Abrasive media

The substrate has to be prepared for cementation. The surface must be clean and free of oxide. Corners and edges are particularly important in diffusion-type coatings sharp edges are usually detrimental. Barrel finishing, ie, tumbling in a barrel with abrasive media, may result in the desired shape. 

Abrasive media cleaning systems, replacing hazardous organic solvents for paint stripping (Davis et al., 1994)... 

The roughness of the abrasion media can vary with the hardness of the plastic. Usually, this is not a critical parameter except where decorative surfaces are important. 

Several sintered glass and glass-ceramic matrix composites obtained from recycled silicate waste have been reported in the literature [100-106]. These are dense or porous products with potential application as building, decoration or architectural materials, such as wall partition blocks, pavements, wall and floor tiles, thermal insulation, fire protection elements, roofing granules and acoustic tiles. Other possible uses include abrasive media for blasting and polishing applications. 

For more thorough flash removal, moderately abrasive material is added to the tmnhhng method described previously. This media deflashing allows both internal and extemsd flash to be removed. The parts to be deflashed are loaded into a sealed container (Fig. 9.16). Once the tumbling starts, an abrasive media material (ground walnuts, hard nylon, polycarbonate pellets, etc.) is directed at the parts vmder air pressing and at hi velocity. Modem deflashers recycle the blast media and use dust collectors to minimize air pollution. 

On suspension duties in abrasive media, the antidpated erosion problem places a limit on the impeller tip speed. In this case, the properties of the agitated medium and the materials of construction place limits on the general appUcabUity of the P/V = scale-up rules. 

A micToabrader can also be used to abrade the material away. The surrounding board areas must be protected from contamination by the abrasion media. 

General service, blow-down service, hquids, gases, steam, corrosives, abrasive media, slurries... 

Rubbing a flat-ended pin against a drum covered with bonded abrasive (sandpaper) is a standard test for metals (ASTM G 141), but it could be used for plastics. The ASTM test standard allows the option of rotation of the pins about their axis. The pins traverse the drum in a spiral path (Figure 3) and a reference pin material is run on the same abrasive in a path parallel to the test pin. Wear is expressed as a wear ratio. The use of a reference sample with every test sample mitigates differences in lots of abrasive media. This test simulates tribosystems with severe abrasion, for example, a plastic snow shovel. In use, it will be rubbed on concrete or simularly abrasive paving. 

Only heavy abrasives can be used in preparing steel surfaces for painting. Lighter abrasive media, such as apricot kernels, plastic particles, glass beads or particles, and walnut shells, are unsuitable for heavy steel constructions. Because of their low densities, they cannot provide the amounts of kinetic energy that must be expended upon the steel s surface to perform useful work. In order to be conunerdally feasible, an abrasive should be ... 

Escaping ice particles cause much less damage to nearby equipment than abrasive media. 

Tapscott et al. [7] and Jermyn and Wichner [8] have investigated the possibility of separating paint particles from a plastic abrasive by sieving. The plastic abrasive media presumably has vastly different mechanical properties than those of the old paint and, upon impact, is not pulverized in the same way as the coating to be... 

The boundary used in these studies was 250 microns material smaller than this was assumed to be hazardous waste (paint dust contaminated with heavy metals). The theory was fine, but the actual execution did not work so well. Photomicrographs showed that many extremely small particles, which the authors beheve to be old paint, adhered to large plastic abrasive particles. In this case, sieving failed due to adhesive forces between the small paint particles and the larger abrasive media particles. 

Webster, M. et al., Solidification/stabilization of used abrasive media for non-structural concrete using portland cement, NTTS Nr. Pb96-111125 (FHWA/TX-95-1315-2), U.S. Department of Commerce, National Technical Information Service, Springfield, 1994. 

Excellent surface preparations for porous surfaces include blasting with oil-free compressed air, abrasive media, or high pressure water (dry time is a must in this case) and grinding or wire brushing followed by oil-free dry compressed air blasting. For nonporous substrates, solvent wiping with clean cloth followed immediately by a clean cloth dry wipe is the preferred approach. 

Mechanical-based paint removal methods involve machineiy and/or manual labor. Two acceptable methods on exterior surfaces are contained water blasting (Hydrosander, 1990) and abrasive blasting with a vacuum arrangement (LTC, 1990 Vacublast, 1990 Zero, 1985). Table 11.3 lists examples ofvacuum abrasive media-cleaning units. Water blasting requires a containment system, which is very difficult to maintain. In addition, clean-up costs are significant, and water damage to interior structures is a possibility. 

Controlled Investigation of Vacuum-Abrasive Media Cleaning... 

A controlled investigation (Gooch, 1991) was conducted at Georgia Tech Research Institute to evaluate different vacuum-abrasive media cleaning on wood, aluminum, galvanized steel, and steel substrates. 

Table 11.6 describes the abrasive media and Table 11.7 gives the results of vacuum-abrasive media cleaning. The test patch size in each case was 3 <- 12 in., and at least 10 patches were tested by each method. 

Abrasive Media Consumption of Media (Ib/ff) Substrate Type Rate of Removal ff/hr Air (mglvc ) Lead Substrate Before/After (mg/em )... 

The consumption of abrasive media was evaluated, but it was soon learned that thousands of square feet of painted surface must be cleaned before determining an accurate mean value for each abrasive material. The averaged Ib/fl figure is listed for each material because no abrasive material was worn... 

The general safety of each vacuum-abrasive cleaning unit is reflected in the lead-monitoring data in Table 11.7. Lead levels near the operator were below the detection limit which means that the working environment is very safe for operators and other workers. With the exception of silica sand, none of the discussed abrasive media pose a potential health problem. Nevertheless, as a precaution, the operator of vacuum-abrasive cleaning equipment should use a dust-filtering (below 1 p.) mask that fits over the nose and mouth. 

The maximum distance from the vacuum cup and the substrate surface is eritieal, sinee it was demonstrated that abrasive media mixed with lead-based paint eould leak into the work area. More specifically, when the vacuum cup was suddenly withdrawn more than 2 in. from the substrate surface, particulate fell on the floor. This potential problem could be eliminated by installing a vacuum pressure switch near the cup so that when the cup is withdrawn more than 2 in. from the surface, air pressure supplying the flow of abrasive media could be shut off... 

A significant benefit of the composite method is a reduction of waste material by virtue of removing the bulk of the paint without having to use abrasive media for residual paint. This advantage reduces hazardous waste disposal costs by as much as 50%. 

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