Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 132 of 331

ELECTROSTATIC PROCESSES/EQUIPMENT The electrostatic application of atomized materials was developed to enhance finish quality and improve transfer efficiency. (See Fig.5.) Presently, there are seven types of electrostatic processes for spray application: Electrostatic air spray atomization Electrostatic high-volume, low-pressure (HVLP) atomization Electrostatic airless atomization Electrostatic air-assisted airless atomization Electrostatic electrical atomization Electrostatic rotary-type bell atomization Electrostatic rotary-type disk atomization Regardless of the electrostatic finishing systems, each has its advantages and lim- itations. What may be suitable for one situation may not be suitable in another. (See Table I.) Electrostatic Air Spray Atomization Electrostatic air spray uses an air cap with small precision openings that allows com- pressed air to be directed into the paint for optimum atomization. Electrostatic air spray is the most widely used type of atomization in the industry today due to its control and versatility. Electrostatic air spray provides very high transfer efficiency by utiliz- ing the electrostatic charge to wrap paint around edges and capture overspray that would have been unusable waste. Standard electrostatic air spray provides transfer ef- ficiencies in the 40 to 90% range depending on the type of material and application. Electrostatic HVLP Spray Atomization Electrostatic HVLP spray utilizes the same atomization characteristics as electrosta- tic air spray technology with slight modifications. When using air HVLP, the pressure of the compressed air at the air cap must be reduced to a range of 0.1 to 10 psi. Trans- fer efficiency is greater when using HVLP spray to lower the particle velocity and at- omize the material thus causing less waste and blow-by of material. Some electro- static equipment can be easily converted or transformed between air spray and HVLP spray technology by simply changing four parts. HVLP spray technology helps meet stringent EPA codes requiring reduced VOCs and waste. Electrostatic HVLP spray provides transfer efficiencies in the 60 to 90% range depending on the type of mate- rial and application. Electrostatic Airless Spray Atomization Electrostatic airless spray technology utilizes the principle of fluid at high pres- sures (500-5,000 psi) atomizing through a very small fluid nozzle orifice. Size and shape of the spray pattern along with fluid quality is controlled by the nozzle ori- fice. Airless spray technology evolved after air spray to aid in faster application rates using higher delivery and heavier viscosities on larger parts. Electrostatic Air-Assisted Airless Atomization Electrostatic air-assisted airless spray technology uses the airless spray principle to at- omize the fluid at reduced fluid pressure with assisted atomizing air to aid in re- ducing pattern tailing and affect pattern shape. Air-assisted airless spray technology offers some of the desirable characteristics of both airless spray and air spray. The de- sirable characteristics being medium to high delivery rates, ability to spray heavy vis- cosities at low velocities, and high transfer efficiency. 131

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