Analysis of the Causes of Printing Color Difference in Spot Color Ink
To enhance the overall effect of packaging, many customers design a large-area spot color when designing packaging. If it cannot be well controlled during the printing process, it will greatly reduce the grade of the product, thereby affecting the product's competitiveness in the market. Therefore, strict requirements must be met in terms of the quality control of raw materials and the technical quality of operators during printing.
- Inconsistent hue in each batch or the same batch
(1) When making the first proof, it is essential to keep a detailed record of the scraper angle and ink mixing ratio.
(2) Before printing, control is crucial as spot color inks are usually mixed by oneself. It is necessary to ensure the deviation of the inks used is minimized and the mixing ratio is accurate. The ink pan, stirring rod, and ink pump must be cleaned thoroughly. The remaining ink from the previous use should be added to the new ink in appropriate amounts, and the scraper and ink viscosity should be adjusted according to the records.
(3) During printing, it is important to control the viscosity of the ink. It is recommended to increase the frequency of manual measurements or equip a viscosity automatic tracking regulator.
- Uneven ink transfer
(1) When mixing colors, try to reduce the variety of inks. If two colors can be used to achieve the desired hue, avoid using three. Inks from different manufacturers should not be mixed. After mixing, stir thoroughly and add an appropriate amount of methyl ethyl ketone for dissolution. When adding the solvent, do so slowly and stir well to prevent the solvent from impacting and causing poor solubility, which can damage the ink structure and lead to transfer issues.
(2) Reduce the scraper angle and pressure (especially for transitional spot colors).
(3) Water streaks. Increase the ink viscosity. Since the spot color plate is usually deeply engraved, generally at 30-35um, low ink viscosity can cause water streaks. It is suggested to use an ink viscosity of 22-20 seconds (Zahn No. 3 cup) at a printing speed of 120-150m/min.
- Skew lines in embossing
(1) Reduce the pressure of the embossing roller.
(2) If the rubber quality of the impression roller is too soft, replace it with a better one with a hardness of 75 Shore.
- Insufficient peel strength and tack
Since spot colors usually account for 40%-60% or even more of the entire packaging, the peel strength of the spot color must be good. It can be controlled from the following aspects.
(1) The surface tension of the substrate must meet the requirements and should not be less than 38 dynes/cm.
(2) Select ink models suitable for the substrate.
(3) Choose solvents with lower surface tension to reduce the overall surface tension of the ink system, thereby improving the wetting of the substrate.
(4) Increase the heat and air volume of the drying oven. If the heat and air volume of the drying oven are too low and the ink volume of the spot color is large, it can cause sticking and insufficient peeling.