Improving Transfer Efficiencies in Coating Operations

Q: How do I improve transfer efficiency in my spray operation?

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:

Inventory — what are the associated costs to purchase individual grounding components and to monitor/maintain inventory levels?
Labor — what are the associated costs to build assemblies in-house, using your own staff, as opposed to buying a complete assembly?
Setup/Cleanup — what are the costs of setup and cleanup?
Effectiveness and Ease-Of-Use — is the grounding method reliable, repeatable, reusable and cleanable?

Purchasing a complete, fully assembled grounding solution is the lowest cost option, for several reasons:

Grounding assemblies can be customized to the exact requirements of your process; each is precisely made and consistent.
The cost of a finished assembly is less than the cost to purchase individual components and build yourself with in-house labor.

No scrap, broken or missing parts. Assemblies arrive ready-to-use.
Less administrative cost to order, stock, receive and count, so fewer SKUs.
A professional evaluation will help optimize your process.

Requirements for an electrostatic grounding clip used in painting processes are somewhat different than other industrial clips. The design goal of an electrostatic grounding clip is to balance functionality, durability and cost. Since electrostatic grounding clips are subjected to harsh environments (such as paint buildup, heat and cleaning solvents) these products need to reliably perform their function for their intended life and withstand cleaning, yet be inexpensive enough to discard and replace in an interval that serves the requirements of the process.

As previously discussed, paint transfer efficiency is important and has many variables. Electrostatic discharge through proper grounding is one of those variables. If properly controlled, the benefits will be realized in a more uniformly coated part and in reduced rework. It's not a question of whether or not to ground your process, but rather how will it be done and the total cost of that method.

There are several effective solutions. Some shops can purchase a standard, prefabricated assembly that is ready-to-use, and others may want to design and specify a custom assembly to perfectly meet the needs of a process, thus specifying each clip connector style and size, select wire type and length.

Another option is to add additional piece of mind with a measuring device that provides instantaneous feedback as to the quality of your ground connection. Although it may appear that a solid ground is established with your part lined up for painting, how do you really know? How many times have you sent parts through the paint booth only to see something less than optimal come out the other side? Chances are, the ground connection you thought you had really didn’t take place. Some devices can now check your ground before the part goes through the paint booth and can even check ground strength from small pieces to large objects like buses and trucks. They are very simple to use and the indicator usually provides immediate feedback to ground strength: green for a good ground, red for a poor ground and blue if no ground exists. Some devices have a siren that will sound if the ground is poor or nonexistent.

There are many methods for addressing electrostatic grounding in metal painting processes, and the selection of the best method for your process is a simple and worthwhile exercise that consumes little of your time, but provides a return in time and cost.

Tim Ulshafer is director of sales at Mueller Electric. Visit muellerelectric.com.