Problems encountered in the use of CTP equipment and solutions
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With the rapid development of CTP technology, prepress technology has been moving toward integration, specialization, and diversification. Since the second half of 2011, through renting and using UV-CTP equipment, the author learned that this equipment offers fast plate making and high printing quality. The processes of exposure, plate making, and printing can be completed in one workflow, reducing intermediate steps, saving manpower, and improving both product quality and work efficiency.
In 2014, the author officially purchased a Kolei UV-CTP device and a Kodak Quansheng 800 CTP device, completely eliminating traditional imagesetting equipment and formally beginning the application of CTP technology.
During this period, I experienced the difficulties of initial operation, the hardships of night shifts, and the satisfaction of solving technical problems. Here, the author summarizes several problems encountered during the use of CTP equipment and their corresponding solutions and shares them with readers.
Equipment Maintenance "Pitfalls"
Equipment maintenance is an essential part of printing production, as equipment performance directly affects production efficiency. In the early stage of using CTP equipment, our company encountered a series of problems due to insufficient understanding of maintenance requirements.
1. Wear of Plate Processor Drive Gears
Once, during normal operation, a burning smell suddenly appeared. After inspection, it was found that the grease and oil in the transmission gears of the plate processor had been depleted. Due to the lack of timely lubrication, the gears suffered severe wear, producing the scorching odor.
In response, our company established a regulation requiring operators to open the plate processor cover every six months and apply grease and lubricating oil to prevent gear wear.
2. Double-Plate Feeding
Double-plate feeding often occurred during the early use of UV-CTP equipment. Careful observation revealed a double-sheet sensor located between the plate feeder and the CTP main unit. During laser exposure, dust is generated. Over time, dust accumulation reduces sensor sensitivity.
If two plates are simultaneously fed into the gripper, the sensor may fail to detect the condition in time, causing improper clamping and potentially damaging the equipment during high-speed operation.
To address this issue, plate-making personnel were required to perform routine cleaning and maintenance. Wipeable areas are cleaned regularly, and compressed air is used for inaccessible locations. Since implementing these measures, double-plate issues have not recurred.
3. Unsuitable Production Environment
CTP equipment has strict requirements for ambient temperature and humidity. In winter and summer, air conditioning must be used to maintain stable workshop conditions. In previous operations, low ambient temperatures caused improper plate processing.
In addition, CTP equipment requires clean, dry compressed air. Good air quality improves performance and extends equipment life. As our company is located south of the Yangtze River, where humidity is high, plate-making personnel are required to remove moisture from air compressors and condensers weekly to meet equipment requirements.
Currently, our company places great emphasis on CTP equipment maintenance. In addition to daily upkeep, professional engineers are invited every six months to service key components such as the laser head and lead screw.
Linearization Curve Calibration
Because different printing presses require different plate linearization curves, curves must be adjusted according to specific press characteristics. For example, the linearization curves for rotary newspaper presses and flat offset presses differ significantly, so the correct curve must be selected for each plate type.
Our company specializes in newspaper printing, which is characterized by high speed, significant dot gain, and susceptibility to background contamination. Therefore, when setting the linearization curve, the measured dot value on the plate should be 2%–5% lower than the designed dot value. During printing, dot gain then brings the actual dot value close to the target.
To ensure plate quality, our company purchased a plate measuring instrument and created control bars on the plate. Immediately after the first plate is produced, dot values at 3%, 50%, and 100% are measured.
Since plate-making conditions change continuously, evaluating plate quality requires actual measurement. For our company, optimal performance is achieved when measured values are approximately 1.5%, 45.0%, and 97.0%, ensuring good dot reproduction and printing durability.
Two Major Plate-Making Problems
Plate making is a critical link between prepress and printing. Once the plate is produced, printing quality and durability are largely determined. Plate abrasion and plate contamination are two common problems that cause significant material and time loss.
1. Plate Abrasion
Traditional PS plates exposed with gallium iodide lamps have high durability, often exceeding 100,000 impressions. Our maximum newspaper print run is 70,000 copies. When CTP was first adopted, plate abrasion appeared at around 50,000 copies.
After repeated testing, the problem was resolved by first reducing printing pressure through roller and packing adjustments, thereby increasing plate durability. After optimizing press conditions, plate-making parameters were adjusted. Through continuous optimization, CTP plate durability was stabilized at 70,000 copies.
2. Plate Contamination
Plate contamination is another persistent problem. Causes include insufficient laser power, low developer concentration, and improper temperature settings.
(1) Laser intensity:
The Kolei UV-CTP allows adjustable laser intensity, which varies with resolution (2400 dpi vs. 1500 dpi). Higher resolution requires lower intensity. The Kodak Quansheng 800 has fixed laser intensity.
(2) Developer concentration and temperature:
Modern CTP processors maintain constant temperature automatically, making developer concentration critical. Concentration is monitored via conductivity. Initially, developer replenishment was based on time and area, resulting in unstable concentration.
After introducing real-time conductivity monitoring, developer concentration was controlled precisely. After repeated testing, optimal conditions were determined to be approximately 72 μS conductivity at 25°C.
A practical method to detect contamination is to place a drop of anhydrous acetone on a non-image area of the plate. If residue remains after evaporation, contamination is likely. Adjustments include increasing laser intensity, developer concentration, or processing time. If necessary, the plate can be reprocessed quickly. As acetone is now strictly regulated, anhydrous alcohol may be used, though with reduced effectiveness.
Weak Development Control
Preventing plate contamination also avoids overdevelopment. Excessive development reduces plate durability and printing quality, causing dot loss, image distortion, and uneven tones. The optimal approach is "weak development," reducing developer concentration and temperature while ensuring the plate remains clean.