White spots (plates are not inked):

If the printing plate can not print graphics, we call it blind version. This may be due to corrosion of the graphic area of ​​the printing plate, or it may be due to the coating area being covered with a layer of hydrophilic rubber or other hydrophilic material. If the graphic area is worn away from the plate, we can simply call the plate wear, and the blind plate is created by the wear of the graphic area.

Blind version usually refers to the fact that there is no ink attached to the plate area. When the graphic area cannot be pasted by the ink roller as in the past, then blinding occurs. With the advancement of plate making technology, the phenomenon of blind printing has been greatly reduced, and the service life of printing plates has been correspondingly extended.

Graphic area wear: If some parts of the graphic area are worn away, this area will become inaccessible again. The blinding phenomenon caused by the wear of the printing plate is the worst case, because there is little room for salvage. It is usually caused by friction with a hard, improperly installed roller; it may also be caused by excessive pressure between the plate cylinder and the blanket cylinder; it may also be due to friction in the ink or paper. Particle friction caused. Sometimes the pigment particles on the paper surface are very hard and abrasive. Paper fibers also have a certain degree of abrasion. When newsprint is used for lithographic printing, a large amount of fiber is often accumulated on the blanket cylinder. This fiber accumulation (also called paper fluffing) causes the printing plate to wear out, which often results in the inability of ink on the graphic area.

Another cause of wear is the smearing of the paper. Sometimes the substrate will be sprayed with a color, and the second color will be printed on the printer. At this time, the dry particles of the first color will wear on the printing plate of the second printing press.

The expansion of the blanket causes its thickness to increase, which can cause an increase in the pressure between the printing plate and the blanket cylinder. Eventually it will lead to plate wear and blinding.

The adhesion of the graphic area to the metal substrate is unfavorable: sometimes a printing plate starts to print well, but there is no ink in the printing process. One of the reasons for this situation is that the adhesion of the graphic area to the metal substrate is not reliable. The coating layer hardened by light irradiation must be firmly bonded to the metal substrate; if painting is used, the painting substrate must be firmly bonded to the coating layer; the ink must be combined with the painting.

If the physicochemical properties of viscose are not good, it will also cause the image area to be partially inaccessible. Sometimes the printer must re-make a printing plate, make some changes in the production process or the use of raw materials to prevent the same problem from happening again.

The graphic area is partially hydrophilic: In the process of printing plate development, the lithographic printing plate is treated with a water-soluble glue solution. Gum arabic or other hydrophilic gums are also often used in this water-containing solution. Hydrophilic glue sometimes adheres to the graphic area of ​​the printing plate. In this case, this part of the graphic area will be accompanied by water rather than ink, the result is that the printing plate graphic part can not be inked, that is, this part of the image can not be printed. In the printing process, there are many situations that cause the hydrophilic adhesive to adhere to the graphic part and cause the printing plate to fail to ink. These situations include:

• The ink layer is too thin when printing. Too thin a layer of ink will make it very easy for a hydrophilic adhesive to penetrate the ink layer and combine with the underlying material. If you want to make the printing color lighter, it is best to add a transparent thick layer above the ink layer, so that the ink layer can be thickened during printing.

· There is too much water in the ink. If the ink contains too much water, the ink in the plate graphic area is easily replaced by the hydrophilic glue.

• The dampening solution contains too much hydrophilic glue. The more hydrophilic glue contained in the wetting solution, the more easily the hydrophilic glue emulsifies in the ink. This makes it easier for the hydrophilic glue to stick to the graphic area of ​​the printing plate.

• The dampening solution contains too much acidic material. The acid will make the hydrophilic glue more hydrophilic, so it will adhere to the plate more easily if there is a chance.

There is also a reason why the printing plate area can not be inked, that is, the problem of dry printing ink plate. If the ink is dry on the plate, this part of the plate is usually not easy to re-ink when printing again. Therefore, when the plate needs to be placed overnight or temporarily stored, it is best to wipe it with a treatment agent before turning off the printing press.

Ink Solvents and Plate Cleaners: Most coating and coating layers that harden on light on a printing plate are extremely resistant to printing inks or plate cleaners. Do not use plate cleaners on positive presensitized plates that have not been plated. If these plates have been exposed to alkaline plate cleaners, they may attack the graphic portion of the plate, resulting in a blind print.

The photosensitive material in UV inks is polar and most easily attacks the coating layer of some printing plates. Therefore, diazo pre-printed printing plates and sensitized printing plates cannot use UV inks, and many photosensitive resin printing plates use UV inks better.

Source: Bison