Investigation and Analysis of the Influence of Ink-Water Balance and Quality in Lithographic Printing

Lithographic printing is based on the principle that oil and water do not mix. The image areas on the plate are oleophilic (ink-attracting) and hydrophobic (water-repellent), while the non-image areas must first absorb water to become hydrophilic and ink-repellent. One of the characteristics of offset printing is that it uses an indirect printing process, with colored reproduction achieved through the transfer of ink dots. Mastering the ink-water balance is critical to ensuring the normal transfer of the print, the depth of ink color, accurate registration, print drying, and preventing ink smudging. Therefore, understanding and controlling the ink-water balance is key to maintaining consistent print quality. A skilled printing operator should not only understand the relationship between ink and water but also be adept at managing the techniques and essentials of ink-water balance.

1. The Relationship of Ink-Water Balance Ink-water balance refers to adjusting the supply of the dampening solution in printing at a specific speed and pressure, controlling the water content in the emulsified ink between 15% and 26%. This balance forms a lightly emulsified water-in-oil (W/O) ink, using the minimum amount of water to counterbalance the ink on the plate.

The most important aspect of ink-water balance is the control of both the dampening solution and the ink. The dampening solution is composed of water and surfactants. It must adequately wet the plate, provide cleaning capabilities, and maintain a stable pH to ensure the plate does not become dirty.

1.1 The Role of the Dampening Solution

1. It forms a uniform water film on the non-image areas of the plate to prevent ink from transferring to these areas and to avoid dirtying the plate. 2.The electrolytes in the dampening solution chemically react with the exposed metal areas of the plate due to wear, continuously forming a new hydrophilic layer to maintain the hydrophilicity of the blank areas. 3.It controls the temperature of the ink on the plate. With modern printing presses operating at increasingly high speeds, the ink heats up during high-speed transfers, increasing its fluidity and causing dot gain. Thus, the dampening solution must help reduce the temperature of the ink rollers and ink when in contact with the plate and inking rollers. 4.It ensures that the image areas neither expand nor contract, maintaining the relative position of image and non-image areas throughout the printing process. Ink typically consists of pigments, fillers, binders, and auxiliary additives. When choosing ink, it’s essential to consider its rheological and optical properties.

The rheological properties of ink are the main criteria for assessing whether it has good printing performance. These properties are largely determined by the binder, although they can be fine-tuned with auxiliary additives. Key indicators of ink rheology include viscosity, tack, thixotropy, yield value, viscoelasticity, and flowability. Understanding these indicators is crucial for selecting the right ink for different printing needs.

Ink and water must coexist on the plate surface in a balanced state to ensure that the printed image carries maximum ink coverage with vivid, saturated colors and clear, sharp dots. Simultaneously, the non-image areas must remain clean. This balance requires adjusting the amount of water and ink during production.

The darkness or lightness of the print image is influenced not only by the amount of ink but also by the water supply and the ink-water balance. If the water supply increases, the image becomes lighter. Conversely, reducing the water supply makes the image darker. Therefore, both ink and water adjustments impact image density. Adjustments should be made based on actual printing conditions, but increasing ink density should not be achieved by simply reducing water supply, as this disrupts the balance and can cause dirty prints in non-image areas.

It’s essential to avoid the situation of both excess water and ink. When there is too much water, and the print appears lighter, increasing the ink supply rather than reducing the water creates a vicious cycle, leading to printing defects such as water and ink overuse, rendering the process uncontrollable.

Experienced printers tend to keep the water supply to a minimum while ensuring the plate does not become dirty. By maintaining a stable balance between ink and water, consistent ink density is achieved throughout the print run. The principle of “small water, thick ink” is often promoted. “Small water” refers to the minimum water required to keep the plate clean, while “thick ink” can only be achieved with minimal water. Excess water causes ink emulsification, making it impossible for the ink layer to remain thick.

2. Factors Affecting Ink-Water Balance Several factors can disrupt ink-water balance during printing, such as machine stops, changes in ambient temperature and humidity, press speed, ink type, paper quality, and plate material.

3. Stable Machine Speed: Machines must maintain a consistent speed. Variations in speed can cause ink-water balance issues. Faster speeds require less water, while slower speeds require more.

4. Stable Temperature and Humidity: Temperature changes affect ink viscosity and flow. High temperatures increase water evaporation, requiring more water. Higher humidity reduces evaporation, requiring less water.

5. Ink Characteristics: Ink’s water resistance determines how much water is absorbed. Inks with strong water resistance require less water.

6. Paper Characteristics: Coarse, porous paper absorbs more water and ink, while smoother, denser paper requires less of both.

7. Plate Type: Different plates have varying water-holding capacities, which influence the amount of water needed.

8. Image Area Size: Larger image areas require more water and ink.

9. Dampening Solution: The pH of the dampening solution should be stable between 4.5 and 5.5.

10. Correct Pressure Adjustment: Precise roller and cylinder pressure is necessary to maintain ink-water balance.

11. Identifying Water Levels Before printing, operators can estimate the water supply based on conditions but must continually assess water levels during production. Common methods include visual assessment of the plate’s reflectivity. Operators also check for:

12. Ink that appears pale and coarse.

13. Water droplets on the ink rollers or fountain.

14. Persistent moisture on the plate after a long stop.

15. Hollow or wavy print dots.

16. Weak or wavy print edges.

17. Watermarks on the blanket cylinder.

18. Noticeable ink color differences after a stop.

19. Effects of Poor Ink-Water Balance on Print Quality Improper ink-water balance can lead to issues like misregistration, color inaccuracies, and defects such as “ghosting” or “blurring.”

20. Affects Registration: Excessive water causes paper expansion, leading to registration errors during multi-color printing.

21. Ink Emulsification: Over-emulsified ink disrupts ink transfer, affecting product quality.

22. Light Ink Color: Excess water reduces pigment concentration, resulting in dull prints.

23. Dirty or Blurry Prints: Excess ink or water leads to smudging and unclear prints.

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5. Tips for Maintaining Ink-Water Balance in Production Operators should ensure:

6. Water rollers are adequately wetted before printing.

7. Water and ink supply stops during idle machine time.

8. Newly installed dampening rollers are properly prepared.

9. Ink is not over-thinned.

10. Water rollers are clean.

11. Ink and water roller pressures are correctly adjusted. In summary, maintaining ink-water balance is essential for producing high-quality prints and ensuring smooth production.