Faraday Cage Effect
One of the main application problems with powder coating is due to the ’Faraday Cage affect’. The Corona field from the gun is attracted to the lowest point of resistance which is the nearest extremities of the part being coated, not into internal recesses. Since the charged powder will follow the field lines, powder will not be attracted into the recesses of the part and it will build up rapidly on the extremities.
Also since the powder particles are all negatively charged, when they are in close proximity to each other they repel like trying to push two south poles of magnets together.
When coating parts that have narrow caps or tight corners the powder is not attracted into these areas due to the Faraday Cage effect. This is made worse by the velocity of the air pushing the powder out of the gun; this funnels into these areas and also pushes the powder away. To overcome this, the operator can reduce the charge output of the gun, the volume of powder and the velocity of the air at the gun nozzle.
The recommended spraying technique is to coat these problematic areas first, moving the gun closer to the surface than normal, then work out to the rest of the part.
Back ionisation can produce a bumpy, orange peel effect on the surface of the powder and is caused by the build-up of electrostatic charge inside the powder film on the surface of the part.
This excess charge can cause the formation of micro craters within the powder film, often called staring, which produce an orange peel affect when the power melts and cures in the oven. This is reduced by not trying to build up the powder film too thick.