When the overprinting is inaccurate during printing, try these methods!

Overprinting is a crucial quality indicator for evaluating the performance of a single-sheet offset printing machine and is a significant factor in determining the yield of printed products. In this article, the author combines personal experience to discuss the main factors affecting overprinting and their solutions in single-sheet offset printing, sharing insights with colleagues.

Improper Adjustment of Printing Press

Misadjustment of Feeding and Positioning Components

Firstly, the paper feeding mechanism is crucial for the entry of paper into the printing unit. Once misalignment occurs during printing, it is essential to check the flexibility and coordination of the paper feeding and separating nozzles, as well as the pressure and tension of the paper feeding belt. Additionally, improper adjustment of the height of the flying fork will lead to mismatches in the lifting amount of the paper stack and the thickness of the paper, causing instability in paper separation and resulting in skewed paper, thus affecting accurate overprinting.

Secondly, the front guides and the paper table should be kept perpendicular, and all front guides should be on the same line. Only when the front guides are stable can the paper smoothly reach the front guides for positioning. If the paper arrives too early, the guides may bounce it back, affecting positioning or causing paper damage. If the paper arrives too late, there is no stable time at the front guides, and before the paper teeth completely grip the paper, the front guides have already left, easily leading to inaccurate paper positioning and, consequently, affecting overprinting accuracy.

Furthermore, the ruler face of the side pull guide should be perpendicular to the positioning surface of the front guide. The height of the side pull guide pressing the paper cover should be about 3 paper thicknesses, and the pressure of the side pull guide paper ball should be appropriate. The pulling distance of the paper is generally 5-8 mm; if the distance is too large or too small, the paper may not be pulled into place, resulting in inaccurate overprinting.

Finally, the paper undergoes a high-speed process on the paper table and then decelerates when reaching the front guides. This is done to eliminate the impact of the inertia of the paper feeding mechanism on the paper. Subsequently, the paper teeth close to achieve zero-speed paper gripping, the paper feeding mechanism accelerates, and the paper is handed over to the drum at a constant speed. If the closing time of the paper teeth is too early or too late, and at this time, the leading edge of the paper is far from the tooth cushion, the edge of the paper is prone to deformation under the action of the paper teeth. This, in turn, causes inaccurate overprinting. In addition, wear and tear of components in the paper feeding mechanism can also affect overprinting due to increased clearances and reduced positioning accuracy.

Uneven Tightness of Drum Grippers and Uneven Height of Gripper Pads

The smoothness of the grippers on the drum of a single-sheet offset printing machine is crucial for the accuracy of overprinting. If the gripper pads on the impression drum or paper transfer drum are uneven in height, it will result in uneven gripping forces of the grippers, affecting their accurate positioning in the next color group. If one or more grippers in a row are too loose, local forced deformation of the paper may occur during the impression process, leading to localized inaccurate overprinting. This type of inaccuracy often comes with paper wrinkling.

Wear of Gripper Shafts

If the wear of the gripper shafts on the two sets of double-diameter impression drums is uneven, it can lead to different gripping forces or axial displacement of gripper shafts with toothed rings. Gripper shafts with lower gripping forces have poorer accuracy in gripping the paper, making it prone to slippage. This can result in inaccurate overprinting, while gripper shafts with normal gripping forces can stably hold the paper. This situation may lead to A/B overprinting inaccuracies.

Inherent Properties of Paper

Plastic Deformation of Paper

For book and magazine printing companies, paper is the primary substrate, and it is a material with plastic deformation. Paper with significant plastic deformation characteristics, when subjected to substantial printing pressure, may exhibit slight expansion in width and length at the trailing edges, causing inaccurate overprinting. This is especially severe in the trailing edge of the paper. The larger the printing pressure, the more obvious the deformation of the paper. Therefore, when printing the same batch of products, to ensure the stability of overprinting, the pressure of each color plate should be as consistent as possible. However, adjustments in pressure may be necessary depending on the specific circumstances. For example, the printing pressure of imported multicolor offset presses often decreases randomly.

Direction of Paper Fibers

The direction of paper fibers is closely related to the accuracy of overprinting. Single-sheet paper should preferably use paper with longitudinal fibers to ensure that the paper’s longitudinal direction is parallel to the axis of the drum. Moreover, for the same batch of products, the fiber direction of the printing paper must be the same to ensure overprinting accuracy.

Peel Force of Paper After printing in the impression area, the paper, gripped by the drum’s grippers, is forced to separate from the rubber blanket. At this point, the fiber structure on the paper is subjected to the action of two opposing forces: the gripping force of the grippers downward and the adhesive force of the ink upward. The direction of these two forces is opposite. During the printing process, if the grippers on the impression drum have sufficient gripping force to ensure that the paper does not shift, the size of the peel force becomes a major factor affecting the deformation of the paper. The amount of deformation depends on several factors.

Properties of the Paper: Paper with high plastic deformation will experience greater peel and stretch deformation, while paper with low plastic deformation will experience less deformation. Therefore, when printing high-end products, it is advisable to choose high-quality paper to avoid the impact of peel forces on overprinting.

Inked Area on the Paper Surface: The larger the inked area on the paper surface, and the thicker the ink layer, the greater the peel and stretch deformation of the paper, resulting in reduced overprinting accuracy.

Ink Viscosity: Ink viscosity is directly proportional to its adhesiveness. Higher ink viscosity leads to greater peel and stretch deformation of the paper, while lower viscosity results in less deformation. To reduce overprinting inaccuracies, ink viscosity can be adjusted appropriately.

Environmental Factors

Under normal circumstances, the temperature in the printing workshop should be controlled between 18 to 28°C, and humidity should be maintained between 40% to 70%. If the temperature and humidity in the printing workshop, as well as the humidity adjustment of the paper, are inappropriate, and if excessive water is used during printing, it can easily lead to overprinting inaccuracies. Overprinting inaccuracies caused by such factors cannot be resolved through mechanical adjustments and may require humidity adjustment of the paper. Specific measures include using hot air to remove moisture when the paper edges wrinkle and using humid air to moisten the paper edges when the paper stack curls due to dryness.

Improper Processes or Operations

Incorrect Operations

Adjustment of Printing Pressure: If the printing pressure is set too high, it can cause the paper to slide within the gripper ranks, and therefore, the printing pressure should be reduced.

Improper Gripper Size: If the paper gripper size is too small or uneven on either side, it needs to be readjusted to achieve uniform gripping.

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Inappropriate Adjustment of Plate Cylinder Packing Thickness: If the packing thickness of the plate cylinder is improperly adjusted, it can result in non-coinciding impressions during overprinting, necessitating recalibration of the plate cylinder’s packing thickness.

Excessive Dampening Solution: Excessive dampening solution on the plate surface can lead to severe paper deformation, requiring a reduction in water usage.

Low Ink Temperature or Excessively Viscous Ink: If the ink temperature is too low or the ink is excessively viscous due to other reasons, adjustments such as adding ink or using a desensitizer to increase ink flowability may be necessary.

Incorrect Installation or Adjustment of the Printing Plate: If the printing plate is installed or adjusted incorrectly, causing plate skew or incomplete installation, recalibration is required.

Improper Processes

Incorrect Plate Imposition: Incorrect plate imposition causing individual image positions to be misaligned requires re-imposition.

Unreasonable Process Arrangement: For example, multicolor overprinting generally should follow the sequence of printing lighter to darker colors, and halftones should precede solid areas. Once the color sequence is incorrect, overprinting inaccuracies are likely to occur.

In conclusion, diagnosing and resolving overprinting faults requires patience. After resolving the faults, it is essential to maintain detailed records. Only through continuous summarization can the causes of overprinting inaccuracies be accurately identified, further reducing the rejection rate, improving print quality, and generating more profits for the enterprise.