Some discussion of dirty prints

We often encounter in the offset printing process where ink should not appear on the prints. Usually we refer to this as dirty and dirty (stained). There are generally two reasons why this occurs during the printing process: one is due to the inadvertent ink on the printing plate; the other is due to the occurrence of paper between the paper and the machine, between the paper and the machine. Contact and friction to transfer the ink of the existing artifacts.

Below we discuss separately:

The first case:

1. The blank part of the printing plate is based on ink. There are several main reasons for this:

1 underexposure when printing, does not fully decompose the resin layer of the blank part, but will lead to ink in the blank part of the printing process;

2 There are dirty spots on the film. When the prints are printed, the dirty spots on the film are sunrayed on the PS plate. In the blank area, there will be unwanted ink spots.

3 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 unfilled photosensitive resin on the printing plate, which may cause ink in the printing process, resulting in dirt on the printing product;

4 In the case of baking, there are often stains on the plate due to baking. These include the wiping of the cotton lint that protects the solution (we recommend the use of a good quality skim-free cotton yarn), and the protective solution will flocculate ( When cleaning the baking solution, it is necessary to pay attention to the use of the protective liquid after shaking;

5 The plate is scratched during printing, baking, and printing on the machine. If the scratch does not harm the resin layer of the graphic and text, use a cleaning paste or moistening powder to wipe it. Generally, the plate will not get dirty. If you have already invaded the graphic part of the plate, you should generally consider replacing the plate.

2. Ink and ink imbalance lead to. This situation is the most common situation in the printing process, which is due to the special nature of the lithographic printing. It is necessary to spread a certain thickness of water film on the printing plate (which actually requires a blank part) to be effective. The resistance to the erosion of the ink produces selective adsorption on the surface of the printing plate. When the amount of water on the plate is too small, or when the area is too small, the ink will come into contact with the PS plate with almost no blocking. This will inevitably cause the blank part of the plate surface to become dirty. This situation generally appears as long as the judgment is accurate (usually such as poor water contact, low water roller speed, low alcohol content, etc. are prone to similar problems), appropriate amount of water, and then adjust the waterway, generally will soon be ruled out .

However, the problem is often not that simple. Similar problems include the fact that the ink is too thin (the ink spreads excessively on the printing plate, which will cause the water film to be unresistable), and the acidity of the fountain solution is too small (the cleaning plate of the fountain solution is too small. And oil resistance is decreased), too much desiccant or remover in the ink, so that the oil in the ink increases. The emergence of such problems should be carefully analyzed, and then remedy the situation, otherwise blindly increasing the amount of water on the layout of the surface will inevitably lead to more new problems.

3. Ink emulsification results. We generally say that the ink and ink balance on the layout is generally of four states: large ink, large amounts of ink, small ink, and small ink. This is the result of four kinds of ink and water balance on the surface of the printing plate. Form, but not the balance of the surface of the printing plate can be sustained 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, and the boundary of the ink and ink is gradually blurred until emulsification occurs. A slight emulsification is necessary ( It facilitates the transfer of offset ink between the ink rollers. However, when the ink is very thin, the emulsification is often violent. The solvent in the ink or ink will break through the boundary of the water and erode to the blank. This will also happen. In the dirty situation, we often call this situation floating. There are many reasons for this type of buoyancy. In addition to the large amounts of ink and water that have just been mentioned, there are also large waters (in fact, of the four types of ink and water balance, only the small amount of thick water is what we really want to achieve. Ink small water, the same can not guarantee the quality of our print). Any of the reasons that can cause serious ink emulsification problems can indirectly lead to the emergence of visceral breakdown. Such as: fountain solution acid value is too high, ink additives (tack-out agent, desiccant, etc.) too much, paper off powder off hair, ink pigment particles are too thick and so on.

4. Plate wear. Printing plate wear is also a major cause of scumming in the printing plate. The worn surface of the plate becomes less and less, which reduces the ability to store water. This makes it easier to get dirty on the worn area. In addition to the normal wear of the plate. Usually this kind of situation is relatively easy to appear in the mouth part of the printing plate, usually because the impact of the combined pressure of the rubber roller and the plate cylinder is too large, and sometimes the pressure of the plate and water roller is too large to jump in the plate neutral. When the surface of the plate is ejected.

The author often encountered such a situation when he opened the machine, that is, the horseshoe-shaped wear often appeared after printing the tens of thousands of sheets in the middle of the printing plate, and the shape was almost imprinted on the paper foot. There was no way. It has been observed that the blanket has the same footprint and is very thick each time the blanket is washed. The analysis may be that the surface strength of the paper is insufficient under the pressure of the presser foot, the sanding, the pressure under the blanket and the water As a result of the run-up, hair loss is more likely to occur, and a portion of the paper sticks to the surface of the rubber. After thick deposits, the printing plate is worn. Through analysis, we replaced the spring on the presser foot with a softer one, and the frequency of the blanket cleaning in the corresponding part of the presser foot also increased accordingly. This phenomenon really disappeared. Also sometimes this problem occurs when the suction of the suction nozzle is too large. (Generally speaking, it will only appear when the paper is very bad)

5. Ink roller mechanical failure. When the contact pressure between the water roller and the ink roller and the printing plate is too large, it will appear in the neutral position of the printing plate. When the water roller falls in the grate, it will cause a partial water supply shortage in the gills. When the ink roller pressure is too high, the same effect will occur, so correct adjustment of the pressure of the ink roller is the key to ensuring the ink balance on the printing plate.

The most common problem in actual production is when the pressure in the middle part of the water (ink) roller is very large, but the pressure at both ends is still very small, so some of the masters are just trying to increase the pressure on both ends. It will cause the above consequences, and it will cause great damage to the water and ink rollers. The roller can often be crushed in a short time. When the pressure at both ends of the intermediate pressure is small, it is necessary to first consider whether the ink roller is in the opposite direction at the time of adjustment, resulting in the inclination of the ink roller, or the both ends of the ink roller and the intermediate diameter due to prolonged use. difference. The use of water roller velvet printing machine should pay particular attention to when sewing the water roller velvet sleeve on both sides of the water roller, do not use too much force, resulting in the water roller on both sides of the wire rope is fine, if so, then by increasing the entire water It is not worth the roller pressure to increase the pressure at both ends of the roller, and it will take a long time for the roller to set.

Second, in the process of paper transport, the contact between printed sheets and printed sheets, between printed sheets and the machine leads to picking up.

1. Dirty between printed sheets.

1 Because the ink layer is too thick, it causes slow drying;

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

3 Ink emulsification seriously causes the ink to dry slowly;

4 When the ink layer is not completely dry, the printing material is too high...

As mentioned above, after all printing is completed, the dirtiness between the printed sheets on the paper receiving table can be avoided by the following means: when the ink layer is thick in real time, proper dusting at the delivery end; About 1000-2000 sheets are added between the paper stacks. Of course, this height can also be changed according to the thickness of the paper; do not apply large impact force on the pile surface before the ink layer is dry (if the operator is sitting un-dried On the pile of paper, using heavy objects to beat on the pile of paper, etc.)

When a four-color offset printing machine prints a large-area field version of a product, the printed matter is often scratched and given dirt by the delivery roller. Since most of the used printing presses (such as manroland, Heidelberg, Komori, etc.) have their anti-adhesive papers (or anti-sticking nets) damaged and their equipments are not fully equipped, it is quite a struggle to deal with sticky problems. Let's talk about our experience in using imported four-color offset printing machines to print large-area field prints for reference.

1). In the printing sequence, try to place the full version on the ground in the fourth group to reduce the damage to the printed product.

2). The fluidity of the ink in the field version should be greater than normal. The combination of red dry oil and varnish is used in winter, and the proportion is about 8%. This kind of ink is used for printing copper plate paper and white cardboard. Ordinary paper printing inks can be blended in normal proportions.

3). When replacing the new release paper still does not solve the problem, it is recommended to install 3 to 5 springs (circumferential direction) with a diameter of about 6 mm on the delivery roller. Without a suitable spring, find a few diameters with a plastic sheath. It is possible to replace the wire with 2mm or so, but the effect is slightly worse.

4). The non-solid part of the printed product is glued to the corresponding part of the delivery roller to a small piece of foam of 5 mm thick, which raises the distance between the delivery 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 the range that does not affect overprinting.

5). When printing with manroland four-color machine, the ground plate is arranged in one or two groups, and the middle transfer tooth row will also touch the dirty prints due to the ink deposit. So long as it is cleaned frequently, it can solve the problem.

6). Some manroland machines only have a few delivery rollers on the delivery rollers. If printing plates are often used, it is best to retrofit them with reference to the home machine's delivery drum and it is easier to do.

2. Contact between the sheet and the machine results.

1 Some parts of the machine are leaking. The most common is Feder's mouthpiece leaking oil, and the oil leaking from the bottom of the machine when the paper passes under the machine.

Usually, the oil leakage from Feida's mouthpiece is very good to judge, so long as the wind pump is cleaned properly. In addition, it is necessary to keep the machine underneath (especially a multi-color machine with two or more colors) clean, and frequently clean the support bar on the paper drive line under the machine, and replace the bent and deformed support bar. Modern imported equipment will have air cushions on the paper path, so ensuring the air vents on the air cushion is an important part of daily maintenance.

2 Some parts of the paper are dirty on their own, or the ink on the sheets is smeared by some machine parts on the paper.

More typical is the hem of some printing presses, such as HDB CD102, when the printed sheets have not yet dried and turned over to print, it is easy to appear, the back of the paper-feeding pendulum during the back of the printed product (this time the back is already printed ) Dirty. In this case, generally there is no good way, only after the printed products are completely dry and printed.

On the printing machines produced by our country's Beiren (08,05), there is a mechanism called the cage (the hollow cylinder with many crossbars) in the delivery direction. It is mainly used to hold the printed sheets, and there are many on the crossbar. The rubber apron, and sometimes the position of the rubber apron, if it happens to be in the graphic part with a large amount of ink, can easily cause the back stain. Cleaning these rubber bands after printing a batch is necessary and often overlooked.

In multi-color printing, the proofs drawn must not only check the ink on the front side of the page, but also often look at the back side for smeared ink. If found to solve the problem in a timely manner.

3 The sideview pulls the ball at the exact position where it was dropped. When turning over, the pattern is transferred between the prints by the ball on the side scale.

When an experienced master prints some prints, he often sees whether the graphic (front, back) on the side of the rules of the paper will be picked up by the ball on the rules. If possible, then according to the operating equipment, try to adjust The position of the version is avoided.

Third, other.

In the printing unit, there are oil leaks, especially three (including printing plate, blanket, and embossing) rollers. The roller shaft head leaks oil. When the oil enters the edge of the printing plate, it will appear like a dirty surface. Dirty. This situation should also pay attention to analysis and processing during operation. I have encountered such a device, the main fault is the shaft seal of the plate cylinder