Application Tips

Causes of Back Ionisation

Back Ionisation occurs when the substrate has reached the saturation point at which no additional powder can be deposited.  During the spraying process, negatively charged powder particles build up on the workpiece until the work has reached saturation.  After this point, the coating layer will repel further particles.  Additionally, election streams will discharge from the surface, disrupting the coating film and creating voids and craters.

Indications that the saturation point has been reached include:

• sprayed powder not being deposited and falling off the work;
• swirl patterns
• powder being attracted to other objects rather than the work.

Avoiding Back Ionisation

As back ionisation acts as a limiting factor for coating thickness, if the final film thickness is too low, that indicates that the electrical earth is insufficient.  Adjustments to alleviate back ionisation therefore are:

• ensuring the electical earth is adequate by ensuring that all connections between the part and ground are clean and intact;
• increasing the gun to part distance;
• reducing the kV at the gun nozzle by 10-20 kV
• reducing the percentage of reclaim in the hopper.