(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
In the offset printing process, we often encounter the situation where there should be ink on the place where there should be no ink on the printed matter. Usually we call this situation as dirty and dirty (stained). There are generally two reasons for this in the printing process: one is due to the ink that should not be present on the blank part of the printing plate; the other is due to the paper between the paper and the machine during the paper transport process Contact and rubbing to transfer the stolen goods with ink.
Below we discuss separately:
The first case:
1. The blank part of the printing plate has a pro-ink foundation. There are several reasons for this:
â‘ Insufficient exposure during printing, the resin layer of the blank part is not fully decomposed, which will cause the blank part to be inked during the printing process;
â‘¡ There are dirty spots on the film. When the printing is done, the dirty spots on the film will be exposed to the PS version, and the undesirable ink stains will also appear on the blank part;
â‘¢The glass of the printing machine is not clean, the printing time is insufficient, and the development time is too short. These are likely to leave undesirable photosensitive resin on the printing plate, which will attract ink during the printing process and cause the printing to be dirty;
â‘£If there is a need to bake the plate, the printing plate will often become dirty due to the bake plate. These conditions include: the cotton yarn wiping the protection liquid is lint (it is recommended to use good quality degreased long staple cotton yarn) When wiping the baking solution, pay attention to shake the protective solution before use);
⑤ The printing plate is scratched in the process of printing, baking, and on the machine. If the scratch does not damage the resin layer of the graphic, use a cleaning paste or moistening powder to wipe it, and it will generally not get dirty. If the graphic part of the printing plate has been injured, it is generally necessary to consider replacing the printing plate.
2. Unbalanced ink caused. This situation is the most common case of dirty printing on the printing process. It is determined by the particularity of lithography. A certain thickness of water film must be spread on the printing plate (actually 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 partly too small, the ink will come into contact with the PS plate with almost no obstruction. This situation will inevitably cause the blank part of the surface of the printing plate to become dirty. In this case, as long as the judgment is accurate (generally similar problems such as poor waterway contact, low water roller speed, low alcohol content, etc.), increase the amount of water appropriately, and then adjust the waterway accordingly, generally it will be quickly ruled out .
But the problem is often not that simple. Similar problems are too thin ink (excessive spreading of the ink on the printing plate will cause the water film to be irresistible), and the acidity of the fountain solution is too small (the fountain plate is used to clean the printing plate) And the oil resistance is reduced), too much desiccant or debonding agent in the ink makes the oil in the ink increase. Such problems should be analyzed carefully, and then treated accordingly. Otherwise, as soon as the plate is dirty, the amount of water on the plate will be increased blindly, which will inevitably lead to more new problems.
3. Caused by ink emulsification. What we generally call the balance of water and ink on the plate surface, there are generally four states, large ink water, large ink thin water, small ink water, and thick ink water. These are the four types that can produce the balance of water and ink on the surface of the printing plate. Form, but not the balance of the printing plate surface can be maintained continuously in these four states. For example, when the ink is thin and the water is large, as the machine runs, the ink is continuously sheared on the ink roller, which will slowly cause the ink boundary to be unclear until emulsification occurs. A slight emulsification is necessary ( Conducive to the transfer of offset printing ink between the ink rollers), however, when the ink is very thin, this emulsification is often violent, and the ink or the solvent in the ink will break through the water boundary and erode to the blank part. Dirty condition, we often refer to this situation as floating dirty. Similar to this situation, there are many reasons for scumming. In addition to the large ink thin water mentioned above, there are also large ink water (in fact, of the four ink-water balance methods, only the thick ink water is small is what we really want to achieve, The ink is small and the water is small, and we cannot guarantee our print quality). Any reason that can cause serious ink emulsification problems can indirectly lead to the occurrence of dirty stains. Such as: the acid value of fountain solution is too high, too many auxiliary agents (removal agent, desiccant, etc.) added to the ink, the paper is powdered and fluffed, the pigment particles of the ink are too thick, etc.
4. Plates are worn. Abrasion of the printing plate is also a major cause of soiling on the printing plate. The surface of the abraded printing plate becomes shallower and less sand, which reduces the water storage capacity, so it is easier to get dirty on the worn part. In addition to the normal wear of the printing plate. Usually this kind of situation is more likely to occur in the gripping part of the printing plate, usually due to the excessive impact of the combined pressure of the rubber cylinder and the printing cylinder, and sometimes due to the excessive pressure of the water and ink rollers on the plate. The surface of the printing plate was smashed out.
When I turn on the machine, I often encounter such a situation that the middle of the printing plate often shows horseshoe-shaped wear after printing tens of thousands of sheets. The shape is almost the imprint of the paper foot. There is no way. Observation shows that each time the blanket is washed, the blanket has the same imprint and is thick. The analysis may be that the surface strength of the paper is not enough. Under the pressure of the presser foot, it is sanded, under the pressure of the blanket, and the water Under the swelling, it is more prone to hair loss, so part of the paper wool sticks to the surface of the rubber, and after thick accumulation, it causes wear on the printing plate. Through analysis, we replaced the spring on the presser foot with a softer one, and the frequency of cleaning the blanket on the corresponding part of the presser foot increased accordingly, and this phenomenon disappeared. Also sometimes, this problem occurs when the suction power of the nozzle is too large. (The above mentioned generally only appears 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 plate jumps in the neutral position. When the water roller falls on the gripper, it will cause insufficient water supply in the gripper part. The same effect will be produced when the pressure of the ink roller is too high, so 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.
The most common problem in actual production is that 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 increase the pressure at both ends with one mind, this is not only It will cause the above consequences, and it will cause great damage to the water and ink rollers, and can often crush the rubber roller in a short time. When the middle pressure is large and the pressure at both ends is small, the first consideration is whether the ink roller makes the opposite directions of the two ends during adjustment, causing the ink roller to tilt, or the diameter and middle diameter of the ink roller are caused by long-term use. difference. When using water roller velvet printing machine, please pay special attention to sewing the water roller velvet sleeves on both sides of the water roller. Do not apply too much force, which will cause the water roller to be stretched by the rope. If so, increase the whole water. It is not worthwhile to increase the pressure at both ends of the water roller by the pressure of the roller, and the rubber roller will also be fixed after a long time.
2. In the process of paper conveyance, contact between printed sheets and printed sheets, and between printed sheets and machines causes smudging.
1. Rubbing between printed sheets.
â‘ Dry slow due to the thick ink layer;
â‘¡Due to the high smoothness of the paper surface or poor absorption;
â‘¢ Ink emulsification is serious and causes the ink to dry slowly;
â‘£When the ink layer is not completely dry, the accumulation of printed matter is too high ...
As mentioned above, basically after printing, the smearing between the sheets on the receiving table can be avoided by the following methods: when the ink layer is thick and real-time, appropriate powder spraying at the receiving end; About 1000-2000 sheets, add partitions between the stacks, 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 stacks before the ink layer is dry (such as the operator sitting on the dry On the pile of paper, use heavy objects to beat on the pile of paper, etc.)
When printing a large-area field product with a four-color offset press, the printed matter is often injured by the take-up roller and is dirty. Since the release paper (or release mesh) of most second-hand printing machines (such as Manroland, Heidelberg, Komori, etc.) has been damaged, and the corresponding equipment is not complete, it is quite troublesome to deal with the problem of sticking. Let's talk about our years of experience in using imported four-color offset printing machines to print large-area field prints for the reference of printing colleagues.
1). In the printing color sequence, the full version should be arranged in the fourth group of prints as far as possible to reduce the link of the printed products being scratched.
2). The fluidity of the field ink should be greater than normal. In winter, red dry oil and ink blending oil are used in combination, and the ratio is about 8%. This ink is used for printing on art paper and white cardboard. Ordinary paper printing inks can be formulated according to the normal ratio.
3). When replacing the new release paper still can not solve the problem, it is recommended to install 3 to 5 springs with a diameter of about 6mm (circumferential direction) on the take-up roller. If there is no suitable spring, find a few plastic-covered, diameter It is also possible to substitute a wire of about 2mm, but the effect is slightly worse.
4). The non-solid part of the printed product is glued with a small piece of 5mm thick foam plastic at the corresponding part in contact with the take-up roller to increase the distance between the take-up roller and the printed product. If the field version is arranged in the first few groups, the thickness of the foam must be controlled within a range that does not affect overprinting.
5). When printing with the Manroland four-color machine, the ground plates are arranged in one or two groups, and the middle paper feed row will also touch the dirty prints due to the accumulation of ink. As long as it is cleaned frequently, the problem can be solved.
6). Some manroland machines have only a few take-up wheels on the take-up drum. If you often print on coated paper, it is better to refer to the take-up drum of the domestic 08 machine to make it easier.
2. Caused by contact between the printed sheet and the machine.
â‘ Some parts of the machine leak oil. The most common is the oil leakage of Feida's mouthpiece, and the oil leakage that the paper gets under the machine when the paper passes under the machine.
Usually, the oil leakage of Feida nozzle is very good to judge, as long as the air pump is cleaned accordingly. In addition, the underside of the machine (especially multi-color machines with more than two colors) should be kept clean, and the support rods on the paper transmission line under the machine should be cleaned frequently, and the bent and deformed support rods should be replaced. Modern imported equipment will have air cushions on the paper path, so ensuring the air holes on the air cushions is also an important task of daily maintenance.
â‘¡Some parts of the paper road are dirty or the ink on the printed sheet is smeared by some machine parts on the paper road.
Typically, some hem-type printing presses, such as HDB CD102, are prone to appear when the printed sheet is turned over before it is dried. The back side of the printed product is handed back to the back of the printing process (the back side is already printed at this time) ) Rubbing dirty. When this happens, there is generally no good way, only to wait for the printed matter to dry completely after printing.
On the printing presses (08, 05) produced by Beiren in our country, there is a mechanism commonly known as a cage (hollow cylinder with a lot of crossbars) in the direction of paper collection, which is mainly used to hold the received printed sheets. Rubber aprons, sometimes the position of the rubber aprons happens to be in the graphic part with a large amount of ink, it is also easy to cause the back to be rubbed. Cleaning these rubber bands after printing a batch of jobs is necessary and often overlooked.
In multi-color printing, not only the color of the ink printed on the front side, but also the ink stain on the back side should be checked frequently. If found, the problem should be solved in time.
â‘¢ There is a pattern at the position where the side ruler pulls the ball. When the printing is over, the pattern is transferred between the printed sheets by the ball on the side ruler.
When printing some printed materials, experienced masters often look at whether the graphics (front and back) on the edge of the paper will be hit by the small ball on the rule. If it is possible, according to the operation equipment, try to adjust the printing as much as possible. The position of the version is avoided.
3. Others.
There are places where oil leaks in the printing unit, especially three (including printing plates, blankets, and embossing) cylinders. The cylinder shaft leaks oil. When the oil enters the edge of the printing plate, a dirty surface will appear. Dirty. This situation should also be analyzed and handled during operation. I once encountered such a device. The main fault was the oil seal of the shaft head of the printing plate cylinder. However, because I did not have time to do maintenance, I had to find a long cloth strip soaked in water and wrapped 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 problem of dirty back. In addition, due to the excessive static content and the excessively long ink filament head caused by the excessive viscosity of the ink, the flying ink phenomenon caused jointly, especially in the first color group, will cause the first color ink to splash on the paper surface being transported on the cardboard , Will also cause ink spots on the surface of the paper.
Water roller fleece has not been cleaned for a long time, and may also leave dirt on the surface of the printed product.
The fountain solution has not been replaced for a long time. The fountain solution contains dissolved ink colorants or emulsified ink in the fountain solution, which can also cause the surface of the printing plate to become dirty.
The author always wants to define a definite limit to the things discussed this time. In fact, it seems that it is no longer necessary. The prints we need are also very simple, that is, where there is ink, there is ink, and the amount of ink is appropriate, there is no ink Don't need ink in the place, this is the effect we need to achieve. It is easy to say and difficult to do. The above mentioned are only some of the problems that can be encountered in actual production. In the production, we must pay attention to the different characteristics of different problems, and we must conduct specific analysis to eliminate them.
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