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Discussion on the contamination of offset printing

      In the process of offset printing, we often encounter the situation of ink where there should be no ink on the print. Usually, we call this situation dirty and rubbed dirty (stained). There are generally two reasons for this situation in the printing process: Contamination Of Offset Printing

      one is that there should be no ink on the blank part of the printing plate; One is to transfer the stolen goods with ink due to contact and friction between paper and paper and between paper and machine during paper transmission.Contamination Of Offset Printing

Contamination Of Offset Printing

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Let’s discuss separately below:

The first case:

1. The blank part of the printing plate has an ink affinity basis. There are several reasons for this:

① Insufficient exposure during printing does not fully decompose the resin layer of the blank part, which will lead to ink on the blank part during printing;

② There are dirty spots on the film. When printing, drying the dirty spots on the film on the PS plate will also cause undue ink dirty spots in the blank part;Contamination Of Offset Printing

③ The glass of the plate printer is unclean, the printing time is insufficient, and the development time is too short. These are easy to leave undue photosensitive resin on the printing plate, which will be ink friendly in the printing process, resulting in dirty prints;Contamination Of Offset Printing

④ When baking plate is needed, the printing plate is often dirty due to baking plate. These situations include wool falling off the cotton yarn wiping the protective solution (it is recommended to use high-quality degreased plush cotton yarn) and flocculation of the protective solution (when wiping the baking plate solution, pay attention to shake the protective solution well before use);Contamination Of Offset Printing

⑤ The printing plate is scratched in the process of printing, baking and computer operation. If the scratch does not damage the resin layer of the image and text part, it will not get dirty by wiping it with cleaning paste or wetting powder. If the image and text part on the printing plate has been damaged, it is generally necessary to consider replacing the printing plate.Contamination Of Offset Printing

       Ink imbalance. This is the most common case of dirty prints in the printing process. Due to the particularity of lithography, a certain thickness of water film must be spread on the printing plate (actually the blank part is required) in order to effectively resist the erosion of ink and produce selective adsorption on the printing plate surface. When the amount of water on the printing plate is too small, Contamination Of Offset Printing

      or local is too small, the ink will contact the PS plate with little obstruction, which is bound to lead to dirt on the blank part of the printing plate surface. In this case, as long as the judgment is accurate (such as poor water contact, low water roller speed and low alcohol content), increase the amount of water appropriately, and then adjust the water path accordingly, it will generally be eliminated quickly.Contamination Of Offset Printing

       However, the problem is often not so simple. Similar problems include too thin ink (excessive spreading of ink on the printing plate will cause the water film to be unable to resist), too small acidity of fountain solution (the cleaning and oil resistance of fountain solution are reduced), and too much desiccant or adhesive remover in the ink, which increases the oiliness in the ink. In case of such problems, we should carefully analyze them and then apply the remedy to the case, otherwise we will blindly increase the water volume of the page as soon as we see that the page is dirty, which is bound to lead to more new problems.

      Ink emulsification. Generally speaking, the ink balance on the layout generally has four states: large ink water, large ink thin water, small ink water and small ink thick water. These four forms can produce the ink balance on the printing plate surface, but not the balance on the printing plate surface can be maintained under these four states. For example, when the ink is thin and the water is large, the ink is constantly sheared on the ink roller with the operation of the machine, which will slowly lead to unclear ink boundary until emulsification occurs. Slight emulsification is necessary (conducive to the transmission of offset ink between the ink rollers).Contamination Of Offset Printing

        However, when the ink is very thin, this emulsification is often intense, The ink or the solvent in the ink will break through the boundary of water and erode to the blank part. This kind of dirty situation will also occur. We often call this situation floating dirty. There are many reasons for floating and dirty in this situation. In addition to the large amount of ink thin water mentioned just now,

        There are also large amounts of ink (in fact, among the four ink balance methods, only small amount of ink thick water is what we really want to achieve. Small amount of ink and small amount of water also can not guarantee our print quality).Contamination Of Offset Printing

         Any cause that can cause serious ink emulsification problems can indirectly lead to dirty faults. For example, the acid value of fountain solution is too high, too many auxiliary agents (adhesive remover, desiccant, etc.) are added to the ink, the paper is de powdered and hairy, the pigment particles of the ink are too coarse, and so on.Contamination Of Offset Printing

      The printing plate is worn. Printing plate wear is also a major reason for the dirty printing plate. The sand mesh on the worn printing plate surface becomes shallower and less, which reduces the water storage capacity, so it is easier to get dirty at the worn parts. In addition to the normal wear and tear of the printing plate. Usually, this situation is more likely to occur in the mouth of the printing plate.

       It is usually due to the excessive impact of the combined pressure of the rubber cylinder and the printing plate cylinder, and sometimes the printing plate surface will be smashed out when the printing plate jumps in neutral due to the excessive plate pressure of the water and ink roller.

      When I start the machine, I often encounter such a situation, that is, the middle of the drag of the printing plate often appears horseshoe shaped wear after printing tens of thousands of sheets, and the shape is almost the imprint of the presser foot. There is no way. It is observed that each time the blanket is cleaned, there are the same marks on the blanket and they are very thick. The analysis may be that the surface strength of the paper is not enough. It is ground under the pressure of the presser foot. Under the pressure of the blanket and the moisture of water, it is easier to lose hair. 

      Therefore, part of the paper wool sticks to the surface of the rubber and causes wear to the printing plate after it is thick. Through analysis, we replaced the spring on the presser foot with a softer one, and the cleaning frequency of the blanket at the corresponding part of the presser foot increased accordingly. As expected, this phenomenon disappeared. Similarly, sometimes, when the suction force of the suction nozzle is too large, such a problem will occur. (as mentioned above, it usually only occurs when the paper is very bad)

      Mechanical failure of ink roller. When the contact pressure between the water roller and the ink roller and the printing plate is too large, it will jump in the neutral position of the printing plate. When the water roller falls in the mouth, it will cause insufficient water supply in the mouth. Similarly, the same effect will be produced when the pressure of the ink roller is too large. Therefore, the correct adjustment of the pressure of the ink roller is the key to ensure the balance of water and ink on the printing plate.

      In actual production, the most common problem is that when the plate pressure in the middle of the water (ink) roller is very large, but the pressure at both ends is still very small, so some masters concentrate on increasing the pressure at both ends, which will not only cause the above consequences, but also cause great damage to the water and ink roller, and often crush the rubber roller in a very short time. 

      When the intermediate pressure is large and the pressure at both ends is small, we should first consider whether the direction of both ends of the ink roller is opposite during adjustment, resulting in the inclination of the ink roller,Contamination Of Offset Printing

       or the difference between the diameter of both ends and the intermediate diameter of the ink roller due to long-term use. For the printing press using water roller fleece, it is particularly important not to apply too much force when stitching the water roller fleece on both sides of the water roller, resulting in the tension of the lines on both sides of the water roller. If so, it is not worth increasing the pressure at both ends of the water roller by increasing the pressure of the whole water roller, and the rubber roller will set over time.

2、 In the process of paper transmission, the contact between printing sheets and between printing sheets, and between printing sheets and machines leads to dirt.

1. Smudging between printing sheets.

① Slow drying due to too thick ink layer;

② Because the paper surface smoothness is too high or the absorption is too poor;

③ Serious emulsification of ink leads to slow drying of ink;

④ When the ink layer is not completely dry, the print accumulation is too high

      As mentioned above, generally speaking, after printing, the scraping between the printing sheets on the paper receiving table can be avoided in the following ways: when the ink layer thickness is real-time, spray powder appropriately at the paper receiving end; 

       Drying rack (add spacer plates between paper stacks every 1000-2000 sheets, of course, this height can also be changed according to the thickness of the paper); Before the ink layer is dry, do not apply a large impact force to the surface of the paper pile (for example, the operator sits on the dry paper pile and uses a heavy object to knock on the paper pile)

       When a four-color offset press prints a large area of field products, the printed matter is often damaged and dirty by the paper receiving drum. Because the anti sticking paper (or anti sticking screen) of most second-hand printing presses (such as Manroland, Heidelberg, Komori, etc.) has been damaged and the corresponding equipment is incomplete, it takes a lot of trouble to deal with the problem of sticking and dirt. Let’s talk about our experience in printing large-area field prints with imported four-color offset press for reference.

1)。 In the printing color sequence, the full plate shall be arranged in the fourth group as far as possible, so as to reduce the damage of the printed matter.Contamination Of Offset Printing

2)。 The fluidity of field version ink should be greater than normal. In winter, it is appropriate to combine red drying oil and ink blending oil, with a proportion of about 8%. This ink is used for coated paper and white cardboard. The ink for plain paper printing can be mixed according to the normal proportion.Contamination Of Offset Printing

3)。 When the problem cannot be solved by replacing with new anti sticking paper, it is recommended to install 3-5 springs with a diameter of about 6mm (circumferential direction) on the paper receiving drum. If there is no suitable spring, it is also possible to find several wires with a plastic sheath and a diameter of about 2mm, but the effect is slightly poor.Contamination Of Offset Printing

4)。 In the printed part, the corresponding parts of the non field part contacting the receiving roller are glued with 5mm thick foamed plastics, and the distance between the receiving roller and the printed product is increased. If the site layout is arranged in the first few groups, the thickness of foam plastics must be controlled without affecting the range of printing.Contamination Of Offset Printing

5)。 When printing with Manroland four-color machine, the plates are arranged in one or two groups truthfully, and the paper transfer tooth row in the middle will also touch the printed matter due to ink accumulation. The problem can be solved as long as it is often cleaned.Contamination Of Offset Printing

6)。 Some Manroland machines have only a few paper receiving wheels on the paper receiving drum. If coated paper is often printed, it is better to transform it with reference to the paper receiving drum of domestic 08 machine, which is also easier to do.Contamination Of Offset Printing

2. Caused by contact between the printing sheet and the machine.Contamination Of Offset Printing

① Some parts of the machine leak oil. The most common ones are the oil leakage from Feida’s blowing nozzle and the oil leakage from the bottom of the machine when the paper is transferred under the machine.Contamination Of Offset Printing

      Usually, the oil leakage of Feida blowing nozzle is very easy to judge, as long as the air pump is cleaned accordingly. In addition, keep the bottom of the machine (especially the multi-color machine with more than two colors) clean, often clean the support rod on the paper transmission line under the machine, and replace the bent and deformed support rod. Modern imported equipment will have an air cushion on the paper Road, so ensuring that the air holes on the air cushion are unobstructed is also an important work of daily maintenance.Contamination Of Offset Printing

② Some parts on the paper are dirty, or the ink on the printing sheet is rubbed by some machine parts on the paper.Contamination Of Offset Printing

      Typically, some lower swing printing machines, such as HDB CD102, are easy to appear when the printing sheet is turned over before it is dry, and the back of the printed matter (at this time, the back has been printed) is rubbed during the swing back of the paper delivery teeth. In this case, there is generally no good way, only after the printing is completely dry.Contamination Of Offset Printing

       On the printing press (08, 05) produced by beiren in China, there is a mechanism commonly known as cage (hollow drum with many cross bars) in the paper receiving direction, which is mainly used to hold up the received printing sheets. There are many rubber rings on the cross bar. Sometimes, if the position of the rubber ring happens to be in the graphic part with a large amount of ink, it is also easy to cause the back to rub dirty. It is necessary and often overlooked to clean these rubber bands after printing a batch.Contamination Of Offset Printing

In multi-color printing, we should often check not only the ink color printed on the front, but also whether there is dirty ink on the back. If found, the problem shall be solved in time.

③ The falling position of the side gauge pull ball happens to have a pattern. When turning over for printing, the pattern is transferred between the printing sheets by the small ball on the side gauge.Contamination Of Offset Printing

       When printing some printed materials, experienced masters often look at whether the pictures and texts (front and back) on the edge of the paper rules will be hit by the small ball on the rules. If possible, try to avoid it by adjusting the position of the printing plate according to the operating equipment.Contamination Of Offset Printing