Die casting aluminum parts in all walks of life has a wide range of applications, the original surface of aluminum die castings are generally burrs, uneven appearance color, easy oxidation discoloration and other phenomena, so aluminum die casting parts in the assembly before application generally need to go through polishing, cleaning, passivation, painting (or powder) treatment, in order to improve the surface corrosion resistance and appearance decoration effect.
The most common surface treatment of die-cast aluminum is painting or powder spraying, but in the production process, there are often problems such as insufficient paint bonding force, paint peeling, foaming and so on.
What are the reasons for the above-mentioned insufficient bonding force, paint peeling, and blistering quality problems? How to solve and prevent it? The following mainly analyzes the surface chemical characteristics of die-casting aluminum, the pretreatment process, and the selection of pretreatment agents:
Chemical characteristics of die-cast aluminum surfaces
1, due to high temperature die casting process, die casting surface inevitably form a layer of oxide film.
2, aluminum die casting process will use release agent (oil), part of the release agent will have chlorinated paraffin.
3. The chemical activity of metal aluminum is high, and the surface is easily oxidized and discolored or moldy, especially in rain or wet weather.
Key Problems to be Solved in Pre-treatment Process of Die-casting Aluminum
1. Remove the release oil or wax components on the surface of die-casting aluminum.
2. Remove the natural oxide film on the surface of die-cast aluminum.
3. Through passivation treatment, a dense chemical film is formed on the surface of die-casting aluminum, and this layer of chemical film has sufficient binding force with paint.
4, in degreasing cleaning, film removal, passivation process can not cause corrosion or hanging ash on the die-casting aluminum substrate.
5. Pre-treatment process of die-casting aluminum:
Degreasing → water washing → activation (film removal) → water washing → chemical film → water washing
How to choose the right pre-treatment agent for die-casting aluminum
1. Selection of degreasing cleaning agent:
Die-cast aluminum is an amphoteric metal, which is easy to corrode with acid and alkali, resulting in ash and discoloration on the surface. Therefore, we must choose a special type of die-cast aluminum degreasing cleaning agent, which will not cause die-cast aluminum corrosion hanging ash.
2. Selection of activation (film removal) agent:
For the pre-treatment of die-casting aluminum, the activation (film removal) process is very important. If it is not properly selected, it is easy to cause discoloration and ash hanging. Once there is hanging ash, the paint adhesion will not be guaranteed.
3, passivation film can choose hexavalent chromium passivation, trivalent chromium passivation, environmental protection chromium-free passivation and other products and processes. From the perspective of environmental protection, it is recommended to choose environmentally friendly chromium-free passivation process.
