Quality Control of Color Copying in the Printing Process (Part 2)

· Graphic quality inspection

The inspection of graphic quality is first of all a check on the content of the graphic text. It needs to check the construction slips and proofs, and check whether there are any errors or defects in the images, characters, symbols, lines, etc. This inspection is relatively simple, as long as the thickness meets the standards and smoothness is fine. What needs to be mentioned here is that the inspection of the content of the graphic and text also includes errors in the failure to grasp the supervision and construction slips and the proofs. In particular, some images that are difficult to distinguish between directions are easily reversed. It is not easy to detect them and it is even more necessary for us to check on each level.

Second, it is the inspection of the image quality. The quality of the image on the plate is the basis of the quality of the print. Without high-quality images on the plate, there is no high-quality print. The image quality check is performed through the inspection of the dots, because the 17i~N images are expressed by dots regardless of whether they are monotone continuous tune or color continuous tune originals. The contents of the inspection are conducted in two aspects.

Image imprinting check. With a visual inspection method, the monochromatic proofs and the corresponding parts of the printing plate are compared to observe if the shade of the printed product is the same. In addition to the overall observation, you can also choose to focus on observations. Observe whether the dot size is consistent with a magnifying glass. The key areas should be selected in the deepest part, the most shallow part, and the most important part of the image content. The hue of the main parts after printing can be adapted to the requirements, and there will be no big deviation in the printed matter. Due to the spread of ink in the printing and the double reflection of light, the printing dots must have a certain degree of expansion. Therefore, the dots on the printing plate should be slightly smaller than the dots on the monochrome proof sheet, and must not be larger than the sample spot. In addition, you can use the test strip to check the depth of the plate.

Check the quality of outlets. The quality of the network is an important guarantee for the quality of the image. With high quality outlets, there will be high-quality reproduction of the tune and high-quality color reproduction. The dots on the plates must be real, the edges should be clear, and the best surrounding is bright and clean. Hairiness on the edges of the outlets will reduce the performance of ink absorption during printing, and the outlets themselves will exhibit unstable conditions. A 10x or higher magnifying glass is usually used to check the dot formation.

· Plate color check

We know that the superimposition of four monochrome versions of cyan, magenta, yellow, and black can be printed as a color image. However, it is not colored for every single color plate. They are the same base, the same form of expression, and the color is reflected by the printing ink. Therefore, it is necessary to identify and review the printing plate colors so as to avoid printing accidents in which printing plate colors and ink colors do not match.

Identify from the point of view. The layout of the yellow version is Oo, and the main color version is 45~. The main color of the character image is magenta, and the main color of the landscape image is blue. Magenta and cyan must have one color as the primary color version, and the other color as the secondary color version. The sub-color version and the black version are in 15o or 75~ arrangement, respectively.

Identification from the layout features. In general, the yellow version has the highest dot coverage in the entire image. The magenta version has the largest dot coverage in the lips, red flags, red clothing, and other parts of the character, and it is in the dots of the human skin color.

Quality and standard

The coverage is also large. Qing version has a high coverage rate in the branches of trees, grasslands and other blue-green landscapes. Black Edition has the smallest network coverage in the general screen, and dot coverage is mainly in the eyes, hair and other black objects.

· Inspection of printed marks

Print marks are designed for print production, post-press processing and quality control. Although they are not official graphic content, they are an important basis for checking the quality of 17i~N.

Gauge, color, which is designed for print production. The gauge line is used for overprinting, also known as the rule line. There are 1 group in the middle of the left and right sides of the printed page, and 1 group in the middle of the trailing area. The color mark is used to record whether the ink color is correct and complete. It is generally set on the edge of the printing page, sometimes set 1 group, and some set a total of 2 groups.

Measurement and control strips, which are composed of color blocks that reflect the geometry of various quality indicators. Commonly used in China, there are GATF test strips and Bruner control strips. The measurement and control strips can reflect the on-site density, dot gain rate, dot expansion direction, relative contrast, overprint rate, gray balance, ink balance and other quality of 17i~N products. In China, the use of measurement and control strips has not been widely used in production. It is believed that with the continuous improvement of printing quality, the role of measurement and control strips will increase.

Postpress marking. This type of mark is set for post-press processing and includes fold lines, corner lines, cutting lines, and other post-press marks. After printing is completed, these marks will not be seen. Polylines are folding position markers. Books, color patches, hot platinum backgrounds, etc. also belong to this category. The corner line is a finished range mark and is also used for cutting marks. The cutting line is dedicated to the cutting, it is suitable for all kinds of slitting.

2. Control of the printing process

In the lithographic offset printing process, the paper, fountain solution, ink and printing plate are in contact with each other, and there are many changes. Inadvertently, it will cause ink unevenness and other ills, which has a great influence on the printing quality.

1 correct signing

Before the printer is officially turned on, it must go through trial printing and color correction operations so that the printing color is the same as the proof color, and after the printing samples are signed, the formal printing can be started. The work of signing the monochrome machine is even more important. It is related to whether the color of the finished image can be faithfully restored. In actual production, although many products are checked by the third-level sample, there is still a considerable part of the final product image due to the inaccurate sampling. This requires the operator, especially the operator of the monochrome machine, to be able to accurately determine the ability of color after each one-color printing.

Signing a sample is a relatively demanding job. In China, because most printing shops do not use measuring instruments, and China's paper size is small, measurement and control strips are used less frequently. Some of them did not play their true role even if they used measurement and control strips. The quality of printed products depends entirely on personal technical experience, which inevitably leads to deviations.

2 Insist on "Three Diligence"

The printing process is completed during the high-speed operation. Therefore, the control of ink-water balance is very critical. Practical experience tells us that efforts to achieve "three services" in printing production are effective quality control measures in the printing process.

Dijun

It is required to agitate the ink in the ink fountain during the printing process. The purpose of the ink is to ensure that the ink fountain can supply ink continuously and evenly. In production, the amount of ink in the ink fountain keeps decreasing as the printing progresses, and when it is less than a certain degree, the ink cannot be delivered to the ink fountain roller in time. Even if the ink is still a lot, due to the long rotation of the ink fountain roller and the specific viscosity of the ink, the ink will form the ink column rotation in the ink fountain. At the same time, when the ink fountain roller feeds ink to the transfer roller, the contact movement between them is opposite. The paper fluff, moisture, and ink on the ink roller are easily transmitted to the ink fountain roller, which will block the ink fountain blade and affect the normal ink transport. In the printing process, the ink fountain is often stirred and the normal ink supply is maintained. This is an important task to make the inks of prints consistent. It must be done carefully.

Diligently look at the water

It is required to look at the surface moisture during the printing process. In lithographic offset printing, it is very important to understand the size of moisture. It is an important condition for realizing the balance of ink and water. To keep the ink uniform, the layout moisture should be kept to a minimum. In actual production, it is necessary to adhere to the operating principle of “ink and water is small”. However, it is not possible to cause dirty plates due to too little moisture. Moisture content may vary due to factors such as indoor temperature and humidity, machine speed, plate thickness, ink and paper properties. Therefore, we must constantly observe changes in the layout of water, take timely measures to make the layout of the water to be controlled, so as to achieve the purpose of controlling the printing quality. In addition, the automatic water supply device should also be checked at any time for its upstream and return water conditions, to avoid the lack of water caused by the lack of water dirty plate, or due to poor return water overflow bucket and affect the normal printing.

Diligent sample

It is required that the quality of printed sheets be checked during the printing process. During the printing process, the operator must often check the newly printed printed sheets. The shorter the inspection interval, the better. This will greatly reduce the scrap rate. Generally it should be checked about 200 times. Diligent inspection of printed sheets is the most basic measure for controlling the quality of printing. In actual production, each time several sheets of printed sheets are taken from the machine's delivery table, two inspections should be performed on the sample table:

Print position accuracy check. Place a stack of new seals on top of the original seal and align them to see if the gauge lines are uniform. Repeat this several times to observe the accuracy of the right and left sides and the tipping line. If there is no alignment (or registration), remove it.

Image quality check. Its main quality is based on printing proofs. Check the shades of the printed sheets against the proofs and whether there are various imprinting defects or malfunctions. The inspection should be focused, carefully, quickly and accurately, and the printed sheet with the problem should be taken out and put in a separate place. If serious problems are found, the machine should be shut down.

He Minli

Source: Guangdong Printing