Discuss some problems of offset printing
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In the offset printing process, ink is often encountered where there is no ink on the printed product. Usually we refer to this condition as dirty and dirty (dirty). There are two reasons for this during the printing process: one is due to the presence of ink on the plate that is not visible in the blank; one is due to the paper transfer process, between the paper and the paper, between the paper and the machine. The transfer of the existing ink is transferred by contact and friction.
Below we discuss separately:
The first case:
1. The blank part of the plate has a close ink base. There are several reasons for this:
1 when the printing plate is underexposed, the resin layer of the blank portion is not fully decomposed, and the blank portion is inked during the printing process;
2 There are dirty spots on the film. When the plate is printed, the dirty spots on the film are exposed to the PS plate, and the dirty spots of the ink should be present in the blank portion;
3 The glass of the printing machine is unclean, the printing time is insufficient, and the development time is too short. These are easy to leave the photosensitive resin on the printing plate, so that the ink will be in the printing process, which causes the printed matter to be dirty;
4 There is a need for baking, often due to the baking plate, the printing plate is dirty, these conditions include: the cotton yarn of the protective liquid is shed (it is recommended to use good quality non-fat long-staple cotton yarn), the protective liquid flocculates (in When rubbing the plate solution, be careful to shake the protective solution first and use it);
5 plates are scratched in the process of printing, baking, and machine. If the scratch does not damage the resin layer of the graphic, use a cleansing paste or a moisturizing powder to wipe the skin. If you have already injured the graphic part of the plate, you should generally consider replacing the plate.
2. Ink imbalance caused. This situation is the most common situation in the printing process when the printing is dirty. It is determined by the particularity of lithography that a certain thickness of water film must be spread on the printing plate (actually required to be a blank part). Resistant to ink erosion, selective adsorption on the surface of the plate. When the amount of water on the plate is too small, or too small, the ink will come into contact with the PS plate with almost no blockage, which will inevitably cause the blank surface of the plate to become dirty. This situation usually occurs as long as the judgment is accurate (generally, such as poor water contact, low water roller speed, low alcohol content, etc.), the amount of water is increased, and then the waterway is adjusted accordingly. .
But the problem is often not so simple. Similar problems are: the ink is too thin (the ink spreads too much on the plate, which will cause the water film to be unresistible), and the acidity of the fountain solution is too small (the cleaning plate of the dampening solution) And the oil resistance is degraded), and the desiccant or the detacking agent in the ink is excessive, so that the oiliness in the ink is increased. Apparent problems should be analyzed carefully, and then the right medicine should be given. Otherwise, blindly increasing the amount of water on the surface of the plate will inevitably lead to more new problems.
3. Ink emulsification results. We generally say that the balance of ink and wash on the layout generally has four states. The ink is large, the ink is thin, the ink is small, and the ink is thick. This is the four kinds of ink balance on the surface of the printing plate. Form, but not the balance of the surface of the plate can be maintained in these four states. For example, when the ink is dirty, as the machine is running, the ink is continuously sheared on the ink roller, which will slowly lead to the ink boundary not being clear until emulsification occurs, and slight emulsification is necessary ( Conducive to the transfer of offset ink between the ink rollers), however, when the ink is very thin, the emulsification is often severe, and the solvent in the ink or ink will break through the boundary of the water and erode to the blank portion. In the case of dirty, we often refer to this situation as floating up. There are many reasons for the occurrence of floating dirt in this case. Except for the large amount of ink and water mentioned above, there is also a large amount of ink. In fact, among the four methods of ink-and-water balance, only the small ink is small, which is what we really want to achieve. The small ink is small, and the quality of our prints cannot be guaranteed.) Any cause that can cause serious ink emulsification problems can indirectly lead to visceral failures. For example, the acid value of the dampening solution is too high, the auxiliary agent (debonding agent, desiccant, etc.) in the ink is excessive, the paper is depilated, the pigment particles of the ink are too coarse, and the like.
4. The plate is worn. Plate wear is also a major cause of dirty on the plate. The surface of the plate that is worn out becomes shallower and less, which reduces the water storage capacity, which makes it easier to get dirty on the worn parts. In addition to the normal wear and tear of the plate. Usually this situation is more likely to occur in the cornice part of the printing plate, usually because the impact of the rubber roller and the plate cylinder is too large, and sometimes the water in the printing plate is skipped due to excessive pressure of the water and the ink roller. When the surface of the plate is pulled out.
When I open the machine, I often encounter such a situation, that is, the printing plate often has a horseshoe-shaped wear after printing tens of thousands of sheets in the middle of the printing. The shape is almost the imprint of the pressing paper foot, and there is no way. It has been observed that each time the blanket is cleaned, the same impression is applied on the blanket, and it is very thick. The analysis may be that the surface strength of the paper is not enough under the pressure of the presser foot, being ground, under the pressure of the blanket and the water. When the run-up is increased, the phenomenon of hair loss is more likely to occur, so that part of the paper wool sticks to the surface of the rubber, and after being piled up, the plate is worn. Through analysis, we changed the spring on the presser foot to a softer one, and the frequency of the blanket cleaning of the corresponding part of the presser foot increased accordingly. This phenomenon disappeared. Also sometimes, this problem occurs when the suction of the nozzle is too large. (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, there will be a situation in which the plate gap jumps. When the water roller falls in the mouth, the partial water supply of the mouth portion is insufficient, and the same is true. The same effect can be produced when the pressure of the ink roller is too large, so correct adjustment of the pressure of the ink roller 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 in the middle of the water (ink) roller is already large, the pressure at both ends is still very small, so some masters increase the pressure on both ends with a single mind, so that not only It will cause the above consequences, and it will cause great damage to the water and the ink roller. The rubber roller will be crushed often in a short time. In the case where the pressure at the two ends of the intermediate pressure is small, it is first considered whether the ink roller is opposite in direction when the ink roller is adjusted, causing the ink roller to tilt, or the diameter and the intermediate diameter of the two ends of the ink roller are caused by prolonged use. difference. In particular, pay attention to the use of water-rolled presses when sewing the water-roller sleeves on both sides of the water roller. Do not use too much force, causing the water roller to be stretched on both sides of the rope. If so, then increase the whole water. It is not worthwhile for the pressure of the roller to increase the pressure at both ends of the water roller, and the rubber roller will be shaped for a long time.
Second, During the paper transport process, contact between the printed sheet and the printed sheet, between the printed sheet and the machine causes smudging.
1. Dirty between sheets.
1 The drying is slow due to the excessive thickness of the ink layer;
2 due to the paper surface smoothness is too high or the absorption is too poor;
3 ink emulsification seriously causes the ink to dry slowly;
4 When the ink layer is not completely dry, the printed matter is piled up too high...
As described above, after printing, the smear between the sheets on the paper receiving table can be avoided by: when the ink layer is thick in real time, the powder is properly sprayed at the delivery end; 1000-2000 sheets, plus a partition between the paper piles, of course, this height can also vary according to the thickness of the paper; do not apply a large impact force to the surface of the paper stack before the ink layer is dry (if the operator sits undried) On the pile of paper, using heavy objects to beat on the pile of paper, etc.)
When a four-color offset press prints a large-area field product, the printed matter is often scratched by a delivery roller. Since the anti-adhesive paper (or anti-adhesive net) of most used printing machines (such as Manroland, Heidelberg, Komori, etc.) has been damaged, and the corresponding equipment is not complete, it is a lot of trouble to deal with the sticky problem. Let's talk about our experience in printing large-area land-based prints using imported four-color offset press for many years.
1). In the printing color sequence, try to arrange the full version in the fourth group to reduce the bumps of the printed products.
2). The fluidity of the solid version of the ink should be greater than normal. In winter, the combination of red dry oil and varnish is suitable, and the ratio is about 8%. This ink is used for printing coated paper and white cardboard. Ordinary paper printing inks can be blended in the normal proportion.
3). When replacing the new release paper still can't solve the problem, it is recommended to install 3~5 springs (circumferential direction) with a diameter of about 6mm on the delivery roller. Without a suitable spring, find a few plastic wrappers with diameter. It is also possible to replace the wire of about 2 mm, but the effect is slightly worse.
4). A corresponding piece of the non-solid part of the printed product that is in contact with the take-up reel is adhered to a small piece of foam of 5 mm thick to raise the distance between the take-up reel and the printed product. If the field version is arranged in the first few groups, the thickness of the foam must be controlled within the range that does not affect the overprint.
5). When printing with Manroland four-color machine, the real version is arranged in one or two groups, and the middle paper-feeding paper row will also touch the dirty printed matter due to ink accumulation, and the problem can be solved as long as it is often cleaned.
6). Some Manroland machine only has a few paper-receiving rollers on the delivery roller. If you want to print the coated paper, it is better to modify it according to the delivery roller of the domestic 08 machine. It is also easier to do.
2. The contact between the sheet and the machine results.
1 Some parts of the machine leak oil. The most common is Feida's mouthpiece leaking oil, and the paper leaks under the machine when it passes through the lower part of the machine.
Usually, the oil leakage of the Feida mouthpiece is very good. Just clean the air pump accordingly. In addition, keep the underside of the machine (especially multi-color machines above two colors) clean, and often clean the support rods on the paper drive line under the machine to replace the bent and deformed support rods. Modern imported equipment will have air cushions on the paper path, so ensuring the air vent on the air cushion is also an important task of daily maintenance.
2 Some parts on the paper are dirty, or the ink on the sheets is pasted by some machine parts on the paper.
Some of the more typical hem-type printing presses, such as the HDB CD102, are prone to appear when the printed sheet is not dry yet, and the back side of the print is in the process of returning the paper (this time the back is already printed) ) Dirty. In this case, there is generally no good way to do it, only after the print is completely dry and printed.
On the printing press produced by Beiren in China (08, 05), there is a mechanism called a cage (a hollow cylinder with many crossbars) in the direction of delivery, which is mainly used to hold up the received sheets, and there are many on the crossbar. The rubber apron, sometimes the position of the rubber apron is also likely to cause the back surface to be dirty if it is in the image portion with a large amount of ink. Cleaning these rubber bands after printing a batch of live is necessary and often overlooked.
In multi-color printing, not only do you always look at the ink on the front side of the sample, but you should always see if there is any dirty ink on the back. If you find that you need to solve the problem in time.
3 The side gauge pulls the ball to the position where there is a pattern. When the printing is turned over, the pattern is transferred between the sheets by the small ball on the side gauge.
When an experienced master prints some printed materials, he often checks whether the graphic (positive and back) on the side of the paper will be picked up by the small ball on the rule. If possible, then according to the operating equipment, try to adjust the printing The location of the version is avoided.
Third, other.
There is a place in the printing unit, especially three (including printing plate, blanket, embossing) roller, the roller shaft head leaks oil, when the oil enters the edge of the printing plate, there will be a similar floating surface of the printed product. Dirty. This situation should also be analyzed and processed during operation. I have encountered such a device. The main fault is that there is a problem with the oil seal of the shaft of the plate cylinder. However, since there is no time for maintenance, I have to find a long cloth strip to soak some water and wrap it on the roller shaft. It really played a big role.
In addition, the electrostatic content of the paper is too large, and it often causes the problem of dirty on the back side. In addition, due to the excessive electrostatic content, and the excessively long ink viscosity caused by excessive ink viscosity, the resulting ink flying phenomenon, especially in the first color group, causes the ink of the first color to splash onto the surface of the paper being transported. It also causes ink dots on the surface of the paper.
Water roller has not been cleaned for a long time, and may also leave dirt on the surface of the print.
The dampening solution has not been replaced for a long time, and the coloring material of the dissolved ink in the dampening solution or the emulsified ink in the dampening solution can also cause the surface of the printing plate to be dirty.
The prints we need are also very simple to say, that is, there is ink in the place where there is ink, and the amount of ink is appropriate. If there is no ink, it is not necessary to ink. This is the effect that we need to achieve offset printing. It is easier said than done, and it is difficult to do it. The above mentioned 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 we must conduct specific analysis to eliminate them.