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In the field of packaging printing, when companies take over the business of transparent film packaging, the white ink undercoating process often faces the problem of insufficient opacity, which is a headache for many practitioners. To effectively solve this problem, we must first thoroughly analyze the core factors that affect the opacity of the ink, which mainly cover the three important aspects of ink color concentration, ink layer thickness, and pigment dispersion, and then carry out targeted experiments and analysis.
1. In-depth analysis of ink color concentration
Increasing the color concentration of the ink is undoubtedly one of the common ways to enhance the covering power of the ink. In the actual printing process, a specially formulated titanium white ink with a color concentration of up to 55% was used (compared to the color concentration of 25% in ordinary white ink, titanium white pigment is known for its whiteness and covering power among many white pigments). However, the printing results showed that although its covering power was better than ordinary white ink, it still failed to meet the standard. This is because a high color concentration means a low solvent content, which in turn leads to faster drying and a high risk of clogging. From a professional point of view, this situation shows that simply relying on increasing color concentration is not a good solution, and we must find another way. For example, looking back at earlier printing processes in history, due to the limitations of pigment and ink formulations at that time, when dealing with similar high-color-concentration inks, we often faced more serious printing quality issues, such as uneven colors and rough printed surfaces. This also provides lessons for our current explorations.
2. Comprehensive investigation and response strategies for ink layer thickness
The thickness of gravure inks is usually between 8 and 15μm. If the thickness of gravure white ink is only 3-4μm, which is lower than the standard value, it will cause many problems. In this regard, we can investigate and solve the problem from the following aspects.
(1) Key points in the production of printing cylinders
Although the engraving depth of the printing cylinders provided by current suppliers has reached the limit (60μm), the pyramid cell technology they use has defects. The pyramid-shaped cells are prone to clogging during the printing process, which leads to poor ink transfer. Technically speaking, the structural characteristics of the pyramid-shaped cells restrict the flow of ink, so although the ink storage capacity has increased, the actual ink transfer has not been effectively improved. In contrast, pyramid-shaped or honeycomb-shaped cells have significant advantages. They not only have a large ink storage capacity, but also transfer ink more smoothly. In addition, the smoothness of the cell edges and the thickness of the chrome plating also have a significant impact on ink transfer. In some cases of technological innovation in printing in the past, there have been significant improvements in ink transfer efficiency due to improvements in the shape of the cells. For example, after a well-known printing company introduced a new honeycomb-shaped cell cylinder, the ink transfer rate increased by nearly 30%, and the printing quality improved significantly. Therefore, for cylinder problems, you can consider using the double printing method to solve them.
(2) The suitability of the impression rubber roller for the substrate
When the impression roller is hard and the substrate is soft, the difference in hardness between the two will result in too little ink transfer in the cells. This is because the ink cannot be transferred fully from the cells to the substrate under the printing pressure due to the hardness mismatch. Just like two gears, if the tooth pitch and hardness do not match, efficient power transmission cannot be achieved. At this time, it is a wise choice to replace the impression roller with a softer one to ensure smooth ink transfer and achieve the desired ink layer thickness.
3. Accurate control of printing pressure
If the printing pressure is too low, the ink in the cells will not be squeezed enough, which will inevitably lead to poor ink transfer. Printing pressure plays a key role in the ink transfer process, just like the heart pumping blood. If the pressure is insufficient, the ink cannot be fully transferred to the substrate. Therefore, appropriately increasing the printing pressure is an effective way to solve this problem, which can promote better ink transfer from the cells and thus increase the ink layer thickness.
4. Surface treatment of the substrate and printing time
If the surface of a film is treated and then stored for a long time, the surface properties will change, resulting in poor ink wetting and adhesion. This is because over time, the surface of the film may adsorb impurities in the air or undergo an oxidation reaction, reducing its affinity for the ink. Judging from some actual production cases, the ink adhesion failure rate of films stored for more than a week will increase significantly. Therefore, printing should be carried out promptly after the surface of the substrate has been treated to ensure that the ink adheres well to the surface of the film, which in turn helps to increase the ink layer thickness.
(5) Balance between ink pigment concentration and viscosity
A high concentration of ink pigments can lead to excessive drying speed, which has a negative impact on ink transfer. There is a complex chemical relationship between pigment concentration and drying speed. When the concentration is too high, the solvent evaporates faster, the ink thickens quickly, and the uniform transfer of subsequent inks is hindered. Therefore, it is necessary to reasonably adjust the pigment concentration and viscosity to find the optimal balance between the two, so as to ensure that the ink can be transferred smoothly during the printing process, while maintaining good drying properties, which is conducive to the stable improvement of ink layer thickness.
(6) Effective treatment of electrostatic problems
Films are prone to static electricity during printing, which can interfere with the ink transfer process. Static electricity can cause ink particles to shift during the transfer process or adhere to the printing equipment, rather than adhering evenly to the substrate. For example, in some dry environments, static electricity is particularly prominent, which can lead to problems such as uneven ink dots and color deviations in the printed matter. Therefore, it is necessary to first eliminate the static electricity on the film, and then use an electrostatic ink absorption device to improve the ink transfer rate and ensure that the ink layer thickness meets the requirements.
(7) Comprehensive consideration of the performance of the printing press
The performance of the printing press itself is also an important factor affecting the ink layer thickness. Printing presses of different models and manufacturers differ in terms of ink transfer, pressure control, and the cooperation between the plate cylinder and the rubber roller. For example, some high-end printing machines are equipped with advanced ink circulation systems and precise pressure control systems, which can better achieve uniform ink transfer and precise control of the ink layer thickness. Therefore, when troubleshooting the ink layer thickness problem, the influence of the printing machine performance should not be ignored, and a comprehensive inspection and debugging of the printing machine should be carried out if necessary.
Although the ink layer thickness increased significantly after the above measures were taken, the opacity of the white ink still did not meet expectations, which prompted us to re-examine the characteristics of the gravure ink itself.
3. The core influence of pigment dispersion and the way to optimize it
The hiding power of ink essentially depends on the ratio of the refractive index of the pigment to that of the binder. When the ratio is 1, the pigment is transparent; if the ratio is greater than 1, the pigment has hiding power. The refractive index of the pigment is affected by the degree of pigment dispersion and the difference in refractive index between the pigment and the binder. Generally speaking, the higher the degree of pigment dispersion, the smaller the above difference, and the better the transparency of the pigment.
The white ink pigment used in the test is titanium dioxide, which is a white crystalline powder. It has the highest opacity of all white pigments, with an index of refraction between 1.84 and 2.55. It is also highly dispersible and has excellent light, heat and alkali resistance. However, gravure printing differs from other printing methods. While other printing methods can achieve the desired opacity with a thin ink layer and low color concentration using ordinary pigments, this is difficult to achieve in gravure printing. This is because the particle size of the pigments in gravure inks must be less than 5 μm, and gravure inks are relatively thinner and more dispersed, resulting in high transparency (i.e., low opacity). If the titanium dioxide used has fine particles and is highly dispersed, it will result in insufficient opacity. If the desired hiding power cannot be achieved by increasing the particle size, then perhaps other printing methods should be considered. For example, in some large-area printed products that do not require high printing precision but have extremely high hiding power requirements, such as outdoor large billboard printing, if gravure printing cannot meet the hiding power requirements of white ink, screen printing can be tried. Its ink thickness is relatively large and the pigment particle size is relatively coarse, which can better achieve a high hiding power effect. However, if the ink is formulated using titanium dioxide with an appropriately increased particle size, the covering power of the white ink will be improved. This method is more efficient and convenient than increasing the ink color concentration and ink layer thickness, and can also reduce production costs. This is because appropriately increasing the particle size can effectively improve the covering power of the pigment without significantly affecting other properties of the ink, and reduce problems such as plate blocking and poor drying that may be caused by adjusting the color concentration and ink layer thickness.
In summary, when solving the problem of insufficient opacity of white ink in the printing of transparent film packaging, it is necessary to comprehensively consider multiple factors such as ink color concentration, ink layer thickness, and pigment dispersion, and flexibly adjust the printing process and ink formulation according to the actual situation in order to achieve the desired printing effect and meet the production needs of packaging printing companies. At the same time, with the continuous development of printing technology, there may be more innovative methods and materials in the future to provide new ideas and approaches to solve this problem. For example, researchers are developing new nano-scale pigments that are expected to achieve higher opacity while maintaining good dispersibility, which will bring new changes and opportunities to the packaging printing industry.
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