A : Technical Requirements for Aluminum Substrate
The basic technical requirements for aluminum substrate: smooth and flat surface, free of scratches, bumps, indentations, non-metallic inclusions, pores, rolling marks and other surface defects; uniform thickness, waviness controlled within 1 mm, flatness less than 2I. The microstructure is fine and uniform without segregation or inclusions, with symmetric and even internal stress distribution. The roughness Ra value is less than 0.28 to guarantee fine and uniform electrolytic graining.
B : Alkali Cleaning
Generally, aluminum plates supplied by aluminum mills carry a certain amount of oil contaminants and oxide film on the surface, which adversely affects the graining process. In production, hot alkali cleaning is widely adopted for surface pretreatment of aluminum plates.
Alkaline solution is mainly used to remove the oxide film and oil contaminants. The conventional degreasing solution is formulated with sodium hydroxide (NaOH), compounded with corrosion inhibitors, sodium gluconate, and fatty acid polyoxyethylene ether surfactants to realize solubilization, complexation and emulsification.
The temperature of the degreasing solution is generally controlled at 55～60℃, and the degreasing efficiency is highly dependent on the solution temperature. Excessively high temperature will cause severe over-corrosion of the aluminum substrate, so strict temperature control is mandatory during production.
The degreasing time shall be determined through comparative tests according to the working conditions of different production lines. For coil production lines operating at a speed of 4m/min, the effective degreasing time must reach 20 seconds to ensure a qualified degreasing effect.
The degreasing solution needs regular and timely replacement. A large number of saponified substances, emulsified substances and aluminum salts will be generated during degreasing, which will reduce degreasing efficiency and hinder the saponification and emulsification of surface oil contaminants. The service life of the degreasing solution can be prolonged with an alkali pump circulation system, while the replacement cycle will be shortened without circulation.
The water film test is a simple and effective method to inspect the degreasing quality. A qualified degreased aluminum plate can form a uniform and continuous water film on the surface. If water condenses into droplets without spreading evenly, the degreasing effect is substandard. Continuous temperature monitoring is required in daily production; insufficient solution temperature will lead to incomplete degreasing and affect the quality of subsequent processes.
C : Pickling
In the production process, a nitric acid pickling step follows alkali cleaning to remove residues and neutralize leftover alkaline liquid. The mass concentration of nitric acid solution is 5%～10%. When preparing the solution, carefully pour nitric acid into water and mix thoroughly. The solution concentration shall be analyzed regularly during pickling, and make-up solution shall be added timely.
D : Graining
Graining is performed to form a grained structure with required roughness. Graining can improve the water affinity and wettability of the aluminum substrate surface to prevent plate scumming. Meanwhile, the grained surface improves adhesion between the printing plate and photosensitive agent, so as to increase the plate run length.
The grained surface consists of numerous peaks and valleys. Different graining structures exert great influence on water retention and adhesion of the photosensitive layer, which strongly affects plate performance. In production, characteristic parameters of the graining cross-section profile measured by surface roughness tester are used for characterization.
