Preliminary Processing of Aluminum Alloy Metal Plates

The Importance of Metal Surface Pretreatment in Screen Printing

Screen printing on metal surfaces necessitates a thorough pretreatment process, often referred to as prepress processing. This essential step ensures that the metal is adequately prepared for the application of printing inks, enhancing both adhesion and aesthetic appeal. Various methods can be employed for pretreating metal surfaces, including degreasing, polishing, drawing, oxidation, sandblasting, and spray painting. The choice of method depends on the specific requirements of the product and the condition of the base materials, allowing for tailored results.

Among the materials commonly used in screen printing, aluminum alloy plates are particularly prevalent. The pretreatment of aluminum alloy surfaces serves two primary purposes: enhancing the decorative quality by eliminating surface imperfections and improving the adhesion of the print coating through a series of preparatory processes. Below are some of the key methods used for pretreating aluminum alloy sheets:

1. Degreasing Treatment

Degreasing is a crucial preliminary step that involves removing any oils or contaminants from the surface of the aluminum plate. This is vital for ensuring that the surface has the necessary affinity for the printing inks. There are several approaches to degreasing, including using fine sawdust to wipe off encapsulant oils or employing organic solvents and alkaline solutions for oils from machining processes. Common formulations for alkaline degreasing solutions include:

• Simple Formula:

  • Sodium carbonate: 50-60g
  • Water: 1000ml
  • Solution temperature: 60-80°C
  • Cleaning time: 2-5 minutes

• Commonly Used Formula:

  • Sodium carbonate: 30g
  • Phosphorus: 50g
  • Sodium silicate: 10g
  • Water: 1000ml
  • Solution temperature: 60-80°C
  • Cleaning time: 2-5 minutes

2. Polishing

Polishing is employed to rectify surface defects and enhance the finish of metal materials. This process can be categorized into mechanical, chemical, and electrochemical polishing. Mechanical polishing involves using cloth wheel polishers and pastes to achieve a smooth surface. Chemical polishing typically utilizes acidic solutions, while electrochemical polishing provides a high-quality finish and reduces oxidation on the aluminum surface.

• Chemical Polishing Solution Example:
  • Phosphoric acid: 75% (w)
  • Nickel sulfate: 2g/1000ml
  • Sulfuric acid: 15% (w)
  • Nitric acid: 10%
  • Solution temperature: 100-115°C
  • Polishing time: 1-15 minutes

3. Drawing and Spiral Processing

Drawing techniques can create various decorative patterns on aluminum surfaces, including straight, random, ripple, and spiral designs. Straight-line drawing involves mechanically creating lines on the surface, while random patterns are produced using high-speed copper wire brushes. Ripple effects are achieved through specific brushing machines, and spiral patterns are formed using rotating polishing tools.

4. Sandblasting

Sandblasting is a method used to create a textured surface with a soft gloss finish. This technique not only enhances the decorative aspect but also improves the adhesion between the printed materials and the substrate. The process is performed in specialized sandblasting machines, where quartz sand is sprayed onto the aluminum surface to achieve the desired texture.

5. Oxidation Dyeing

Oxidation dyeing is a common application method for aluminum substrates, particularly in the production of signage. The choice of aluminum material is crucial; high-purity aluminum and certain alloys are preferred due to their compatibility with the oxidation process. Oxidation can be classified into chemical and electrochemical methods, each with specific formulations and conditions that affect the quality of the oxide film produced.

• Example of Sulfuric Acid Anodizing:
  • Sulfuric acid concentration: 150-200g/L
  • Anode current density: 0.8-1A/dm²
  • Operating temperature: 15-25°C
  • Time: 40-60 minutes
  • Voltage: 18-24V

These pretreatment processes are essential for ensuring that metal surfaces are adequately prepared for screen printing. Not only do they enhance the visual appeal of the final product, but they also significantly improve the durability and adhesion of the printed inks, leading to a higher quality end result. As the screen printing industry continues to evolve, understanding and implementing effective pretreatment methods will remain critical for achieving optimal results on metal surfaces.