Poor adhesion of film label ink and color difference issues

With the continuous development of China's economy, consumers have increasingly higher demands for product packaging, and more and more film materials are being used in adhesive label printing. At the same time, as the usage of film materials increases year by year, the production volume of adhesive labels is also gradually rising. Traditional flexographic printing has been gradually phased out due to its low production efficiency, and more and more companies are adopting flexographic printing for production. Since film materials have higher printing requirements compared to paper materials, many problems often arise during the actual production process. In this article, the author shares some common issues and solutions of film material in the flexographic printing process for readers' reference.

PART 01

Ink Adhesion Problems

Because film materials do not absorb ink like paper materials, printing on film materials often results in poor ink adhesion or ink rub-off. The currently accepted industry standard is the tape test method, which involves using 3M810 or 3M610 tape applied to the printed surface. After peeling off the tape 30 seconds later, if no ink is removed or only minor ink loss occurs, the print is basically acceptable. However, if a large area of ink is removed, it is deemed unacceptable. The common reasons for ink rub-off on film materials can be summarized into three aspects.

01

Low Surface Energy of Materials

Many film materials have relatively low surface energy. Generally, the surface energy of film materials should reach at least 38 dynes/cm to be printable. But does reaching 38 dynes/cm guarantee no ink rub-off? Based on the author's experience: no. Usually, the ink adheres more firmly to the surface of film materials when the surface energy reaches above 42 dynes/cm, greatly reducing the risk of ink rub-off. Film materials with surface energy between 38–42 dynes/cm can be normally printed, but there is no guarantee that the ink will securely adhere to the surface. Therefore, the best solution is to perform inline corona treatment during printing, as the surface energy of film materials treated this way generally reaches above 42 dynes/cm.

One point that needs to be specially noted is that there are many coated film-type adhesive materials on the market. The purpose of the coating is to solve the issue of printing ink adhesion. Therefore, most coated materials have very good ink adhesion. However, once a material is coated, the Dyne value can no longer be used as a reference. Many coated materials have Dyne values below 38, yet still exhibit good ink adhesion. At the same time, regardless of whether it is inline or not, coated materials in principle no longer require corona treatment, because the corona process can actually damage the existing coating of the material, leading to poorer ink adhesion.

02

Ink and Material Compatibility

As mentioned earlier, low surface energy of the material can lead to poor ink adhesion. In fact, to address this problem, many ink suppliers have introduced inks specifically designed for low surface energy materials. These inks can effectively solve the issue of ink picking on low surface energy materials. However, there is no universal ink in the world. If ink suppliers need to provide such matching solutions, printing companies can send the materials to the ink suppliers, who will conduct tests to determine which existing inks are compatible with the materials and recommend the most suitable ink for printing the materials to the printing companies.

In addition to using specialized inks, many ink suppliers also provide specialized primer inks. These inks are colorless and transparent, and when applied to the material surface, they can significantly improve ink adhesion, making them another good option. However, the surface gloss of materials coated with primer will change noticeably, so printing companies need to pay attention to whether this change is within the acceptable range for their clients.

03

Process Issues

Some labels require large-area printing with multiple colors overlaid. In such cases, the order of color application becomes particularly important. The author believes that a reasonable sequence is to print smaller areas first and larger areas later, as much as possible allowing the ink to print onto the material surface rather than onto another layer of ink. This is because most inks, especially UV inks, contain silicon components. After printing and drying, they form a shiny surface layer, which is also why the gloss of printed images is often very high after using UV inks. However, this shiny layer is not favorable for the adhesion of other color inks, which means that the next color printed on the already dried ink layer is more prone to peeling off.

In addition, the adhesion of inks of different colors on the surface of the same material may also vary. The author has encountered such a case: for the same type of film material and using the same equipment for on-site printing, red and yellow inks adhered very well, while blue ink had poor adhesion, coming off entirely when peeled with tape after printing. The reason for this situation, as analyzed by the author, is the difference in the composition of inks of different colors in bonding with the material. Therefore, when a printing company encounters this situation, it can try testing with inks from other brands.

Here, the author also wants to point out another issue: UV inks have post-curing properties. During label printing, if the freshly printed product is tested immediately, the ink adhesion will seem poor. However, after placing it for a few days and testing with tape again, the ink shows significant improvement in adhesion, which is due to the post-curing nature of UV ink. In principle, UV ink should dry instantly when exposed to ultraviolet light and should not have post-curing issues. However, in actual production, incomplete UV exposure often occurs, mainly because many printing companies continue to use UV lamps beyond their service life for cost reasons (generally, companies replace the lamp only when the ink cannot dry completely).

Once a UV lamp exceeds its service life, its power gradually declines during the curing process, making a 'false dry' phenomenon likely. This means the ink appears dry on the surface, but the underlying ink is actually not dry. Therefore, if a product is tested immediately after printing, the ink may experience severe shedding. But after being left for a period, the ink further dries thoroughly due to environmental factors, and adhesion will greatly improve upon retesting. Thus, sometimes if a batch shows poor ink adhesion, do not immediately decide to scrap it; it is better to leave it for a few days and test again, as the adhesion may significantly improve. The author has used this method to help companies recover substantial economic losses.

PART 02

Color Difference Issues

Operators who have worked in flexographic printing for a long time often encounter the following problem: with the same process, the same equipment, and film materials provided by the same supplier but from different batches, there can be obvious color differences in the printed products. This issue is particularly troublesome for technical staff: nothing appears to have changed, yet the colors differ noticeably. How should this be resolved?

In fact, even if the materials are provided by the same supplier, different batches can have different surface energies. Therefore, when encountering this problem, you can first use a magnifying glass to observe whether there are obvious changes in the dot size of the products printed in this batch compared to the previous batch. If the dot size changes significantly, there will definitely be more serious color differences. Sometimes the dot size of several colors changes, and sometimes only one color's dot size changes. In the latter case, I believe it is not closely related to the material and may be due to plate aging and printing pressure. If it is the former, then it's necessary to consider whether the surface energy of the two material batches is different, causing the ink to shrink on the surface of the material.

At this point, if there are leftover materials from the previous print, you can print under the same conditions to determine whether the color difference is caused by the material batch difference. If there is no previous material, you can consider using inline corona treatment to see if the color difference improves.