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Abrasive Material

Blasting is carried out with abrasive materials such as chilled iron grit, steel or aluminium oxide grit. Sand or other substances containing free silica must not be used, as anyone exposed to dust from it could develop silicosis.76

In a factory, blasting operations should be carried out in a suitable enclosure or room to protect other personnel from injury and nearby machinery from damage. The ‘blast room’ should be provided with an efficient system of exhaust ventilation, preferably of the down-draught type. During the blasting operation super abrasive material rebounds from the surface of the article with a high velocity. Consequently the operator must be given special protective clothing such as gloves, apron and leggings. A helmet supplied with fresh air at a positive pressure is also necessary to protect the blaster from both flying particles and harmful dust.82

Because of the friction between the finely divided particles of grit and the blasting hose and nozzle, discharges of static electricity occasionally take place. It is advisable to earth the blasting hose and nozzle.

The sand blasting technique is based on blasting an abrasive material in granular, powdered or other form through a nozzle at very high speed and pressure onto specific areas of the garment surface to give the desired abraded look. A straighter surface and less effect can be obtained with the sand blasting process than with the sanding process, and sand blasting can be done in less time. For this reason, it is more advantageous in terms of costs. However, silicon grains that are located in the sand can cause silicosis disease. The sand blasting process is now prohibited in most countries because of its negative effect on human health (Suglobal Tekstil, 2013; Paul and Naik, 1997a; Paul and Pardeshi, 2003).

Very hard surfaces can be studied by the abrasion of the surface with a sheet of abrasive material, such as silicon carbide or carborundum paper. At this point a number of different methods may be used to analyze the abraded material. In essence, any solid sampling technique that is capable of handling fine powders—KBr pellet, diffuse reflectance, ATR, photoacoustic, etc.—may be used to study the material. An interesting variant is to use diffuse reflectance to study the abrasive (see the reference to the silicon carbide method in Section 4) for the residual material.

Materials used as abrasives include both natural minerals and synthetic products. Abrasive materials can be considered as cutting tools with geometrically unspecified cutting edges that are characterized by high hardness, sharp edges, and good cutting ability. The sharpness of abrasive grains may be described in terms of edge radius and apex angle. As grain size increases, the percentage of sharp apex angles decreases, indicating a deterioration of grain cutting ability. In addition, cutting ability depends on specific features such as grain structure and cleavage, which are connected with the ability of cutting grains to regenerate new sharp cutting edges and points.

The choice of abrasive for a particular application may be based on durability tests involving impact strength, fatigue compression strength, dynamic friability, and resistance to spalling which occurs under the influence of single or cyclic thermal stress.

The abrasives industry is largely based on five abrasive materials; three are considered to be conventional abrasives, namely silicon carbide (SiC), aluminum oxide (alumina, Al2O3), and garnet. The other two, namely diamond and cubic boron nitride (CBN), are termed superabrasives.

A primary requirement of a good abrasive flow is that it should be very hard; but hardness is not the only requirement of an abrasive. The requirements of a good abrasive are discussed below. The decision to employ a particular abrasive will be based on various criteria relating to workpiece material, specified geometry, and removal conditions.

Cutting fluids should be used wherever possible in grinding to achieve high material removal rates coupled with low wear of the grinding wheel.

Mechanical polishing is done on a rotating disk covered with a felt, and is sprayed with a very fine abrasive materials (e.g., aluminum oxide or magnesium oxide). Because of, not creating grooves on the samples, brush or the felt that is employed here, should be thick and have no hard particles especially dust. At the entire period of polishing, it is necessary to smear the felt with suspended aluminum dust in water frequently. Grading of the aluminum oxide particles is different and we usually polish hard materials such as steel and cast iron with coarser aluminum powder rather than the soft metals like aluminum and lead. The alumina is in two forms: allotropic alpha (hexagonal) and gamma (cubic). The polishing properties of these two forms are different, so that alpha alumina acts faster in abrasion of metals and is more suitable for rough polishing, while gamma alumina prepares a polished surface with the high quality, so it is apt for the final polishing. Sometimes, it is utilized magnesium oxide for polishing aluminum and its alloys. This substance absorbs the carbonic gas of the air and produces carbonate. Therefore, aluminum oxide is usually used in the metallurgy laboratory. A disk with a velvet coating and diamond powder is being used to obtain a mostly polished surface and devoid of any grooves. Often, it is better to use an appropriate lubricant such as alcohol for synthetic diamond paste, to remaining free or rising up the power of cutting powder of diamond particles, and also to increase the life of velvet coating.

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