The consistency of raw materials is the fundamental prerequisite for achieving uniform color blending, requiring strict control from both ink and substrate perspectives. As the core carrier of color presentation, ink batch stability directly impacts blending results. It is essential to purchase inks from the same brand and model to avoid discrepancies caused by pigment particle size, resin content, or solvent ratio differences across manufacturers or batches. Before use, inks must be pre-treated: thoroughly stir bucketed inks, especially those stored for more than three months, as pigment sedimentation may cause stratification. A mechanical stirrer should be used at 500–800 rpm for 15–20 minutes to ensure full integration of pigments, resins, and solvents. Care must be taken to prevent excessive air entry during stirring, as trapped air can cause bubbles during printing, resulting in uneven color layers. Substrate surface characteristics also influence color adhesion. For paper labels, surface smoothness and absorbency must be checked, while for film-based labels (such as PET, PVC), surface tension must meet standards (≥38 dyne). If surface tension is insufficient, inks may show pinholes or sagging; in this case, corona treatment is necessary to enhance adhesion and ensure even spreading.
A scientific color-matching process is the core to achieving uniform blending, following the logic of “sample testing – batch adaptation – precise fine-tuning.” During sample testing, ink proportions should be calculated according to design values (e.g., Pantone codes) and weighed with an electronic scale accurate to 0.01 g, avoiding estimation errors. For example, when blending red labels with a 7:3 ratio of red to yellow ink, weigh exactly 7.00 g of red ink and 3.00 g of yellow ink. Mix in a dedicated container using a stainless steel spatula in one direction for at least 5 minutes until no visible streaks or color blocks remain. Apply the blended sample evenly on the same substrate as the final label, place it under a standard D65 light box for 10 minutes, and check color tone and uniformity after partial solvent evaporation. If deviations occur, adjustments should follow a “small amount, multiple times” principle—for example, if the result is too yellow, add no more than 0.1 g of red ink per adjustment, remix, and recheck until the color deviation is within ΔE ≤ 1.5 (an industry-accepted tolerance). For batch blending, scale up proportions based on the sample while maintaining consistent stirring strength and duration—for instance, if a 5-minute stir was used for a sample, a 50 kg batch should be stirred for 30–40 minutes, with samples taken every 10 minutes to ensure uniformity.
Accurate equipment parameter adjustment is key to ensuring consistent color transfer. In flexographic printing, the anilox roller—responsible for ink transfer—must match ink properties. Fine-pigment inks require 300–400 LPI anilox rollers, while coarser pigments suit 200–250 LPI rollers, ensuring uniform cell filling and ink transfer. Rollers must be inspected before installation; clogged cells must be cleaned with alkaline solutions and soft brushes to avoid uneven transfer. Doctor blade pressure and angle should also be optimized: the blade angle should be set between 30°–45°, with pressure at 0.15–0.2 MPa. Excessive angles or pressure can cause local roller wear and streaks, while insufficient pressure leads to leftover ink accumulation and smudging. Drying temperature and speed must match ink type. For solvent-based inks, drying should be set at 60°–80℃, synchronized with printing speed (e.g., drying speed 30 m/min for printing speed 30 m/min) to avoid premature skinning or sagging.
Dynamic process control is critical for preventing color deviation. Environmental stability is essential: temperature should be kept at 20°–25℃, relative humidity 50%–60%. High humidity can cause paper labels to warp and absorb unevenly, while low humidity accelerates solvent evaporation and leads to ink drying issues. Ink viscosity should be checked hourly. Solvent-based inks should remain at 18–25 seconds (Ford cup No.4), and water-based inks at 25–35 seconds. If viscosity rises, add diluent (e.g., ester solvents for solvent inks, deionized water for water-based inks) in amounts no more than 5% of total ink, then stir thoroughly. For multi-color overprinting, printing pressure and speed must be consistent across units, with registration accuracy kept within 0.1 mm. Misregistration can cause uneven color overlap, ghosting, or gaps, requiring immediate plate and tension adjustments.
Quality inspection and traceability systems form the final safeguard. A dual mechanism of “in-process sampling + finished product full inspection” should be established. During in-process checks, for every 500 meters of labels produced, 10 samples should be taken. Under a standard light box, both visual comparison and instrumental testing should be performed. For visual comparison, place the sample next to the standard color card at a 30 cm distance, observing at a 45° angle to check for local color variations, streaks, or blotches.
