Offset printing in quality control of PS plate making

4 baking version and glue

Baking plate

Before the baking, remove the platen dirt completely to prevent the baking plate from being removed. The dirt should not be removed: the glue should not be excessively thick, so that it will not appear watery traces, which will cause difficulty in inking. At the same time, it will prevent foreign matter from remaining on the plate and be dirty: according to different Plate material requirements, adjust the baking temperature and time; baked plate should be naturally cooled, to avoid deformation of the plate base; secondary development to remove the plate protection adhesive to avoid ink emulsion or ink when printing. Roasting plates can increase the resistance to printing, but baking problems are often encountered in the actual work.

(1) The rubber track appears on the printing plate

The rubber track is often caused by improper protection of the wiping agent. The glue path left over due to the high concentration of the protective agent or uneven graze will harden on the printing plate after baking and will not dissolve in the developer. Ink absorption. In this case, a slightly thicker alkaline solution than the normal developer can be used again. In the baking plate, the high temperature causes the PS plate to be deformed and bent, and the plate is often scraped while the plate is being fed. Scratches are generated during printing. When the plate is printed, the dirt can be used to remove the dirt.

(2) Light red after baking

The light red color is due to a trace amount of photosensitive layer on the blank portion of the plate, which is changed after baking. If it is treated with developer, it will not get dirty and if it is processed, the light red color will remain on the layout. It must be dirty. This condition can be treated with a dirt remover.

(3) dark brown or dark brown after baking

Under normal circumstances, the photosensitive layer is reddish brown after baking. If the color is dark brown or dark brown, it indicates that the baking temperature is excessive or the time of the fine plate is too long. This version is over-baked version, any degree of photosensitive resin layer of carbonization, darkening and brittle, the result will reduce the PS version of the print resistance.

(4) Dark green after baking

This version of the color indicates that the baking temperature is low or the time is short, the photosensitive resin is not completely polymerized, and its abrasion resistance, corrosion resistance, solvent resistance, and ink affinity have not been maximized. At this time, the second baking version can be performed. To achieve the best results.

(5) Dirty after baking

The reasons for this are as follows: The dust, impurities and marks on the pre-baked plates are not completely cleaned in time. After these batters are baked, they will be firmly absorbed on the plate, causing the plates to become dirty.

2) gluing

Gluing is the last process of PS plate making. That is, a layer of gum arabic is coated on the surface of the printing plate to make the hydrophilicity of the non-graphical blank part more stable, and protect the plate surface from being stained.

3) Extracting ink

The application of developing ink and sizing is a graphic that applies the developed black ink to the printing plate, which can increase the adsorption of the ink to the graphic, and also facilitate the checking of the printing quality. If the ink is dirty from time to time, use turpentine oil to wash off the ink and re-extract the ink.

5 printing environment and machine maintenance

(1) The printing studio environment should be clean, especially the printing plate glass, must be wiped clean before working, and the developing machine should be regularly cleaned and maintained.

(2) Light source and illuminance should be checked regularly. Voltage, grid inspection, 380V (360-400V).

(3) The storage place of the PS plate should be installed in an air-cooled dark room or a place where the temperature is low and dry, especially avoiding the water tank. When using the PS version, be careful not to bend or scratch. Take care not to touch the surface with hands covered with oil or developer.

6 Printing

The PS version plays an important role in printing. The PS version should meet the following requirements: flat layout, uniform thickness, dimensional stability, non-deformable, no dirt on the surface, scratches and flaws, can firmly adhere to the photosensitive layer, has better The hydrophilicity and wear resistance should have high resistance to printing. In the printing, problems related to the PS version may arise due to problems such as printing machinery, technology, and materials.

1) Low print resistance

The reason why the printing plate is not resistant to printing is very complicated: there are operating factors such as too thick developer and excessive development time. There are also PS body quality problems, such as inappropriate photosensitizers. If the film-forming agent content is too low, the sensitivity falls and the print resistance is low. If the content of the photosensitive resin is too low, the wear resistance and alkali resistance of the photosensitive layer will be reduced, and such a PS plate has low resistance to printing. The production and drying of the PS plate play a decisive role in improving the printing plate's resistance to printing. When the printing process is operated in accordance with data, standardization and standardization, the production and drying of printing plates are particularly important. It strongly advocates the digitization, standardization, and standardization of drying, ensuring the quality of platemaking as much as possible, and improving the printing plate's resistance to printing.

2) The layout is non-graphically dirty

That is, the non-graphical part of the pro-inking problem, the main reasons are:

(1) The chemical composition of the fountain solution is not properly configured to erode the hydrophilic layer of the blank portion. Solution: Replace the dampening solution and use a dampening solution that is properly configured according to the manufacturer's instructions.

(2) The roller sleeve of the reversing water roller is dirty or worn, damaging the blank surface. The solution is to clean or replace the reversible water roller cover and protect the plate with wet cleaning.

(3) Excessive inking causes ink to expand when performing halftone printing, ultimately sensitizing non-graphic areas. Solution: Use thinner ink for printing.

(4) Abrasive sand on the blanket is retained and the printing plate is worn. Solution: Clean the blanket and replace it with a good paper: Add anti-stacking agent to the fountain solution; minimize the amount of water: reduce the pressure between the plate and the blanket, if there is an impression roller, then The pressure between the impression cylinder and the paper should be checked; if the build-up occurs mainly in the first printing unit, the amount of water should be increased to help remove debris from the paper.

(5) Aluminum base plate oxidation. The oxidized dirt looks like a lot of ring-shaped spots. It tends to occur in areas that have long been in contact with the moist, water-dependent roll. Solution: Follow the recommended plate handling procedure. Store plates in a dry place. During the plate making process, a cheesecloth is used to quickly dry the plate. In the printing process, such as parking, should rely on the printing roller to leave, open the empty car until the plate is dry.

(6) The water roller or the ink roller in the ink system works incorrectly, resulting in printing with a circumferential or axially dirty bar. Solution: Check and adjust the pressure, cleanliness, geometric accuracy, and overall condition of each roller in the ink system.

(7) Pre-exposure (grayed) occurs in the pre-print stage of the plate. Solution: Keep the plate in a light-proof box: Install appropriate security lighting at the platemaking site.

(8) The ink is too soft or too oily. Solution: Contact the ink manufacturer to adjust the ink formulation to suit the requirements.

(9) Problems with fountain solution. Excessive fountain solution will cause the imprint to be snow flakes; the ink color will appear solid in gray and the color will darken in the field; in the zoomed-up state, the unevenness in the field will be found, filled with tiny white spots. Excessive fountain solution emulsifies in the ink. When the ink film is split, the water droplets are exposed, and these water droplets hinder the transfer of the ink to the paper. Solution: Control dampening fluid, pay attention to reduce the amount of water supply.

3) Partial wear on the PS plate

The main reason is that there are too many resins in the fountain solution and too much acid in the fountain solution. Solution: Reconfigure the fountain solution and check the pH.

4) There is white fiber on the plate

This is mainly due to the fact that the paper fluff is too much or that the roll fabric is peeled off. Solution: Clean the plate.

5) The graphic part cannot be properly inked

The main reason is that there is a cured resin or correction liquid in the printing plate graphic area, so only the hydrophilic is not ink: In addition, there is a short or water-immersed ink on the blanket cylinder, printing plate and blanket. . Solution: Wipe the plate with a damp cloth to reduce the amount of water used to protect the minimum amount of water needed to clean the non-graphical parts.

6) Graphic loss

The reason is that the pressure between the rubber roller and the plate cylinder is too great. Solution: Check and adjust the appropriate printing pressure.

7) The ink on the plate after baking is too slow

This is mainly caused by too thick a thermosetting layer. Solution: Wash with phosphoric acid solution or wash with plate cleaner.

8) Scratches on the plate

The reason is that the outer layer of the paper roll before the machine has a small sand, or small sand grains embedded in the surface of the printing roller. The solution: The paper roll on the machine will be divided again to remove two or three layers of paper and clean the rubber roller.

9) Cracked plate

The PS version may have cracks along the bend line at the end of the plate or the end of the plate. The main reason may be that the plate is not installed in the correct size. Does not evenly cling to the drum body. This may be due to inaccurately bending the plate, the plate material moving on the plate bender, or the platen being very worn; in addition, the platen plate is not cleanly sewn, and the residual ink makes the plate not mounted properly, the platen or plate Severe stress. (Text / Qi Cheng)