A. Skill Requirements for Aluminum Plate Substrates
The basic skill requirements for aluminum plate substrates are that the surface must be smooth and flat, without scratches, dents, impressions, non-metallic inclusions, pores, or rolling marks; the thickness must be uniform; waviness must be controlled within 1mm; and flatness must be less than 2I. The structure should be fine and uniform, without segregation or slag inclusions, and internal stress distribution should be symmetrical and uniform. The roughness Ra value should be less than 0.28 to ensure fine and uniform electrolytic sand grains. At present, many substrate materials used in China are 1050A, 1060, etc.
B . Alkali Cleaning
The aluminum sheets supplied by aluminum processing plants generally have a certain amount of oil on the surface. Oils and oxide films affect the treatment of the sanded surface, so hot alkali washing is commonly used to treat aluminum sheets in production.
Alkaline solutions are generally used to remove the oxide film and oil. A common formulation for degreasing solutions includes NaOH as the main component, along with corrosion inhibitors, sodium gluconate, and surfactants such as polyoxyethylene ether of fatty acids for solubilization and chelation.
The temperature of the degreasing solution is generally controlled around 55–60°C, and the degreasing effect is related to the solution's temperature. If the temperature is too high, it can cause severe corrosion of the aluminum sheet surface, so temperature control is necessary.
The degreasing time must be determined carefully through comparison, as it is related to different production lines. For a coil production line, it depends on the line speed. Taking 4 m/min as an example, the degreasing time should be about 20 seconds to ensure effective treatment.
It is important to replace the degreasing solution promptly, because a large amount of soap, emulsions, and aluminum salts are generated during the degreasing process. Their presence affects the degreasing effect and prevents the saponification and emulsification of surface oils on the aluminum sheet. When the degreasing effect is found to be weak, the solution must be replaced in time. Generally, if the solution is circulated using an alkali pump, it can last longer; without circulation, the replacement time is shorter.
A simple method to check the degreasing effect is using water. On a well-degreased aluminum sheet, water spreads evenly across the surface. If water droplets remain on the surface, it indicates poor degreasing. The temperature of the degreasing solution should be monitored regularly, as too low a temperature 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 water and mix thoroughly. During the pickling process, the concentration of the solution should be regularly monitored and supplemented in time.
D. Roughening
Roughening is intended to create a sand-grain structure with the required roughness. Roughening can improve the aluminum plate substrate's affinity for water, increase the substrate's wettability, and prevent dirt on the plate surface. At the same time, a roughened surface can enhance the adhesion between the plate and the photoresist, thereby improving the print durability of the plate.
The roughened sand grains consist of numerous peaks and valleys. Different sand-grain structures significantly affect the adhesion of water-retentive photosensitive layers and greatly impact the use of PS plates. The production usually uses a surface roughness tester to generate characteristic parameters from the cross-sectional profile curve of the sand-grain roughness to represent it.
