Prepress Design Considerations

1. Size Deformation Processing of Imposition Files

For "gravure and soft" PE film flexible packaging, registration issues are generally not a concern. As long as the job size and uniformity in the circumferential direction of the plate cylinder are considered, an appropriate imposition method and matching plate cylinder size can be selected. Proper deformation treatment should also be applied to the imposition document, typically elongating the circumferential dimension by 0.5% to 2% and reducing the axial dimension by 0.3% to 1.2%. This helps ensure accurate printing after stretching during the process.

2. Handling of Small Text, Thin Lines, and Barcodes

"Gravure and soft" flexible packaging products require careful attention when dealing with small text, thin lines, and barcodes. Normally, the small Yang characters in gravure soft packages should be thickened by 0.01mm, and individual small Yin characters need to be finely processed. Barcodes should be adjusted by 0.02~0.03mm. When switching to flexo printing, the approach is reversed to prevent thin lines from expanding or fine negative lines from becoming too large.

To ensure barcode readability, testing different colors, scale sizes, and anilox rolls under flexographic conditions can help determine the best processing data. For example, a barcode scaling range of 90% to 110%, with a left safety distance of 4mm and a right safety distance of 3mm, along with a thickness of 0.07~0.1mm, is suitable for anilox rollers with 500-800 lines per inch. Black and dark blue are preferred for optimal visibility. Using a 1016-line anilox roll (BCM value of 2.0), a positive line width of 0.018mm and a negative line width of 0.053mm can be achieved, but this is close to the limit of what's possible.

3. Trapping and Registration

Modern satellite flexo printing machines offer high registration accuracy, which should be fully utilized for "gravure and soft" flexible packaging. The offset value for flexo-printed flexible packaging is typically between 0.1 to 0.3 mm, depending on the specific product. The trapping value can be determined based on the layout image distribution and imposition design.

It’s important to focus on circumferential registration. If axially spliced products can be arranged out of alignment in the layout, it can improve registration and reduce bar issues.

4. Field and Dot Gradient Processing

Gravure printing doesn’t face dot gain issues, but flexo printing must address them, especially when fields and dot gradients appear together. A comparative analysis of monochrome dot gradients illustrates the differences between the two processes.

In the gravure process, a 150 lines/inch plate with a minimum dot size of 1% can print both solid areas and dots seamlessly. Controlling the plate depth (35–50μm) ensures the required density in the field, and the transition from gradual to absolute network is smooth and stable for mass production.

In flexo printing, CDI plates are used with 150 lines/inch screens and a minimum dot size of 1%. Solid density is mainly determined by the anilox roller’s ink storage capacity (BCM value), along with the plate material and ink. Flexo printing faces dot gain, where the minimum dot size increases by 5–8% at 1%. Jumping directly to 0% can create visible gaps. High-line anilox rollers are needed to match small dots, but they store less ink, limiting gradient density. Hence, 150 lines/inch and 1% minimum dot are rarely used in flexo printing for flexible packaging.

Testing on hygienic bags using 120 and 150 lines/inch plates with a 3% minimum dot size and 1016 lines/inch anilox rolls showed that flexo proofs were close to gravure proofs, but with limited ink loading and shallow images. To improve this, companies can use spot colors, shallow mesh layouts, and multi-color overprints. They should also consider 120–150 lines/inch plates, anilox rolls with more than 1000 lines/inch, and HD Flexo technology. Additionally, solvent-based inks currently used in flexo have higher color density than gravure inks, and improving this could significantly enhance flexo packaging quality.

Plate Making Precautions

The CDI plate-making process has been widely used in China’s flexo packaging industry for over a decade, with a mature process and stable plate quality. Flexo packaging requires thin plates, typically 1.7mm or less. The 1.14mm laser plate is the industry standard, capable of producing 175 lines/inch plates with 1% minimum dots. However, few companies dare to use it due to insufficient hardware and challenges in process control.

Printing plates should have a flat surface, no residual solvents, moderate viscosity, proper relief height, sufficient exposure, and sharp, stable dots. Plate durability is also crucial. In the coming years, high-screen plates and new screening technologies will raise the bar for plate lifespan. It’s recommended that suppliers consider introducing thinner plates from abroad for preliminary testing.

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