Comparison and Evaluation of Flexo Platemaking Process-One-to-One Flexo Platemaking Process

Keywords: flexographic plate traditional platen digital UV imaging laser engraving CDI

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Printing plate is one of the five major elements of printing. The quality of the printing plate is directly related to the quality of the final printing product. Based on the rapid development trend of flexographic printing, this paper classifies and compares and evaluates current major flexo platemaking processes.

Traditional negative platemaking process

For many years, negative films have been used to make photopolymer flexographic printing plates. At the time of plate making, a vacuum is drawn to bring the negative film into close contact with the printing plate, and ultraviolet light (ie, UV light) forms a graphic portion on the printing plate through the transparent portion of the film.

Photopolymer flexo plates are generally made of extremely soft materials and can adhere well to a smooth base surface. In order to avoid halation due to bubbles formed during vacuuming, negative emulsion surfaces of the negative film are required to have a very low gloss. In addition, the maximum density of negative film must be above 4.5. Otherwise, too much UV light is transmitted through the dark part of the negative film for a relatively long exposure time, resulting in blurring of the edges of the graphic and interest image parts.

Over the past few years, the performance of chemically-based sensitized films has continued to improve and has now reached a relatively high standard. However, due to the reduction in the use of photographic film, manufacturers do not plan to spend more on technological innovation. Only - it is worth looking forward to is a thermal film technology. This film is designed for a complete dry process, eliminating the need for developer during production. Although thermal film is currently limited to fine-copy and high-resolution applications, manufacturers believe that thermal film is expected to reach the standard in the general film field in the near future.

The biggest problem with the use of negative flexo printing is the compensation for dot gain. The tone of highlight regions is extremely easy to lose, resulting in a marked lack of image clarity in these regions. In order to solve this problem, people then propose the following improvement method, the second plate making process.

Improved negative platemaking process

The improved negative plate process can improve the dot gain compensation only by the RIP or the front-end prepress system. The goal is to be able to generate the same points on the plate as the CTP, while still keeping the gradation range as wide as possible. This process makes use of the fact that the size of the laser beam produced by the chipper is generally somewhat skewered. In this way, when the photographic film is exposed, the size of the blank portion used to generate the graphic portion is slightly smaller (energy dispersal, energy value is less than the value of the exposure field). This is the main reason that affects the size of outlets.

However, in order to cope with this situation, there is an advantage in reducing the size of the text part. That is, you don't have to deal with digital files (such as using the Curves curve tool), but you can also keep the entire tone. In general, the longer the plate exposure time, the more likely it is to get smaller dots on the plate. However, disadvantages exist. With the increase of the light intensity of the machine, the linearity of the machine will not be stable enough. In this case, the entire process depends to a greater extent on the properties and quality of the photographic film raw materials.

Another point needs attention, that is, the relationship between the network and its neighboring outlets. Specifically speaking, whether a small dot screen can be recorded on a printing plate basically depends on whether it is adjacent or relatively independent of other large dots. If it is the former situation, it is prone to the loss of small outlets. In the traditional plate making process, this is related to the photopolymerization process of the material under vacuum, and it cannot be avoided.

Mask-based computer-to-plate (LAMS/Mask-based)

The CTP process (computer-to-plate process), which is now widely known in the flexo platemaking process, uses a photopolymer flexo plate with a mask coating. The plate material is coated with a dark opaque coating called LAMS (Laser Ablation Mask System). This coating replaces the photographic film in the conventional process and is irradiated with a thermal laser beam. The irradiated portion is peeled off on the stencil; subsequently, the raised portion of the printed image is polymerized and cured by means of ultraviolet light.

The LAMS CTP system has been used successfully for many years in the flexo platemaking process. The considerable profits it brings offset the relatively complex structure of the system and higher acquisition costs. Compared with the traditional process, the graphic portion of the printing plate made of LAMS CTP has a steeper shoulder and more contrast. Especially when copying small positive or negative texts, this process is unmatched. Fine dots in the highlights can also be generated very well. This is also the advantage of flexo printing. The last point, but by no means the least important point, is that the generation of outlets has nothing to do with the surrounding outlets, which can guarantee a good prediction of the effect of the plates produced.

It may seem complicated to first image a precoat exposure, but doing so does have its advantages. Since the two imaging steps are separated (first imaged on the mask coating and then cured by UV light), the composition of the two parts of the plate can be better customized as needed. The pre-coating can be optimally configured according to the performance of the printing machine, and the photopolymerizable layer can be optimally configured according to the printing characteristics. It is this characteristic that paves the way for the development of CTP in flexo printing.

Direct laser engraving process

The direct laser engraving process is a one-step imaging process. The electronic image data is written to the printing plate by the laser beam, and unnecessary portions on the printing plate are evaporated, resulting in the formation of the raised text portion to be printed. This process does not require subsequent steps. The plates used for engraving are mainly various rubber plates, but now polymeric plates and photopolymer plates can also be used.

The direct laser engraving platemaking process belongs to a dry platemaking process. All costs of the rinsing step can be eliminated in the dry lithography process, which can result in profits. But what really makes a profit is the engraving equipment used, because it determines the productivity and quality that can be achieved.

According to the manufacturer's analysis, the laser and plate materials used are still in the stage of improvement. The number of network cables that can be reached is now 481/cm (122 l/in), which can sometimes be slightly higher.

Most direct laser engraving systems use a CO2 laser beam. Using a few hundred to several kilowatts of power can yield acceptable levels of quality while maintaining lower costs. However, experts also pointed out that for higher resolution requirements, CO2 laser technology development space is not too great. The CO2 laser beam has reached its limit of 1270 dpi and can generate 481/cm (1221/in) network lines. Obviously, in order to achieve higher quality requirements, other laser technologies are needed. However, experience has proven that it will be very expensive.

Direct digital UV imaging plate making process

With the digital UV direct imaging platemaking process, it is expected that traditional photopolymer flexographic plates can be used. The direct digital UV platemaking process is not a one-step platemaking process because the resulting photopolymerizable plate also requires subsequent processing of it. Even so, because this is a new process, its application prospects are still worth watching. However, its practicality cannot be determined at present.

Conclusion

In general, the LMS CTF system can best meet the requirements of higher quality and high production efficiency; the direct digital UV plate making process is still in the development stage and may be a better choice; the direct laser engraving plate making process is Within the reach of the best can provide very good results; and purely traditional photopolymerization process due to the sensitization film based on a variety of shortcomings.