Common problems and solutions of offset printing of Cosmetic boxes

Common problems and solutions of offset printing of Cosmetic boxes

Principles of Printing Cosmetic Boxes-Process-Technology: How to determine the edge of the mouth when loading the plate? How to determine the width of the jaw edge of the printing plate when loading the plate? What is a printing positioning system... The following will introduce you to the common problems and solutions in packaging offset printing. The following content is organized by Xiaobian for your reference.

Common problems and solutions of offset printing of Cosmetic boxes

When installing the edition, how to determine the edge of the mouth?

Which side of the original plate is used as the mouth edge should be considered from the needs of printing and post-press processing.

①Considering the specification limitations and positioning accuracy of the printing machine, the long side of the original rectangular plate should be used as the mouth edge.

②From the perspective of overprinting accuracy, since the deformation of the paper on the gripping edge is small, and the deformation of the paper on the trailing edge is large, the side with high overprinting requirements on the original plate should be used as the gripping edge.

③From the point of view of saving paper, since the position of 1cm wide on the mouth edge cannot be printed, sometimes the paper format is very tight, and this part of the white edge needs to be fully utilized, then the blank edge on the finished product or the paper tongue edge of the gluing box should be placed. When in the mouth.

④From the point of view of post-press processing, the choice of gripper edge should be beneficial to die-cutting processing.

How to determine the width of the mouth edge of the printing plate when installing the plate?

When installing the plate, the distance between the image of the original plate and the mouth of the plate is required to be within a certain range. Suppose the distance between the image on the original plate and the mouth of the printing plate is c, the distance between the image on the original plate and the mouth of the paper is a, and the distance between the mouth of the paper and the mouth of the printing plate is b, then c=a +b. The size of b is determined by the position of the plate cylinder relative to the impression cylinder and how much the front gauge brings the teeth and paper. The influence of adjusting the front gauge on b is very small. The position adjustment amount of the plate cylinder relative to the impression cylinder is different, generally about 5cm.

The drying plate, that is, when c has been determined, adjust the size of b to change the front and rear position of the printed image on the paper, commonly known as "borrowing a cylinder". However, if the position of the image on the paper needs to be adjusted by "borrowing the cylinder" during printing, it will obviously affect the production efficiency. A more reasonable approach is to choose a roughly centered b0 value for a certain type of printing press. If you want the image to be printed on the paper at a distance a from the edge of the mouth, determine the image when printing according to c=a+b0. Distance c from the gripping edge of the plate. Every time the position of the original plate relative to the gripping edge of the printing plate is determined in this way, there is no need to "borrow the cylinder" during printing. If there are corner lines on the original plate, the distance between the original plate and the mouth of the printing plate is usually determined by using the corner line as a reference.

What is a printing positioning system?

From the original to the printed matter, it has to go through color separation, imposition, and copying. Shaiban, printing and other production processes. In order to ensure that the color separation images can be accurately overprinted in the fastest way in the production of each process, the matching of positioning pins and positioning holes can be used to achieve the purpose through a series of operations. This system is called a printing positioning system. Since the desktop system is now generally used for layout production, which can automatically output accurate color separation negatives for overprinting, the positioning system is mainly used in the printing and printing process.

What does the positioning system consist of?

The positioning system is mainly composed of printing plates. It consists of a negative punching machine, a positioning pin and a positioning pin on the plate cylinder.

What are the benefits of using a positioning system?

①It greatly shortens the time for version installation and version adjustment. If the printing plate can reasonably arrange the position of the original plate relative to the printing plate. And the correct use of the positioning system, then the printing machine can basically achieve the purpose of registration without changing the version between the various color groups, which greatly improves the production efficiency.

②The printing plate no longer needs to be pulled repeatedly, which reduces the overprinting inaccuracy caused by the stretching and deformation of the printing plate, and improves the printing quality of the product.

How to use the positioning system correctly?

At present, the multi-color printing and printing positioning system is adopted in China. The correct use method is as follows:

①Cut the polyester film base, the width should be smaller than the distance between the image on the original plate and the mouth of the printing plate, and use a punching machine to punch the polyester film base.

②Use the punched edge of the polyester base as the benchmark for the gripping edge of the printing plate, and use the oblong hole in the middle of the polyester base as the centerline of the printing plate, and fix the negative of the first color version on the base. Note that the film base should be attached to the non-medicated film side of the negative film.

③ Put the polyester film base of the first color version of the fixed number on the positioning pins, and register the second color version negative film with the first color version negative film. At the same time, put the second polyester film base on the positioning pins, and fix the second color version of the negative film and the second polyester film base together under the condition that the second and first color version negatives are in register. Process the remaining negatives in the same way as the second-color negatives.

④ Punch holes for each printing plate to be printed, align the negatives of each color plate and the positioning holes of the printing plate with positioning pins, and then print the plate.

⑤When installing the plate, first make the plate clamps at the mouth of the plate cylinder of each unit retreat to the same reference position, and hang the two U-shaped holes on the printing plate on the positioning pins, so that the U-shaped holes and the positioning pins are closely connected. Then tighten the plate clip on the trailing edge first, and then tighten the plate clip on the mouth side.

What are the requirements for the exposure light source for the PS version?

①The spectrum of the light source of the printing machine should be consistent with the photosensitive spectrum of the PS version of the sensitizer. The photosensitive spectrum of the PS version is about 350-450nm, and the light source is required to have a high-energy peak in this wavelength band to shorten the exposure time and achieve a good exposure effect.

②The illuminance of the light source should be uniform. The illuminance meter can be used to detect the illuminance of each point in the middle and periphery of the printing table, and the illuminance unevenness of each point on the layout is required to be less than 15%.

③The power of the light source should be around 3000W, and the voltage fluctuation value should be less than 10%. Before exposure, the light source should be preheated to ensure stable light intensity during exposure.

How to determine the exposure time of the PS version?

The exposure time of the PS version mainly depends on the photosensitive range of the PS version. Sensitivity and spectral characteristics of the light source, the strength of the light source and other factors. Therefore, every time you change the PS version of a different brand or change the light source, you need to use the measurement and control strip to conduct a series of tests on the PS version to determine the correct exposure time.

The test method is as follows: choose a transmission gray scale with a continuous density level difference of 0.15 and a Brunel or other similar test strips with small dots and thin lines in the positive image, and use the two test strips to gradually increase on the PS version. Expose the exposure time, record the corresponding exposure time of each piece, observe with a magnifying glass after development, check each test strip that is exposed, and the exposure time corresponding to the qualified test strip can be used as the normal printing time. When making ordinary mesh-adjusted products, the first 4 levels of the gray scale are required to be dried; 1% of the 6 dots are dried, and 2% of the dots are exposed; 11μm wide thin lines are retained, and 8μm wide thin lines are dried out. .

How to adjust the exposure time of PS version according to specific conditions?

①There are no dots, only thicker text. The original version of lines and solids can increase the exposure time, remove the small dirty spots and stains, and reduce the time for cleaning.

②If the negative is stored for too long, the blank part turns yellow, or after manual imposition, there are two or three layers of blank film bases superimposed together, which can slightly increase the exposure time; the original plate with insufficient density in the solid part can reduce the exposure time.

③The pressure of the proofing machine is lower than that of the printing machine, so the increase of the proofing dots is smaller than the increase of the printing dots, so the exposure time of the proofing plate should be less than the exposure time of the printing plate. For the proofing version, 0.5% of the outlets are generally removed, and 1% of the outlets are retained.

Strictly speaking, for the color separation plate making of layered drafts. When determining the tone reproduction curve, the difference in dot gain in various situations should be considered and compensated. In fact, the standardization of this process in China is not very good, so it should be based on the production conditions of the respective printing factories. Adjust the exposure time. If the dot increase rate is large during the printing process of our factory. Exposure time can be increased. Depending on the substrate to be printed, when printing the printing plate of offset paper, the exposure time should be longer than that of printing the printing plate of coated paper.

Why do you need to pump air when printing and exposing?

Taking the production of the positive pattern PS plate as an example, when printing the plate, if the film surface of the negative film is not in close contact with the photosensitive coating of the PS plate. If there is a gap in the middle, light will be obliquely shot from the edge of the blank part of the negative to the photosensitive coating directly under the graphic part, causing the edge of the graphic part to see light decomposition. Make the image blurry. Dots become smaller, small dots and fine lines are lost.

Because one side of the printing frame is transparent glass, and the other side is a rubber pad. When the vacuum pump draws out the air in the printing frame, the air pressure inside is low, and the rubber pad will press the PS plate under the action of the external atmospheric pressure. The original negatives face the glass and are tightly bonded together.