About the causes and solutions of knife line in the process of gravure printing


Published on:

2017-04-21

Gravure printing often has many quality failures, of which the generation of knife line is one of the most common failures, the generation of knife line to scrap the product, resulting in economic losses. In this paper, from a professional point of view, combined with the actual production experience, the five major factors affecting its production are analyzed and summarized in detail, and the corresponding solutions are proposed.

About the causes and solutions of knife line in the process of gravure printing

Gravure printing often has many quality failures, of which the generation of knife line is one of the most common failures, the generation of knife line to scrap the product, resulting in economic losses. In this paper, from a professional point of view, combined with the actual production experience, the five major factors affecting its production are analyzed and summarized in detail, and the corresponding solutions are proposed.

 

A. Ink factors

1, the impact of ink viscosity

Ink viscosity should be matched with the printing speed, ink viscosity is too high, the adhesion of the printing plate cylinder is too strong, not conducive to scraper to scrape the layout of excess ink clean, the chances of printing knife line is high. Printing process to regularly measure and control the ink viscosity, generally every 20 minutes to 30 minutes to measure once. This type of knife line in the print roughly presents two forms.

(1) the first kind of knife line is a micro-flow filament-like, generally does not drag too long, but intermittently appear in the print, micro-flow filament-like knife line at the beginning of the point is generally thicker than the end of some, in the actual production if not carefully observed, this micro-flow filament knife line is easy to ignore. This filament-like knife line is often the root cause of quality accidents, must be paid attention to.

Solution:

There are several ways to solve this flowing filament-like knife line, which is also commonly used by printing staff, but also according to the actual situation to determine.

① will be a 125px wide, 375px long, 140μm thick soft blade grinding very smooth, lightly pick at the appearance of the knife line, under normal circumstances, this tiny flow of filament-like knife line will be improved. This operation is usually done with the machine running, which requires a familiar operator to complete.

②The most fundamental solution is to add the appropriate amount of compatible solvent to the ink to reduce the viscosity of the ink, ink viscosity is reduced, the small flow of filamentary knife line will generally be eliminated. This is because, after the addition of the melt ink viscosity is reduced, the flow of the corresponding increase in the reason, so the small flow of filamentary, knife line and the ink's liquidity becomes poor also a certain relationship, please note.

(2) the second kind of knife line to print the pattern as the starting point, but the knife line concentration, and more rough, and very regularly appear in the same location of the print, and often continue to the next color, this knife line is easy to find. The reason for this knife line is that the ink viscosity is too high, the printing plate on the network transfer is not good, the ink within the additives with the change in ambient temperature and destroyed, especially when the temperature is too high, the ink surface will form a thin layer of dry solid layer, a long time between the printing plate and the ink scraper will be accumulated between the ink layer will be scraped up the top, resulting in this knife line plot.

Solution:

First of all, adjust the angle of the scraper knife, in general, the angle between the scraper knife and the printing plate is 60 °, but some times the scraper knife and the printing plate angle increased to about 65 °, so that the pressure of the scraper knife also increased accordingly. At the same time to the high viscosity of the ink to add the appropriate amount of the corresponding additives or solvents to reduce the viscosity of the ink, increase the fluidity of the ink. Add solvent should be stirred with the direction of the solvent to the ink, another solution is to put the stirring roller in the ink tank, with the rotation of the plate cylinder stirring roller also rotates accordingly. There to prevent the ink surface caking, but also to reduce the knife line generated by a method. The above two solutions used in conjunction with each other, in the absence of special circumstances, the above concentration of varying thickness of the knife line will be effectively curbed, it must be noted that the angle of the ink scraper should not increase too much to prevent damage to the card cylinder.

2, the impact of ink dryness

Ink drying too fast, prone to crusting phenomenon, if the crust is not dissolved in the ink cycle process, once in contact with the ink scraper knife will adhere to the knife mouth or knife bottom, resulting in printing knife line. If the ink circulation system is used in the open ink tank, more likely to occur in the ink crust, caking phenomenon. In order to reduce the chances of such knife line failure, to often stir the ink tank in the ink; as needed to add the appropriate amount of slow drying agent; such as conditions, you can also add a sealed cover, can effectively control and reduce the printing knife line failure.

3, the impact of the coarse and fine degree of ink particles

The fineness of the pigment particles have a direct impact on the frequency of the knife line, for example, the blue ink used in the pigment particles of high hardness, even after full grinding is difficult to reach a very fine granularity, in the printing process is easy to damage the scraper or affect the effect of scraping ink, printing blue ink often knife line failure. The finer the ink, the better the quality of the print, the chances of knife line will be reduced accordingly. There are strict requirements for the ink particle size coarse, usually requires ink fineness of 15mm, ink fineness can not meet the requirements, the rough particles will put the scraper top, thus producing knife line, while too rough ink particles on the scraper, the printing plate damage are great, the ink thickness should be moderate, too fine is not good.

After a long period of observation of such a knife line, found that the ink particles rough resulting in the knife line on the print is not the same, the main surface for three forms.

(1) the first kind of knife line on the print irregular thickness of the knife line, this knife line from the printing pattern part has been extended to no pattern at. Knife line is longer, and with the squeegee swing left and right, the knife line also moves with, sometimes a, two, sometimes many. The solution is to adjust the position of the ink scraper up and down, and at the same time should observe the print has no color shades away from the phenomenon.

(2) the second kind of knife line is very regular, appearing in the same printing plate on the printing pattern part, and is in the same position, with the squeegee's left and right swing, basically no big change, this is because the ink in the coarse particles top up the squeegee, the squeegee's left and right swing simply can not drive it, if the appearance in the middle color, the next color ink will be added to these knife line fill, that is, the next color at the knife line ink.

(3) the third knife line is easy to appear in the last color, in the plastic film printed usually appear in the white version, such a knife line or two will run through the entire print, from the front of the print to observe, if the knife line is not rough, not easy to find, from the back of the print to observe the knife line can be found throughout the formation of "traces", this "trace" in the composite will undoubtedly appear, especially in the composite coating!

Solution:

Whether the above kind of knife line, the solution is to stop printing, filtering ink, usually the first, the second color used to filter the ink before printing on the machine, filtering mesh selection 140 mesh metal screen, after a long period of practice, has been effective, worthy of peer learning and reference.

4, the impact of ink activity

In the ink production process, generally add a certain amount of active agent. Plate cylinder chromium plating also has a certain activity, if the ink and plate cylinder activity is too high, in the printing, the active material will be firmly adhered to the cylinder, the ink scraper is also difficult to scrape it clean, thus forming a knife line. At this time, the first printing plate cylinder cleaning, to remove the active substances on its surface, while adding active mitigants in the ink to eliminate knife lines.

5, the impact of ink quality

Ink storage for too long, the solvent balance changes, causing the ink solvency force is reduced; or ink blending, improper use, causing chemical reactions, will make the ink precipitation. In the printing process, the precipitate in the ink is prone to accumulate in the ink scraper knife mouth caused by the knife line, the need to replace the ink.

Second, the printing plate factors

1, the impact of the layout of the smoothness

The printing plate is divided into two kinds of manual engraving and electric engraving plate, manual engraving intaglio layout is not smooth, easy to appear knife line, electric engraving intaglio surface is smooth, the chance of knife line is much lower than the manual engraving intaglio. In the manual engraving intaglio printing, the first time on the machine before the use of 1000 # water-resistant sandpaper to polish the layout, the specific practice is: the printing speed will be opened faster, empty machine operation, sandpaper back and forth when polishing, but not too much force, otherwise it will damage the network cavity. After grinding with a soft cloth to dry the roller will be on the printing machine again, the knife line phenomenon is reduced a lot. Need to pay attention to: this work needs to be served by experienced personnel.

In addition, gravure printing plate if the polishing when the burr failed to polish clean, in the printing process, the layout of the burr parts of the ink is not easy to scrape clean, serious damage to the scraper, thus forming a knife line. For the printing plate surface roughness caused by the printing knife line, available 1200 purpose fine sandpaper gently polish the plate, but in general it is best not to polish the plate to avoid damage to the plate.

2, the impact of chromium layer hardness

Plate making, if the chrome plating is too long, the plate surface hardness of the chrome layer is too high, easy to damage the ink scraper, the printing will appear on the variable knife line. On the contrary, if the chrome plating is too short, the plate surface chrome layer is too soft, the hardness of the plating layer is not enough, easy to damage under the action of the squeegee, the printing will appear on the fixed knife line, this knife line sometimes, also known as the "hidden knife line" often appear in the non-printing pattern, this knife line only in the reverse side of the fluorescent lamp irradiation is easy to find, for This knife line, there are two solutions: one is in this case, with 1000 # water-resistant sandpaper on the plating gently grinding, generally will be improved, but often printed after a period of time, and have to appear. Therefore, it is best to return the plating treatment, more serious when it is necessary to re-plate; base two is to adjust the ink scraper, increase the pressure of the scraper, while blowing wind to the layout, also received good results.

3, the impact of the cleanliness of the layout

Plate grinding process generated by the debris can not be completely removed and attached to the plate, the printing process will damage the plate and ink scraper, thus giving birth to the knife line. Before printing on the machine, it is best to use the solvent to clean it thoroughly, to remove oil and impurities to reduce the birth of printing knife line failure.

4, the impact of mechanical processing precision

Poor mechanical processing accuracy of the printing plate cylinder can also lead to printing knife line failure, and difficult to detect or not easy to check, so it is easy to be ignored, the actual operation, you can check on the machine. In addition, should also strictly control the installation accuracy of the printing plate, so as not to exceed the concentricity error, at high speed printing on the scraper knife raw too much impact and cause the occurrence of printing knife line failure.

Third, the scraper factors

1, the hardness of the scraper knife

The hardness of the scraper knife should be lower than the hardness of the chromium layer on the surface of the printing plate. If the hardness of the scraper knife is too high, will accelerate the wear and tear of the printing plate, resulting in scraping: If the quality of the scraper knife is poor, in a relatively short period of time will wear, resulting in the layout of the ink failed to scrape clean, the knife line, at this time need frequent knife adjustment, knife replacement.

2, scraping ink knife mouth gap

When the ink scraper knife mouth damage gap, printing will appear on the regular left and right moving knife line, this knife line is relatively easy to identify. If the ink scraper knife on the gap is relatively small, available oil stone or fine sandpaper sanding to smooth and smooth. If the gap is larger, it is necessary to replace the ink scraper knife. When replacing the knife, the pressure blade and knife slot must first be thoroughly clean of ink accumulation, so as not to affect the smoothness of the installation of the ink scraper knife, and the ink scraper knife to reveal the length of the pressure blade part to be consistent, about 5mm or so. In the printing process, some ink scraping knife needs to be worn with the printing plate for a longer period of time before reaching a good ink scraping effect, when the available metallurgical sandpaper will scrape the edge of the ink scraping knife grinding smooth, in order to make the ink scraping knife to achieve a better scraping effect, and is conducive to the improvement of raw efficiency.

3, scraper knife installation flatness

Ink scraper knife installation of the flatness of the poor will also cause printing knife line. Therefore, the installation of the ink scraper knife to ensure good flatness, so that the scraper knife force is uniform.

4, the pressure and angle of the scraper knife

Scraper knife pressure and angle of the printing knife line also has a great impact. Printing also according to the layout of the degree of ink scraping and printing inking situation, the appropriate adjustment of the angle and pressure of the ink scraping knife to achieve good ink scraping effect, thereby reducing the printing knife line failure.

Installation of the scraper should be installed with the scraper under the flat plate frame in the small plate between the protection of 0.5mm-1.2mm, the scraper An too prominent, the scraper is easy to deformation, wavy shape, affecting the effect of scraping ink, thus causing quality accidents, if the installation is too short, the ink is very easy to scrape out, but the version of the knife marks will be a lot, and as long as the knife is loose and then printed out the pattern becomes thicker or lighter, can not be used repeatedly.

5, the scraper knife grinding and selection

Ink scraper knife grinding good or bad and knife line has a great relationship. Grinding is appropriate, not easy to produce knife line, otherwise it is easy to produce knife line. Grinding, usually choose oil stone grinding, and then 1000 # water-resistant sandpaper with 6 # oil grinding smooth, the back of the knife gray grinding into 25 ° oblique mouth, if the scraper produces a serious gap, it should be replaced with a new blade, but also should pay attention to the blade if it is imported soft single sex, it is not easy to produce knife line, if it is a domestic blade, it is easy to produce knife line. The blade must have a certain elasticity, hard surface without elasticity is the main cause of the knife line. There are other reasons such as, scraping the excess ink on the printing plate, and the occurrence of fog affect the quality. General copper hardness between 200HV-300HV, chromium hardness between 800HV-900HV, scraper hardness should be selected between 450HV-(900 + 300)/2, so that not only does not hurt the printing plate, but also does not leave knife marks, while also reducing the chances of generating a knife line. The tip of the squeegee should be selected to have a rounded tip, which can protect the printing plate. Because the plate is round, the scraper is rounded, not to damage the printing plate, but also easy to scrape the ink clean. Keep printing without pattern at the clean and not stagnant ink, if the tip of the knife is flat, due to the flat very sharp both hurt the printing plate and affect the printing, scraping ink is not net produce knife line.

6, the pressure of the scraper

Scraper pressure is too large will also produce knife line, the solution is to reduce the pressure, while the appropriate blowing wind to the page, will also effectively reduce the production of its knife line.

Fourth, the solvent factor

In the printing process, if the solvent is used improperly, so that the ink is not fully dissolved, sticking together, once adhered to the scraper, it will form a knife line. This knife line is generally a dark line, sometimes not. The use of gold ink printing field color block is more common, because the gold ink is made of gold powder, gold oil and solvent blending, metal particles are heavy, easy to precipitate, if the solvency capacity of the solvent used is poor, coupled with inadequate mixing, metal particles will be unevenly distributed and accumulate together. If stagnant at the blade of the ink scraper, a thicker dark line will be formed. This dark line is characterized by: the appearance of a period of time will automatically disappear, and then formed again, seriously affecting the quality of printing. This type of knife line in the printing is difficult to eliminate, the need to start from the root cause of the work from the preparation before printing, the deployment of ink when using the appropriate solvent, fully stirred to fully dissolve, printing to maintain good circulation of ink.

V. Environmental factors

If the operating environment is dusty, Fa'ee into the ink will lead to the production of knife line. The workshop should first isolate the printing unit, reduce the flow of personnel, the room is equipped with air conditioning facilities and dust removal equipment, when going to work, the first color sequence and the second color of the ink filter before printing on the machine. Dust removal at work, special sanitation personnel to clean up at the end of the day, the environment must be well maintained.