The five "silver bars" in flexo printing

The ink bar is also called a taping bar. It refers to a streak that is darker or lighter than the normal color on the printed matter. It is common in offset printing and gravure printing. Flexographic printing ink bars and offset ink bars, gravure ink bars cause and solutions are different. This article focuses on the causes of the flexographic printing ink bar and the solutions.

First, the gear stick

The gear ink stick is the ink stick that is caused by the bad meshing between the printing plate roller gear and the impression roller gear during the printing process. Its initial performance is that the gear sticks appear successively at low speed commissioning pressure. The distance between the two sticks is just the distance between the two teeth. After the normal production at the speed increase, the number of sticks decreases, but the color at the sticks appears. Shallow, imprinting phenomenon is not true. At this time, the operator will often increase the pressure on the plate, but will cause serious "double eyelid" phenomenon and the pressure of ink bars.

There are mainly the following reasons for generating gear sticks:

1, due to processing or long-term wear, resulting in the printing plate roller gear accuracy is not high or the overall gear is not round, can not be tightly meshed with the impression roller gear, resulting in vibration, passed through the plate cylinder to print and reflect.
2. There are dry ink and dirt in the teeth of the gear, affecting the normal meshing between the plate cylinder gear and the impression cylinder gear, causing one or more striations. The color of the ink bar is lighter than that of a normal printing graphic, and the position of the ink bar on the print is not fixed and it moves back and forth.
3, the printing plate precision is not enough or not round, in the engagement with the impression roller, the contact pressure between the printing plate and the substrate is not uniform, the pressure is large, the color dark ink bar; pressure is small, the color of the lighter .
4, the accuracy of both ends of the plate cylinder bearing and shaft is not enough, such as the hole of the bearing does not have the heart, the rolling elements in the bearing wear, the bearing inner circle has crossed, the shaft is not straight or the shaft surface scratches, etc. May cause ink sticks.

For the gear roller, the following measures can be taken:
1. The gear is not round or worn. If it cannot be repaired, the gear with high precision can be re-machined.
2. The cylindrical surface of the cylinder is not round, and it can be lightly polished with a fine sandpaper on the non-circular surface.
3, replace the bearing problem.
4. If the surface of the shaft is worn, it can be ground with fine sandpaper; for the curved shaft, it can be repaired by a special processing factory.

Second, the pressure of the ink bar

Pressure stick The ink stick caused by incorrect adjustment of plate pressure or anilox roller pressure. The thicker streaks on the printed matter may be several times that of the geared ink bars; the color is darker or shallower than the geared ink bars.

1. Since the halftone pattern is on the same plate as the fine text, when debugging plate pressure, either consciously increase the pressure to ensure that the small text on the plate is compacted; or consciously reduce the pressure to Make the outlets not deformed and maintain a rich level.
2. The running roller of the anilox roller is not clean or has poor lubrication. The slider that adjusts the roller pressure of the anilox roller does not operate smoothly in the slideway, resulting in the pressure of the anilox roller being too large or too small. When it comes into contact with the printing plate during the printing process The pressure is not uniform, forming an ink stick.
3, ink drying too fast, in debugging the plate roller pressure or anilox roller pressure, the ink has not yet been transferred to the substrate, has been dried on the plate or anilox roller. The operator mistakenly believes that it is too small and continues to increase pressure, resulting in improper pressure adjustment and causing ink bars.

In the above situation, the following measures should be taken:

1. Fine text and halftone images can be printed separately, so that under pressure debugging, the appropriate pressure can be adjusted according to the characteristics of the printing plate graphic and text to avoid the appearance of ink bars.
2, thoroughly clean the slide, coated with a special lubricant to increase the flexibility of the slider in the slide, thus ensuring accurate adjustment of the anilox roller pressure.
3. For fast drying inks, add a suitable amount of retarder to reduce the drying speed, ensure the ink is transferred smoothly, and avoid the influence on the normal debugging printing pressure.
4, the operator properly reduce or increase the speed, in order to reduce the impact of pressure on the ink bar.

Third, the triangular gear caused by the ink bar

The main motor drives the gear in the gear box through the drive shaft, and the gear outside the gear box drives the rotation of the anilox roller through the meshing of the triangular gears that are installed together with the three gears. Due to the long operation time, the screws tightening the triangular gear may be loosened, causing the gears to have gaps between the gears and causing vibrations. The transfer of the anilox roller produces ink bars on the printed matter.

If this happens, check whether the position of the triangle gear is changed and whether the fastening screw is loose and check whether the oil delivery pipe is unblocked and whether the oil can be delivered normally.

Fourth, printing plate and double-sided tape caused by the ink bar

Different plates have different printing characteristics, and some plates are prone to ink bars when they are printed; some plates are less prone to ink bars when they are printed. Double-sided adhesive tapes can also cause difficult-to-eliminate ink bars due to their different structure or improper use.

For these problems, a variety of plates and double-sided adhesive tapes can be selected for experiments and comparisons to find suitable plates and double-sided adhesive tapes for different products.

V. Vibrating ink bars

Because the die-cutting workstation is working, it will cause machine vibrations, which will cause the printing pressure to change suddenly and change slightly, resulting in the production of ink bars. When a big shock occurs, the operator can take the following measures:

1. Reduce the height of the back of the die cutter blade without affecting the die cutting quality.

2. Place one or two layers of transparent tape on the back of the blade to cushion the vibration generated by die cutting.

3, rotating the paper guide roller to a suitable position, so that when the vibration of the cutting knife, the plate cylinder just contact with the impression cylinder at the seam to reduce the impact on the printing pressure.

The causes of ink sticks may be unilateral or multifaceted. In the settlement, it should be carefully investigated, comprehensively taken into consideration, and measures taken in various aspects to eliminate dark bars and improve product quality.

(Henan Golden Mango Printing Co., Ltd. Huang Guangwei)

Reprinted from: "Printing Technology"