Print Overprinting Failure Analysis (I)

In evaluating the quality of printed products, the two most commonly used standards are color and overprint. Overprinting generally refers to the fact that colored graphics are transferred to the specified geometrical position of the substrate. The accuracy of the overprinting of color graphics conforms to industry standards. For some printed products, such as sophisticated maps, navigation maps, etc., overprinting not only meets the above basic requirements, but also the graphic and text must meet the required size. Generally, overprinting accuracy is not limited to different colors on the same print product, but the positive and negative sides of the same print product should meet certain standards between the same batch of print products.

From the material preparation, plate making, and printing workflows, almost every link can cause overprint failures. There are many reasons for the failure of overprinting in actual work. The following author makes a simple analysis of these reasons and proposes some solutions for reference.

Reason for making plate

1. Imposition

With the increasingly widespread application of CTF processes, the failure of overprint due to imposition has become less and less, but it is still rare in manual imposition. Once a failure occurs, you can only re-impose and print. However, as long as the operation is carefully done, it is basically possible to avoid faults in overprinting.

2. film

The film deformed during storage.

3. Printing

Overprinting failures are not common in a print, and they often occur in two or more prints. It is recommended to print the paper first in multiple printings. The conditional company can use the dryer, so that the printing accuracy can be better guaranteed.

4. Baking plate

If the baking time of the PS plate is too long or the temperature is not uniform during baking, the plate will become soft and the tensile strength will be reduced. After the machine is affected by the tension, it is easily deformed, resulting in failure of overprinting.

Material reason

The materials mentioned here mainly refer to paper. The main components of the paper are cellulose and hemicellulose, which contain a large amount of hydroxyl groups. With the porous structure of the paper, water can easily bind, adsorb, and infiltrate into the paper through chemical bonds. Therefore, the paper has strong water absorption capacity and because the fibers absorb water. After the rapid expansion will determine the paper's shape and size instability.

Bad paper

Analysis of reason: Offset printing is different from other printing methods. Water must be involved in the printing process. If paper with poor water resistance is used, or if the pulp is used, the mash contains a large amount of hemicellulose, and the hydroxyl content in the hemicellulose. Papers with a higher content than cellulose), due to their poor stability in shape and size, will undergo severe deformation during the printing process, resulting in failure of overprinting.

Solution: The most direct method is to replace the paper. When it is necessary to print with poor quality paper, the paper can be printed on the water before the formal printing, artificially increase the water content of the paper (commonly known as "run water"), so that the paper has a pre-deformation process, so that in formal The deformation of the paper during printing is much smaller.

2. Paper has tight edges or flounces

Solution: Paper humidity conditioning, so that the paper edge and the middle of the water content of the same, and try to make the paper moisture content slightly higher than the relative humidity of the workshop, it is recommended that the use of mandatory humidity control method.

3. Paper is electrostatically charged

Analysis of the causes: During the transportation process, the mutual friction between the papers or the change of its own volume can easily cause the paper to be electrostatically charged. Paper with static electricity usually causes difficulties in paper feeding, and overprinting cannot be guaranteed.

Solution. Appropriately increase the relative humidity of the workshop, generally the humidity should be maintained at 55% to 65% at the workshop temperature of 20°C, and try to ensure the workshop temperature. Constant humidity is also an effective means to prevent static electricity.

Mechanical reason

There are many reasons for overprinting due to mechanical reasons, but nothing more than the following two categories.
One is limited by the accuracy of the overlay that the machine itself can achieve. The allowable value of overprint error for imported printing presses in China is 0.01mm, and the allowable value for overprint error of domestic printing presses is 0.02mm. Once the overprinting error exceeds 0.07mm, it indicates that the performance of the machine itself is extremely poor, and it is impossible to print out accurate prints. . In the face of overprinting accuracy, the operator is generally powerless. Therefore, the size of the overlay error of the machine itself is the premise of printing the exact print.

One is improper operation in the production process, improper debugging of the machine, or overprint failure caused by the machine wear.