A:Skills Requirements for Aluminum Sheet Substrates
The basic skill requirements for manufacturing aluminum sheet substrates are: the surface should be smooth and flat, free of scratches, dents, press marks, non-metallic inclusions, pores, or rolling marks. The thickness should be uniform, the waviness controlled within 1 millimeter, and the flatness less than 2I. The structure should be homogeneous and dense, without segregation or slag inclusion, and internal stress should be symmetrically and evenly distributed. The roughness Ra value should be less than 0.28 to ensure fine and uniform electrolytic grains.
B:Alkali Cleaning
Generally, aluminum sheets supplied by aluminum processing plants have a certain amount of oil on their surface. Oil and the oxide film affect the treatment of the sanded surface. In PS plate production, hot alkali washing of aluminum sheets is commonly used for surface treatment.
Alkali solution is generally used to remove the oxide film and oil. A commonly used degreasing solution formulation is: NaOH with corrosion inhibitors and sodium gluconate, and surfactants such as polyoxyethylene fatty acid ethers for solubilization, complexation, and other surface-active functions.
The temperature of the degreasing solution is generally controlled at around 55–60°C, and the degreasing effect is related to the temperature of the solution. If the temperature is too high, it can cause serious corrosion to the aluminum plate substrate, so it is necessary to control the temperature.
The degreasing time must be carefully determined. This is because it is related to different production lines. For a coil production line, it depends on the speed of the production line. For example, at 4 m/min, the degreasing time needs to reach 20 seconds to ensure its effectiveness.
It is necessary to pay attention to timely replacement of the degreasing solution, because many saponification products, emulsions, and aluminum salts are generated during the degreasing process. Their presence affects the degreasing effect and prevents the saponification and emulsification of oil on the surface of aluminum plates. When the degreasing effect is found to be weak, it is necessary to replace it promptly. Generally, if the solution is circulated using an alkali pump, the degreasing solution can last for a longer time, whereas without circulation, the replacement time is shortened.
A simple way to check the degreasing effect is to use water. On aluminum plates with good degreasing, water can evenly spread and disperse; if water forms beads on the surface after being poured, it indicates poor degreasing. The temperature of degreasing should be constantly monitored; if the temperature of the degreasing solution is too low, it may result in incomplete degreasing.
C:Acid Pickling
In the production process, after alkali washing, there is a nitric acid pickling step, the purpose of which is to remove the residues after alkali washing and neutralize the remaining alkali solution. The mass concentration of the nitric acid solution is 5%–10%. When preparing it, care should be taken to pour the nitric acid carefully into the water and mix thoroughly. During the pickling process, the concentration of the solution should be monitored frequently and replenished in time.
D:Roughening
Roughening is aimed at creating a sand-grain structure with the required roughness. Roughening can improve the aluminum plate substrate's affinity for water, increase the wetting of the plate surface, and prevent the plate from getting dirty; at the same time, the roughened surface can also enhance the adhesion of the plate to the photosensitive agent, thereby improving the printing plate's printing durability.
The coarsened sand grain is composed of countless peaks and valleys. Different sand grain structures have a significant impact on the adhesion of the photosensitive layer for water retention and can greatly affect its use. In production, the characteristic parameters of the sand grain rough cross-sectional profile curve, made using a surface roughness tester, are used to characterize it.
