Currently, there are primarily three types of printing methods used in the field of self-adhesive label printing in China: letterpress printing, flexographic printing, and PS plate printing. Among them, letterpress printing is still the mainstream due to its relatively thick ink layer, which results in bright and vivid colors. However, letterpress printing may also encounter various problems during daily production. Today, we will discuss some of the more common problems, their causes, and solutions.
The so-called "ghosting"refers to the uneven thickness of ink transferred onto the printing plate during the printing process, resulting in an uneven printing effect. Ghosting often occurs on products with large blank areas in the middle of a pattern because the large area of blank space corresponds to a large concave area on the printing plate. When the ink transfer roll corresponds to this area, the ink is not transferred to the printing plate, resulting in an excessive amount of ink on the roll in that area. If this ink is not evenly distributed, it will be transferred to the printed product and cause partial color distortion, resulting in ghosting.
Therefore, it is clear that ghosting in letterpress printing is mainly caused by uneven ink transfer. To eliminate ghosting, the ink transfer needs to be made even. Here are several ways to ensure even ink transfer:
Add auxiliary ink rollers
One or two small ink rollers can be added to the plate cylinder of the printing press equipment to make the ink easier to distribute evenly. Many devices are currently equipped with ink rollers for this purpose, so it is not difficult to add them. If the equipment does not have matching ink rollers, you can also consult with the supplier to customize them. The process of adding ink rollers has already been matured, so it is easy to operate.
Reduce ink viscosity
Reducing the viscosity of the ink can also have a good effect on even ink distribution besides adding ink rollers. This is because the ink is easier to distribute evenly when the viscosity is reduced. Generally, some ink thinner or defoaming agents can be added to the ink, with a proportion of no more than 5%, and the ink can be pulled out into a 15-20cm ink line after being added.
It should be noted that ink has a thixotropic property, which means that the viscosity of the ink will decrease when it is agitated in the ink fountain. Therefore, the proportion of ink thinner should be controlled according to the actual situation to avoid low viscosity of the ink affecting the printing effect.
Deep ink and shallow printing
A large amount of special color ink is needed in letterpress printing. When a large area of blank space needs to be printed with special color, the color of the special color ink can be made darker to reduce the ink supply during the printing process, thereby achieving the effect of deep ink and shallow printing, as a thinner ink layer is easier to distribute evenly and avoids ghosting.
The problem of blurred printing is also relatively common in letterpress printing. Blurred printing refers to the phenomenon where the dot patterns printed are unclear and merged together, seriously affecting the printing effect. The following are the main causes of blurred printing:
Worn dot patterns on the old plates
In the production process of letterpress printing, the printing plate can be reused. If some old printing plates have been used too many times, the dot patterns in some areas may be too worn and require a large amount of pressure to produce clear patterns. However, due to the large pressure, areas with less wear and tear will also be affected, resulting in blurred printing. In this case, it is recommended to make new plates.
Excessive ink amount
Letterpress printing is a waterless printing method that does not require a dampening solution. In addition, the printing plate and the ink roller are in direct contact, so if the ink amount is too much, it will directly cause blurred printing. In this case, the problem can be solved by controlling the ink amount reasonably.
If the viscosity of the ink is too low, it will also lead to blurred printing. This kind of blurred printing is often widespread, covering both the highlight and shadow areas. To solve this problem, the ink needs to be replaced or new ink can be added to the ink fountain to increase the viscosity until the viscosity is appropriate, and the problem can be resolved.