Reasons for ink dots in printed matter (1)

Feb. 09, 2023

1. The blank part of the printing plate has an ink-friendly base. There are several reasons for this:

①Insufficient exposure when printing the plate, the resin layer of the blank part is not fully decomposed, which will cause the blank part to be inked during the printing process;

② There are dirty spots on the film, when the dirty spots on the film are printed on the PS plate, there will also be undue ink dirty spots in the blank part;

③ The glass of the printing machine is not clean, the printing time is insufficient, and the developing time is too short. These are easy to leave unnecessary photosensitive resin on the printing plate, which will attract ink during the printing process and cause the printed matter to be dirty;

④ In the case of plate baking, the printing plate is often stained due to the baking plate. These situations include, the cotton yarn that wipes the protective liquid loses hair (it is recommended to use good quality degreasing long-staple cotton yarn), and the protective liquid flocculates (in the When wiping the baking plate liquid, pay attention to shake the protective liquid before use);

⑤ When the printing plate is scratched during the process of printing, baking, and loading on the machine, if the scratch does not damage the resin layer of the graphic part, use plate cleaning paste or wetting powder to wipe it, and generally it will not be dirty. If the graphic part on the printing plate has been damaged, it is generally necessary to consider replacing the printing plate.

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2. Caused by ink imbalance. This situation is the most common situation where the printed matter is dirty during the printing process. It is determined by the particularity of lithographic printing. A certain thickness of water film must be spread on the printing plate (in fact, the blank part is required) to be effective. It resists the erosion of ink and produces selective adsorption on the surface of the printing plate. When the amount of water on the printing plate is too small, or the local area is too small, the ink will contact the PS plate with almost no obstruction, which will inevitably cause the blank part of the printing plate surface to be dirty. In this case, as long as the judgment is accurate (generally such problems as poor water contact, low water roller speed, low alcohol content, etc. are prone to similar problems), increase the amount of water appropriately, and then adjust the water circuit accordingly, generally it will be eliminated quickly. .

But the problem is often not so simple. Similar problems include, too thin ink (excessive spread of ink on the printing plate will cause the water film to be unable to resist), too little acidity of the fountain solution (the cleaning of the printing plate by the fountain solution and oil resistance), too much desiccant or detackifier in the ink, which increases the oiliness of the ink. When such problems arise, we must carefully analyze them, and then prescribe the right medicine. Otherwise, we will blindly increase the amount of water on the page as soon as we see that the page is dirty, which will inevitably lead to more new problems.

3. Caused by ink emulsification. What we generally call the balance of ink and ink on the printing plate generally has four states, large ink and large water, large ink and thin water, small ink and small water, and small ink and thick water. These four states can produce ink balance on the surface of the printing plate. form, but not the balance of the surface of the printing plate can be continuously maintained in these four states. For example, when the ink is thin and the water is large, with the operation of the machine, the ink is constantly sheared on the ink roller, which will gradually cause the boundary of the ink to become unclear until emulsification occurs, and slight emulsification is necessary ( It is beneficial to the transfer of offset printing ink between the ink rollers), but when the ink is very thin, this emulsification is often severe, and the ink or the solvent in the ink will break through the water boundary and erode the blank part, and this will also occur In the case of being dirty, we often refer to this situation as floating dirty. There are many reasons for floating dirt in such a situation, except for the thin ink mentioned above, there are also large ink and large water (in fact, among the four ink-ink balance methods, only thick ink and small water are what we really want to achieve. The ink is small and the water is small, and the quality of our printed products cannot be guaranteed). Any reason that can cause serious ink emulsification problems can indirectly lead to dirty faults. Such as: the acid value of the fountain solution is too high, there are too many additives (adhesive remover, desiccant, etc.) in the ink, the paper is powdered and lint-free, the pigment particles of the ink are too thick, etc.

4. Plates are worn. The wear of the printing plate is also a major reason for the dirty printing plate. The surface of the worn printing plate becomes shallower and less sandy, which reduces the water storage capacity, so it is easier to get dirty on the worn part. In addition to normal wear and tear of the printing plate. Usually, this kind of situation is more likely to appear in the jaw part of the printing plate, usually due to the excessive impact of the rubber cylinder and the printing plate cylinder, and sometimes jumping in the neutral position of the printing plate due to the excessive pressure of the water and ink rollers. When the surface of the printing plate was smashed out.

When the machine is turned on, we often encounter such a situation, that is, the middle part of the printing plate is often worn out in a horseshoe shape after printing tens of thousands of sheets, and the shape is almost the imprint of the presser foot. There is no way. After observation, it was found that the same imprint was on the blanket every time the blanket was cleaned, and it was very thick. The analysis may be that the surface strength of the paper is not strong enough. Under the pressure of the presser foot, it was sanded, and under the pressure of the blanket and water With high moistening, it is more prone to shedding, so a part of the paper wool sticks to the surface of the rubber, and after the thick accumulation, it will cause wear to the printing plate. Through analysis, we changed the spring on the presser foot to a softer one, and the cleaning frequency of the blanket on the corresponding part of the presser foot also increased accordingly, and this phenomenon really disappeared. Also sometimes, when the suction of the suction nozzle is too strong, this problem will also occur. (The above mentioned will generally only appear when the paper is very bad)

5. Mechanical failure of the ink roller. When the contact pressure between the water roller and the ink roller and the printing plate is too large, it will appear that the printing plate jumps in neutral. When the water roller falls on the jaw, it will cause insufficient water supply in the jaw. The same effect will be produced when the pressure of the ink roller is too large, so adjusting the pressure of the ink roller correctly is the key to ensure the balance of ink on the printing plate.

The most common problem in actual production is that when the pressure of the middle part of the water (ink) roller is already very high, but the pressure at both ends is still very small, so some masters concentrate on increasing the pressure at both ends, so that not only It will cause the above consequences, and it will cause great damage to the water and ink rollers, and the rubber rollers will often be crushed in a short time. If the middle pressure is high and the pressure at both ends is small, first consider whether the direction of the two ends of the ink roller is reversed when adjusting, causing the ink roller to tilt, or the diameter of both ends and the middle diameter of the ink roller are caused by long-term use. difference. For printing machines using water roller fleece, pay special attention not to use too much force when sewing the water roller fleece cover on both sides of the water roller, which will cause the two sides of the water roller to be stretched thin. If this is the case, then by increasing the water roller It is not worth the pressure of the roller to increase the pressure at both ends of the water roller, and the rubber roller will also set shape over time.

Others. There is oil leakage in the printing unit, especially the three cylinders (including the printing plate, blanket, and embossing). The surface of the product is dirty. This situation should also pay attention to analysis and processing during operation. The author once encountered such a device. The main fault was the oil seal of the shaft head of the printing plate cylinder. However, since there was no time for maintenance, I had to find a long piece of cloth to soak some water and wrap it around the cylinder shaft. It really played a big role.

In addition, the electrostatic content of the paper is too large, which can often cause the back of the paper to be stained. In addition, due to the excessive electrostatic content and the excessively long ink filaments caused by the excessive viscosity of the ink, the phenomenon of ink splashing, especially in the first color group, will cause the ink of the first color to splash on the surface of the paper being transported. , can also cause ink dots on the surface of the paper.

If the water roller fleece has not been cleaned for a long time, it may also leave dirty marks on the surface of the printed matter.

The fountain solution has not been replaced for a long time, and there is dissolved ink color in the fountain solution, or the emulsified ink is in the fountain solution, which can also cause floating dirt on the surface of the printing plate.

RX packaging always wants to make a definitional limit on the things discussed this time. In fact, it seems that it is unnecessary. The printed matter we need is also very simple to say, that is, there needs to be ink where there is ink, and the amount of ink is appropriate. Where there is ink, there is no need for ink. This is the effect we need to achieve in offset printing. It is easier said than done, and the above-mentioned problems are only some of the problems that can be encountered in actual production. In production, we must pay attention to the different characteristics of different problems, and conduct specific analysis to eliminate them.