Problem 1: Coating defects in films can lead to ink dropout, and the causes are typically divided into two main categories. For coated films, poor adhesion of the ink may occur due to insufficient firmness of the surface coating. On the other hand, uncoated films often suffer from ink adhesion issues because of the weak surface fastness or poor ink compatibility. To address ink dropout caused by these coating problems, several solutions can be applied. One approach is to use a varnish printing process on the surface layer, which helps protect the ink and prevent it from dropping off. Another method involves reprocessing the surface through corona treatment, ensuring that the dyne value is at least 38 mm/m. If the dyne value is below 36 mm/m, ink adhesion is likely to be compromised. Applying a primer or an ink enhancer with strong adhesion (typically around 3% concentration) can also improve ink adhesion. Additionally, some synthetic papers with sub-surface coatings may not be suitable for testing with 3M tape due to their paper-like characteristics.
Problem 2: When the printing surface of a film has undergone corona treatment, achieving good ink wetting usually requires a dyne value above 38 mm/m. If the value drops below 36 mm/m, ink dropout becomes more likely. Moreover, the dyne value tends to decrease over time, leading to ink loss during the printing process. This issue essentially stems from surface tension problems. To resolve ink loss due to this, one solution is to use a satellite rotary press with online corona treatment, which can enhance the surface energy of the material. Alternatively, using an intermittent printing machine with a surface primer coating can improve adhesion. If the dyne value fades, corona treatment should be performed during printing. However, it's important to note that the dyne value doesn't always fully reflect the adhesion between ink and material. The fading cycle typically lasts 3-6 months, so it’s advisable to minimize long-term storage of raw materials.
Problem 3: Ink drying and tension during the printing process can also cause ink dropout. This includes situations where the ink layer is too thick, leading to incomplete drying; UV lamps with insufficient energy, aging lamps, or aged reflectors causing inadequate curing; or overly dry ink leading to material deformation and ink peeling. To solve these issues, three approaches are commonly used. First, reduce the ink thickness by using deeper ink shades or minimizing the amount of ink applied. For example, spot color fields can be matched with stronger colors, and less ink can be used in official prints while increasing UV lamp output and reducing printing speed to ensure proper drying. Second, check the condition of the UV lamp tubes and clean the reflectors regularly. Domestic UV lamps typically last 500–1000 hours, while imported ones can last up to 1000–2500 hours. It's recommended to clean them every six months. Third, adjust the UV lamp power to avoid over-irradiation, especially on large areas, as excessive exposure can cause film curling, deformation, and reduced ink adhesion.
Problem 4: The compatibility between ink and material can also contribute to ink dropout. For instance, if the ink has high molecular tension, it may not form a uniform layer on the substrate, or if expired or low-quality domestic inks are used, ink may easily peel off. In such cases, switching to inks specifically designed for the material—such as film-compatible or light-fast inks—is recommended. Always consult the ink manufacturer for the best match. Before using expired inks, conduct test prints with simple text or small logos. For large-area solid or full-dot patterns, it's better to use high-adhesion, reputable inks like Hangzhou Hua ink.
In summary, there are eight key factors that can lead to ink dropout:
- Poor surface coating fastness in coated films
- Poor ink fastness on uncoated films
- Low dyne value (below 38 mm/m) on corona-treated surfaces
- Dyne value degradation over time (3–6 months)
- Thick ink layers combined with insufficient UV lamp power
- High ink tension causing uneven curing
- Use of expired or low-quality inks
- Over-drying or excessive UV irradiation
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