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In offset printing for packaging applications, large areas of solid base color are common, necessitating thick, consistent, and uniform ink laydown. Ink color consistency encompasses two key aspects: consistency across consecutive printed sheets and uniformity across different regions within a single sheet. The latter is particularly critical and serves as a primary indicator of print quality. However, due to the inherently thin ink film in offset printing, challenges such as streaking, back trapping, and uneven density-typically darker at lead edges and lighter at trailing edges-are frequently encountered. As a result, achieving a dense and uniform appearance when printing large solid areas remains technically demanding.
To successfully produce large solid color areas using offset printing, both high-precision printing equipment and optimized process controls are essential.
1. Avoid fine details within solid spot color backgrounds
The standard halftone or solid ink film thickness in offset printing is generally inadequate for rendering large solid color areas effectively. This limitation becomes more pronounced on substrates with insufficient surface smoothness, often resulting in perceived ink thinness and mottling. For solid spot color printing, increasing ink feed leads to thicker ink films on the plate, which in turn raises the risk of ink spread into non-image areas. Therefore, during prepress design, it is advisable to avoid placing fine text or delicate line work within solid color fields. Where small text is necessary, black type is preferred due to its bolder strokes, which reduce the likelihood of ink fill-in or smudging.
2. Improved uniformity through double printing (double-banking)
If a page contains both a black background and a color halftone image, then there must be both large areas of solid color and halftone images on the black page. This determines that the black ink cannot be printed too thickly. Without specially creating a black background page, to achieve a visually thick black background, about 40% of the screen mesh can be added to the black background area of the cyan page. Since the black ink is still black when it is overlaid on the cyan screen mesh, the result is that the ink layer becomes thicker. By using this method, a relatively satisfactory thick effect can be achieved without printing the black ink too thickly.
For some product layouts, in addition to having color images, certain deep solid color backgrounds are also required, such as a bright red background or a deep blue background. At this time, a flat screen printing base layer can be added to the corresponding area of the background color on a certain color plate, and then the solid color can be printed on top of it.
The advantage of using two printing passes to create a large area base color is as follows: Firstly, while controlling the thickness of the ink layer on the base color plate, the ink layer obtained through the double printing is thicker; Secondly, it can significantly reduce the adverse effects of ink film, paper dust, streaks, etc. on the printed image.
III. The layout should be designed to facilitate even ink distribution.
Try to avoid having long-shaped gaps in the middle of the actual printing area that run along the rotation direction of the printing cylinder. Due to the limited ink transfer capacity of the printing machine, when printing, the upper and lower ends of the gaps along the rotation direction of the printing cylinder will have darker ink colors, which seriously affects the aesthetic appearance of the picture. If this gap is not within the shape of the finished product, consider filling it with the same solid area as the surrounding areas, or, if possible, rotate the entire pattern by 90 degrees. When making the printing plates for packaging products, since multiple patterns are often overlapped and placed together, the connection areas often have long-shaped blank parts. These blank parts should be filled with the same solid area as the surrounding areas. Although some ink is wasted, it is beneficial for the ink color to be uniform.
Four. Printing is the Most Crucial Step
The high precision of the printing machine is a prerequisite for printing a uniform solid color over a large area. If a uniform solid color can be printed with a relatively low printing pressure, it indicates that the precision of the offset printing machine is high. The processing precision of drum bearings, transmission gears, as well as the reliability of clutch pressure mechanisms, ink distribution mechanisms, etc. are particularly important.
1. The magnitude of printing pressure
When printing on a substrate, a higher printing pressure is adopted. On the one hand, this helps ensure that the printing surface makes full contact, resulting in a dense and thick print; on the other hand, it promotes the pressurized penetration of the ink on the paper, accelerating the solidification and drying of the ink.
2. Stability of printing pressure
During the rolling process of the printing cylinder, the rubber cylinder, and the impression cylinder for one complete cycle, if the printing pressure changes, it will inevitably lead to a change in the ink transfer rate, ultimately resulting in uneven ink color on the solid areas.
3. The elasticity of the rubber blanket
Over time, the rubber blanket will gradually age. As a result, the ink on the surface of the rubber blanket and the surface of the paper cannot fully come into contact, leading to poor ink transfer and problems such as uneven and insufficiently thick solid areas.
Furthermore, the tighter the rubber blanket is stretched, the more the rubber layer thins and the more severe the elastic loss becomes. For air cushion rubber blankets, it may even cause the air cushion layer to completely fail, which is not conducive to achieving uniform ink color.
4. Drum lining
If the liner for the plate cylinder and the rubber cylinder is not properly installed, it will cause the printing surface to have significant slippage. The inconsistent sliding amount on the printing plate surface will result in ink streaks, thereby affecting the uniformity of the solid ink layer.
5. Printing plate
Full-page printing with solid ink should normally require a separate solid ink printing plate to be exposed. However, it is also possible to perform full-page printing using an old printing plate. This approach can reduce production costs and improve efficiency. However, there are slight differences in the adsorption and transfer capabilities of the photosensitive resin in the graphic part of the PS plate and the alumina in the blank part. As a result, the ink layer corresponding to the graphic part of the old printing plate on the printed product will be slightly darker than the ink layer corresponding to the blank part of the old printing plate. For strong colors, such as dark blue, it is best to use a completely blank printing plate or install the old printing plate upside down (with the front side facing in and the reverse side facing out) for printing. For weak colors, such as printing yellow and applying gloss varnish, since it is difficult for the human eye to distinguish such small differences, the old printing plate left over from the previous printing can be directly used.
6. Ink Mixing
The ink used for printing solid areas has a slightly greater fluidity than the ink used for printing halftone images. If the ink fluidity is too low, it is difficult to spread evenly among the ink rollers, which may result in the ink color becoming darker at the beginning and lighter at the end. With greater fluidity, the ink will quickly level out before drying, which is beneficial for increasing the coverage of the solid areas and achieving a uniform and thick ink color effect. The worse the paper quality is, the more necessary it is to enhance the ink fluidity when printing solid areas.
The adjustment of ink fluidity mainly involves regulating the viscosity of the ink. Generally, low-viscosity No. 6 mixing oil is added to dilute the ink. However, the dosage must be strictly controlled as well, because if the viscosity is too low, the ink is prone to emulsification.
When printing on copperplate paper, in order to prevent the backside from getting dirty, a small amount of desiccant or release agent needs to be added to the ink.
7. The ink transfer property of the ink roller
The ink rollers may become smooth due to aging or improper cleaning, and their ability to adsorb and transfer ink will be weakened. Especially for the ink application rollers, if they cannot adsorb sufficient ink, when they first transfer ink after contacting with the printing plate and the graphic elements, due to the low ink storage capacity, the dark tones and solid areas on the printing plate have already absorbed most of the ink on their surfaces, and the transfer rollers cannot immediately obtain sufficient ink to replenish it. Therefore, during the second ink transfer, the ink received by the dark tones or solid areas of the image is much less than that during the first transfer. Although there are four ink application rollers for ink transfer, the ink color phenomenon of being darker at the beginning and lighter later still occurs. Therefore, the ink application rollers, especially the first two, must be selected with good surface ink transfer performance.
8. Adjustment of the pressure of the ink roller
The pressure between the ink rollers and between the ink rollers and the printing plate should be adjusted appropriately. If the pressure is too low, the ink transfer is poor. Especially if the contact pressure between the ink transfer roller and the transfer roller is too light, the ink transfer roller will not receive sufficient ink, resulting in a phenomenon where the ink color on the plate is lighter at the beginning and heavier at the end; while if the pressure is too high, it will accelerate the aging of the ink rollers. Especially if the pressure between the ink transfer roller and the printing plate is too high, impact and jumping will occur at the nip point, and ink streaks will appear. The pressure between the same ink transfer roller and the transfer roller should be slightly less than the contact pressure between it and the printing plate. This is to ensure that when the surface linear speed of the transfer roller and the printing plate are not consistent, the ink transfer roller can run synchronously with the side with higher pressure, that is, the side of the printing plate, so that the surfaces of the ink transfer roller and the printing plate have as little sliding as possible.
9. Pressure adjustment of the water roller
If the pressure between the water rollers or between the water roller and the printing plate is too small or too large, it will lead to poor water transmission, causing the printing plate to get dirty. Excessive pressure between the water roller and the printing plate, or excessive pressure between the water roller and the water transfer roller, may cause white streaks.
10. Adjustment of the transfer roller
To achieve the best ink distribution effect, the axial movement of the ink roller is generally at its maximum position. Being in this position also facilitates the operator to quickly adjust the axial ink volume on the plate evenly. Some printing machines allow the ink roller to move axially after adjustment. To achieve a better ink distribution effect, the ink roller can be allowed to move slightly axially. By adjusting the starting point of the axial movement of the ink roller, the circumferential distribution of the ink can be changed. It is difficult to determine which starting position is definitely the best one. Generally, it can only be determined by gradually adjusting the starting position of the axial movement to find a position that is most conducive to uniform ink color.
11. Controlling the effect of emulsification on ink color
When printing the solid base color, the emulsion consumes a lot of water, resulting in a large water supply. At both ends of the solid base color area, especially when the base color part approaches or reaches the edge of the paper, the areas outside the paper neither have ink and water emulsification nor do they absorb water from the paper blank part. As a result, excessive water will enter the solid base color area at both ends of the paper along with the movement of the water roller, causing the base color ink layer at both ends of the paper to be overly emulsified, the ink color to become lighter, and it may also cause the back to get dirty. The reasonable approach is to try to use a water blowing device or attach a paper strip on the water drum roller to reduce the water supply at both ends of the printing plate, and control the degree of emulsification throughout the entire plate surface.
