The selection, installation and adjustment of the doctor blade in gravure printing have an important influence on the quality of the printed matter. The following discusses some views on the application of squeegee blades in gravure printing and some issues involved.
1. Selection and installation of doctor blade
High-speed gravure presses generally use imported steel scrapers with high elasticity, whose thickness is 150 ～ 180μm.
When installing the squeegee, pay attention that both sides of the squeegee should be longer than 10-20mm of the printing plate to avoid the ink of the printing plate splashing on the rubber roller. The steps and main points are as follows:
1. The distance between the lining and the back of the knife is about 10mm. The distance between the liner and the blade is 5-7mm. However, according to the specific situation, the distance can also be increased appropriately (that is, installed as a soft blade), or reduced (installed as a hard blade).
2. Put the new blade behind the lining, insert it into the knife slot and tighten the back screw. The screw should be tightened from the middle of the blade first, then gradually outward, and the two sides should be tightened alternately. Here is a trick to avoid blade warpage: when tightening the screw, it should be completed two or three times, and it cannot be done in one step. While tightening the screw, hold a rag to clamp the blade and lining and pull it sideways, so the assembled knife is relatively flat. Only a flat blade can guarantee the uniform ink volume of the printing plate.
3. Select the appropriate sharpening tool and sharpening method.
When using a sharpened knife, if the whetstone is rough, it is not suitable for sharpening a new knife. According to practical experience, it is more appropriate to grind the knife with No. 1 emery paper, which can grind both new blades and old blades with large defects. It is okay to grind the new knife with No. 0 gold steel sandpaper, but it is difficult to grind well if it is used to wear the old blade with a larger notch. If sometimes it feels rougher to grind the blade with No. 1 emery paper, then use No. 0 emery paper to grind it slightly. There are generally two ways to sharpen the knife: One is to fold the sandpaper into a V shape, but pay attention to fold several layers of sandpaper to prevent the blade from cutting through the sandpaper and hurting your hands. This method is more efficient, the angle of the cutting edge is easier to control, but it is not safe; the other is to use two hands to grind the knife at both ends of the sandpaper, which is very safe, but the angle of the cutting edge is not very good control. Blades that are too blunt and too sharp have a negative impact on print quality.
2. Reasonably adjust the doctor blade
Squeegee pressure, sharpness, and contact position with the plate all have an impact on print quality. According to actual needs, reasonable adjustment of these factors can solve many problems in printing.
1. Pressure ADL doctor blade imported from Japan, the factors that affect the pressure of the doctor blade in Japan ADL ink are the size of the cylinder pressure, the difference between the hard and soft blades, the angle at which the doctor blade crosses the printing plate, etc. The greater the slope, the higher the pressure The bigger. Excessive pressure will reduce the transfer rate of the ink and cause greater wear on the printing plate and blade; too small pressure is prone to dirty plates or scraper lines.
2. Sharpness of the knife
The sharpness of the knife depends on whether it is a new knife, the blade angle when sharpening, the type of sandpaper (stone) used, etc. The appropriate sharpness of the knife should be effective to scrape off the ink without generating wire. Too sharp scrapers are often prone to traces of wire, while at the same time wearing plates.
3. The distance the squeegee touches the printing plate
The reduction of the distance between the blade and the printing plate is helpful to improve the ink transfer rate, but it is easy to produce dirty plates; on the contrary, it can effectively solve the dirty plate and avoid some small line marks, but it will reduce the ink transfer rate.
4. Flatness of the knife
The flatness and warpage of the knife mainly depend on the installation method. Of course, it may also be related to the adhesion of foreign objects in the slot of the knife holder or on the blade and the lining. When the flatness of the knife is poor, some bead-like marks will appear on the edge of the printing surface, or some intermittent line marks will appear from time to time. Some people think that the above phenomenon will only occur if the ink is rough and the ink flow is not good. In fact, this situation can also occur when the knife is uneven.
5. The movement of the doctor blade
The movement of the doctor blade to the left and right has an important effect on reducing the doctor blade line, improving the doctor blade utilization rate, and reducing the abrasion of the plate. If the squeegee moves abnormally, you should try to improve it.
When line marks appear in printing, one is causing plate damage, and the other is that the scraper is not sharp. However, there is another reason that foreign objects are stuck behind the blade. The solution is to move the squeegee back and forth up and down in order to get rid of the sticky objects. If you can't get rid of it, you can only stop and wipe the squeegee.
Another problem is related to the squeegee, and many people misunderstand that it is caused by ink. This refers to the occasional appearance of dozens of irregular line marks of varying lengths, which look like they are piled up by ink. It is different from the scraper line. It is not continuous, but sometimes there is no time, or the printing port is only hanging in the river. The 1/2 or 1/3 part appears, which is actually caused by the pressure of the blade being too low or the blade is not sharp enough.
Several indicators of the above scraper have both differences and similarities. When solving specific problems, one of them can be used, and sometimes it needs to be comprehensively applied to completely solve the problem. For example, when there are some intermittent ink lines, it can be solved by increasing the pressure, or by sharpening the knife; when the dirty version that is more severely punished, it needs to be combined in several aspects; first, use a sharp blade, second To increase the pressure, the third is to increase the distance between the blade and the contact surface.
When the squeegee is seriously worn and cannot be ground with sandpaper or oilstone, it can be honed with a special sharpener so that it can be reused several times, greatly reducing the cost of the squeegee.
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