Hello everyone, I'm Qiang Yongsheng. I'm delighted to communicate with you all through this column. In daily production, we often encounter many technical problems. Whenever I successfully solve a technical challenge, I like to document the causes and solutions for future reference. Unconsciously, I've accumulated considerable production experience. I want to share these experiences with you through this column, hoping to provide some reference for fellow professionals. At the same time, I hope this can serve as a catalyst, encouraging other colleagues who read this content to share their valuable experiences gained from production processes. Given my limited experience, there may be biases in the article, and I welcome constructive criticism from fellow professionals.
Currently, pressure-sensitive label printing equipment in China primarily consists of flexographic and letterpress machines, with UV inks being the dominant choice (accounting for 70%~80% of pressure-sensitive label printing applications according to statistics), supplemented by water-based inks. Although UV inks offer advantages such as rapid curing and easy drying, many issues still arise during daily production. In this article, I'll share my experience in solving problems related to poor ink adhesion and peeling.
Poor ink adhesion and peeling are common problems in pressure-sensitive label printing. If not detected promptly, these issues can cause significant economic losses, as labels with peeling ink are essentially defective products. If an entire batch suffers from peeling problems, the losses can be substantial. Currently, the industry's standard method for testing ink adhesion is the 3M tape test. This involves applying 3M 610 or 810 tape to the printed surface for 30-60 seconds, then peeling it off to check for ink transfer. So, what causes poor ink adhesion and peeling?
01 Over-Drying of Ink Leading to Crystallization
During daily production, operators often set UV lamp power higher when printing large solid areas with heavy ink coverage, fearing incomplete drying. This can lead to over-drying of the ink, causing crystallization.
Crystallization refers to the rapid drying of ink during printing, forming a hard, shell-like surface similar to a crystal. Crystallized ink becomes brittle, and if subjected to external force, it's likely to crack and peel off.
Once ink crystallization occurs, if the product requires bleed cutting, ink is prone to peel off at the cut edges of the graphics. Under magnification, this peeling typically appears jagged.
Therefore, in daily production, we must pay special attention: UV lamp power is not necessarily higher the better; rather, the lower the power while ensuring proper ink drying, the better. On one hand, this helps avoid excessive energy waste, saving electricity costs for the enterprise. On the other hand, it prevents ink crystallization due to over-drying.
02 Insufficient UV Lamp Power Leading to False Drying
During production, if UV lamp power is too high, it may cause ink crystallization. However, if UV lamp power is too low, problems can easily arise. The most typical issue is "false drying" of ink.
False drying mainly occurs during printing of large solid areas. Due to heavy ink coverage and thick ink layers, if UV lamp power is insufficient, only the surface layer of ink dries, while the inner ink remains incompletely cured. This results in ink resembling a "sandwich cookie" – hard on top and soft underneath.
During tape testing, the ink is easily lifted by the tape. This phenomenon is particularly prone to occur with darker inks, such as black or blue. This is because darker inks have stronger hiding power. If the ink layer is thick and UV lamp power is insufficient, UV light may fail to penetrate the ink surface to reach the inner layers.
This false drying phenomenon is not easily detected during production. It's often discovered during later die-cutting or packaging stages, and sometimes even during end-user application. Therefore, it's recommended that operators perform tape tests on the beginning and end of each roll of semi-finished printed products during printing to identify and address issues promptly. Additionally, if batch false drying is detected, to avoid losses, a secondary UV drying method can be employed: increase UV lamp power and pass the printed semi-finished products through the UV lamps again. If the problem persists, letting the semi-finished products sit for a few more days can help, as UV inks have post-curing characteristics; sometimes, adhesion improves after a few days.
03 Material Surface Strength Issues
Some pressure-sensitive materials, especially paper-based ones (like writing paper materials), have strong ink adsorption capabilities but insufficient surface strength themselves. During tape testing, the tape often lifts both the ink and the face material, causing delamination of the substrate. This is not caused by poor ink adhesion but by the insufficient tear resistance of the substrate carrying the ink. Therefore, tape testing is not recommended for testing ink adhesion on such low-surface-strength pressure-sensitive materials.
04 Excessive Ink Additives
During daily production, operators often add a certain amount of additives to ink to improve printability, making the ink perform better during printing. However, if additive proportions are not properly controlled, problems can arise.
Thinners and tack reducers are common ink additives. Their purpose is to reduce ink viscosity and enhance flow. Generally, these additives must be added in specific proportions. For example, the thinner proportion should be 5%~10% of the ink used. Exceeding this ratio makes the ink too thin and can lead to poor drying. Therefore, it's crucial to strictly follow the manual's proportions when adding auxiliary additives.
It's important to note that some operators add excessive amounts primarily because the ink feels too thick during mixing. Even after adding thinner, the viscosity remains high, leading them to add more. In reality, ink itself has thixotropy; under shear force, the originally thick ink becomes thinner. During printing, ink is constantly agitated by the ink fountain roller, gradually thinning in the process. So, if excessive thinner is added during ink mixing, it may feel appropriate at the time, but once on the press, the ink becomes even thinner and more fluid as it's agitated in the fountain. Therefore, there's no need to add excessive thinner. If thinner is overused, it can easily lead to poor ink drying.
05 Ink and Material Mismatch
For some film-based pressure-sensitive materials, their surface energy is relatively low, preventing ink from adhering well to the material surface. Generally, material surface energy should reach at least 38 dynes to be suitable for printing. For ink to adhere firmly to the material surface, the material surface energy should reach at least 42 dynes.
For film-based pressure-sensitive materials with lower surface energy, two solutions can be adopted to ensure firm ink adhesion during printing:
First solution: Apply a primer coat. Before printing, apply a layer of primer to the material surface. This primer increases the material's surface energy, allowing ink to adhere firmly. Note that primers are mostly light yellow. Materials coated with primer will experience changes in color and gloss. Therefore, primer coating is more suitable for graphics with large solid areas.
Second solution: Use ink compatible with the material. Some ink suppliers develop inks specifically for low-surface-energy materials. These inks can adhere firmly even when printed on low-surface-energy materials. We can send the material to be printed to suppliers for testing and request ink recommendations compatible with that material.
Of course, in recent years, with equipment upgrades, inline corona treatment has become standard on many machines. Performing corona treatment on low-surface-energy materials during printing is also a good solution. Corona treatment serves two purposes: First, it creates micro-pits on the material surface via electric arcs, allowing ink to adsorb and fill these pits. Second, it generates substantial electrical charges on the material surface, which have an adsorptive effect on ink, making the ink layer more stable.
Corona treatment typically shows significant effects only on relatively soft film materials like PE or PVC, and has limited effect on harder materials like BOPP or PET. Therefore, the most suitable solution should be configured based on actual circumstances.
06 Poor Ink Drying
The typical service life of UV lamps is 800~1000 hours. If UV lamps exceed their rated lifespan, they may still operate, but their power output is affected. UV lamps past their service life experience varying degrees of power reduction. When printing large solid areas, this can lead to incomplete ink drying, and incompletely dried ink has reduced adhesion.
Therefore, operators must accurately record the usage time of each lamp. Once a lamp exceeds its rated service life, special attention is required. Avoid using that set of UV lamps for drying products with large solid areas. Also, pay close attention and promptly replace lamps if poor ink drying is observed.
Conclusion
The six issues discussed above represent common causes of poor UV ink adhesion and peeling in pressure-sensitive label printing. By understanding these root causes and implementing the corresponding solutions, printing operations can significantly reduce defects, improve product quality, and minimize economic losses. Regular testing, proper material selection, and careful process control are essential for maintaining consistent ink adhesion performance.
Post time: May-18-2026