An analysis of the causes of ghosting in a four-color offset press

Our factory has a P24880-01B lithographic offset printing machine (manufactured by Beiren Group). Once there was a bad situation where the printed product gradually produced ghost images during the offset printing process. According to the layout analysis of the printed products, we judged that ghosting was produced in the Three-color group, and the emergence of ghosting is gradual, not in the printing process bursts.

Ghosting, commonly known as double-printing, has two basic causes. It can be summed up in two basic ways: one is axial ghosting, in which the imprinting of the ghosting is nearly parallel to the axis of the drum, and the main causes are: :

1. Offset precision, bearing wear;

2. Improper color lining;

3. Axial displacement of plate cylinder or blanket cylinder occurs;

4. The shaft heads, bushings, and wall plate holes of the plates and blankets are worn out, resulting in large gaps and large vibrations during operation.

5. The water roller and the ink roller are not properly moved axially;

6. The paper is not flat, creating flounces or curls.

The other is the circumferential ghosting, where the footprint of the ghosted part is perpendicular to the axis of the drum. The reason is:

1. Improper roller lining;

2. The blanket on the blanket cylinder is not tight;

3. Roller pressure is not in place;

4. Out of the drum or bearing wear or the gap between three rollers;

5. The diameters of blanket and plate cylinders are significantly different;

6. The bite force of the impression cylinder bite is too small or the tooth pad wears;

7. For multicolor machines, the positioning between the units is not acceptable.

As mentioned earlier, we analyzed the subjective judgment of failures and thought: This ghosting fault belongs to a fault that occurs gradually. Based on this, it can be analyzed that the cause of the failure is due to the wear of the parts (if the failure is sudden, other factors should be considered). As far as the wear of the parts is concerned, the above-mentioned causes are analyzed and judged, and the inspection is focused on the wear of the roller gears and bearings, and the insufficient bite force of the biting paper teeth of the impression roller row or wear of the tooth pads. Therefore, starting with routine inspections, we first checked and standardized the pressure on the teeth and dental pads. After such treatment, the quality has improved, but it is unstable and ghosting still exists. For this purpose, we further examined the key parts of the cam roller, which is related to the opening and closing of the teeth of the impression cylinder, which has a diameter of 32 mm and is mounted at a position on the edge of the roller in the direction of the end face. When it was removed and inspected, it was found that both the roller and the roller had been seriously worn out. We know that the paper biting action of the bite paper teeth is caused by the contact transmission between the cam roller and the cam surface. Because the cam roller wears inside and outside, the cam roller is unstable in the contact rotation with the cam. This kind of instability inevitably causes the trajectory of the cam roller to rotate with the cam surface to form an abnormal state. This abnormality is obviously It directly affects the pressure of the teeth on the roller and the bite paper bite. Because the pressure of the bite paper is not normal, it is inevitable that the teeth will be displaced during the biting. In the offset printing process, the displacement of paper transfer is one of the main reasons for the ghosting failure of printed products. The crux of the problem lies in this. After we replaced the cam roller, the ghosting fault was also eliminated.

In this four-color offset printing press, the small cylinder, the impression cylinder, the transfer cylinder, and the plate cylinder are all equipped with such a cam roller. In use, wear of the cam roller is inevitable, and wear of the roller is inevitable. , Decrease in precision, is extremely sensitive to the bite paper pressure of bite paper teeth. Once worn, the precision decreases, and ghosting faults of printed products often appear. Therefore, in offset printing, we should pay attention to the accuracy of the cam roller at these critical locations when ghosting begins. In the maintenance of institutional equipment, maintenance and inspection of this part must not be neglected, as far as possible to prevent accidents.