Five Key Standards for Assessing Printing Ink Quality

Posted on 04-09-2024 by

Drying Speed

The drying speed of ink typically affects printing quality and can lead to various printing issues, such as causing dirty backsides of prints or even hindering the smooth progress of web printing. Users commonly express that the drying speed of domestic ink significantly impacts print quality. Issues such as the occurrence of defective prints and waste due to inadequate ink drying, as well as the considerable influence of season and climate on drying speed, cannot be ignored.

Printability

In modern printing, there is a wide variety of substrates and auxiliary consumables, each with significant performance differences. Therefore, the requirements for ink have become more complex. Companies generally attach great importance to the printability of ink to ensure its suitability for machine printing and compatibility with various consumables and substrates. They often maintain relationships with a few fixed ink suppliers to communicate their special requirements. For example, when using conventional offset printing ink to print coated paper, the colors on the printed images are bright, and the gloss is good. However, when the same ink is used to print lightweight coated paper, transparency issues may arise, and glossiness decreases. Yet, there seems to be no dedicated printing ink available for lightweight coated paper on the market. Therefore, ink manufacturers are hoped to develop corresponding inks based on different substrate materials, especially the commonly used new materials like lightweight coated paper.

Additionally, the choice of ink may vary depending on the equipment used by newspaper printing plants. For instance, up-web printing machines have higher requirements for ink performance as they apply ink from the top. Currently, domestic newspaper inks do not fully meet requirements in terms of properties such as rheology and viscosity. Printing equipment using down-web printing also faces issues like ink emulsification and dirt, with newspapers requiring high standards for images and headlines. Therefore, ink manufacturers need to adjust ink properties according to actual situations.

Stability

For printing plants, the stability of ink also affects print quality. Some companies have used foreign-brand inks produced domestically. Although the ink formula is the same as imported products, differences in production conditions and management levels between domestic ink factories and the original ink manufacturers result in slight discrepancies in ink performance of the same brand and model. For example, using the same red ink and proportions may result in noticeable color differences. There is still a considerable gap between domestic and imported inks in terms of color intensity, environmental friendliness, and drying speed. Domestic inks often have poor flowability, higher viscosity, and may require the addition of anti-set-off agents. Printing plants hope for improvements in ink viscosity to gradually replace imported, expensive inks. Newspapers, in terms of printing effects, require inks with high viscosity and low tackiness and good transfer properties, especially considering the higher drying speed requirements for high-speed printing presses.

Color Density

Color density refers to the saturation of colors. Nowadays, newspapers are no longer simply black text on white paper but include more colorful images and graphics, increasing the demand for higher color density in offset printing inks. Ads, images, and headlines on color newspapers need to stand out, requiring relatively higher color density in inks. However, many newspaper inks currently do not meet the desired color density. To ensure printing density, printing plants have to increase ink quantity or enhance dot percentages, inevitably leading to ink stacking and increased ink usage. Excessive ink usage can also cause dot gain. Typically, rotational offset printing dot gain should be kept below 25%. Exceeding this can result in loss of detail and excessive dot gain, especially noticeable in images. Moreover, insufficient saturation of black ink used to be a significant issue in previous years, though it's less of a concern now. The reason for the low color density of black ink is that printing plants, for cost-saving reasons during regular text printing, opt for lower-priced, slightly inferior black ink. As there is demand for such ink, some ink manufacturers produce low-quality inks, affecting overall ink quality.

The low ideal color density of domestic inks is attributed to several factors: Firstly, most ink factories do not produce pigments themselves but purchase domestically produced pigments, which still have a quality gap compared to imported ones. Secondly, to save costs, ink factories use fewer pigments, resulting in insufficient color saturation. Another crucial factor is that even if pigment quality and quantity meet requirements, national standards stipulate that ink particle size should not exceed 15μm. Finer grinding of pigments results in brighter ink colors. If pigment processing is inadequate, such as inappropriate particle size after grinding, it affects ink color density. Of course, the quality of printing ink is not only related to ink but also to plate making, printing equipment, ink-water balance, and other technical conditions, making ink quality the prerequisite and guarantee for improving printing quality.

Overprinting Capability

The requirements for ink trapping properties in newspaper printing mainly include two aspects: color sequence and ink viscosity variation. Currently, the standard color sequence for newspaper printing is cyan-magenta-yellow-black, with viscosity gradually decreasing to avoid ink pulling. Some large ink manufacturers have recognized the importance of adjusting ink viscosity values according to printing color sequences and often communicate with ink suppliers to timely adjust ink properties.