Application of Flexographic Proofing Press in Evaluating Color Reproducibility of Flexographic Inks

This article primarily discusses the use of a flexographic proofer to evaluate the color reproducibility of flexographic inks. It introduces the role of the proofer in simulating production conditions, as well as how to prepare samples by setting parameters such as pressure, speed, and anilox rolls. Color measurements are then taken using a spectrophotometer/densitometer to calculate indicators such as color difference, solid density, and dot gain, thereby assessing the accuracy of the ink color. The article also cites an example of water-based ink used for corrugated boxes, illustrating the color difference between the undiluted ink and the letdown ink. Finally, it emphasizes the importance of standardized operation and proper cleaning of the proofer to minimize color deviations in mass production.

Introduction

Flexographic printing is widely used in corrugated boxes, flexible packaging, labels, and other fields, and ink color reproduction is the key to evaluating the quality of prints. The proofer simulates mass production conditions and quickly evaluates flexo inks. This paper focuses on the application of proofing machine in evaluating the color reproduction of inks, and analyzes its scientific nature and operational points.

The role of the proofer

The proofing machine can control the pressure, speed, and anilox roller parameters to prepare flexible proofs on the substrate. By comparing the proofing sample with the standard color chart or target color value, the color saturation, hue deviation, and overprint effect of the ink can be evaluated. The proofer also verifies the adhesion and drying properties of the ink on the specified substrate.

Key points of experimental design

First, set the proofing parameters according to the standard regulations (such as GB/T 26461 or ASTM D7305): the number of anilox roll lines (in terms of line/inch), the angle of the scraper (about 45°), and the printing pressure (0.1-0.3 mm gap). The ink viscosity should be controlled at 25°C for 4-6 seconds (with Zahn 4# cup). The substrate was pre-humidified, and the samples were dried in a constant temperature and humidity (23°C, 50% RH) chamber for 24 h before being measured.

Color measurement method

The L*, a*, b* values of the samples were read by the CIE L*a*b* system using a spectrodencimeter, and the color difference value ΔE was calculated_ab = √[(L*₁−L*₂)² + (a*₁−a*₂)² + (b*₁−b*₂)²]。 Usually ΔE_ab ≤ 3 can be considered an acceptable difference. The field density (D) should also be recorded_max) and the branch expansion rate (T = D_m−D_f）。

Key evaluation indicators

Indicators	Description
Chromatic aberration ΔE_ab	Combined deviation between the sample and the standard color value, ΔE_abThe smaller it is, the higher the reproducibility
Field density	reflects the ink coverage; The fluctuation of field density ≤ 0.05 is stable
Expansion of outlets	measuring the reproduction of intermediate tones; The enlargement value varies with the number of reticulated threads and pressure
Overprint rate	The ink compatibility during multi-color overprinting was detected and compared with CIE Lab values
Gamut volume	Using the CIELAB color gamut, the proofing should cover more than 85% of the nominal color gamut

Practical application cases

Taking flexo ink for corrugated boxes as an example, the original ink and diluted ink (dilution ratio 1:4 to 1:9) are tested by proofing machine. Under the same proofing conditions, the ΔE of the original ink_abis 1.8, diluting the increase in the expansion rate of outlets due to increased transparency, ΔE_abincreased to 3.2. Accordingly, the ink formula can be adjusted to improve the reproducibility of small and medium-sized outlets.

Influencing factors

The number of anilox rollers of the proofing machine (such as 200-400 line/inch) determines the ink transfer volume; The higher the number of lines, the thinner the ink layer, and the color saturation decreases. Excessive printing pressure (>0.4 mm) will cause dot deformation and hue shift. The solid content and dispersion of pigments in inks also directly affect color reproduction.

Standardize operational suggestions

Ensure the cleanliness of the proofing machine, and thoroughly clean the scraper and anilox roller with a special solvent after each use. Calibrate the spectrometric density meter to the reference whiteboard. At least 3 parallel proofs, take the average. Record ambient temperature and humidity and ink batches for easy traceability.

Summary

The proofer provides a reproducible testing platform for the evaluation of color reproduction of flexo inks, helping operators determine the match between ink and substrate. By standardizing the measurement of parameters such as color difference, density and dot expansion, the risk of color difference in mass production can be effectively reduced and the consistency of printed products can be improved.

Cite the article

1. GB/T 26461-2011 Quality requirements and inspection methods for flexographic printed materials.

2. ASTM D7305‑08 Standard Practice for Evaluating the Reproducibility of Flexographic Printing.

3. Flexo Ink Color Management Practice, China Packaging Federation Technical Materials, 2022.