Analysis of the causes of ink sticks in offset printing
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The ink stick is a phenomenon in which the dot points in the axial strip-like area appearing on the printed matter are increased or decreased.
In the daily work of offset printing, if the problem of ink sticks on printed products is often a headache. Because the cause of the ink stick is multi-faceted, it needs more comprehensive analysis and judgment to deal with it effectively.
In general, the ink sticks we can encounter in our work can be roughly divided into four types: vibrating ink sticks; gear-shaped ink sticks; roller ink sticks; and ink sticks produced by other reasons.
Let's categorize it below:
First, the vibrating ink stick
The vibrating ink bar means that the contact pressure between the printing plate, the blanket, the paper, and the impression cylinder changes instantaneously after the abnormal vibration of the machine occurs during the printing process (may increase instantaneously or may become instantaneous). ), resulting in dot deformation.
Blanket: The role of the blanket in the printing process is not only to increase the transfer rate of the ink to the paper, but also to absorb the machine vibration generated in the imprint. When the blanket is hardened, it is impossible to effectively absorb the vibration generated during the operation of the machine. When the vibration is expressed on the printed matter, the problem of the ink stick may occur. Many domestic printing presses in China use soft pads because of the low manufacturing precision and the large vibrations that occur during the printing process.
Poor oil supply: When the moving mechanism of the machine is poor in oil supply, it will have a certain impact on the smooth running of the machine. When this effect is large enough, it will cause the vibration of the machine. The transfer of the dot is affected to some extent, resulting in local dot deformation and eventually a dark ink stick.
Poor embossing force: When the roller pressure is too large, the vibration of the machine will occur during embossing, resulting in a rubber bar. Especially when the plate cylinder and the blanket cylinder just enter the embossing area (at the mouth), if the pressure is too large, the contact between the blanket cylinder and the impression cylinder will be affected by the vibration, and a large, regular ink will appear. Bar; when the blanket cylinder and the impression cylinder have just entered the embossed area from the cornice, if the pressure is too large, the vibration bar will appear in the contact area between the plate cylinder and the blanket cylinder, and then transferred to the rubber cylinder. On paper.
Bearing wear or pressure instability: When the roller shaft head is running out of oil or the machine wears natural wear for a long time, the gap between the roller shaft head and the bearing is increased, and the mismatch is poor. When the pressure is pressed, the ink stick marks are caused by the tremor. In this regard, it is advisable to make the rubber inner liner into a trapezoidal shape to reduce the impact during the imprinting state and to reduce the degree of the ink stick. In addition, if the roller shaft or bearing is seriously worn, it should be considered to repair or replace the new bearing to eliminate the ink trace.
The loosening of the grounding screw: the grounding screw is the foundation of the whole printing machine. If the foundation is unstable, of course, the smooth running of the machine will not be discussed. In the installation and commissioning of the machine and the maintenance of the machine once a year, it is necessary to specifically check the anchor screws and tighten the loose screws in time to ensure smooth operation of the machine.
Other reasons that can cause vibrating ink bars in printing are:
Vibration of the transfer roller; vibration caused by the impression of each roller; vibration caused by opening and closing of each paper cutter; improper motor rectification; uneven rotation; loose chain; loose bolts on the main part of the machine; Heart; roller imbalance; gear eccentricity and other reasons.
Second, the geared ink bar
The cause of the ink stick: the roller gear is worn, the gear gap is too large, or foreign matter accumulates between the gear teeth. When the drum gear wears, the gap or the foreign matter adheres between the teeth, during the operation of the machine, the drum gears cannot be precisely meshed, so that the linear speed between the rollers is not consistent. In addition, the backlash is too large, and the drum is also prone to chattering and shaking, forming a printed bar mark, causing a trace of the ink stick that is similar to the gear pitch. There is a foreign body obstacle in the backlash, and it is easy to cause the drum to tremble due to the phenomenon of "top teeth", so that the lateral layout of the printed product forms a strip of ink stick. In this regard, the tooth gap should be reduced by re-adjusting the center distance between the rollers; for gears with severe wear, new gears should be replaced; if there is foreign matter in the teeth, it should be cleaned up to eliminate the effect of the ink bar.
The reasons that can cause the gear bar are combined as follows:
Gear wear; bearing preload is too large or too small; accuracy is not high; tooth surface and tooth end chamfering is improper; weekly pitch error; tooth error tooth surface smoothness is not high; gear skew; improper backlash; excessive tension of driving belt Or insufficient; foreign matter on the tooth surface; foreign matter accumulates at the root of the tooth.
Third, the roller ink bar
There are two types of ink bars produced by rollers. One is a white bar (also called a water bar), and the other is a white bar (for convenience, we will refer to them here as a ink bar). The former is generally due to water. The problem with the roller is caused by the problem of the ink roller. However, the cause of the roller, whether it is a water roller or an ink roller, is basically the same.
The most common cause of ink sticks in daily production is that the roll pressure is too strong.
When the contact pressure between the rubber roller and the printing plate is too large, during the operation of the machine, the inking roller is pressed against the printing plate, and the impact is easily caused to cause chattering, so that the frictional force increases the streaks. If the pressure of several inking rollers is too large, the density of the bar marks will be greater. In this regard, the contact pressure between the rubber roller and the printing plate should be adjusted to maintain uniform and moderate contact conditions, and the ink bar can be better eliminated after eliminating excessive resistance. In addition, when the pressure between the inking roller and the inking roller is too large, the iron roller is pressed against the rubber roller, and the temporary temporary deformation caused by the elasticity of the rubber roller causes sliding friction on the printing plate to cause the ink bar. Moreover, since the pressure of the ink receiving roller and the ink roller and the printing plate are different, it is easier to cause the surface linear velocity to be out of synchronization to cause the ink bar. Therefore, the pressure between the rubber roller and the tandem iron roller should be adjusted moderately to prevent the rubber roller from being squeezed, and the resistance and friction force are increased to easily cause the ink stick to occur. There is also a case where the rubber roller has a bad core, is not straight, and is prone to ink sticks. This is because during the rolling process of the machine, the contact pressure between the rubber roller and the printing plate is inconsistent, and when the machine rolls, the local friction is aggravated and the bar-shaped bar marks are caused. In this regard, the high-precision ink roller should be replaced for printing the full-page and the textured version.
Similar to the ink bar caused by excessive roll pressure, there are some reasons for the roller ink bar in printing, the principle is basically similar to the ink bar caused by excessive roll pressure. Such as: roller bending; roller expansion, cracking; poor cleaning; ink roller, water roller aging; ink roller shaft wall hole is not concentric; bearing, gear and other wear; roller arrangement problem; roller bearing fulcrum problem.
Fourth, other ink bars
In addition to the three cases mentioned above, the reason why the ink bar sometimes appears in the printing production is really difficult to think of. Let us introduce some reasons that may cause the ink bar for reference.
material:
The plate-making paper is wrinkled; the liner under the blanket has wrinkles; the thickness of the plate is not uniform; the thickness of the blanket is not uniform; the remaining residual varnish;
Machine aspect
Wear of the drum and the bearing; the clearance of the bearing support is too large, the preload is too large; the bearing is skewed;
Use
The plate cylinder or rubber roller lining is too large: if there are two cases, the strip line will be caused by the inconsistent surface line speed, and the impact of the drum will be caused by the drum when the embossing is made. Friction is generated to cause a trace of the ink stick. In addition, sometimes the ink stick is easily caused by the imprinting of the bite portion, which is not obvious (the pressure is light), and the paper strip is too much. In this regard, the thickness of the lining should be accurately adjusted, and the minimum printing pressure should be used to make the ink layer uniformly transfer, so as to maintain the uniform line speed between the rollers as a prerequisite. When adding a paper strip to the bite area, it should be carried out by gradually adding the pad. Do not pad more at once to avoid the appearance of the ink stick trace.
The blanket is loose. If the blanket is just changed, the stretching becomes loose during the printing process, and the tension is not tightened in time, so that sliding friction is likely to occur during rolling, and thus a strip of ink stick marks appears. Therefore, when the blanket is slack, measures should be taken to tighten it to prevent the printing of the ink stick.
Fifth, Judgment and elimination after the occurrence of ink stick
As we mentioned at the beginning of the article, the general occurrence of ink sticks is very troublesome. The reasons for the occurrence of bar marks are various, and the situation is sometimes complicated. There are one reason, and there are also a variety of adverse conditions coexisting; there are equipment parts and components worn out, and there are also improper operation methods and adjustments. Moreover, there are not many opportunities for general ink bars to appear in production, and it is difficult for a general operator to accumulate a lot of experience in this area.
However, after careful analysis, we will find that there are certain rules for the generation and elimination of ink sticks.
1. The color of the part of the ink stick is lighter than the normal part. Generally, the problem may be larger on the water roller.
2. When the ink position of the ink stick is dark, give priority to whether the ink roller mechanism is normal.
3. When the ink stick is more obvious at the mouth of the mouth, when the drag is less and less obvious, the water roller or the ink roller should be given priority.
4. When the full version has deep and shallow streaks with a width of about 1 cm, it may be a problem with the gears (the most likely cause is the wear of the gears or the problem of gear spacing).
5. When the width of the ink bar is relatively wide and the position on each print is relatively fixed, it may be caused by the wear of the roller bearing or the excessive pressure of the roller.
6. When the ink bar is wider, its production may be related to the vibration of the machine.
7. If the appearance of the ink stick is very regular, it may be caused by the periodic vibration of a certain mechanism. Further judgment can be made based on the spacing and position of the ink bar.
8. If the appearance of the ink bar is completely irregular, it may be caused by loose or worn parts of the bolt where it is needed.
In fact, after understanding the causes and laws of the appearance of the ink bar, it will be much easier to solve the problem after the ink stick problem occurs in production. Focusing on the maintenance and maintenance of equipment in daily production, scientifically and rationally using and adjusting the machine to prevent and avoid the chance of ink sticks, is the best way to deal with the problem.