Painting Q&A: Incorporating Electrostatics
How to incorporate electrostatics into an application process.
Q. What do I need to do different in my application process to use electrostatics?
A. Electrostatic applicators are available in a wide range of configurations for both manual and automatic applications. Manual applicators are typically handheld spray guns which use atomization technologies such as air spray, air assisted airless or airless technology. Automatic applicators can be machine or robotically mounted and the available technologies include air spray, air assisted airless, airless technology and rotary atomization. Regardless of the technology and method, to use electrostatic applicators, you will need to be more vigilant when it comes to earth grounding, maintenance and coating material formulation.
The fundamentals of the electrostatic process involve applying a negative electrical charge to the coating material as it is being atomized. The negatively charged particles are then attracted to a work piece which is at ground potential. The benefit is increased transfer efficiency (causing less waste). Electrostatics also “wrap,” or the attract the coating to edges and the back side of the workpiece, increasing efficiency in the application process.
To sustain the integrity of an electrostatic system, ensure that the applicator’s voltage is maintained and that the workpiece being sprayed is grounded. According to National Fire Protection Association, the resistance between the object being sprayed and true earth ground should be 1.0 mega ohms or less, providing suitable electrostatic attraction and permitting the charge from the particles to safely dissipate. This level of grounding applies to all objects in the spray booth and applies to non-electrostatic application.
In most finishing systems, the workpiece is hung from a conveyor using load bars and hooks. It is important to ensure that the contact points are clean so good ground contact can be made through the various attachment points. A meg-ohm meter (often called a “megger”) is used to test ground and typically outputs 500–1000 volts; this is required to overcome contact resistances through various attachments. A standard vom (volt ohm meter) may give inaccurate readings.
Most coating materials can be applied electrostatically. The main consideration for an electrostatic process is to monitor the resistances of the coating material. For manual applications, the recommended resistance is between 0.1 mega ohms–∞. For automatic applications, the recommended resistance is between 0.05 mega ohms–∞. The primary concern is that the material is not so conductive that the voltage applied at the applicator follows the fluid column to the grounded fluid supply, dissipating the charge to ground.
Coating materials with lower resistance can be applied electrostatically using items such as coiled fluid tubes and “short stopper” hoses. Waterborne coatings, which have no resistances, can also be sprayed electrostatically using an isolated fluid supply or a voltage block system.
Originally published in the October 2015 issue.
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