Gravure Fault Statistics and Typical Failure Analysis

1) The adjustment of the squeegee knife When the squeegee is installed, it is necessary to adjust the axial parallelism: adjust the preliminary angle of the squeegee so that the blade is lightly pressed on the plate, and adjust the left and right sides of the blade to ensure the printing. This ink is uniform, so that it can accurately adjust the squeegee pressure and angle filling, which is an important part of the impact on printing quality.
In general, under the same angle, the greater the pressure of the blade, the better its ink-scraping performance, but the pressure of this assembly to operate the plate, the pressure is too small and the ink is not scraped, will cause the knife line, so the pressure of the blade should be abrupt, even if It should also increase slowly when pressurized.
Under the same pressure, the larger the angle of the blade is, the better the squeegee performance is. Similarly, if the angle is too large, the squeegee and the plate will operate seriously. If the blade is too small, the squeegee and the ink will be easily drawn.
Since the angle of the squeegee and the two affect each other, we can sum them up into one physical quantity—the pressure P between the squeegee and the plate, which is more intuitive.
After printing for some time, the contact area S between the squeegee and the printing plate will be increased due to wear. At this time, the knife line phenomenon will occur, and some of the squeegee blades should produce a large squeegee pressure F. According to the formula P=F/S, it is difficult to judge the change of pressure, we have to judge it. When the knife process exceeds MAX, the pressure P will decrease, that is, excessive knife will make the knife line problem more serious. Because the process of pressing the knife is a pressing process, the squeegee pressure is lower than the limit value, generally around 3-4Kg/cm3, and the angle is between 50 and 70 degrees. When the knife line appears, the angle may be steeper. Above 60 degrees, the pressure is greater.
After the scraper is used for a period of time, it must be polished again, according to certain rules. First of all, clean the blade holder and then expose it with a hard blade clip soft blade. Note that the distance between the two ends should be the same. When sharpening, the knife holder was clamped on the table, polished with whetstone, and polished at an angle of about 40 degrees, and then roughly grinded with 360 water sandpaper, and then finely ground with 1000 fine sandpaper. When the wear is severe, the blade must be replaced.
If the cutting lines appear to be relatively regular and obstruct the movement of the squeegee, it can be concluded that the squeegee is mainly caused by the adhesion of the squeegee and the blade, and is solved by grinding or replacing the squeegee and the foreign material on the back of the blade.
2, ink ink is the bearer of the print pattern reproduction, the central is composed of pigments, binders, fillers and additives, and its influence on the knife line is also outstanding.
Intaglio printing inks can be divided into: engraved gravure inks and gravure inks. Among them, the engraving and gravure printing ink is mainly used for printing valuable securities. However, the problem of the knife line in this article is mainly in the packaging field, so it is unnecessary to discuss it. Let's talk about gravure ink.
Photogravure ink is a volatile drying type. It is a typical liquid ink with good flow properties and low viscosity. It is generally composed of colorant, solid resin, volatile solvent, and additives. Some of the fineness, viscosity, dryness, hardness, and activity of the table are greatly influenced by the knife line. The following discusses the properties of the gravure ink in combination.
1) Fineness If the fineness of the ink is not enough, that is, the material is too large, it may be embedded in the scraper or damage the scraper, resulting in a knife line. The fineness of the ink itself is determined by the manufacturer. The ink manufacturer should be flexible, have no chemical reaction with the resin binder used, and have good flowability and properties during the development of the color. The printing factory should check the fineness of the ink when purchasing the ink, and use the fineness of the scraper to measure it with the detection method suitable for the conditions of the factory. Generally, the fineness of the gravure ink is about 15um. The smaller the fineness is, the smaller the cutting line is. favorable. In fact, the national factor is not only this step, the ink is mixed in the use of impurities, long-term use of the ink caused by long-term contact with the air oxidation of the resin cross-linked to form coarser particles will produce a knife line. For the long-term use of the tip regularly filter the ink in the ink tray or filter into the metal wire mesh in the ink circulation system, the recycled old ink should be filtered before use, the ink should be replaced when the ink is flocculated, and some help can be added. The solvent is thoroughly stirred to evenly disperse the particles. If conditions permit, a closed ink jet device can be used, which can greatly reduce the ink and air contact.
2) Viscosity The viscosity of the ink is too high, which can cause it to increase its impact on the blade, so that the blade cannot completely clean the ink. The viscosity of the ink has a lot to do with the solvent. Gravure ink uses volatile solvent, and in order to reduce the affinity of the ink to the printing plate, the resin is not precipitated in large quantities, and a solvent with strong dissolving power and quick volatilization is used. As printing progresses, the solvent in the ink continuously evaporates, causing the viscosity of the ink to rise. For this purpose, attention should be paid to changes in the viscosity at any time during printing, and the viscosity controller or artificially replenished solvent should be used according to the loss of the solvent in order to maintain the stability of the viscosity. In practice, flow cups can be used to measure viscosity. Gravure prints are generally used with a Zahn stream cup. According to the size of the opening, they can be issued in several sizes. A reasonable choice of flow cups has a great influence on the crudeness of the viscosity measurement.
3) Drying nature Gravure ink is a volatile drying type, so its dryness is also the volatility of the solvent. When the ink is excessively dry and the solvent volatilizes too quickly, the friction between the printing plate and the blade is relatively increased, resulting in a scraper It is difficult to scrape off the ink on the layout to cause the knife line, but too late drying, that is, the slow evaporation of the solvent, causes the non-graphical portion of the trace ink to transfer to the substrate to form a plate, so the evaporation speed of the solvent is moderate. In the analysis of viscosity, it was mentioned that a solvent with a quick volatilization was used, and there was a contradiction in that it was possible to solve the problem of excessive drying by adjusting the pressure and angle of the doctor blade. The gravure ink was also selected as a solvent with high solubility and fast evaporation. For better, ethyl acetate, ethanol, etc.
4) In fact, this link can be attributed to fineness. For pigments with high hardness, such as red and blue, even if they are opened, they cannot reach a very small particle size, and they are easily scratched and damaged. This inevitably results in a knife line. Then, when using red and blue inks, it is necessary to set a fine degree of fineness. When buying inks, try to provide suppliers with finer inks, and then pre-select the preventive measures to prevent other factors from affecting inks. Fineness, with attention to the condition of the blade, guarantees the integrity of the blade.
5) Activity In the ink production process, a certain amount of active agent is generally added. There is also a certain activity after the plate cylinder has been chrome plated. If the activity of the ink or the plate cylinder is too high, the active material does not adhere strongly to the plate cylinder during printing, and the scraping blade is also very difficult to scrape, thereby forming a cutting line. In this case, the knife line is meteor-like or microfilament-like. There is a certain degree. At this time, the plate cylinder can be washed and cleaned first to remove the active material on the surface, and then the surface active agent can be added to eliminate the knife . When using this additive, it is preferable to use an ink circulation system so that it is repeatedly used.
The quality of the ink has a great influence on the knife line. When the conditions permit, it is better to use higher ink. To ensure the proportion of fresh ink, just pay attention to the series of measures mentioned above for purchase and use. The knife line problem is It is not difficult to solve.
3. Environmental conditions It is very important to maintain a good environment in gravure printing workshops. The environment here mainly refers to dust and temperature and humidity.
1) Dust and dust will produce thin knife lines, and the plastic film will generate electrostatic adsorption during printing. This dust will return from the film to the printing plate cylinder during printing. When the dust collects in the ink, it will adhere to the blade and create a knife. line. To this end, static eliminators should be installed. Above the plastic substrate film, anti-static screens or electrostatic belts should be suspended to increase the humidity in the air to reduce electrostatic adsorption.
Dust may also originate directly from the air, which requires a good hygienic environment in the printing shop: the printing presses are placed, the workshop is isolated from the outside world by wind curtains, the number of people in the workshop is controlled, wet ground is often used to mop the ground, and the ground is used exclusively Paint treatment to prevent dusting. In the maintenance of the machine, it should also be noted that the ducts and other unattractive parts are usually cleaned to prevent dust accumulation. For the dust in the ink, a filter device can be added to the ink circulation system to clean the ink tank and the ink tank. When the old ink is used, under the premise of ensuring quality, 1/3-1/2 old ink can be used, but it is better not to use the old ink completely, and to filter it with a metal wire net with more than 120 mesh before the old ink.
2) The temperature and humidity conditions are actually included in the previous article. Here is a detailed analysis.
The temperature is too high, the solvent volatilizes too quickly, the solvent balance is unbalanced, and the drying is excessive; the ink solution is poor, the viscosity becomes large, and the ink scraping is difficult, both of which can easily cause the knife line.
Therefore, the gravure environment should be kept as close as possible, install air filters and dust content determination, install air conditioners to maintain the environment temperature and humidity stability.
Through the above analysis, we can see that there are many factors in the production of knife lines, and are related to each other, such as plate cylinder and squeegee, squeegee and ink, ink and environment, knife line also often with the version of contamination, paste version, etc. Phenomenon occurs at the same time, so it is often difficult to judge when the fault is caused by various factors, the solution can not be specified, but as long as the different characteristics of the knife line, through the judgment method mentioned above, grasp the major aspects of the main spring Influencing factors and taking corresponding solutions can eliminate it. Of course, the most ideal state is to proceed from the factors that create the knife line involved in this paper to reverse thinking, and take all measures to prevent it from occurring to reduce the incidence of knife lines.