The Most Effective Method for Controlling the Use of Spot Color Inks

Posted on 05-15-2024 by

Currently, many printed materials use spot color inks to ensure consistent color quality. In anti-counterfeiting printing, special security spot color inks are often used to enhance anti-counterfeiting effects. In map printing, due to process and material limitations, spot color printing is frequently employed, sometimes using over 20 colors. However, compared to four-color inks, the consumption of spot color inks is much smaller, making large-scale production by ink manufacturers unfeasible, leading to high costs for spot color inks. As a result, most spot color inks are mixed by the printing companies themselves. Given the low consumption, the mixed ink often exceeds the actual usage, leading to leftover ink that, when stored, can degrade in performance. For printing companies, leftover ink is a significant issue, greatly affecting production costs and cannot be ignored.

To address this problem, the fundamental solution is to accurately calculate the ink usage in printing. Some ink manufacturers or suppliers provide rough methods for estimating ink usage. For instance, the Dutch company VANSONINK and the British company Vanson offer their own ink calculation methods. Domestic counterparts have also summarized some methods through practice. Hengsong will provide you with a method to calculate ink usage (under the offset printing method, with the printing machine working normally, and the quality of the printed products meeting standards).

To determine the ink usage, the first step is to analyze the factors affecting the ink usage and the calculation methods.

Printing Area

The printing area and ink layer thickness determine the ink volume and consequently the ink usage. The size of the printing area is determined by the original image. However, determining the actual image area to be covered by each spot color is not easy. For regular images with known dot percentages (flat screens), the actual printing area can be easily obtained. But for irregular images with rich gradation, how can the actual printing area be calculated? Here, we introduce a method—the digital image processing method, with the following steps:

1. Digitally capture and store the original image in the given area. For analog planar images, the input device processes it into a digital image stored in the computer; this step can be skipped for digital images.

2. Process the collected original information, identifying and summing all pixels that meet the requirements.

3. Output the sum of the calculated results, which represents the actual dot area needed for printing.

Some printing companies use another practical method: weighing the copied pattern and a cut piece of paper (250px x 250px) to calculate the image area as follows:

Image area = (100 * weight of copied pattern) / weight of the paper

Ink Layer Thickness

Ink layer thickness is also a decisive factor for ink usage. The complexity and variability of printing conditions and substrates cause uncertainty and unevenness in ink layer thickness. With continuous advancements in high-precision and high-reliability offset printing machines, when the quality of printing materials is guaranteed, the maximum error in ink layer thickness during printing will not exceed 0.058μm, thus having a relatively small impact on total ink usage. Therefore, generally, we can consider the ink layer thickness in the printing environment mentioned above to be uniform.

The ink layer thickness range is not arbitrary. Generally, in a specific printing environment (printing machinery, substrates, inks, and printing room temperature, and humidity), there is an optimal ink layer thickness that ensures full ink coverage, maximum solid density, and print contrast, while minimizing dot gain and achieving optimal gradation reproduction for stable print quality.

There are two common methods to measure ink layer thickness: a common one is the weighing method, which compares the weight difference of a substrate before and after printing, divided by the area of the printed section.

Ink Density

Ink density is often measured in g/cm³ and can be calculated using an ink suitability tester or an ink distributor. Some ink suppliers provide this parameter.

Print Run

This is the total quantity of prints required.

Substrate Characteristics

If only considering paper substrates, they can generally be divided into coated and uncoated paper. The ink absorption, surface smoothness, and structural strength of the paper will affect ink usage. These factors are suitable for qualitative analysis or added to other factors for quantitative analysis due to the difficulty of precise quantification.

Printing Equipment Characteristics

The characteristics of the printing equipment affect ink usage mainly in two ways: one is the impact of printing pressure and speed on ink layer thickness and dot gain; the other is the minimum ink volume the printing machine can handle, which is the ink needed to cover the mechanical surfaces in the printing ink path and a certain amount of ink reserved in the ink fountain to stabilize the ink supply.

Ink Waste During Printing

During printing, besides some ink being lost in the ink transfer process and on the plate and blanket cylinders, some ink is also wasted during proofing, which needs to be considered. The waste amount can be quantified based on actual printing conditions.

Pre-stored Ink

Due to various unknown factors, printing companies usually mix extra commonly used spot color inks for storage. This parameter is determined by the printing company itself.

Conclusion

To determine the two main factors—printing area and ink layer thickness—a quantitative analysis is needed to determine their values, and verification may be required. Ink density and print quantity generally have minimal errors. Substrate characteristics and printing equipment characteristics are more suitable for qualitative analysis. Ink waste rate and pre-stored ink are related to the actual printing volume. Although some factors cannot be precisely determined, their impact on the overall ink usage calculation is minimal, with errors controlled within a very small range.

With the known parameters, the required spot color ink usage can be calculated using the following formula:

. Ink usage for one print (g) = Printing area * Ink layer thickness * Ink density

. Total ink usage = Ink usage for one print * Print quantity + Minimum ink volume for the press + Ink waste

Given the variability of substrate characteristics, printing equipment characteristics, ink waste, and pre-stored ink, these parameters are adjustable based on actual conditions.

In summary, controlling the amount of spot color ink used is a complex and intricate process. To save on spot color ink costs for printing companies, Hengsong is perfecting a color mixing center aimed at reducing ink costs, providing scientific and quantitative computer color matching solutions for our printing partners.