Comparison and Analysis of Exposures of CTP Plates Using Circular Spots and Square Spots

1, the question is raised
Since its inception, the CTP system has shown its unique advantages. The CTP plate has the advantages of fast plate making speed and high imaging quality, and further pushes the printing to digital. In a few years, the CTP system has been widely used.

With the wide application of CTP technology, there are more and more manufacturers of CTP systems. All the well-known manufacturers in the world are involved in the production of CTP systems into their own business, including many famous companies in the printing industry such as Agfa, Creo and Heidelberg. Moreover, the technologies used have their own advantages. The shape of the laser spot used in the exposure process is one of them. A square spot is used in Creo's imagesetter, while the company's imagesetters such as Agfa and Heidelberg use circular spot exposures.

In the above manufacturer’s respective product descriptions, some say that Fang Guangdian has special advantages, and some say that there is no difference between the round light spots and the square light spots, and the opinions vary. The description of the business can not avoid the suspicion of advertising, printing companies in the process of selecting equipment should also be based on their own needs to determine, then the author will elaborate their own views.

2, circular spot and square spot technology
Because of the different shape of the laser beam, the circular spot and the square spot are different in the exposure of the plate material, and the nature of the spot is also different. Specifically, the following points are generally included.

(1) The cell exposed to the exposure of the recording grid is a recording grid, and the shape of the recording grid is generally a square in the design of digital screening.

The commonly used laser spot is a circular spot, which cannot fill the entire recording grid, and the corners cannot be exposed.

Obviously, due to the failure to expose the part, the record grid can not be fully exposed, resulting in a lower tonal value than the screen design value, the network point will have a certain loss in the process. And the edges of the dots in the grid made up of record rasters are not smooth enough.

The square spot is exactly the same as the record grid because the shape and the record grid are exactly the same.

Therefore, in the process of the CTP outputting the amplitude modulation network, the percentage of the halftone dots of the printing plate obtained by the circular spot exposure is generally smaller than the percentage of the halftone dots in the digital screening; and the dots obtained by the square spot exposure are almost the same as the number. The design value in the net is the same.

(2) The nature of the exposure point
The traditional CTP laser is a laser tube that emits a light spot for actual exposure, so the imaging point inevitably inherits the Gaussian energy curve of a single laser tube. The circular spot with a Gaussian energy curve has a wide edge halo. Due to the edge halo, the edge of the circular dot formed on the printing plate is blurred, and the delicate level of the image cannot be faithfully reproduced.
The characteristics of a square spot are completely different from those of a circular spot. Its energy curve is no longer a Gaussian curve, but an approximately trapezoidal shape. In this way, there is little halation around the square spot and the energy distribution is very even. The edges of the square dots created on the plate are very neat. The use of a square spot CTP plate produces an extremely sharp spot with an edge precisely controlled at ±O. 2 (four) m (even if the backend: 10% change in the Chinese version of the condition), the expansion rate of the 10 lJm-sized FM network can be controlled within 4%. Can delicately copy delicate images.

The principle of square spot formation is to emit a laser beam through a group of laser tubes in parallel. The laser beam is superimposed on the lens system. The lens system evenly distributes the energy of the laser, and then the energy is evenly distributed through a “light valve”. The laser beam is scattered into square spots.

The square spot technology is a new technology, its advantage is to make up for the lack of circular spot, such as the dot overlap due to the printing plate to expand the Internet point, the edge of the network is not sharp, difficult to reproduce the fine level. The square spot can be exactly matched with the edge of the grid, reducing dot gains generated during the plate-making stage, and accurate representation of delicate levels in place.

When using a square spot, plate making can be well reproduced by a computer, especially when using a frequency modulation network.

3, the effect of light spot shape on the entire printing process and adjustment

In the case of circular spot printing, dot spot enlargement may occur, and in order to correct this deviation, a method of linearizing the spotted dot output is usually adopted. The linearization of outlets is achieved by adjusting the exposure of the imagesetter, the amount of dot enlargement of each step, the development temperature, and the development time to achieve the same purpose of output and computer production.

Most RIPs and some software have linearization capabilities, so this can be done very easily. The linearization of a CTP system usually takes the following steps:

1 Determination of development conditions

The correct setting of the developing conditions is directly related to the density of the printing plate, the correct reproduction of the dots, and the size of the fog. The test can be performed according to the description of the middle wash kit and the film to determine the optimal development conditions. Here, it is mainly determined by the length of the development time and the level of the development temperature.

2 Adjustment of the maximum recording density value

The maximum recording density of a printing plate refers to the density value of the 100% dot area on the printing plate. The maximum density of the printing plate is directly related to the density of printed solids. Therefore, it is closely related to the printing quality. It is very important to control the density of the plate in the field. .

3 Linearization adjustment

The plate was subjected to test exposure at a suitable laser intensity and developed under standard developing conditions. Then use the plate density meter to measure the dot area ratio of the obtained printing plate and input it into the linearization setting dialog of RIP or related software. The RIP or related software will automatically correct the dot. After the linearization is performed, the dot output is generally within 2%, and the output is basically the same as that produced by the computer.

Due to the different effects produced by different spot shapes during plate making, detailed descriptions have been given in the previous section. However, from digital files to printers printing products, the entire process of printing includes many links. In addition to plate making, there are pre-press graphic information processing and printing. Therefore, considering the effect of light spots on the print quality, we must comprehensively compare other factors.

There are mainly two types of dot enlargements in color print copying. They exist in different processes: first, dot enlargement in the output process (laser photocopy output, direct plate-making output, etc.); second, dot enlargement in the printing process . The dot enlargement of the output process is related to the dot type of the dot, the number of screens, and the exposure amount. The dot enlargement of the printing process is related to the shape of the dot, the number of lines added to the ink, the amount of ink supplied, and the printing pressure. Since the enlargement of outlets will result in the loss of tone, it is necessary to try to keep outlets within a narrow range.

In order to achieve the minimum dot gain in printing, almost all devices need to be linearized in print production. However, even after the linearization of the printing press, the dot enlargement in the printing process is still obvious. The ISO standard reflects the curve of dot expansion in different printing environments.

It is worth noting that the production of this curve is the result of a large number of tests under different printing conditions using a traditional PS (negative or positive) version. The determination of this curve makes the results of the entire printing process predictable. Sex, printed products also have a rating standard, it is worthy of note is that the traditional method of making PS version of the recording unit after exposure is circular, so the use of circular spot can apply this standard, the standardization of the printing production process And quality control is a great help.

However, with the use of a square spot, the situation has changed. Due to the difference in the shape of the printing unit of the printing plate, the dot expansion of the square recording unit and the circular recording unit in the printing process must be different, but since the square spot is a comparison The new technology has not been widely used at present, and related research has not yet reached a satisfactory conclusion. Therefore, there is no corresponding standard. Therefore, the output of printing with a square spot CTP plate becomes unpredictable. Therefore, from the printing point of view, especially when using the amplitude modulation screen printing, the circular spot is more conducive to quality control, can predict the results of printing, circular spot is more conducive to printing.

4 Conclusion

Fang Guangdian technology is a new technology developed in recent years. It has advantages in the making of plates, but there is still some way to go in the research of the other party's light spot in the entire printing process. However, it can be predicted that the square spot technology has great vitality.

For printers, no matter which type of equipment is used, the goal is to obtain the best prints. The printing crystals with good reduction quality and small batch differences are the pride of the printing companies. Therefore, the specification of the entire process is equally important. When purchasing the corresponding equipment, the author believes that it is necessary to make a correct assessment of the company's operation standardization, quality control, and technical foundation based on its own production conditions. It is very worthwhile for companies with powerful capabilities in various fields to try new technologies. Advocated.

Source: Yantai Printing Network

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