What to Do When Offset Printing Encounters Ghosting and Ink Bars?

When using offset printing machines, issues such as ghosting and ink bars often arise. Here, we will analyze the causes of these problems and provide solutions for readers’ reference.

Paper Tape Tension

Paper tape tension is a crucial factor in ensuring print quality on offset printing machines. Inappropriate tension can lead to unstable paper tape movement or cause rubber blanket slippage, resulting in ghosting.

The causes of paper tape tension faults can generally be categorized as follows:

1. Malfunction of tension detection devices. In such cases, personnel should promptly check the working condition of tension detection and sensing devices to ensure their sensitivity and reliability.

2. Inappropriate current supplied to the magnetic powder brake. When the maximum width of the paper roll placed is 787mm, the current supplied to the magnetic powder brake should generally be between 0.8 to 1.3A.

3. Unstable tracking speed of tension controllers. In this situation, tension controllers should be reconfigured or replaced with new ones.

4. Aging or clumping of magnetic powder in the magnetic powder brake. In such cases, the magnetic powder should be replaced.

Printing Cylinder and Transmission

1. Large clearance between eccentric sleeves and bearings

After eccentric sleeves and bearings wear out, the clearance increases, allowing the cylinder, eccentric sleeves, and bearings to move freely. When the machine operates unstably or vibrates, the cylinder may jitter, leading to ghosting or ink bars. Therefore, worn eccentric sleeves and bearings should be replaced promptly. When replacing bearings, attention must be paid to fitting bearing spacers and pressure caps properly. After a period of use, it is necessary to check and adjust the bearing clearance.

2. Non-parallel printing cylinders or unreasonable center distance

Printing cylinders should be parallel. For machines with shoulder plates, the contact between the shoulder plates should be good, and the shoulder plates should be kept clean. For machines without shoulder plates, the clearance between the shoulder plates should meet requirements, and both sides of the shoulder plates should be consistent.

3. Large axial runout of cylinders

Operators should pay attention to checking and adjusting the axial runout of cylinders to ensure that the amount of runout is within the specified range.

4. Poor meshing of cylinder gears or other transmission gears

The main drive gears, cylinder gears, and ink distribution gears should ensure accuracy, good meshing, and clean tooth surfaces. Worn gears should be replaced promptly. If the cylinder transmission gears are worn, the clearance is too large, the meshing is poor, or there are foreign objects on the tooth sides, it will significantly affect the linear speed and smooth operation of the cylinder, leading to ink bars. The degree of wear of cylinder gears and the clearance between gears and shaft heads can also cause vibration due to unstable transmission, resulting in ghosting. In actual production, the faster the printing speed, the more and more severe the ghosting. Higher machine speeds increase the likelihood of machine vibration and drift of certain components. Especially when the printing cylinder and the plate cylinder or ink rollers operate unstably, and the tension of the paper tape is unstable, the probability of other printing faults such as ghosting increases, without any specific pattern. Therefore, regular maintenance and timely overhaul of the machine are essential to maintain its performance.

Printing Materials

The rubber blankets selected for printing should have good ink transferability and elasticity, with uniform thickness and hardness. Printing paper should be absorbent, have good tensile strength, uniform tension, flatness, consistent thickness, and good rewinding properties. The viscosity of ink and other properties should match the paper and meet the requirements of printing color. In addition, unreasonable or mismatched liners for printing plates and rubber rollers may cause slippage on the impression belt, resulting in ghosting. Particularly, excessive pressure on the rubber roller can cause significant deformation of the rubber blanket, poor repositioning after each transfer, or slippage between the rubber roller and the paper, resulting in ghosting.

Usage Aspects

1. High printing pressure

Reasonable printing pressure is crucial to avoid ghosting. With high printing pressure, the deformation of the rubber blanket increases, increasing the likelihood of relative slippage between the rubber blanket and the paper. Especially when the rubber roller rotates after being compressed, the rubber blanket tends to lag behind and then rebound to a certain extent, which easily causes ghosting. Therefore, the pressure of the plate cylinder and the rubber roller should be matched, minimizing printing pressure while ensuring print quality.

2. Inappropriate pressure of the plate (water) rollers

Excessive pressure between the plate (water) rollers and the printing plate can cause the rollers to bounce at the gap of the cylinder, resulting in instantaneous changes in linear speed and disrupting the uniformity and continuity of ink film. The indentation width between the plate roller and the printing plate should be 4 to 6mm. Regular checks should be conducted on the fixing conditions and bearing wear at both ends of the plate roller to prevent bouncing.

3. Poor ink-water balance

Minimal water usage should be employed while ensuring print quality.