Paper mill experts help you analyze the problem of paper printing powder

Author: Bai Tao

Printing off powder. It is during printing, due to the external force, the paper surface paint, fibers or dust are pulled up by the blanket, or the impurities in the ink path are transferred to the blanket, resulting in the partial transfer of the graphics to the paper, which seriously affects the quality of the finished product. A printing phenomenon.

In offset printing, printing powder loss is one of the few quality problems that are easy to occur, which has caused greater problems for printing plants and paper mills. For this reason, printing plants and paper mills have how to avoid the occurrence of powder loss It took a lot of effort, but it could not be completely eliminated from the paper and printing aspects.

What should I do if powder falling occurs? From the perspective of the paper mill, we have taken many measures in the production process to avoid the occurrence of powder loss, such as improving the surface strength of the paper and increasing the dust suction device of the rewinding machine. Increasing the frequency of cutter replacement has achieved relatively good results.

From the perspective of the printing house, how to adjust the printing process and conditions to reduce and avoid the problem of powder loss for paper that has already occurred? Through our years of experience in handling customer complaints and communicating results with customers, respectively Aiming at the problem of powder dropping in different states, we summarized the causes and how to take remedial measures from the printing process after the powder dropped. I hope to share it with the printing plant. Please correct me if there is a fallacy.

First define two nouns: original point and transfer point (as shown in Figure 1). The original dots are the spots on the first printed sheet where the powdering problem occurs, and there is no ink in the middle; and the transfer dots are the spots generated on the subsequent prints, which have an irregular ring shape with ink in the middle.

Figure 1

Catheter section

The duct joint is a non-fibrous substance that cannot be completely removed from the pulp. It is relatively brittle and hard, and has a low binding force with fibers and paint. It is easier to be pulled up when subjected to external forces. His appearance is more regular, showing an irregular rectangle or square (as shown in Figure 2), about 1mm in size.

Figure II

During printing, if it is found that the powder form on the printed matter belongs to the duct section, the following methods can be adopted, which may be improved:

● Reduce the corresponding ink viscosity;
● Reduce printing speed;
● Increase the cleaning blanket frequency;
● Replace a new blanket;
● Reduce the printing pressure of the pink color sequence and increase the printing pressure of the previous color sequence;
● First print the original paper with varnish, stick the catheter section off, and then print normally.

2. Dedusting

Dedusting is due to the coating between the paper surface. The bonding force between the coating and the fiber is insufficient, a powder falling phenomenon that occurs when the coating is pulled up during printing. Its state is irregular (as shown in Figure 3).

Figure three

The reason for this kind of dusting may be related to insufficient surface strength from the perspective of paper; from the perspective of printing, it may be related to factors such as excessive ink viscosity, excessive printing pressure, and excessive printing speed.

When the powder falling phenomenon such as de-powdering occurs, the method of treating the powder falling of the catheter section type as described above may be adopted, and there may be a certain degree of improvement.

3. Paper wool paper powder

If paper wool paper powder adheres to the surface of the paper and adheres to the blanket during printing, it will also cause the image and text to not be transferred to the surface of the paper normally, forming spots, and its shape is irregular and fibrous (Figure 4).

Figure 4

The main reason for the generation of paper wool and paper powder is that the cutting knife is relatively blunt. When cutting, the fiber is not cut but pulled off, and at the same time, some fibers are pulled out and fall onto the paper surface. Under normal circumstances, this type of dusting occurs mostly on the four sides of the printed matter, and the slightest part is the most serious. To determine whether there is paper dust on the paper, you can use a dry black cloth to wipe it on the surface of the paper, and then see if there is white powder on it. If there is, it means that there is paper dust on the paper, otherwise it is not.

It is found that there is paper dust paper powder on the paper, you can take an empty print on the monochrome machine first, stick the paper dust paper powder, and then print normally, there will be a great improvement. If it is a printing factory's own cutting, it must shorten the sharpening cycle to avoid the generation of paper dust.

4. Mopi

If the ink is dry and skinned. There is foreign matter in the ink or the water system is contaminated to generate foreign matter. During the printing process, it adheres to the printing plate or blanket, and it will also cause spots on the printed matter. The original dots cannot be found on this kind of powdered printed matter, the paper coating has not been damaged, and the spots are all in the shape of transfer points. For such powder falling, the ink path and water path must be thoroughly cleaned, including the ink tank water tank, the ink transfer roller group and the water transfer roller group. Plates and blankets, and then replace with new ink and fountain solution.

For the above types of powder falling, if it is not possible to accurately determine which type is based on the shape of the spots on the printed product, you can also use transparent tape to stick the foreign objects on the blanket and observe the shape and composition under the microscope Identify the specific cause, and take corresponding countermeasures for different types to reduce the occurrence of powder loss and minimize the loss.

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