Analysis of the causes of viscera on the PS plate from four aspects

In the offset printing production, the dirty on the PS plate is a frequently occurring problem. This article only briefly analyzes the structure features, printing process, printing process and printing materials of the PS plate.

PS version structure features

The ordinary PS plate is composed of an aluminum plate base, an aluminum oxide layer, and a photosensitive layer. Before the PS plate is coated with the photosensitive liquid, the aluminum plate should be pre-treated, including electrolytic sanding, anodizing, and sealing.

1. Electrolytic grit

The purpose of electrolysis is to form a sand mesh on a smooth aluminum plate so that the graphic part of the plate has a good adsorption base.

The part can be evenly wetted with moisture, thereby forming a closed water film layer.

Electrolyzed grit mostly uses HCI and HNO3 mixture as electrolyte. Practice has proved that fine, uniform and deep sand is conducive to improving the resolution of the printing plate, can reproduce the fine level of graphic and text, and establish a good adsorption base.

2. Anodizing

The purpose of anodizing is to generate AI2O3 film on the surface of the aluminum substrate to improve the printing plate resistance and the hydrophilicity of the non-graphical part. The thicker the oxide film, the stronger its wear resistance, but if the thickness of the oxide film increases, the elasticity of the film will decrease, and the rigidity will increase. This will make the film brittle, and it will be easily cracked in the high-speed printing process, resulting in dirty printing plates.

3. Sealing hole

After electrolytic treatment, there will be uniform and deep sand on the plate base. If direct sensitized adhesive is applied at this time, the plate will be absorbed firmly by the photosensitive adhesive. After exposure and development, it will not be completely detached, making the plate non-available. The graphic part is oleophilic, so it is easy to get dirty on printing. Therefore, sealing treatment is required to reduce the sensitivity of the sand. Sealing treatment mostly uses silicate solution to fill the pores on the aluminum base before coating the photosensitive liquid.

Factors affecting the printing process

1. exposure

The amount of exposure is the key to obtaining the right optical density on the PS plate. Excessive exposure will cause the original dots to shrink after printing and the dots will be lost. This will cause the lipophilicity and abrasion resistance of the dots in the graphic text. Reduce, not easy to show the gradient of the level; exposure is too small, easy to make the photopolymer resin light decomposition is not complete, dark tone, white paste, and even cause difficulty in development, causing non-graphic part of the dirty. The formula for calculating the exposure amount is:

E=I·t

During exposure, depending on printing requirements and product characteristics, the light intensity I and the exposure time t should be controlled so as to obtain a suitable exposure amount E.

2. development

When the developer concentration is too high or the temperature is too high, the development will be accelerated, and the photosensitive layer with no light will be dissolved, so that the dots on the printing plate will be reduced, the ink receptivity will be decreased, and the graphic text will be imaginary. phenomenon. If the developer concentration is too low or the temperature is too low, the development time will be prolonged and the development will be incomplete, resulting in a dirty print plate.

Therefore, in the development process, the developer should be formulated in accordance with the manufacturer's instructions for use with the PS version, and the development temperature should be controlled at 20 to 23°C.

3. Printing environment

Stain marks on the plate of the printer, dirt on the film, tape used for the imposition, and excessive rule lines that have not been removed will hinder the normal passage of light. If they are not found and repaired in time, the development will be incomplete and then printed. Dirty in the process.

At the same time, the PS plate was not properly preserved and was exposed to natural light. A slight photochemical reaction took place in the photosensitive layer, causing fog on the plate surface and incomplete development, resulting in non-graphical and oleophilic smudging.

To maintain a good environment for printing, use safety lights in the interior of the printing plate, and do a good job of saving the film and PS plates, which can effectively reduce the impact of environmental factors on the development quality.

4. Baking plate

In order to improve the printing plate's resistance to printing force, it is sometimes necessary to bake the plate. Before the baking plate, the dirt may not be completely removed or the baking liquid may be smeared on the plate. . Therefore, carefully check the layout before baking to ensure that there is no dirt. Baking temperature is 180 ~ 200 °C, baking time to 3 ~ 5 minutes is appropriate.

Factors in the printing process

1. Mechanical damage to the printing plate

In the high-speed printing process, due to the effect of the printing pressure, the non-graphical part of the printing plate is worn away, causing the surface energy to drop, the water storage volume to decrease, causing the non-graphic part to be oleophilic and dirty; the graphic part is due to wear, Lipophilic hydrophobicity will change in the opposite direction, emulsifying ink emulsification, resulting in floral version. Therefore, on the basis of ensuring a good reproduction of the graphic, a small printing pressure should be used as much as possible.

In addition, excessive printing pressure not only destroys the basic and non-graphic parts of the printing plate graphic part, but also reduces the printing plate's resistance to printing.

2. Mechanical wear

(1) Wear between printing rollers

The rubber roller lining is used improperly and the deformation is large. During the printing process, the linear speed difference on the surface of the three rollers is increased, which aggravates the wear of the printing plate.

Blanket cleaning is not done properly, paper lint, powder fall off or paper peeling and build-up on the blanket cause local bulging and abrasion of the printing plate. Improper patching of the blanket's bruises will also cause the plate to wear out.

(2) wear of water roller and ink roller

During the printing process, the abrasion of the surface of the printing plate due to the axial turbulence of the plate roller and the plate roller should be eliminated as much as possible, and the water roller and the fountain solution should be kept clean to minimize the stickiness of the water roller velvet. Ink. Otherwise, in the process of contact with the printing plate, the lipophilicity of the blank part of the printing plate will increase, causing the blank part to become dirty.

Impact of printed materials

1. Paper effect

If the printability of the paper is poor, such as a serious lint-free phenomenon, or impurities in the paper, will increase the wear on the printing plate.

The ideal pH value of printing paper should be neutral, but at present, our country mainly adopts alkali pulping, and uses NaOH as a cooking agent, so most papers are slightly alkaline. If the alkalinity is too large, it will neutralize the acidic dampening solution of the non-graphic part of the printing plate, weakening the hydrophilicity of the non-graphical part of the hydrophilic adhesive layer. If it is not timely supplemented, it will inevitably cause non-graphical part of the hydrophilicity. Sexual decline.

2. Effect of ink

The ink consists of pigments, binders and auxiliary materials. The dispersity of ink pigment particles for printing, that is, the requirement for the size of the pigment particles is relatively high. In general, the dispersion degree should be less than 1 μm. If the pigment particles are large, the wear of the printing plate will be aggravated during the ink transfer process, and the hydrophilic layer of the non-graphic part will be destroyed, resulting in dingling, and the pigment will also accumulate locally, causing a paste plate. If there is too much dry oil in the ink, the ink will dry too fast and it will also accumulate on the plate, causing a paste plate.

3. Effect of fountain solution

Offset printing is based on the hydrophilicity of the non-graphic part of the printing plate, and the graphics and text are partly oleophilic, and the ink transfer is achieved based on the principle of oil-water repulsion. The alumite film formed during the substrate treatment is an amphoteric oxide. If the dampening solution is acidic or alkaline, it will increase the corrosion of the plate sand and cause the plate to be dirty. Therefore, in the printing process, the pH of the fountain solution should be controlled between 5 and 6, and the amount of fountain solution should be controlled so that the content of the fountain solution in the emulsified ink is between 15% and 26% to ensure the ink balance.

The reason for the dirty on the PS plate is not a single one. It is the result of a combination of factors. Understanding the reasons that may lead to the scumming of the PS version will help users to find out the root cause of the problem through analysis and reasonably reduce and eliminate the problem.

Source: Printing Technology Jinan Hanzhang Printing Co., Ltd. (He Shiying)