Electrostatic spraying

One method of powder coating application is electrostatic spraying. Powder is applied with an electrostatic spray gun to a part that is at earth (or ground) potential.

Before the powder is sent to the gun it is fluidized to:

• separate the individual grains of powder and so improve the electrostatic charge that can be applied to the powder
• ensure that the powder flows more easily to the gun

Powder coating guns

There are at least three types of electrostatic guns in use:

• Corona charging guns where electric power is used to generate the electrostatic charge. Corona guns can be either internal or external charging.
  
• Tribo charging guns where the electrostatic charge is generated by friction between the powder and the gun barrel.
  
• "Bell" charging guns where the powder is charged by being "flung" from the perimeter of a "bell".

Not all powder is applied using guns. One system makes use of electrostatic tunnels.

Because, in this application process, the powder particles are electrostatically charged, they tend to wrap around to the back of the part as they move towards the air offtake system. This can improve coverage of the part. In addition by collecting any powder which completely misses the part, and recovering it (by filtration), material usage efficiency can be increased to around 95%.

The powder stays attached to the part as long as some of the electrostatic charge remains on the powder. To obtain the final tough, abrasion-resistant coating, the coated items are placed in an oven and heated to temperatures ranging from 160ºC to 210°C.

For a thermosetting powder there are four stages to full cure.

MELT > FLOW > GEL > CURE