What are the processes for powder spraying or powder coating of aluminum extrusions?

Two types of powder coating processes in aluminum extrusion parts:

High voltage electrostatic spraying and Tribo electrostatic spraying or coating in aluminum extrusion parts.

• High voltage electrostatic spraying:

  
  

High-voltage electrostatic powder spraying is a coating method in which powder coating particles are charged or polarized due to corona discharge and aerodynamic action of a certain electric field strength, and are adsorbed on the surface of the workpiece.

It is based on the physical principles of electrostatic induction, tip discharge, corona discharge and electric polarization. In high-voltage electrostatic spraying, high-voltage static electricity is supplied by an external device or a built-in high-voltage electrostatic generator.

The aluminum extrusion parts are grounded first when spraying. Under the action of purified compressed air, the powder coating enters the electrostatic powder spray gun from the powder feeder through the powder conveying pipe. The head of the powder spray gun is equipped with a metal ring or a pole needle as an electrode, and the edge of the end of the metal ring is sharp. When sprayed from the electrostatic spray gun head, the captured charge becomes a charged powder, which flies to the grounded workpiece under the action of airflow and electric field, and is adsorbed on the surface of the workpiece.

In the process of powder electrostatic spraying, the force on the powder can be divided into the powder's own weight, the thrust of the compressed air, the attractive force of the electrostatic field and the dipole force.

The powder overcomes its own gravity with the help of the thrust of the air, the attractive force of the electrostatic field and the dipole force, and is adsorbed on the surface of the aluminum extrusions, and then solidifies to form a solid film.

• From the perspective of electrostatic powder adsorption, it can be roughly divided into three stages:

In the first stage, the negatively charged powder flies to the workpiece along the direction of the electric force line in the electrostatic field, and the powder is uniformly adsorbed on the workpiece surface of the positive electrode.

In the second stage, the attraction of the workpiece to the powder is greater than the mutual repulsive force between the powders, so the powders are densely stacked to form a powder layer of a certain thickness. The negative charge is difficult to leak, and only a small part of it neutralizes the positive charge of the workpiece (the resulting current density is about 1μA), and most of the negative charge is stored in the deposited powder, making it difficult to match the aluminum extrusion. The electrostatic attraction is formed between them, so that the powder can be adsorbed on the surface of the aluminum extrusion for a long time.

In the third stage, with the continuous thickening of the powder deposition layer, the repulsive force of the powder layer to the flying powder particles continues to increase, when the attractive force of the aluminum extrusion to the powder is equal to the repulsion force of the powder layer to the powder , the powder that continues to fly will no longer be adsorbed by the aluminum extrusion, which is called the self-limiting effect of the powder.

This effect makes the powder layer on the easy-to-spray parts on the aluminum extrusions reach a certain thickness and does not increase, while the difficult-to-spray parts can continue to absorb powder, so that the aluminum extrusions can obtain a coating with a uniform thickness.

However, it should be pointed out that in the third stage, when the negative charges in the coating accumulate more and more, the electric field strength becomes larger and larger, until it rises to the strength where the air is broken down, and the phenomenon of back ionization occurs.

At this time, the coating on the aluminum extrusion is damaged, and after heating and curing, an uneven coating film will appear, forming defects such as orange peel, honeycomb and pinholes. Therefore, the process parameters should be strictly controlled to avoid such defects.

• Tribo electrostatic spraying:

Select the appropriate material for the gun body. When spraying, the powder is charged by friction with the inner wall of the gun and the inner wall of the powder conveying pipe under the push of compressed air, and the charged powder particles leave the gun body and fly to the aluminum extrusion.

When approaching the aluminum extrusion, due to the electric charge of the powder coating itself, a local electric field is generated and adsorbed on the surface of the aluminum extrusion with the opposite charge.

In tribostatic systems, the gun body usually uses an electronegative material. When two objects are rubbed, the material with the weaker negative electrode generates a positive charge, and the material with a stronger negative electrode generates a negative charge.

When spraying, due to the collision between powder particles and the friction between the powder and the gun body made of strong negative material, the powder particles are positively charged, while the inner wall of the gun is negatively charged, and this charge is introduced into the earth through the grounding cable. .

The positively charged powder particles fly to the aluminum extrusion part under the action of air flow and are adsorbed on the surface of the aluminum extrusion part. After curing, a film layer is formed, so as to achieve the purpose of powder coating.