Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 127 of 331

application methods ELECTROSTATIC SPRAY PROCESSES BY JOEL RUPP, ERIC GUFFEY, AND GARY JACOBSEN TW RANSBURG ELECTROSTATIC SYSTEMS, TOLEDO, OHIO PRINCIPLES OF ELECTROSTATICS Electrostatic Theory Electrostatic finishing got its start in the early 1950s. Coatings engineers needed an application method that would significantly increase transfer efficiency and reduce finishing costs. They reasoned that particles and objects with like charges repel each other, and objects with unlike charges attract each other. The same would apply to charged spray coatings and a part to be painted. They discovered that by negatively charging the atomized paint particles and positively charging the workpiece to be coat- ed (or making it a neutral ground), an electrostatic field would be created that would pull paint particles to the workpiece. (See Fig.1.) With a typical electrostatic spray gun, a charging electrode is located at the tip of the atomizer. The electrode receives an electrical charge from a power supply. The paint is atomized as it exits past the electrode, and the paint particles become ionized (pick up additional electrons to become negatively charged) An electrostatic field is created between the electrode and the grounded work- piece. The negatively charged paint particles are attracted to the neutral ground. As the particles deposit on the work piece, the charge dissipates and returns to the pow- er supply through the ground, thus completing the electrical circuit. This process accounts for the high transfer efficiency. Most of the atomized coating will end up on the part. The degree to which electrostatic force influences the path of paint particles de- pends on how big they are, how fast they move, and other forces within the spray booth such as gravity and air currents. Large particles sprayed at high speeds have great momentum, reducing the influence of the electrostatic force. A particle's direction- al force inertia can be greater than the electrostatic field. Increased particle momen- tum can be advantageous when painting a complicated surface, because the mo- mentum can overcome the Faraday cage effect — the tendency for charged paint par- ticles to deposit only around the entrance of a cavity. (See Fig. 2.) On the other hand, small paint particles sprayed at low velocities have low mo- mentum, allowing the electrostatic force to take over and attract the paint onto the workpiece. This condition is acceptable for simple surfaces but is highly susceptible to Faraday cage problems. An electrostatic system should balance paint particle ve- locity and electrostatic voltage to optimize coating transfer efficiency. Electrostatic Advantages The main benefit offered by an electrostatic painting system is transfer efficiency. In certain applications electrostatic bells can achieve a high transfer efficiency exceed- ing 90%. This high efficiency translates into significant cost savings due to reduced overspray. A phenomenon of electrostatic finishing known as "wrap" causes some paint particles that go past this workpiece to be attracted to the back of the piece, further increasing transfer efficiency. Increased transfer efficiency also reduces VOC emissions and lowers hazardous waste disposal costs. Spray booth cleanup and maintenance are reduced. Coating Application Any material that can be atomized can accept an electrostatic charge. Low-, medium- 126

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