First, the knife line caused by particulate impurities
1. Foreign particles
For example, the paper scraps and ink skin contained in the film are brought into the gap between the scraper and the plate roller surface, and they cannot partially lift the ink scraper through the scraper port, so that more ink bodies are transferred to the On the substrate. In addition, multiple fine knife lines may be generated side by side, as shown in Figure 1.
During the printing process, the squeegee can be moved back and forth, up and down, swing out the sticky foreign matter or use a bamboo stick to directly remove the squeegee blade (must be completed by a skilled operator to avoid unnecessary losses due to improper operation) If necessary, stop the machine and clean it thoroughly.
At the same time, because some of the dead corners of the ink in the ink tank, the resin is not precipitated due to poor circulation, crystals and agglomerates of different sizes are formed, or ink skins are formed on the surface due to excessive drying (ink skins are easy to produce in summer, and spring and autumn are both There are many cases of ink crystallization and agglomeration due to the influence of temperature in the season.) With the circulation of the ink, a part of the ink crystal or the ink skin is brought to the blade by the rotating plate cylinder, and it is broken by the doctor blade to form an irregularity. Brushed. So it must be improved from the aspects of ink circulation and filtration.
2. Smaller hard particles
Such as: ① the ink itself is poorly ground, resulting in hard particles. (The fineness of general ink should be ≤15μ)
② Coarse particles produced by agglomeration, agglomeration, and coarsening of pigment particles in ink
③Dust in the environment falls into the ink.
The knife line caused by such particles generally has the following characteristics:
① The length of the knife line is long, the thickness is basically the same, and it has continuity. Because it is a small particle, the resulting knife line is also thin
②In some cases, the blade line swings left and right as the scraper moves in series. On the other hand, the direction is very regular, appearing in the printed pattern part of the same printing plate, and at the same position, there is no substantial change with the left and right swing of the scraper. This is because the coarse particles in the ink lift up the scraper, which cannot be driven by the swinging of the scraper.
③ In addition to the case of the solid knife line, there is another case where the fine particles are sandwiched between the blade edge and the ink in the dot is squeezed away. There is no ink transfer there and another layer of printing color is exposed or compounded when printing only white. After the base color of the substrate is exposed
Therefore, the old ink must be filtered with a wire mesh of more than 100 mesh before use, and the ink that has been used for a long time should be re-filtered before use, and the environment of the workshop should be maintained to prevent dust from falling into the ink and generating knife lines.
The filtering work of the ink circulating in the ink tank is also very important. It can well filter out the resin crystals and foreign impurities formed during the use of the ink. The mesh should be finer. The method is to use the ink in the ink tray or the outlet. Put a stocking under the ink pan.
Second, the knife line caused by the gap of the scraper
The basic characteristics of the knife line caused by the small notch of the scraper are: first, it moves left and right with the left and right serial movement of the scraper; second, it always appears at the damaged part of the scraper; The fourth is that when the blade gap is a small defect, it often appears as a thin knife line that appears intermittently. Due to the possible fluctuations in the blade pressure, the thinness of the ink across the fine knife wire varies, and some places may not be transferred to the printed product, so it does not appear as a uniform, consistent, continuous solid knife on the printed product. line.
The factors that cause the small gap of the scraper are:
1. When the hard particles pass through the blade edge, it may cause the blade edge to be damaged, and the blade will form a small gap.
2. The scraper is too sharp and sharp, and it is easy to scratch the layout when the layout is floating. The scratched layout in turn makes the scraper form a small gap and generate a cutting line; this is why the blade has a cutting edge during assembly. One side must be facing up.
3. In addition, if the layout is rough and has burrs, it is also easy to cause a small gap in the scraper and generate a knife line.
4. If you change to a printing plate with a different pattern, you should re-sharpen or replace it with a new one, so as to avoid scraping the ink with a locally worn scraper, which will produce fine knife lines during printing.
Third, the knife line with a clear band starting point
There is a clear band-shaped starting point, then it gradually becomes filament-like and disappears from the layout, and swings left and right with the scraper moving
This kind of knife line is obviously formed by the thick ink particles in the ink that lift up the doctor blade, and we can clearly find the ink marks formed by the coarse particles being crushed under the shear force between the doctor blade and the plate. Then, under the recovery of the scraper pressure, it gradually changed from a band shape to a filament shape and disappeared from the layout. Such coarse particles caused by pigment agglomeration (sometimes caused by poor dissolution of auxiliary agents in the ink) are generally difficult to filter out with a wire mesh, and the ink can only be replaced.
4. Intermittent ink thick knife line [can't be wiped off with a bamboo knife, commonly known as "spitting ink" "
1. At the beginning, it is filament-like, gradually becoming band-shaped (the widest point can reach 1 ~ 2mm width), and then gradually shrinks into a micro-filament, disappearing from the layout, without obvious starting and ending points.
2. Swing left and right as the scraper moves.
3. The length is generally within 2m, and they are intermittent (several meters to several hundred meters apart).
In general, such cutting lines are prone to appear in blank areas within 1 to 2 cm of the layout with obvious vertical color blocks close to the pattern.
Reason analysis: Firstly, the wear parts of the scraper are mainly concentrated on the dots. Under the observation of a magnifying glass, it will be found that the edges of the dots are actually relatively rough, so the degree of wear of the scraper is different in the graphic and blank parts. In this way, under the series motion of the scraper, the worn scraper part swings to the blank part, forming a weak link for scraping the ink in the blank part. The scraper cannot be closely adhered to the layout here to create conditions for the passage of the ink body. The situation of endless scraping (the ink fluid partially lifts the scraper and passes through it) is initially only a small part, but when the ink fluid passes through the gap between the scraper and the plate, there is a large Hydraulic pressure, so that more ink fluid continues to pass through the blade to become a band, and then gradually changes from a band to a microfilament and disappears under the action of the rebound pressure of the blade. Because the scraper lifts up a very soft ink gel, the scraper can drive it to swing left and right.
Such line marks are actually formed by the scraper's inability to scrape the ink on the plate, and then are formed by the hydraulic force of the ink colloid. Sometimes the printing plate with a different layout is replaced. This kind of cutting line also appears when the scraper is not replaced or is poorly ground, and the position of the outlet on the layout is relatively concentrated. The layout is relatively rough, which will cause uneven wear of the scraper and also cause this kind of knife line.
The following treatment methods are generally used for such line marks:
1. When the viscosity and solubility of the ink are adjusted correctly, use fine sand (800 #) to wipe the layout of the line, so that the scraper can easily wipe out the ink, which can slow down the chance of such a knife line.
2. For new or unplated version, if the problem is serious, and such knife line appears again after sharpening or shortly after tool change, you should consider whether there are burrs on the edge of the screen dot, which may cause excessive and rapid wear of the scraper. of.
3. The tool change or discarding is a more direct and effective method.
The above analysis is the cutting line when the layout is special. In addition to considering the abrasion factors of the scraper in special parts, some basic factors should also be considered. If the knife line of this shape appears full, you should consider whether the following control points are appropriate:
1.Viscosity of ink
The scraper scrapes off the remaining ink on the layout. The scraper must penetrate the ink layer. The higher the viscosity of the ink, the higher the viscosity of the ink. The force of the scraper penetrating the ink layer to scrape off the ink is relatively high. The increase in viscosity under the same conditions increases the difficulty of scraping the ink with a doctor blade.
2.The wear of the ink scraper and its elasticity
The blade edge of the scraper should be ground to a 25 ° oblique arc, so that it will not hurt the layout when it comes into contact with the layout, and can also function as a blunt blade, and the scraping effect of the scraper is good. With the use of the scraper, the contact width between the cutting edge and the plate surface increases, the ink scraping becomes worse, and it should be ground in time.
The elasticity of the scraper. Because the electric engraving rollers are uneven at the micro level, when the high-speed operation is performed, the scraper relatively floats on the surface of the plate. If the scraper has good elasticity, the scraper can quickly return to its original position after floating on the plate. The ink effect will be good.
3.Fineness of ink
Ink with rough particles has a greater impact on the squeegee during high-speed printing, it is easier to lift the squeegee from a part, and the fluid is more likely to form a cutting line through the blade edge. In fact, high-end inks rarely have intermittent, varying thickness knife lines.
Causes of rough ink:
①The fineness of the ink itself is not up to standard.
② Poor solubility of solvents to resins or auxiliaries leads to agglomeration and poor dispersibility of ink pigments, and the ink returns to coarse.
③ In the use of polyurethane ink, pay attention to the influence of solvent balance on the solubility of the ink. In the case of imbalance in solubility, it may cause the coarseness of the ink and the poor fluidity.
When the friction resistance of the layout is low or the pressure of the squeegee is too large, the layout is easy to form obvious vertical lines. The existence of these stripes actually forms a local gap between the squeegee and the layout. When the squeegee floats on the layout, the ink fluid is easy. Through these gaps, a knife line is formed. For the full version of the intermittent cutting line gradually changed from a filament to a ribbon, generally by polishing the plate, most cases can be resolved, this is because the surface finish is improved, the scraper is more likely to scrape off the remaining ink .
5.The running accuracy of the squeegee
The imbalance in the rotation of the plate roller will cause an impact on the scraper, so that the scraper cannot always be close to the plate surface, so the ink fluid can easily pass under the high-speed running state, and the scraper is lifted from a part to form a knife line.
Affecting the running accuracy of the plate roller are:
① The transmission accuracy of the machine itself.
② The dynamic balance of the plate roller.
③ Assembly accuracy.
6.Influence of squeegee angle and pressure
If the angle of the squeegee is too small, the impact force of the ink fluid on the squeegee is greater in the direction of the contact point between the squeegee and the plate. The fluid is easy to lift the squeegee blade from the local area during ink formation to form a cutting line. The squeegee pressure required for high-speed printing is greater than the squeegee pressure for low-speed printing.