June 10, 2022

Basics of abrasive cleaning (aka sandblasting)

Legend has it that sandblasting was first discovered by Benjamin Tilghman, an American inventor, who noticed the effect of windblown sand on windows in the desert when he was a general in the military. . At the time, sand was the most common abrasive used in this process, hence the name sandblasting. Over the past 50 years, additional materials have been adapted for the material cleaning process.

Today the terms blasting and abrasive blasting define the process more precisely, as the stripping material can include any number of products, such as coal slag, garnet, glass beads, shells nuts and corn cobs.

Jim Deardorff, owner of Superior Coatings, a sandblasting and painting company in Chillicothe, Missouri, stresses that sandblasting can be used on virtually any part of the tractor, given the right mix of media material, pressure air, volume and sandblasting nozzle. .

Here are some of the basics of component selection.

The compressor
“The air compressor is the most important component of the blasting process,” says Deardorff. “It provides the volume of air and pressure needed to move the abrasive media through the hose and blast nozzle with sufficient speed to remove scale, rust or aged coatings from the target surface. “

For cabinet blasting, 3 to 5 cubic feet per minute (cfm) may be enough, he says. For larger jobs, a range of 25 to 250 cfm may be required.

When choosing a crucible or blasting cabinet, there are two types to choose from: suction feed and pressure feed, says Deardorff.

Feeding systems
Suction feed systems work by siphoning abrasives directly into the sandblasting gun. This relies on supplying air from the compressor to the sandblasting gun to create a vacuum. When the gun is triggered, the abrasive is drawn into the sandblast gun feed line. The escaping air then carries the abrasive to the target surface.

“In contrast, pressurized supply systems store the abrasive in a container or pot,” he says. “The pot operates at a pressure equal to that of the material pipe. A control valve positioned at the bottom of the pot doses the abrasive in a high speed air flow. The air flow then carries the abrasive through the blast hose to the work surface.

The sandblasting nozzle is the device used to maximize the impact speed of the sandblasting abrasive. Although there are several types of nozzles, there are four common ones.

• Straight bore nozzle creates a tight pattern for spot cleaning or cabinet sanding. It is generally used for cleaning small parts.

• A venturi nozzle is the best choice for high output cleaning of large areas. It is important to note, however, that when blasting at high pressure (100 psi or more) the abrasives can reach speeds of over 500 mph.

• A double venturi blowing nozzle can be considered as two nozzles placed end to end. Air intake holes in the nozzle body allow air from the compressor to mix with atmospheric air. This venturi action increases the cfm and also increases the size of the explosion model. Deardorff notes that a dual venturi nozzle is the best choice for low pressure cleaning. This is because the suction action of the air intake holes has the ability to transport large amounts of heavy and dense abrasives through the material pipe at low pressure.

• A fan nozzle produces a fan pattern which is used for blowing large flat surfaces. The fan nozzle requires more cfm air volume to operate.

Deardorff says the nozzles are also available with a choice of coating materials, including aluminum, tungsten carbide, silicon carbide, and boron carbide. Of course, the choice depends on your budget and the rigors of the job. Just keep in mind that media consumption increases with the wear of the nozzles.

All about abrasives
Factors that affect abrasive performance are as follows.

• Resistance to dirt, corrosion or aged coatings to remove.

• Composition and sensitivity of the surface.

• The quality of cleaning required.

• The type of abrasive.

• Cost and costs of disposal.

• Recycling potential.

The abrasive is the part of any sandblasting process that actually does the cleaning job. There are four major classifications for abrasive materials.

• Natural abrasives include silica sand, mineral sands, garnet and specular hematite. These are considered consumable abrasives and are primarily used for exterior sandblasting.

• Man-made or fabricated abrasives, such as glass balls, aluminum oxide, silicon carbide, steel grit and plastic backings, are reusable and can be used in systems that allow recovery and the recycling.

• Abrasive by-products – such as coal slag, which is a by-product of coal-fired power plants – are considered the most widely used abrasive after silica sand.

• Non-metallic abrasives are generally classified as organic materials. These include glass marbles, plastic carriers, and types of kernels such as corn on the cob, wheat starch, pecan shells, coconut shells, and nut shells. Organic abrasives are used when minimal damage to the surface is required.

Shape and hardness
Other considerations when choosing an abrasive are physical form and hardness.

“The shape of the abrasive will determine the quality and speed of the sandblasting process,” Deardorff notes. “Angular, sharp, or irregularly shaped abrasives will clean faster and etch the target surface. Round or spherical abrasives will clean parts without removing excessive amounts of the base material.

Hardness, on the other hand, affects not only how quickly it cleans, but also the amount of dust produced and the rate of degradation, which also has a direct effect on recycling potential.

The hardness of an abrasive is classified by a Mohs index – the higher the number from 1 (talc) to 10 (diamond), the harder the product.

It is not uncommon to use some type of media mix. In fact, Deardorff developed one himself several years ago. Sold under the trade name Classic Blast, it is a special blend of aluminum oxide, ground black walnut shells and its own blend of other materials including garnet.

Using the product in a closed-top sandblasting jar, which uses a vacuum to suck the medium into the chamber, he says it can reduce pressure to as much as 35 pounds while cleaning fragile parts without damaging them. He often demonstrates the effectiveness of his sandblasting method by removing paint from an aluminum can still filled with liquid.

Since the nutshells in the mixture tend to polish the sheet metal as it helps remove paint, Deardorff says the stripped surface is also less prone to rust than if other types of media are used. The walnut shells also mitigate the impact of the more aggressive material in the mix.