Q: How do I improve transfer efficiency in my spray operation?
powder coating spray application
A: There are several factors that impact transfer efficiency, including the spray equipment being used — both air and fluid pressure — as well as ambient temperature, humidity and moving air in the paint area. Additional factors include the actual shape and size of the paint target, and the spray finishing methods, including atomization and the individual or robots operating the spray applicator. Finally, the electrical conditions of the applicator, paint material, metal target and holding fixture/carrier is very important and impacts both the paint transfer efficiency and the paint quality of the component.
Common sense would tell you that a metal object (such as an auto chassis, bus chassis or vehicle frame) would be automatically grounded as it sits on a metal carrier of some sort as it travels through the paint line. But today’s chassis and frames are pretreated with an undercoat or primer that are extremely rugged, as they seek to extend the life of a vehicle by standing up to harsh environmental elements such as moisture, chemicals, road hazards, heat and cold, to name a few. This undercoat and primer is extremely thick and creates a solid shell around the chassis and frame. The thick shell not only provides solid protection but also provides a great insulator between the vehicle and metal frame, which, in turn, breaks the quality ground connection that once existed.
As a result of the interrupted ground, transfer efficiency measurements are looking more compromised than ever. With the cost of today’s paints, the thought of paint not landing on its intended target is not a welcome one. Unfortunately for the paint line manager, this is a good way to be in the spotlight for all the wrong reasons. It may sound simple, but all you have to do is reestablish a good solid ground like the one you used to have.
The proper grounding of electrostatic charges in the metal painting process is crucial not only for paint transfer efficiency but also for the minimization of part scrap and rework, which can add significant cost to the process. With this, more attention has been directed at optimizing paint process grounding and it’s been made a part of process improvement program objectives.
A target with an electrostatic charge causes paint to flow unevenly — or even repel paint — thus reducing the transfer efficiency and increasing the potential for part defects. If the electrostatic polar charge of the paint and metal component are the same, the two materials will repel. When this occurs, paint spray will be propelled unevenly around the target, creating uneven patterns and inconsistencies. A metallic base amplifies this effect. When an electrostatic grounding issue occurs during paint application, the painted metal component may need to be reworked, which is a costly and time-consuming event.
Each grounding method — charged paint/target, grounding clip assembly, and more — improves paint transfer efficiency to varying degrees and, thus, the quality and process consistency of the painted plastic components. The next question to asked is ‘Are the rework savings realized through effective electrostatic grounding being eroded by the costs to operate and maintain those systems?’ Costs to be considered include: