Misprint analysis

Overprinting is not one of the key factors that cause print quality problems. This paper analyzes the reasons for overprinting inaccuracies in printing.

The accuracy of overprinting is a key aspect of quality issues in all print production and is particularly important in the printing process. The author analyzes and summarizes the problems that have occurred in printed matter during the printing process, hoping to communicate with everyone.

First, the size of the printing plate and semi-finished products match

1. Unprinted semi-finished product sizes are matched to the plate. In order to ensure the accuracy of overprinting, the machine personnel should carry out the necessary sampling (upper, middle and lower) of the paper before picking paper to see if its specifications are consistent with the size of the printing plate. Especially when the season changes and the temperature and humidity of the working room are unstable, sufficient attention should be paid. If more paper sizes are found to exceed the allowable tolerance range, they must not be printed, and they must be re-humidified to avoid overprinting and paper feeding problems.

2. The printing plates used in each printing press should be inspected carefully before installation. All the printing plates are required to have the same size and size. Under the possible conditions, the thickness should be the same. This is very good for overprinting.

Second, the overprint caused by changes in the printing plate is not allowed

The metal plates used for printing have a certain degree of ductility. After they are mounted on the rollers, they are subjected to various pressures during the imprinting process and the erasing process, and inevitably produce different degrees of deformation. Especially the changes in length and width can affect the accuracy of overprinting.

1. The relationship between the thickness of the printing plate and the change in the surface size of the printing plate.

The printing plate will surely deform to a certain degree when it is installed. However, when the machine has a large roller curvature (usually the roller diameter is larger than 450mm), when the central angle formed by the plate and the roller is larger, the dimensional deformation of the printing plate surface is Does not meet the "Hooke's Law." Under a reasonable tension condition, the thicker the plate, the greater the surface deformation.

When the plate is mounted on the roller, it must be bent. When a metal object bends and deforms, a neutral layer is created, the inside of the surface is compressed, and the outer surface of the surface is stretched. This will cause the front and back of the printing plate to have different radii, and the external surface is subject to greater tension, causing it to "crack" and cause the size of the surface of the printing plate to elongate.

This deformation value is related to two factors: First, if the drum radius is constant, it increases as the thickness of the printing plate increases. The second is that if the plate thickness does not change, it will increase as the radius of the roller decreases.

2 to reduce the printing plate deformation measures.

From the above analysis, it can be seen that the change in the printing plate due to various factors after installation on the roller is unavoidable and will have a certain influence on printing. However, various measures and technical measures can be taken to minimize this adverse effect.

(1) The following principles should be followed when loading the plate: Make the position as accurate as possible when loading the plate: Use the minimum tensile force as appropriate (according to plate and roller fit): Reduce the number of times of pull plate and school plate is very It is necessary and beneficial; uniform thickness plate should be used.

(2) Problems to be noticed when correcting plates.

In the printing process if you need to adjust the position of the printing plate, you should follow this principle in the operation: the direction is clear, rather not much, rather accurate data, do not use force. In this way, the position of the corrected printing plate tends to be accurate, or it may easily cause repeated adjustments. First, it wastes time and secondly, it causes unnecessary tensile deformation.

In addition, before the plate is calibrated, the stress should be relieved—the friction between the back of the plate and the surface of the roller should be minimized.

Only in this way, the plate can be displaced smoothly under the action of pulling force (or force) to achieve the intended purpose. Otherwise the plate will be stretched and deformed, or even broken.
Determine the stress: Any force that hinders the plate from moving in the required direction is stress. If the diameter of the cylinder is large and the central angle between the plate and the cylinder is small, the printing plate can only be moved in a slight amount by one side, and only the one-way pull plate screws can be loosened. However, when the amount of movement is large, all related pull-top (top plate) screws must be loosened. Otherwise, the diagonal resistance of the plate will produce a “twisting phenomenon”, which will not only achieve the purpose, but will make it more difficult. Version produces torsional deformation. Therefore, when the printing plate is moved circumferentially, it is necessary to take into account the influence of the axial printing plate screws, and when the printing plate is moved axially, it is necessary to take into account the influence of the peripheral plate screws.

Third, the overprint caused by paper deformation is not allowed

Paper is the substrate, and its dimensional change is a common cause of overprinting. The main reason for the change in paper size is due to changes in the paper's water content caused by changes in the paper's moisture content caused by environmental temperature and humidity, resulting in inaccurate overprinting.

The change in the moisture content of the paper is the main cause of paper deformation and misprinting. Paper is a thin layer of material that is interwoven with natural plant fibers and is highly hydrophilic. In addition, papers formed by interweaving fibers have numerous capillary pores of different thicknesses, and have a capillary adsorption effect on water. Therefore, paper is a material having high water absorption. Due to its special structure, the paper not only absorbs moisture when it comes in contact with the paper, but also has the ability to absorb (moisture) water from the moist air and to drain (dehumidify) air.

After the paper absorbs water, it expands, and the straight-line size and area of ​​the paper increase, and the mechanical strength decreases. After the paper is dehydrated, it shrinks, and the straight-line size and area of ​​the paper are reduced in all aspects, becoming stiff and brittle. These changes are caused by the plastic deformation caused by the change in the moisture content of the paper. Therefore, to control the deformation of the paper, the moisture content of the paper must be controlled, and the water content of the paper must be uniform in order to guarantee the accuracy of overprinting.

Fourth, blankets and gaskets on the impact of overprint

Embossing blankets and liners are the main materials for obtaining good prints, and their tightness, thickness, and evenness will have a certain impact on overprinting.

The thickness of the lining: The thickness of the selected lining depends on the characteristics and model of the printed product. Reasonable choices can be made according to specific circumstances.

The flatness of the lining: The lining of the embossed package consists of blankets, paper, liners, etc. Each material may not be absolutely flat. When choosing, you should make the error itself as small as possible, and do not overlap to make the error concentrated in one part. This can make the thickness error complementary and avoid local bulges or depressions in the total thickness. Otherwise, if the errors are added together more than O.05mm, the printed sheet after compression will be subject to certain errors. Therefore, the necessary measurements should be made in order to grasp accurate data when changing pads.

The tightness of the lining: Since the blanket is an elastomer, it has a certain degree of plasticity. When subjected to an external force, its thickness and length will change. Therefore, changes in the tightness of the rubber have an effect on the actual radius of the impression cylinder. The change in thickness will cause a change in the printing pressure, which is accompanied by the inaccurate overprinting of the printed sheet. This phenomenon is strongly expressed in the two corners of the edition.

When the elasticity of the blanket is uneven, it is divided into the following two situations:

The first is overall looseness. This type of overprinting is not allowed. It appears on the printed sheet as a “paper crepe” phenomenon that increases from the print head (mouthpiece) to the end of the print sheet. The overprint error is weak to strong. The reasons are: loose rubber, resulting in mechanical wear, failure, damage; after a new replacement rubber, after a period of time printing, natural stretching, causing relaxation; improper clamping screw plate, causing the rubber hole screw hole exclusion.

The second is partial elasticity. When there is partial elasticity in the rubber sheet, the entire sheet surface of the printed sheet will be ununiformly pressed, and if it is severe, partial overprinting (uneven deformation) or pleats will occur. The reasons are as follows: The shape of the rubber cloth is not standard, that is, it is not rectangular: the screw holes at both ends of the plate are not parallel; the screws of the individual plate screws are not tightened: the splint is deformed or bent; the head and tail of the rubber cover (edge ​​of the roller action surface) Cracking: The error in the lining thickness is gathered in one place.

Fifth, the impact of mechanical strength on overprint

In the previous discussion, the misprints caused by printing plates and papers were not allowed. In addition, the printing machinery we use will cause overprinting from external factors, and it is the main aspect of frequent failures in normal printing. Because, the paper starts from the "paper separation" on the transporting table, and is sent to the receiving warehouse after the imprinting to get the whole process of the printed sheet. If any part of the sheet fails, it may cause overprinting. In the following, the possible adverse effects of mechanical factors will be analyzed and discussed.

1. The paper separation mechanism should coordinate - the paper starts neatly.

The “separation” process is the starting point for the transition from a stationary state to a moving state. Therefore, the parts of the paper separation mechanism should coordinate and operate, and the paper should be crisp and neat when it is separated from the paper basket. Here, we only discuss the rationality of the separation paper brush and the nozzle rubber apron. If it is used improperly, it may become a paper feeding failure. The incentive.

(1) Brush. Its role is to control the height of the stack of paper (of course, should be coordinated with the nozzle), the bottom of the single sheet of paper sucked by the nozzle paper "brush" fall. Therefore, apart from the quality, the key is whether the position is reasonable or not. The upper and lower positions should be 3~5mm above the stack, and the front and rear positions into the stack should be less than 8mm (this is only a general data, of course, oblique and flat brushes also have The difference, the specific circumstances can be slightly different). The purpose of doing so can make the height of the nozzle relatively high, to prevent friction with the pile of paper when the suction nozzle "return", causing the top of the paper to start the position of disorder and the rubber apron curl, the paper suction deformation. On the other hand, it is convenient for loose paper mouthpieces to blow dozens of sheets of paper on top, which is conducive to the normal separation of paper.

(2) rubber aprons. Its purpose is to increase the area of ​​paper suction, should be based on the size of the printing paper size, thickness and thickness, the use of different rubber aprons, to prevent double suction from excessive suction. It is worth noting that when adjusting the position of the suction nozzle, keep it parallel to the plane of the paper tray or slightly “raise the head”. Do not lower your head and avoid the rubber apron from moving the single sheet that will be sucked up. In addition, in the work should be careful not to contact with oil substances, to prevent deformation, resulting in sucked paper start position is not correct.

2. The "conveyer drive" must be accurate - it must be handled correctly.

The so-called "conveyer drive" is the transfer paper feeding nozzle and the guide rubber tire transfer process. It is the "starting line" of paper from stillness to movement, and it is the first "co-transportation", which is directly related to the quality of paper transport. Therefore, it is necessary to be careful and accurate when adjusting the movement relationship.

3. The delivery process should be smooth - can not be out of control.

After the paper is conveyed onto the paper feed table, the "paper feeding process" until the positioning is not to be lost. It is necessary to perform regular maintenance on the transfer table to ensure its good uniform smoothness, otherwise it will cause misalignment when the paper is transported, resulting in misregistration and even tearing.

4. Positioning mechanism work stable - good performance.