In recent years, there has been an increase in the number of color prints with negative characters on the layout. The failure of pasting offset lettering often occurs. In case of this fault, if the fault is not correctly analyzed, the cause of the fault shall be found and eliminated as soon as possible, which will inevitably lead to more detours and increased consumption. Here are some views on this issue for reference only.
1 After the negative characters paste the printing plate, first check whether there is a problem with the printing plate.
Most of the yin characters are in large "field", and the yin characters or letters, patterns are large and small, and the lines and strokes are thick and thin. Pasting often occurs in small strokes and lines. Because the contrast between "field" and blank words is large, if you do not observe carefully, you can not see the problem of plate making, which is easy to be ignored.
In addition to the original version, the causes of paste in plate making are:
① PS version manufacturing quality:
First, the PS version is too shallow, too fine and has little water storage;
Second, the sealing time of some PS versions is not enough, and the extremely small holes in the pages are not sealed, which makes the pages too attractive;
Third, the sand mesh of regenerated PS plate and the river sensing layer are uneven. With the above PS version for print printing, due to the hydrophilicity of the negative characters, poor water storage capacity and easy pollution, it can not resist the "field" ink film spreading, resulting in paste.
First, the exposure time is not enough or the developer concentration is low;
Second, due to the lack of attention to the indoor light during the printing process, the PS plate will produce fog under the natural light, which will bring difficulties to the development, resulting in the fat sensitivity of the lettering part, the poor lipophilicity of the "field" part and the graphic part, and the weakening of the ink absorption performance of the "field" part when printed on the machine. In order to achieve the ink film thickness, the ink amount of the ink bucket must be increased, which is more likely to cause the lettering paste.
First, the PS plate with high quality standardization is used to make printing plates;
Second, the exposure should be sufficient and the development should be thorough during the printing operation;
Third, the indoor lighting in the printing process should not use light sources with strong light, the windows should be covered with ink cloth, and the doors should be double doors with color bands spaced.
In addition to plate making factors, too thick ink film during printing is also the main reason. Because of the high smoothness and low absorbability of the color printing surface, the slightly thick ink film is easy to spread into the negative text, resulting in a paste failure.
There are two situations when the ink film is too thick:
First, the ink film is too thick due to "small water and large ink". After formal printing, it is found that the ink film is too thick. Increase the water volume, and then adjust the ink amount slightly to eliminate it;
Second, the ink film is too thick due to "large water and large ink". This is because the ink discharge amount of the ink bucket is adjusted too much before the official printing, the water supply is increased wrongly after continuous printing, or the ink film is thinner when the printing starts, and the ink discharge amount is increased wrongly, leading to "large water and large ink", a vicious cycle, serious ink emulsification, reduced viscosity, and extremely easy to paste.
① In daily operation, we should always summarize experience to improve the technical level. Before formal printing, we should calculate the amount of ink in the ink bucket correctly. We would rather use less ink than too much ink. If we are inexperienced, we can use blotter paper to debug to achieve an appropriate ink film thickness without paste.
② Since the water supply of the "field" page cannot be small, in order to avoid the ink film being too thick due to "large water and large ink", the PH value of the liquid medicine can be reduced (the acid value can be increased), or an appropriate amount of surfactant can be added to reduce the water supply. In case of large amount of water and ink and ink emulsion, the emulsified ink on the ink cleaning roller shall be cleaned in time, and then the ink quantity shall be reduced and the ink shall be printed again;
③ When mixing ink, the color of duplicate color and light color ink prints should be slightly darker than the original. For the three primary colors, the ink with strong coloring power should also be selected. The "deep ink thin printing" measures can also avoid the ink film being too thick.
3 In addition to the above two main reasons, the following factors can also cause the typeface to paste when printing:
① The printing pressure is too high, mainly because the printing pressure is too high;
② The fluidity of ink is too large;
③ Poor water resistance of ink particle group or pigment;
④ Too much dry oil is added during inking;
⑤ Aging of inking roller;
⑥ The inking roller is too soft and bent, and relative slip occurs with the plate cylinder during operation;
⑦ Excessive pressure between inking roller and printing plate;
⑧ The water content of the plate is too small or the water roller is too dirty, and the pressure between the water roller and the printing plate is too light or too heavy;
⑨ The blanket is not tight;
⑩ Improper drum lining;
? Paper PH value is greater than 7 (slightly alkaline), easy to dilute the concentration of liquid medicine;
? The paper is seriously de powdered;
? In summer, due to the rapid drying of the PS plate, the plate was not coated with protective glue when the machine was stopped, and re printing would cause the plate to paste.