Offset Overprint Accuracy Related Questions (1-9)

Offset printing takes color printing as the main subject, overprinting accurate issues has great significance for quality and economy. Multi-color products, even if there are partial inaccuracies, will also destroy the quality of the entire screen. At the same time, it will make the network overlaps and inaccurate, the imprinting will be hard, the level will be unclear, and heavy batches will be scrapped. Or the problem of overprinting was not solved in time and production could not be carried out. In particular, maps and other high-precision multiple side-by-side prints are required. Accurate printing is the decisive key. Imprinting is not allowed, everything else is busy!

1. What is the concept of overprinting in the offset printing process?

The color printing plates made by color separation will be different in the angle of the screen pattern, and will be successively overprinted according to the order of the printing colors. This will eventually become a reproduction with the same level of tone as the original, which is called the printing overprint reduction process, or the color reproduction of the dot. . Overprinting is the basis for lithographic offset printing to achieve color reproduction.

In the color reproduction process, there is a problem with overprint accuracy. Under normal circumstances overprinting accuracy is reflected in two aspects: one is the entire print sheet radial (that is, up and down direction) overprinting is not allowed and axial (to go in the direction) overprinting is not allowed; the other is the partial overprinting of graphic images is not allowed There are many factors that cause overprinting.

2. What is the exact tolerance of overprinting?

Absolute overprinting accuracy does not exist in production practice. It always produces a certain amount of error. This kind of error is not unlimited, and it has a certain allowable tolerance period range, which is called overprint tolerance.

When determining the allowable tolerance value, it is based on the human visual observation force. Theory and practice have proved that the subtle ability of the human eye to distinguish can reach o. 10mm, at the same time, the human eye can recognize very slight differences in hue. Within this range, the human eye basically does not feel the change in tone value. Therefore, the allowable tolerance for overprinting is generally specified as 0.10 mm.

Overprint tolerances generally fall into two categories: one is the overprint error, and the other is the platen tolerance, which is:

Overprint Allowance Tolerance=Overprint Error Ten Rule Error=0.10mm

3. What is the exact process principle of overprinting?

Color trapping printing requires each graphic in the same plane (paper and other watermarks). The specification of overprinting when overlaying is to make the overlapping of each color image point in the specified position accurately, so as to organize the color and restore the color to the original. According to such a principle, the basic principle of overprinting is the whole process of optical combination, that is, the additive color method of color light and the subtractive color method of color material, so that the graphic network points realize the juxtaposition and coincidence of the correct positions, and finally the unique functions of the dot network. To organize colors and restore color.

4. What is the difference between the accuracy of overprinting and the rules of platemaking?

(1) The registration error mainly refers to the registration accuracy of the offset press manufacturing, but there are many factors that affect the registration accuracy in the printing production. For example, in the printing process, the machine and the paper work together, if the machine is old and worn out. Severe or new aircraft are subject to collision during transportation or due to installation, etc., all affect the accuracy of registration. The registration error can be further divided into: continuous paper registration error—refers to the image registration error produced by a single-color or multi-color sheet-fed unit; the resulting paper-sheet registration error occurs mainly in Single-sheet multi-color printers; printed sheets cause image registration error during the transmission of each unit. Error reference:

The maximum allowed value of the paper registration error: 0.02-0.05mm

The maximum allowable error of the paper-handling register is: 0.02-0.03mm

(2) Platemaking rule error. It refers to the sum of the errors resulting from the specification of various images and the accurate placement of images in the process of making images such as photography, imposition, and printing, and because they are all based on the rules; Platemaking rules error. The maximum permitted tolerance of the sum of its rules must not exceed 0.05 mm.

5. What is the relationship between overlay accuracy and plate tension?

The offset printing plate is thin and light. In the case of trapping printing, errors in registration can occur due to changes in its shape and volume, resulting in inaccurate overprinting.

The plate is placed flat, and the two ends are subjected to a force to produce an irreversible shape and a radial volume expansion and deformation of the roller, which is called a stretch deformation of the printing plate.

The tensile deformation of a printing plate is determined by the physical properties of the metal plate, the ductility. Such as: zinc plate, aluminum plate and copper and chromium double copper plates, etc., under a certain tension. Produce significant ductility and stretch deformation, so the printing plate with text and graphic, in the role of tensile force radial elongation, axial narrowing, the surface of the text will inevitably elongate, its elongation value Image registration error.

Under the action of the tensile force, the elongation value of the metal plate is proportional to the tensile force and length, and is inversely proportional to its cross-sectional area and elastic modulus. Production practice proves:

(1) The stretching deformation of the printing plate is caused during the pull-out process when the platen is tightened by the platen and the typography.

(2) The value of the plate's tensile deformation is affected by two major factors: one is the tension of the tight plate, that is, the greater the tensile force, the more severe the deformation; the other is the thickness of the plate, that is, the thicker the plate, the more the plate is stretched. The smaller the deformation;

(3) The longer the length of the printing plate is, the larger the stretching value is, and it is more obvious in the machine with large printing area.

6. What is the relationship between the accuracy of overlay printing and the thickness of the media?

Since the plate has the characteristics of tensile deformation, its characteristic rule is: the thicker the plate, the smaller the stretching deformation. In order to ensure the accuracy of overprinting, the common practice is always to use a thicker plate to produce the first or second color version. In terms of overprinting, due to the increase in the number of printing, the size of the paper will also increase, and the size of the image on the corresponding sheet will also increase with the increase in the number of overprints. Therefore, a thicker plate is used to print a relatively small or constant graphic size at the beginning of printing to accommodate the radial extension of the paper, ensuring that the overlay is accurate. Because of the use of the zinc plate, the zinc plate must be ground once for each printing, and all the images and texts will be worn off; Zinc version body due to the number of grinding and other reasons, there are thin points, easy to use. The current use of PS version is more, its thickness is generally about 0.3mm, so it is not possible to use thick plate material, just the chance of a reborn version, only the thickness of the PS version.

7. What is the relationship between the exact registration and the size of the drum radius?

The influence of the curvature of the printing plate loaded on the roller, the size of its positive pressure, inversely proportional to the concise diameter, and inversely proportional to the friction generated. Due to the above relationship, the stress on the various dots of the printing plate is not uniform, so the distribution of the stretching values ​​produced on the printing plate is also different.

(1) The larger the wrap angle (indicating that the drum radius is small), the greater the force required to tighten the printing plate, the greater the friction force generated;

(2) The smaller the rolling hemisphere, the larger the wrap angle of the printing plate, the larger the stretching value, the more uneven the stress distribution of the particle, and the distribution of the stretching deformation value is more concentrated at both ends of the printing plate (ie, Bite and drag);

(3) The larger the simplified radius, the smaller the wrap angle occupied by the printing plate, the higher the value of the shell stretch value, the more evenly distributed the stress distribution of the particles, and the even distribution of the stretch value larger than the wrap angle.

(4) The roller radius is infinite, then the stretching phenomenon is in line with: the stretching value is distributed very evenly (in fact it is lithographic stretching. In order to make the printing plate graphic catch up with the stretched paper, a drawing machine can be made by oneself and the printing plate will be Place it in the plane and stretch it. This kind of elongation will play a very good role in catching overprints.)

In production, the more common abnormal tensile deformation (axial bite edge deformation, axial arc deformation, axial torsional deformation, radial drag edge tensile deformation, radial bite edge tensile deformation, Radial stretching deformation) all reflect the unfavorable serial layout, normal pressure, and friction caused by plate pull and rule finding. These deformations will inevitably cause changes in the shape and size of the layout graphic. The value of the deformation will cause the misprint of the sheet in the overlay. In general, the plate deformation value allows the maximum registration error to not exceed 0.10 mm. Otherwise, the color shift due to overprinting is not allowed.

8. What is the relationship between the misprint and the thickness of the base?

Take a new printing plate, the thickness of the plate itself is 0.70mm as the standard value thickness, do not add tree pad paper when loading the plate, and take another plate of the same graphic and text, the thickness of the plate itself is 0.30mm, in the same — When inserting the machine, 0.40mm of spacer paper is required. The two plates had a lining value of 0.70mm on the roller and overprinted each other, excluding other variable factors, and found that the graphic and text size of the former was slightly smaller than the latter's graphic size. Generate a certain amount of error.

Due to the different values ​​of the thickness of the plate itself, the size error of the picture and picture appears in the overlay, which is called the plate thickness error.

The thickness error of printing plate base is often a problem in the printing process, so it is used by the process. The reason for this is as follows: the thick base is wrapped on the slip, and its back is affixed to the slip, the friction force is large, the pull force is only concentrated on the two ends of the mouth and the drag edge, and the whole printing plate is not completely Stick on the roller. The uneven distribution of stress among all the mass points, except for the natural stretching, the tensile deformation is small, it shows a rigid body in the printing, and there is almost no extrusion deformation, so the surface textural elongation is small, basically stable . The thin plate base and the pad plated paper plate are installed on the roller. On the one hand, the friction coefficient of the tree paper is small and the corresponding friction force is small. After the plate is pulled, the stress distribution of each particle is uniform, and the bending can be better. Adhere to the roller, so it's stretch deformation is great. For this reason, its graphic size is slightly larger than the thick format.

It is precisely because the thick and thin plate base can produce the graphic and text size error in the printing process. Therefore, in the printing process, the thickness of the base plate is selected and the color sequence is gradually reduced. That is, the first and second color selection thicker version of the base, the rest of the color version choose thin base printing, to avoid overprinting errors.

9. What is the relationship between the accuracy of overprinting and roll reduction and cushioning?

Due to the increase or decrease in the improper control of the underlay, the equivalent of the corresponding drum radius should be out of balance. This would lead to unequal arc lengths and central angles per unit time, resulting in a change in the size of the graphic image. This kind of change on the one hand caused overprinting is not allowed, on the other hand, can also be used to adjust the radial image text overlay error due to various factors.

When the plate cylinder radius is greater than the radius of the blanket cylinder, the plate size is smaller than the paper size. When the plate cylinder radius is smaller than the radius of the blanket cylinder, the size of the printing plate is larger than the size of the plate.

In order to change the error value of teletext extension or contraction, the method of changing the thickness of the tube joint