CTP imaging technology and its possibilities

Faced with various CTP devices that are constantly appearing on the market, people tend to focus on the performance of the device while ignoring its fundamental imaging technology. This article will discuss this issue with everyone.

With the introduction of computer-to-plate, many new exposure machines, light sources, and exposure designs have appeared on the market. In practice, what is the difference between different CTP systems? Which type of laser and plate are used separately? What are the advantages and disadvantages? This article outlines the above issues.

Design of CTP Exposure Machine

The structure of the CTP system can be divided into the following different designs:
Inner drum design (single beam system);
External drum design (multi-beam system);
· Platform design (single beam system);
·Platform design (multi-beam system);
Internal and external drum mixing (multi-beam system).

light source

Energy sources available in the printing industry can be divided into gases, solids, UV lasers, and laser diodes. The energy distribution goes from visible light in the UV region to the infrared region. Obviously, different types of lasers have different wavelengths.

It can be said that the laser diode (heat-sensitive 830nm; violet laser 405nm) is a light source that the printing industry likes to use.
Internal Drum Single Beam System: Photopolymerized plates and silver salt plates (400 to 650 nm) were imaged using the existing 3 lasers.
External drum multibeam system (up to 512 beams): The thermal plate is tensioned on a rotating drum. Thermal laser diodes (800 to 830 nm) are mounted in a lens component or diode component. The system images various thermal plates such as Kodak Poly Optical's DIIP, ELectra Excel, and ThermalNews plates.
· Platform-type single beam system: Photopolymerization plates and silver salt plates are imaged using the existing three types of lasers, and the two plates react to laser light having a wavelength of 400 to 640 nm.
· Platform-type multi-beam system (up to 512 beams): The thermal plate is imaged using a thermal laser diode (830 nm). Systems such as Kodak Polyray's TH100 and TH180 Newsetter use their multi-beam heads to image thermal plates. The road leading to the "processing-free" plate has been opened.
Internal drum/outer drum hybrid multi-beam system: 64 or 128 thermal laser diodes (830 nm) along with an optical system are provided on the drum where the system rotates. The thermal plate was imaged at rest (830 nm) in the inner drum. All types of thermal plates can be used.
In summary, different laser types and plates are available for all exposure machine designs. In particular, it is important to point out that only a thermal system can image “process-free” plates.

Unlike the laser spot laser spot

Some single-beam or multi-beam systems use circular dots when imaging. For circular laser spots, there is a difference between addressability and resolution. Due to unexposed gaps between small laser pixels, incompletely covered and unprintable dots may be produced. In order to prevent this problem, overlap imaging is often used. However, due to the overlapping of larger laser spots, inaccurate reproduction of the gradation values ​​(the Yin and Yang lines are never as wide). For example, to solve this problem, some systems now use a modulator to split the laser beam into 256 or 512 square points with an energy-balanced distribution.

Printing plate

The CTP system uses a silver salt plate, a photopolymerization plate, and a thermal plate.

Silver Salt and Photopolymer Versions: This first generation of digital plates is imaged with visible light. The silver salt version only requires lower exposure energy, and its disadvantage is that the structure of the dots is not clear due to the reflection of light on the substrate, and the silver discharge processing cost is high. The photopolymerization plate also requires only lower exposure energy, and the resulting reaction is completed by reheating. The disadvantage is that, in order to ensure the number of prints, the use of a minimum amount of exposure results in a higher dot gain rate (8% to 10%), and a certain cost is required to clean the developing machine.

• Thermal plate: The advantages are high imaging quality, fast development speed, wide reheating temperature range, high energy of the developing machine, and low discharge processing cost. After the printing plate is on the machine, it can be opened quickly, resumed quickly after the restart, good ink absorption and strong solution resistance. If water-soluble chemicals are used, they are ecologically friendly and can be baked.

Photopolymer and thermal plates are incomparable: 1000 times higher exposure energy than the thermal plate makes the pixels clear and dot gains minimal. The resolution is up to 60 lines/cm, the processor is easy to clean, it has high resistance to printing force, and the dots will not increase in the copy.

• Process-free printing plates are divided into four major categories: ablation-type plates, controlled ablation plates, plate-in-machine plates, convertible polymer plates, or discolored plates.

1 Ablation means that the coating material is vaporized by laser irradiation and eroded. A suction device is required for this CTP system. Plates used: eg Agfa-Mistral, Presstek-Anthem, etc.
2 Controlled ablation plates do not require a suction device. The plates used include Fuji-Brillia LD-NS, Agfa-Thermolite, and the like.
3 On-machine platemaking: Agfa-Thermolite, Heidelberg Saphira, KPG-Thermal.
4 convertible polymer plates: Asahi-Thermal, KPG-Thermal SP.

Dot-matrix-sensitivity versus photothermographic systems In photoreactions, the extent of the reaction depends only on the number of photons.
In the heat-sensitive reaction, energy affects in a short time. The thermal plate has a fixed energy threshold that can only be imaged when it reaches or exceeds this threshold.

Quality assurance

In order to monitor production, at a minimum, inspection and measurement tools should be available: lighting magnifiers, test rigs for measuring the accuracy of RIP, CTP, and process adjustment, test plates, and plate densitometers (CCD camera systems are used whenever possible).
To be sure, only a properly calibrated CTP system can guarantee standardized operations.
CTP is the basic condition of FM screening. FM screening replaces different amplitude AM outlets with the same size of outlets. The number of outlets per unit area varies according to the tone value of the copy. The FM network performs its spatial distribution in accordance with the exact prescribed mathematical rules.

For example, the Bayreuth printing factory has long recognized the advantages of using FM dots in daily printing production. All web thermosetting offset products and 80% of sheetfed offset products are printed on the FM network. Commonly used dot sizes are 10, 20, and 25 microns. Sheet-fed offset printing is not suitable for FM screen printing due to the coarse pigment particles when using paper with strong ink absorption and metallic ink or fluorescent ink. The plant uses a baked KPG-DITP thermal plate and Creo's Staccato FM network. The FM network has the following advantages: Significantly improve the printing quality (especially for less good quality); the best image quality; smooth transition (no dot structure interference); good color tone (no moiré); fine level conversion is good. In combination with the thermal printing plate, a smooth dot plane is created with no visually visible particles. In short, the tone value is stable, improving process reliability.

summary

The future of thermal technology is full of promise. It will become a process-free and environmentally friendly way of making plates, and it can also be directly imaged on a printing press. In particular, the thermal efficiency of the thermal technology is high, and in the case of increasing the resistance to printing forces, a predictable production process can be achieved. More importantly, thermal CTP and FM network points can achieve new quality requirements for offset printing

Source; "Digital Printing"