Analysis of Ink Repellence on PS Plates

Posted on 05-16-2024 by

I. Storage Environment and Shelf Life of PS Plates

Currently, there are nearly 20 domestic manufacturers of PS plates in China, and some recycled PS plate producers have also started manufacturing PS plates using aluminum base plates. The standardization management and production scale of these manufacturers vary, as do their sales progress and inventory periods. In the context of a market economy, printing companies have a wide range of channels for purchasing PS plates, sometimes acquiring plates that have been stored for over a year. This is a major cause of ink repellence issues with PS plates. Additionally, some printing companies' warehouses or plate exposure workshops are not suitable for storing PS plates, such as environments that are too humid, have strong light, high temperatures, or are subject to acid and alkali corrosion. These conditions can cause PS plates to lose their effectiveness, resulting in abnormal exposure and development, poor ink receptivity, and difficulties in inking or complete ink repellence during printing.

II. Sand Grain Condition and Photosensitive Layer Quality of PS Plates

Even if the storage period of the purchased PS plates is not long and the storage and usage environments meet the requirements, ink repellence issues can still occur. This is mainly due to poor quality in the electrochemical sand grain treatment and the photosensitive layer. Specific reasons include:

1. Electrochemical Sand Grain Treatment: The production process involves surface treatment of the aluminum base plate before coating the photosensitive layer, with the electrochemical sand grain process being crucial. Improper treatment can affect subsequent processes and the quality of plate exposure and printing. The sand grain consists of numerous peaks and valleys. If the valleys are too deep and the peaks too high and steep, it becomes difficult to coat the photosensitive solution evenly. After exposure and development, the peaks may not be adequately covered by the photosensitive layer, causing inking difficulties.

2. Photosensitive Solution Quality: If the photosensitive agent in the solution is insufficient, the resulting synthetic resin will have poor alkali resistance and ink receptivity. The photosensitive emulsion's poor alkali resistance causes fine details to erode and shrink during development. Extending the development time further reduces the ink receptivity and printing durability of the PS plate.

3. Storage of Photosensitive Solution: High temperature and humidity in the storage area, or an overly long storage period, result in poor ink receptivity when the solution is used for coating.

4. Insufficient Curing and Thin Coating: An insufficiently cured or overly thin photosensitive layer, or a smooth coating surface due to various reasons, can lead to poor inking if not properly treated after exposure and development.

III. Plate Exposure and Printing Operations

In addition to the quality and storage of the PS plates, improper operations during plate exposure and printing can also cause ink repellence issues.

1. Low Density of Original Image: If the original image's density is too low or if it does not adhere tightly to the glass during exposure, the photosensitive layer may be compromised, reducing its alkali resistance.

2. Overexposure and Overdevelopment: Overexposure, excessive development, or a high concentration of sodium hydroxide in the developer can thin the photosensitive layer, reducing ink receptivity.

3. Prolonged Light Exposure: If the plate is exposed to light for too long after exposure and development without proper inking, the photosensitive layer can decompose, turning a light blue color and reducing ink receptivity.

4. Incomplete Removal of Protective Agent: During the second development after baking the plate, if the protective agent is not completely removed, it can cause ink emulsification and reduce the ink receptivity of the image areas.

5. Ink Adjustment and Water Balance: In winter, thick ink requires the addition of ink oil to thin it, which increases water usage. Excessive water can emulsify the ink, reducing ink receptivity, especially if the photosensitive layer is thin or the density is low.

6. Degraded Protective Glue: If the protective glue used during plate exposure or temporary storage degrades, incomplete removal before printing can cause ink repellence.

7. Reusing Plates: Plates reused after printing without proper cleaning and sealing can have ink repellence or dirt issues due to residual substances on the plate surface.

8. Wear and Tear: Repeated friction from water rollers, ink rollers, rubber cylinders, and paper dust can damage the photosensitive layer, reducing its oleophilicity and increasing its hydrophilicity. Excessive dampening solution can form a water film on the image areas, preventing inking.