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abrasive
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Blasting with a suitable media can remove dirt, mill scale, rust or previous coatings from a substrate, providing a surface profile that gives good coating adhesion. The blast media will vary dependent on the surface to be blasted and the quality requirements of the blasted product. Typically used media includes sand, steel shot, grit and glass bead. The media is delivered to the part surface at high velocity to impact the soils and cut them away from the metal surface. The blasting equipment used to deliver the media may be air-blast or turbine-blast. Hand held air-blast systems are very dependent on the concentration of the operator and quality may vary. Blast cabinets are often suction-feed systems that draw particles into the spray gun by induced vacuum and accelerate the media it with a metered stream of compressed air. There are also pressure-blast systems that use a pressurized vessel to deliver the media. Pressure systems are capable of higher nozzle velocity that can provide much faster cleaning of the surface than a suction system.
Blast cabinets' function similar to any booth designed for containment of oversprayed material. Negative pressure within the cabinet is maintained with a fan that draws air into the enclosure through a suitable filter. Typically, this exhaust system will use a cyclone separator to remove the dust and fine particles from the air stream and recover the media for reuse. The scrap material that is separated out of the air-stream is collected for disposal in a container attached to a dust collector. This scrap material should contain a small percentage of the heavier, reusable media to indicate that the fan pull is sufficient to prevent the build-up of fines in the recovered blast media. A vibratory screener can be added to the process to further refine the recovered material and maintain consistent particle size. Turbine-blast systems use high-speed turbine wheel with blades. The media is metered to the center of the wheel where it is fed onto the blades, which sling the particles at the surface being blasted. These systems are more energy efficient than air-blast systems because theydo not use compressed air for delivery. Abrasive blasting is most often used for preparation of metal surfaces of heavy structural parts, particularly HRS weldments. It is a very good way of removing the encrustations and carbonized oils that are characteristic of this type of product. Blasting operations can be manual or automated and they can be installed as part of a conveyorized powder coating system or as a batch process. The blasting device may be a nozzle type or a centrifugal wheel type. As previously stated, nozzle blast systems require compressed air for delivery of the media while a wheel system uses centrifugal force. Even though the compressed air is an added cost, it may be necessary to direct nozzles into hard to reach areas of a part. The blast area must be enclosed to contain the blast media and dust. In addition to cleaning, a blasted surface can create a very good anchor pattern for a coating. Different blast media can be used to vary the profile created on the metal surface. Less aggressive media will remove most soils without cutting too deeply in the metal and leaving a visible texture on the metal surface. More aggressive media can be used to cut stubborn encrustations, such as red oxides, but it will leave more texture on the surface. A blast system does not require as much space as a spray washer that uses chemical cleaning and it does not generate any wastewater. For these reasons, mechanical cleaning may be the only treatment required for finishes where initial paint adhesion is required. However, mechanical cleaning alone will not provide undercoat corrosion resistance or extend the life of the finished product. Blast cleaning standards depend on the quality requirements of the surface. Published documents clearly define quality grades of blastcleaned steel surfaces. Pictorial standards were originally developed by the Swedish Corrosion Committee and later adopted by the Steel Structures Painting Council (SSPC) and other organizations. The principal four grades of blasting endorsed by the SSPC are: . White Metal Blast: Removal of all visible rust, mill scale, paint, and foreign matter. Used for conditions where corrosion resistance is very important and the environment is highly corrosive. . Near White Metal Blast: Blast cleaning until at least 95% of all surface area is free of all visible residues. Used for harsh environments where product is exposed to heavy usage. . Commercial Blast: Blast cleaning until at least two-thirds of the surface is free of all visible residues. For applications where tightly adhering contaminants are allowable on the surface; for products with lower quality standards and non-corrosive environments. . Brush-off Cleaning: Blast cleaning of all except tightly adhering residues of mill scale, rust, and old coatings, exposing numerous evenly distributed flecks of underlying metal. Acceptable in noncorrosive environments where long-term coating life is not expected. Blast Media In selecting a specific media it is helpful to understand some of the materials used and how they compare. Blast media can be made of natural material such as silica, sand, mineral sand, flint, garnet, zircon, and other mineral products. It can be made of some natural byproducts such as walnut shell or corncob. And it can be manufactured of a variety of metal and non-metal compositions such as steel, iron, aluminum oxide, silicon carbide, plastic, wheat starch, and glass bead. In selecting a media, the comparative features that are the most important size of the product, how well it will cut, how well it will recycle and how much it cost. It is also important to know if there are any health and safety issues, such as lung problems associated with silica, and if the media will leave by-products on the surface, such as oils from walnut shells. Iit is a good idea to test different media to have a visual idea of the effect that they will have on the part. Hard grit media such as aluminum oxide will cut faster and deeper than soft, angular media such as plastic or agricultural grit. Mineral, ceramic, or metallic grit media are used in air-blast systems. Iron and steel media are more often used in turbine-blast equipment. Materials that are more prone to fracture, reflected in the chart by friability, are not good materials for recycling. Recirculation of thesematerials will produce wide variations in the surface condition.
Powder Coaters Manual
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June 14, 2012