To pass the powder from the powder application system first we need to fluidise the powder so that it can go through the application system and apply a uniform coating on the article. For this we need to pass moisture free clean air from powder containing hoppers. main methods which can be used to apply the powder after fluidized.
In spray application the powder transported from the fluidized hopper to the spray guns via tubes.
Overall application is controlled by the adjustment of air and powder.To control the fill thickness of coating this is one parameter to be used. The application of powder to the articles is based on the principal of attraction between objects with different charges. The charged air and powder particles from the gun are attracted to an earthed substrate. If the powder is not charged properly it will not adhere to the surface of the metal.
To charge the powder their is to methods
The most common application method is electrostatic spray which is done by Corona Spray Gun. in this gun up to 100 kv voltage is passed through an electrode t tip of the corona gun. Due to this air is ionized and an electric field is created between electrode and earthed object. Powder molecules flow to the earthed substrate where they are held in place by the electrostatic attraction. Film thickness is controlled by air pressure and powder concentration.
In tribo gun powder particles are charged by friction when they pass through a PTFE tube. The powder particles lose electrons during this process and become positively charged. The charged powder particles flow by air towards the earthed object. The charged powder cloud creates an electric field and an electrical attraction between the powder and the object. To achieve a desired coating thickness, the gun operator can adjust the volume of powder and air pressure of the gun.
Main difference between tribostatic and electrostatic guns is that tribostatic guns have lower electrical field strengths between the gun and the substrate in comparison to electrostatic.So they perform better when you coat a typical product.
The object is pre-heated to a temperature above the melting point of the powder in a fluidised bed process. Pre heated objects then entered in the cloud of powder which is just above the fluidized bed of powder. The powder melts and forms a coating onto the pre heated surface. After that object is removed from the fluid bed and passed through another oven to fully cure the powder. The film thickness is controlled by the preheating temperature of the object and time of entering and removing the object from a fluidized cloud of powder.
