During the newspaper printing process, various issues often arise alongside ink transfer, which can directly or indirectly affect the quality of the final product. For example, problems like ink overprinting can cause images to bloom, while ink penetration may lead to smudging and poor print clarity. Additionally, flying ink and ghosting that occur during the process can significantly impact the overall quality of the newspaper.
Image Blooming
Image blooming is a common issue in newspaper printing, primarily caused by ink overprinting. The color reproduction effect of newspaper images also depends on the overprinting performance and the overprinting rate. The overprinting rate, also known as the ink's receiving power, measures how well different ink layers adhere to each other. A higher value typically means better overprinting results. When measuring the density of a single-color ink, a color filter is used in the densitometer through subtractive color mixing, with the filter color usually complementary to the ink being measured.
Overprinting methods include dry overprinting and wet overprinting. Dry overprinting involves applying the next color after the previous ink has dried, while wet overprinting occurs when the second ink is applied before the first one is fully dry. In newspaper printing, where the time between colors is very short, the ink films are often in a wet state, leading to wet overprinting. To improve the overprinting rate in wet conditions, it's important to control the ink’s adhesion and viscosity. Typically, the viscosity order for each color group follows: first color > second color > third color > fourth color, taking into account ink film thickness and printing sequence. Since black is the base in most newspaper printing, the standard order is green → magenta → yellow → black.
Smudging and Print-through
Smudging and print-through are common issues in newspaper printing, often resulting in unclear or blurred images. This can be due to excessive ink on the paper roll or triangular plate, or issues with ink penetration. Cleaning the paper roller and triangular plate usually resolves the first problem, but the latter requires more detailed troubleshooting.
Newsprint has a relatively smooth surface, and during the embossing process, ink relies on the capillary action of the paper under pressure. This includes both pressure penetration and free penetration. If not properly controlled, this can lead to ink offset or bleeding, which reduces print quality. Experimental data shows that penetration depth increases with imprinting time, decreases with higher ink viscosity, and rises with increased printing pressure.
Additionally, maintaining a proper ink-water balance with minimal water usage helps reduce ink emulsification and improves drying. Controlling dampening solution amounts, measuring relevant parameters, and selecting suitable paper can all help speed up ink fixation during the printing process. These steps are crucial in addressing smudging and print-through issues.
Flying Ink
Flying ink is a frequent problem in high-speed newspaper printing, causing environmental pollution and health risks for workers. Factors affecting flying ink can generally be categorized into mechanical, environmental, and ink-related causes.
Mechanical Factors
The faster the printing speed, the more likely ink filaments will break, leading to severe electric double-layer damage. To reduce flying ink, it's essential to control the printing speed. For small roller printing machines, speeds are typically kept around 600 rpm. Using the principle of homologous repulsion, adding a contact roller or installing a weak metal wire can also help minimize the issue.
Moreover, the thicker the ink layer, the more ink participates in splitting and breaking, increasing the severity of flying ink.
Environmental Factors
At a constant temperature, higher humidity reduces the charge carried by ink filament fragments, thus reducing the flying phenomenon. Increasing workshop humidity, improving ventilation, and ensuring fresh air circulation can effectively reduce flying ink in real-world printing scenarios.
Ink Factors
Larger ink particles result in more filament splitting and a more noticeable flying effect. Table 4 illustrates the impact of ink particle size on flying ink. In practice, using ink with smaller particles or adding an appropriate plasticizer can help reduce the occurrence of flying ink.
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