have oils, silicones, or other release agents on its surface that prevent proper ink adhesion. Thoroughly clean the substrate.
- Low Surface Energy: Some materials, like certain plastics, have very low surface energy, making it difficult for inks to wet and adhere. Surface treatment methods like corona or plasma can be necessary.
- Incompatibility: The UV ink or coating might not be chemically compatible with the substrate. Always check manufacturer recommendations.
- Insufficient Cure: As mentioned earlier, an under-cured ink will naturally have poor adhesion.
- Exhaust Issues: If the UV system’s exhaust is not functioning correctly, it can lead to a buildup of ozone or other byproducts that interfere with adhesion.
Troubleshooting Steps: A Systematic Approach
When faced with UV curing problems, a methodical approach is your best friend.
- Isolate the Problem: Determine exactly where the issue is occurring. Is it on every print, or only in certain areas? Does it happen with all jobs, or just specific inks/substrates?
- Check the Obvious: Start with the simplest potential causes. Are the UV lamps on? Is the press running at the correct speed? Is the substrate clean?
- Verify Settings: Double-check all press and UV system settings. Lamp power, exposure time (related to press speed and lamp proximity), and exhaust levels are critical.
- Consult Manufacturer Data: Refer to the technical data sheets for your inks, coatings, and substrates. They often provide crucial information about compatibility and optimal curing conditions.
- Test Systematically: Make one change at a time and observe the results. This helps pinpoint the exact cause. For example, try reducing press speed by 10% and see if curing improves.
- Use a Radiometer: This is an indispensable tool for measuring UV intensity. Regularly monitor your lamps to ensure they are performing as expected.
- Cleanliness is Key: Maintain a clean printing environment. Dust and debris can cause a host of issues. Clean UV lamps and reflectors regularly to maximize their output.
By understanding these common issues and employing a systematic troubleshooting process, you can ensure your UV curing systems perform optimally, delivering high-quality prints consistently across your label, flexo, offset, and narrow web operations.
