Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 131 of 331

Fig. 4. Typical electrostatic hand gun application. not be charged electrostatically. Solvent-borne materials paint resistivity will vary from one material to another. When spraying solvent-borne coatings with electrostatics,it is critical to measure and monitor the resistivity of the paint being sprayed. Materials that are too conductive, (very low resistance,often referred as "hot") will also drain some or all of the electro- static voltage off to ground. This will greatly reduce the electrostatic effects on the paint particle. On the other hand, when using materials with a very high resistance, often referred to as "dead," the paint particles will not readily accept the electrostatic charge and the transfer efficiency will be very poor. Coating suppliers can easily formulate their solvent-borne materials tobe within a specific resistivity range. The optimum resistivity may differ depending on the tool used for application. For example, with an electrostatic disk or bell, the optimum resistivity range is between 0.05 and 1 megohms on a (Ransburg) paint resistivity meter. An electrostatic spray gun however, can effectively spray coating materials be- tween 0.1 to 00 megohms of resistance. Another example is the No. 2 Process on-site electrostatic spray gun.This gun re- quires a more precise paint resistivity because it relies solely on the electrostatic charge to atomize the coating materials. The paint used with this gun must read be- tween 0.1 to 1 megohms on the (Ransburg) paint test meter to work properly. Another key element in the electrostatic process or charging of paint particles is particle size. Large particles sprayed at high speed have greater momentum and reduce the influence of the electrostatic force.Increased particle size and momentum can be an advantage when coating complicated surfaces because the momentum can overcome the Faraday cage areas (where paint particles are attracted to the edges of a work piece while avoiding inside corners and recessed areas). On the other hand, small paint particles sprayed at low velocities have low mo- mentum, thus allowing the electrostatic force to take over and attract the coating ma- terial to the target object. This condition is acceptable for simple surfaces but is high- ly susceptible to Faraday cage problems. 130

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