White spots in printed adhesive labels are a common issue during the printing process. Mr. Qiang Yongsheng, the Technical Service Manager at Dongguan Ande Label Material Co., Ltd., has gathered insights from years of customer feedback and summarized the main causes behind white spots in label printing, along with practical solutions to address them. Here's a detailed breakdown.
In general, four primary factors contribute to the occurrence of white spots: rough material surfaces, powder dust, static electricity, and environmental dust in the workshop. This article will explore each cause and provide effective remedies.
1. White spots caused by a rough material surface
Currently, most self-adhesive label printing in China is done using two main methods: letterpress and flexographic printing. Both of these techniques require a relatively smooth surface on the label material. For example, when using offset printing on coated paper materials, the surface roughness should be less than 1.4 microns. However, for letterpress or flexographic printing, the surface roughness must be under 0.7 microns to prevent white spots during the print run.
This is because the printing pressure used in letterpress and flexographic methods is much lower than that of offset printing. If the material surface is too rough, some areas may not receive ink properly, resulting in visible white spots—especially in large-area dot graphics. Under high magnification, you can clearly see that the white spot area has no ink at all. This happens because the ink on the printing plate cannot reach the lower parts of the uneven surface. Another clear sign of this issue is that the white spots tend to be large and often appear as noticeable patches across the entire print.
To resolve this type of white spot problem, consider the following solutions:
1. Replace the material with a smoother option. If the material’s surface is not smooth enough, it's a quality issue that can be addressed by requesting better-quality materials from your supplier. However, replacing materials might not always be feasible due to tight production schedules. Printing companies often have limited time and need to prepare thoroughly before starting a new batch. If the new material doesn't work well, the preparation effort could be wasted, which negatively affects productivity.
2. Alternatively, you can apply a thin layer of primer or a diluent onto the material's surface before printing. These additives are typically colorless and won’t significantly affect the final print color. After application, they help fill in surface imperfections, improving the material’s smoothness and reducing the likelihood of white spots. This method is often more efficient and cost-effective, especially when material replacement isn't an immediate option.
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