Principle of electrostatic spraying

1. The difference from high-voltage electrostatic spraying

Tribostatic spraying does not require a high-voltage electrostatic generator used in high-voltage electrostatic spraying, and does not need to establish a high-voltage electrostatic field between the powder Spray Gun and the workpiece. The key is that the gun body of the powder Spray Gun should be made of strong negative material. When two materials with different negative charges rub against each other, the weak negative material will be positively charged, while the strong negative material will be negatively charged. When using powder coatings with weak electronegative film formers, during the spraying process, the collision between powder coating particles and the friction between the powder and the gun body made of strong electronegative materials will make the powder coatings positively charged. And the inner wall of the gun produces negative charge (this negative charge can be introduced into the earth through the grounding cable). Therefore, when a suitable strong electric negative material is selected as the gun body of the powder Spray Gun, the powder coating will rub against the inner wall of the gun and the inner wall of the powder delivery pipe when it moves in the gun body under the push of compressed air, and bring positive Electric charge, after the positively charged powder coating particles leave the gun body, they will fly to the grounded workpiece under the push of the air flow and be adsorbed on the surface of the workpiece. The loose coating film formed by adsorption will form a uniform, continuous and smooth coating film after curing treatment. It can be seen that, unlike high-voltage electrostatic spraying, the charge on the powder coating is not provided by the external electric field at this time, but is carried by the friction between the powder coating and the gun body.

2. Electronegativity of materials

The order of electronegativity of polymer materials from weak to strong is roughly: polyurethane < epoxy resin < polyamide < polyester < polyvinyl chloride < polypropylene < polyethylene < polytetrafluoroethylene. Most of the above-mentioned polymer materials can be used as film-formers in coatings, so charged particles can be formed when powder coatings made of them rub against gun bodies made of other polymer materials, and the relative distance between powder coatings and gun body materials The greater the difference in charge negative, the better the charging effect of the powder coating, on the contrary, the smaller the difference in relative charge negative, the worse the charging effect of the powder coating, so polytetrafluoroethylene is often used as the material of the friction Spray Gun body, and polyurethane is used. , Epoxy resin is used as the film former in the powder coating, the purpose is to obtain better charging effect.

3. Characteristics of the electric field formed by tribostatic spraying

The electric field intensity of the space electric field formed by the charged powder coating particles itself depends on the charge density of its space and the geometry of the electric field, that is, it depends on the charge amount of the powder particles, the proportion of the powder in the gas-powder mixture and the distance of the nozzle of the Spray Gun. Jet graphics. The air-powder mixture ejected from the nozzle of the Spray Gun, due to the diffusion effect of the air flow and the repulsion between the powders with the same charge, its volume gradually increases, so the charge density decreases, and the electric field strength gradually weakens. Moreover, the direction of electric field weakening is consistent with the direction of airflow movement, and the force direction of powder is the same as the direction of airflow movement. This is the characteristic of the space electric field formed by tribostatic spraying. Just because its electrostatic field is not formed by a high-voltage external electric field, there is no Faraday shielding phenomenon of lack of power lines in the concave part of the workpiece due to uneven distribution of power lines between the positive and negative poles of the electric field.

4. The process of friction electrostatic spraying

The principle of friction electrostatic spraying is shown in Figure 6-3. The powder coating in the powder supply barrel is suspended and dispersed by supplying compressed air to the fluidized bed, and then the negative pressure is generated by ventilating the Venturi pump, and the powder coating is sucked into the friction Spray Gun. During the friction process with the friction Spray Gun body The powder coating is positively charged and sprayed to the workpiece through the nozzle, absorbing on the surface of the workpiece and forming a coating film. Since there is no electric force line effect formed by the external electric field, powder coating particles will not accumulate on the outer edge of the sprayed workpiece and can enter the depressions and other difficult-to-coat parts of the workpiece. .

The driving force for the powder coating to move to the workpiece is mainly the thrust of the air. Under the action of the air thrust, the powder coating can reach various parts of the workpiece at different angles, and can adhere well to form a dense coating. As mentioned above, since there is no external electric field, tribostatic spraying can well overcome the Faraday shielding effect and can improve the primary powder feeding rate of the workpiece and reduce the amount of powder recovered.