A Blasting Challenge

Article From: Products Finishing,

Posted on: 4/1/1997

Users of abrasive blasting technology have a number of media and equipment choices. Is there one product that can do it all?

You may need to blast clean delicate aircraft parts, strip the paint off a 1957 Chevy or even prepare cookware surfaces for a Teflon® coating. You may think you need
different media for each job; but you do not.

Aircraft parts require a softer abrasive, such as wheat starch, because of the parts' delicate nature. The abrasive cannot become embedded in the part, warp it or otherwise damage the part.

Paint and rust can be stripped off the steel body panels of a 1957 Chevy using a coarser abrasive. However, the abrasive cannot be so large that it damages the substrate. Restorers must also be concerned with the heat that can be generated from the process. They need to choose a media that will not generate much heat, which could warp the auto body.

Abrasive blasting is used to finish, clean or prepare a variety of metals and plastics. Abrasives are formed into various shapes and graded into sizes by screening them through sieves. The numbers given to the grains' size approximates the mesh of the control screen. Abrasives are also graded as to hardness using the Mohs scale. Diamond is the hardest at 10; aluminum oxide rates an eight to nine and polishing/buffing rouge is around 5.5 to 6.5.

Recently, worker health and safety issues have become more important, particularly where abrasives are used to remove toxic paints and coatings. Not only does the process create dust containing toxic particles, but the end result forces companies to treat the abrasives along with the hazardous waste.

Common applications of coarse grains are blast cleaning and preparation of large steel structures. Finer grains are used in finishing applications such as paint stripping, cleaning and product finishing. Micro-grit grains are used within polishing and buffing compounds.

Fine abrasive blast materials used in industrial finishing operations vary. Traditional fine abrasives include glass beads and fused aluminum oxide in sizes smaller than 150 microns. Aluminum oxide is reclaimable and widely used in industrial cleaning and finishing. Since it is hard enough that it can become embedded in certain substrates, it may be blended with glass bead media to create a less aggressive composite material. Glass bead itself is a softer medium and applicable to a limited range of substrate hardnesses.

Plastic media are typically used in applications where damage to the substrate must be minimal. Plastic media are forced at high velocity through a nozzle at the painted surface. Because the beads are abrasive, the paint or grit is dislodged from the surface on impact. There are three types of operations, a blast cabinet, blast room or glove box. The hardness of the media, blast pressure, distance from the surface to be stripped and angle of approach must be selected to optimize the effectiveness of a plastic blast media operation.

Wheat starch can be used in plastic media blasting machines. It can also be used for aircraft maintenance. One potential problem is its high susceptibility to moisture. Once wet, wheat starch may no longer be effective for stripping even if dried.

Carbon dioxide pellets can also be used to clean and/or strip components. It does have some environmental and maintenance advantages. CO2 pellets remove paint by impact-flushing rather than abrasion. The pellet breaks apart into a shower of smaller dry-ice particles that mushroom out and create a lifting and shearing effect. Because the CO2 pellets sublime after impact, no media separation or disposal is required.

Crystalline ice blasting has also been proposed as a blasting process for removing coatings. Like the CO2 process, ice crystals remove the coating by fracturing rather than abrasion. The ice is crushed and graded to specific sizes and injected from a nozzle at 70 to 140 psi of air pressure at the surface. Water quantities generated total about 25 to 40 gph.

Despite the number of choices, none is perfect. However, Conversion Technologies, Hazlet, New Jersey, took up the challenge to create a loose-grain blast medium that was reasonably priced, reclaimable, environmentally acceptable and capable of preparing a range of substrates quickly and cleanly.

This was not an easy challenge. Reclaimability is a complex issue. Highly reclaimable steel grit granules tend to round off during use, slowing cleaning speed. Moderately reclaimable abrasives, such as aluminum oxide and garnet are also rounded upon impact. Also, this media can become embedded in the substrate. Non-reclaimable abrasives are brittle, dusty and clean slowly.

Conversion Technologies and scientists at Alfred University's Center for Advanced Ceramic Technology, Alfred, New York, went to work to develop an abrasive that was fast, durable, lightweight, reclaimable and environmentally friendly. The "ideal" formulation would be a resilient, durable aluminosilicate that displayed fracturing properties, which created fresh cutting edges and higher particle counts during continued blasting. The fracturing process would continue until the particles were small enough for capture by a dust collection system.

The development of Alumaglass™ took more than three years. During this time, various formulations were tested and evaluated. A Scientific Advisory Board was assembled to guide the company's research and development team through the final formulation and application testing and manufacturing planning stages.

The composition contains approximately 20 pct alumina and 54 pct silica, which provides the fracture characteristics. Other components include calcia and soda. This formula yields a highly efficient, low-density abrasive that achieves excellent cleaning speeds at low energy use.

It has an engineered hardness of Mohs 6-7. It comes in an array of grit sizes, so it can be used to finish steel, aluminum, bronze, brass, copper, stainless, fiberglass and plastic substrates.

The blocky angular shape of the medium allows it to fracture into sharp, effective granules that continue to blast effectively through repeated cycles. Depending on grit size and applications, multiple re-use cycles are possible.

The particles have a relatively low specific gravity of 2.7. Because of this, less energy is needed to generate compressed air for blasting. There are also more working particles per pound.

The product contains no free silica and tends to be low dusting, which creates a better blasting environment for worker health, visibility and performance. It is chemically inert, non-rusting, non-magnetic and non-flammable. The product, in its coarsest form, resembles broken green glass; in its finest form, talcum powder.

Alumaglass has been used in the aerospace industry, mold cleaning, powder coating preparation and removal and automotive industry for cleaning, preparation and finishing. It can be used in a dry blast cabinet, blast room or wet blasting setup. In dry blasting, media are collected using an air cyclone separator that keeps the working particles circulating within the blasting system.

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