Metal Finishing Guide Book


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coating materials and application methods SPRAY APPLICATION PROCESSES BY JERRY P. HUND JPH FINISHING CONSULTANTS, WEST CHICAGO, ILL. The spray application of coatings to protect and decorate products has undergone considerable change since its inception some hundred years ago. The Industrial Revolution has given us an unlimited supply of new products to finish, along with an ever-increasing variety of coating materials to use on those parts to provide them with protection and decoration. Presently, we have four major processes of spray applications: (1) air atomization-conventional air spray and high-volume, low-pressure (HVLP) atomization, (2) airless atomization, (3) air-assisted airless atomization, and (4) electrostatic atomization, which can be combined with any of the three previously mentioned forms, or used with rotational atomizers. Regardless of the finishing system, all have their advantages as well as their limitations. What may be suitable for one situation may not be suitable in another. To select properly which system is best suited for your needs, a review of the advantages and limitations of each process is in order. Table I lists advantages and limitations for each process (see also Figs.1–3). CONVENTIONAL AIR SPRAY Conventional air spray (siphon/gravity/pressure feed) is the oldest system, coming into prominence during the 1920s with the growth of the automobile and furniture industries. It remains today as the finishing system most widely used by industry. Advantages Conventional air spray lays claim to two basic advantages over the other methods as follows. Control. This is the most controllable process available. The spray operator, when properly trained, can control the spray pattern from a fine dot to a large production-type spray pattern. This permits the spraying of small or large areas without changing guns or nozzles. Also, the degree of atomization can be controlled. This process provides the finest degree of atomization available in a hand-held system. Needless to say, it is the choice for those who want the best quality finish possible. Versatility. This process affords the operator the ability to spray the widest range of coating materials. It is also the easiest system to operate and maintain. Just the fact that the process has been around so long has resulted in a huge inventory of equipment, plus volumes of knowledge regarding application techniques. Disadvantages On the other hand, conventional air spray has a low level of transfer efficiency. Often, more material is wasted than is actually deposited on the part. This condition is usually aggravated by excessive pressure and poor operator technique. Air spray also consumes large amounts of compressed air (7–35 cfm at 100 psi). 168

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