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Prepress knowledge: matters needing attention in the use of film
In the process of printing, phototypesetting film is a commonly used material. To ensure optimal performance and avoid issues during production, it's important to follow these key guidelines:
1. While general photographic film can be handled in normal lighting conditions, it's crucial to strictly adhere to safety procedures when loading the film. Place the film into the designated loading box, secure the cover, and tighten the screws on both sides of the top. Ensure that the film exits the box evenly, with both edges parallel and approximately 1.5–2 cm wide. This prevents the film from falling out and ensures consistent exposure.
2. Before using each batch of film, it's essential to test its base density and dot linearity. Based on the results, adjust parameters such as laser focus, exposure time, and development/fixing temperature and duration. This helps maintain standardized and efficient production, reducing the risk of defects.
- **Film Density**: There are two types—low density and high density. Low density refers to the density of unexposed areas after development, typically around 0.05. The lower this value, the clearer the image. High density is the solid density at 100% exposure. For coated paper, high density should be at least 3.5, ideally between 3.5 and 4.5, to ensure good transfer of image information. On newsprint or offset paper, due to higher ink absorption, the required high density may be slightly lower.
- **Dot Linearity**: This ensures that the percentage of halftone dots set on the computer matches what is printed on the film. For example, 50% dots on the screen should appear as 50% on the film, with no more than 1% error. In color printing, even small errors can cause noticeable distortion. Therefore, it’s recommended to check dot linearity daily to maintain quality.
3. Adjust the concentration and temperature of the developer and fixer solutions accordingly.
- When using domestic developers like Abbey SH-1000, dilute the solution to a ratio of 1:4 to 1:3. If the temperature is too high, the developer may evaporate or oxidize quickly, leading to excessive fog and low density. The ideal temperature range is 34–36°C. Always measure density and check linearity once the temperature stabilizes.
- For fixing solutions, the typical dilution ratio is 1:4. Too high a temperature can cause rapid evaporation or oxidation, increasing haze and reducing high density. The recommended fixing temperature is 28–32°C, with a fixing time of 30–35 seconds. This helps prevent damage to the film and maintains clarity.
4. Laser phototypesetting films often have an anti-halation layer that may peel off during fixing and washing, causing light blue spots on the film. Adding a small amount of hardener to the fixer can help eliminate this issue. Keep the washing tank clean by draining and drying it daily, wiping it with clean cloth before adding new water. At least once a week, thoroughly clean the water roller to prevent buildup and contamination.