Flexographic printing has a reputation for being hard to control — and most of that reputation comes from skipping the prepress testing step. Unlike offset or gravure, flexo’s elastic plates, variable impression pressure, and anilox ink metering demand data-driven compensation before a single production plate is made. Here’s how to do the testing and what to compensate for.
Why Flexo Needs Prepress Testing
Three things make flexo different. First, if you prep flexo files the same way you’d prep offset or gravure, the print comes out washed-out, missing highlight dots, and suffering severe dot gain. Second, the plate mounting process alone introduces cumulative error: even with the cylinder held to 0.015mm concentricity and the mounting tape within ±0.015mm thickness tolerance, the combined error reduces overall registration precision enough to require extra impression pressure — which amplifies dot gain. Third, ink is metered through an anilox roller whose cell geometry determines everything about ink transfer, and that geometry must be matched to the specific job.
Prepress testing answers three questions before production plates get engraved: What’s the real dot gain on this press with this ink and this substrate? What anilox specification delivers the target density? And how much does the plate stretch when mounted?
Test Plate: Measure Before You Compensate
A test plate is a small engraved plate carrying diagnostic elements that runs on the production press before final plates are made. It contains:
- Line gauges: Progressively narrower lines to establish the thinnest reproducible rule. Any standalone line thinner than 0.1mm risks bending under impression pressure.
- Text blocks: Character samples at decreasing point sizes to determine minimum legible type.
- Dot scales: Tint patches at known dot percentages — typically 1%, 2%, 3%, 5%, 10%, 25%, 50%, 75%, 90%, 95% — to measure actual dot gain at each level and identify the highlight cutoff point where dots disappear.
Measure the printed test sheet with a spectrophotometer. Plot the measured dot area against the digital file values — this is the dot gain curve. The curve becomes the compensation map: if the file specifies 50% but prints at 68%, the next job’s files get compressed so that 50% in the file corresponds to 50% on press.
For line work, the key measurement is the Bar Width Reduction (BWR) value for barcodes — how much the printed bar width exceeds the digital bar width. Subtracting this value from the file prevents unreadable barcodes. And don’t treat standalone lines the same as grouped lines (text): a single thin line bends more easily under pressure than the same line weight in a character stroke.
Test Anilox: Dial In the Right Cell
Anilox cell angle, line count, cell shape, depth, and width collectively determine cell volume — which is ink volume. For laser-engraved ceramic anilox, the line count typically runs 4 to 5 times the screen ruling. But ink viscosity, adhesion characteristics, and substrate surface all shift what works, which is why the recommended ranges are wide and the only reliable approach is to test.
A test anilox roller is a trial roll with candidate specifications. The plate maker selects the test parameters based on the original artwork type, press model, substrate, and target ink coverage. The test plate is then run with the test anilox on the production press. The printed result determines the final engraving specifications for the production anilox. Without this step, you’re guessing at the most expensive consumable on the press.
Three Pre-Compensations Every Flexo Job Needs
1. Plate Elongation
Mount an elastic photopolymer plate onto a cylinder and it stretches around the circumference. The printed image comes out longer in the machine direction than the file. Three compensation methods:
- At the design stage: Measure the elongation percentage and subtract the corresponding dimension from the original artwork in the machine direction.
- In prepress software: Apply a single-axis scaling command before color separation — shrink the file in the circumferential direction by the measured stretch factor.
- Drum-based plate exposure: When the plate exposure drum has the same diameter as the print cylinder, the stretch is built into the imaging geometry.
2. Dot Gain
Dot gain in flexo has two components: mechanical (the physical spreading of the dot under impression pressure) and optical (light scattering in the ink film and substrate — a visual phenomenon, not a physical one). Mechanical gain is what you compensate for.
The test plate dot scale gives you the gain curve. Feed that curve into color management software for automatic correction, or apply it manually in Photoshop using curves adjustment. What matters is that the gain is measured, recorded, and systematically compensated — not eyeballed at the press.
3. Trapping (Choke and Spread)
In multi-color packaging graphics, text and image elements often sit on colored backgrounds. The foreground color and the background knockout appear on separate plates. When the press shifts — and it will — a white hairline appears between them.
The fix: expand the foreground slightly (spread) or contract the knockout slightly (choke). The rule is universal: extend the lighter color into the darker color. Both operations are standard in Photoshop and trapping plugins. For flexo, where total registration error is larger than offset, the trap width needs to be correspondingly wider.
References
- Wikipedia: Flexography: Comprehensive overview of flexographic printing including plate materials, anilox metering, impression mechanics, and prepress workflow differences from offset and gravure.
- Wikipedia: Dot Gain: Explanation of mechanical and optical dot gain mechanisms, measurement methods, and compensation strategies in halftone printing.
- Wikipedia: Trapping (Printing): Fundamentals of choke and spread techniques, trap width determination, and implementation in prepress workflows for flexo and offset.
- Flexible Packaging Association (FPA): Industry resource covering flexographic printing technology, quality control standards, and best practices for flexible packaging converters.
- Wikipedia: Anilox Roller: Description of anilox cell geometry, engraving methods, line count selection, and the relationship between cell volume and ink transfer in flexo printing.