On the quality control of prepress film

2022-09-19
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Talking about the control of prepress film quality

I have been engaged in practical operation, technical support and quality management in the newspaper Phototypesetting room for many years, and have experienced many cases of printing quality accidents caused by film quality problems, deeply feeling the impact of film quality control on plate making, printing and other subsequent processes. Now, based on years of practical experience and theoretical discussion, it is summarized as follows and shared with you

with the development of prepress technology, the tasks of newspapers and printing houses are becoming more and more important. However, due to different standards and requirements, the film quality output by each unit is also uneven, which directly affects the printing quality. It can be seen that to ensure the printing quality, the output quality of the film must be controlled to make the film quality meet the printing requirements

film quality control includes optical quality control and process quality control

I. optical quality of film

the optical quality indicators of film include field optical density, fog density, point blackness, point virtual halo, etc. in addition, the appearance quality program of film will automatically analyze the test curve quantity, which should also meet the requirements

1. The meaning of field density, fog density, point density and point virtual halo

(1) field density

offset printing process needs to completely transfer the points on the film (including the field) to the printing plate, so the covered parts on the film (i.e. image and text points) cannot pass through the amount of light and cause the photosensitivity of the printing plate photosensitive layer, so the points (including the field) need to have sufficient light blocking ability. The field density is based on the average field density of 2 Measured by the crushing of waste plastics

(2) fog density

fog density can be regarded as the density with exposure equal to zero. That is, the transparent part (blank part) on the film must have enough light to pass through, so that the photosensitive layer of the printing plate is fully photosensitive and dissolved during development, so the film must have enough light transmission capacity. The fog density is measured by the average density of the blank area of the film

(3) point blackness, point virtual halo

the ideal point should be absolutely clear, and the blackness is equal in the plane covered by the point. In fact, due to the scattering and refraction of light, the points formed by different films have the phenomenon of virtual halo edges in varying degrees (that is, the blackness of the point edge is less than the blackness of the center of the point). Due to the existence of the point virtual halo edges on the film, the size of the points on the P5 version and the points on the film is changed because the light blocking ability of the point edge area is weaker than that of the point center area during printing, Thus causing differences in graphic and text levels. In order to reduce this change, generally speaking, the smaller the edge of the point virtual halo, the better, that is, the edge of the point should have enough clarity

point blackness is expressed by the concept of point center density; Point halo is expressed by point edge density

2. Detection and control standard of optical quality

at present, the optical quality of film is mainly identified by testing the optical density of various indicators. When measuring, ordinary transmission densitometer is used to measure the field density and fog density, micro densitometer is used to measure the central density of points, and point test strip and magnifying glass are used to measure the edge clarity of observation points

because the field density, fog density, point blackness and point virtual halo of the film mainly depend on the film base used by the film and the photosensitive performance of the film photosensitive film, the Phototypesetting Center cannot be controlled. In order to ensure the field density and haze of the film, when choosing the film supply, the Phototypesetting Center can conduct repeated tests according to the photosensitive performance of its own Phototypesetter and select the appropriate film

according to the theoretical knowledge and the basic conditions and quality requirements of printing, in practical application, the film quality should meet the following standards that existing enterprises aim at the recycling link:

measure the point center density with the field density. If the field density value of a large area is greater than the transparencies film base density by more than 3.5, the point center density can be considered to be qualified

the optical quality of the film depends on the output quality of the hair arranging system itself, the performance of the film material and; In our practical work, we should try our best to use various means to improve these factors and meet the requirements of industry standards to the greatest extent, which will be conducive to the stability and improvement of quality

II. Process quality control of film

process quality of film, including the number of target lines selected during output, the accuracy of point shape, angle, point percentage, and the accuracy of four-color overprint. These characteristics of film are closely related to the printing process and printability, that is, corresponding to different printing materials and replication priorities, different parameters should be relatively selected for output. If the optical quality of a film is very good, and the output parameters are improperly selected, the printing quality will also be poor. Obviously, the process quality of the film is inappropriate, and it cannot be considered as a high-quality film

1. number of EYELINES

number of EYELINES: that is, the number of unidirectional parallel lines in unit area. The higher the number of lines and the smaller the dot, the finer the print can be. However, it is not that the higher the number of lines is, the better. It should be selected according to the requirements of printing and material technology. The conclusion of theoretical calculation shows that thin lines are much more sensitive to changes in printing than thick lines. When the paper quality fails to meet the corresponding standards, the machine accuracy is not high, the ink texture is poor (coarse particles, etc.), and the printing pressure is large and the dot expansion rate is high, it is easy to cause the thin film to appear the phenomena of plate pasting and color running, so the number of lines should be consistent with the actual print. For example, the target of newspaper printing is about 100 lines/inch. If the printing suitability conditions are good, such as the paper quality is high and the process error can be effectively controlled, it can be higher accordingly. At present, the number of color newspapers printed in Hong Kong and Taiwan is higher than that in the mainland, and can be as high as 133 lines/inch

it is worth noting that when the resolution of the imagesetter is certain, the mesh number is inversely proportional to the level of the image. If the number of lines is high, the level is correspondingly less, otherwise, it is more. Generally, newspaper printing should ensure that each color version has at least 100 layers. In this way, if the mesh count is 100 lines/inch, the resolution of the imagesetter should be at least 1000dpi. Generally, the resolution of Phototypesetter used for newspaper publishing should be between 1524 and 2540

2. point shape

point shape, that is, the geometry of a single point. The influence of point shape on the copy effect is not as obvious as the number of meshes, but different point shapes have the phenomenon of angle and grade jump due to pressure changes at different adjustment values (point percentage). Accordingly, for the key adjustment range to be copied, the point shape with grade skipping phenomenon in this range should be avoided as far as possible. The jumping of points with different shapes is usually: the square point is at 50%, the diamond (oval) point is at 35% and 65%, and the circular point is at 78.5%. Therefore, when dealing with different images, we should give due consideration to the characteristics of points. When choosing, we should select and keep a high profile to avoid the skip area of points

3. Angle

angle refers to the angle between the vertical line and the horizontal line in the center of the point, which reflects the arrangement direction of the points. In monochrome printing, the human eye has the best visual effect on the angle of 45 degrees. In multicolor overprint, due to the inaccuracy of dot overprint, there may be "moire" phenomenon. Choosing appropriate angle combination is one of the methods to reduce moire

4. output point percentage accuracy

point percentage accuracy refers to the error between the point percentage on the film after output and the point percentage set before output. Ideally, the percentage set during front-end processing is accurately output to the film. But in fact, there are inevitable errors, which must be strictly controlled. If the dots on the film expand to a large extent, it will inevitably cause the overall picture to be dark, dull and not refreshing after printing. The error is mainly caused by the quality of hair arrangement, the performance of photosensitive materials and; Medium washing process and other factors, of which the quality of Phototypesetter is the most important factor

the above indicators are closely related to the printing process and printing suitability, which determines whether the ideal printing reproduction effect can be obtained. High quality film should be of both optical quality and technological quality, and it is also one of the necessary conditions for high printing reproducibility

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