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Abrasive Blasting Media Guide

Abrasive Blasting Media Guide

Abrasive blasting uses mechanical energy to hurl particles at high speeds against metallic and non-metallic surfaces, removing paints and other organic coatings.

Air Pressure Equipment used for abrasive blasting

The common methods for the abrasive blasting process include air pressure and water pressure. These two methods simply use the force of compressed water or air to expel the media at the surface.

Water Pressure Equipment used for Abrasive blasting

The centrifugal wheel method is a less common technique. With this method centrifugal and inertial forces are used to give the media speed and energy.

Abrasive selection is key to a successful coating job. The surface effects produced with various abrasives can range from deep cutting to gentle scouring of the surface. Important factors to consider in selecting an abrasive include

  • Type of surface to be cleaned
  • Shape of the structure
  • Type of material to be removed
  • Profile, breakdown rate of the abrasive
  • Hazards associated with the use of the abrasive
  • Potential damage to equipment located in the repair area

Abrasives commonly used for stripping include steel grit, aluminum oxide, garnet, and glass beads. Steel grit creates a rough surface profile on the substrate that aids coating adhesion. Because it is so hard and durable, steel grit can be reused, and it generates the least amount of waste per unit of surface area stripped. To maximize the reuse of steel grit, companies must keep the blast media dry to avoid rusting. Aluminum oxides are considered to be a multipurpose material that is less aggressive and less durable than steel grit, and it results in a smoother surface profile and less removal of substrate material. Garnet and glass beads are the least aggressive abrasive and often are used in a single-pass operation (i.e., the abrasive is not recycled). Use of garnet and glass beads is most suitable for preparation of soft materials that are easily damaged, and for maintenance of the dimensional tolerance of the part.

Moh’s Hardness Scale for Abrasive Media
Walnut Shells 2.5-4.5
Clear-Cut 2.0-2.5
Polyester Type I 3
Urea Type II 5
Melamine Type III 4
Glass Beads 5.0-6.5
Silica Sand (Quartz) 8
Garnet 10
Zirconia 11
Aluminum Oxide 12
Silicon Carbide 13

Companies can use abrasive blasting to remove paint and corrosion products from larger metal structures in the field (field stripping) or from smaller metal structures in a hanger, booth, or blasting cabinet.

Outdoor blasting can be performed in an open area. Operators must wear self-contained breathing equipment in order to be protected from the stripping dust. After blasting, the used abrasive can be shoveled or vacuumed from the area and processed through the reclaimer. Some systems combine dust control and abrasive recovery by including a vacuum collection pickup device with the blasting nozzle. Abrasive blasting in cabinets is often performed using manual blast cabinets and automated blasting chambers to remove paint from parts. The abrasive is fed into the cabinet or chamber and directed against the part being stripped. Used abrasive and removed paint are then pneumatically conveyed to a reclaimer. Reusable abrasive is separated from the waste and fines (broken-down abrasives and paint chips) are collected in a dust collector.

Plastic Media Blasting

Plastic media blasting (PMB) is an abrasive blasting process designed to replace chemical paint-stripping operations and conventional sand blasting. This process uses soft, angular plastic particles as the blasting medium. PMB is performed in ventilated enclosures such as small cabinets (a glove box), a walk-in booth, a large room, or airplane hangers. The PMB process blasts the plastic media at a much lower pressure (less than 40 psi) than conventional blasting. PMB is well suited for stripping paints, because the low pressure and relatively soft plastic medium have a minimal effect on the surfaces beneath the paint.

Plastic media are manufactured in 6 types and a variety of sizes and hardness. Military specifications (MIL-P-85891) have been developed for plastic media. The specifications provide general information on the types and characteristics of plastic media. The plastic media types are

  • Type I Polyester (Thermoset)
  • Type II Urea formaldehyde (Thermoset)
  • Type III Melamine formaldehyde (Thermoset)
  • Type IV Phenol formaldehyde/Clear Cut (Thermoset)
  • Type V Acrylic (Thermoplastic)
  • Type VI Polyallyl diglycol carbonate (Thermoset)

Facilities typically use a single type of plastic media for all of their PMB work. The majority of DOD PMB facilities use either Type II or Type V media. Type V media is not as hard as Type II media and is gentler on substrates. Type V media is more commonly used on aircraft.

After blasting, the PMB media is passed through a reclamation system that consists of a cyclone centrifuge, a dual adjustable air wash, multiple vibrating classifier screen decks, and a magnetic separator. In addition, some manufacturers provide dense particle separators as a reclamation system. The denser particles, such as paint chips, are separated from the reusable blast media, and the reusable media is returned to the blast pot. Typically, media can be recycled 10 to 12 times before becoming too small to remove paint effectively. Waste material consists of blasting media and paint chips. The waste material may be classified as a RCRA hazardous waste because of the presence of certain metals (primarily lead and chrome from paint pigments). An alternative solution to handling the potential hazardous waste is to recycle the media to recapture the metals. Reusing the plastic blasting media greatly reduces the volume of spent media generated as compared to that generated in sand blasting. When compared to chemical paint stripping, this technology eliminates the generation of waste solvent. PMB is also cheaper and quicker than chemical stripping. The U.S. Air Force and airlines have found PMB effective for field stripping of aircrafts, but PMB could also be used to strip vehicles, ships, and engine parts.

As with any blasting operations, airborne dust is a safety and health concern with PMB. Proper precautions should be taken to ensure that personnel do not inhale dust and particulate matter. Additional protective measures should be taken when stripping lead chromate- or zinc chromate-based paints, as these compounds may be hazardous. Inhalation of lead and zinc compounds can irritate the respiratory tract, and other paint compounds are known to be carcinogenic. Inhalation of paint solvents can irritate the lungs and mucous membranes. Prolonged exposure can affect respiration and the central nervous system. Operators must wear continuous-flow airline respirators when blasting operations are in progress in accordance with OSHA requirements. PMB systems can range in cost from $7,000 for a small portable unit to $1,400,000 for a major facility for aircraft stripping.

Black Beauty Abrasives

Black beauty abrasives also known as coal slag, black diamond, black blast and boiler slag is an inexpensive media. It is one of the safer forms of abrasive media, containing less than 1% or no silica. Black beauty media also produces little dust, however, may release hazardous air pollutants (HAP) into the surrounding air.

Black Beauty abrasives are made from crushed liquid coal slag from utility boilers. The abrasive contains iron (Fe), Aluminum (Al), Magnesium (Mg) and Calcium (Ca). The media comes in sharp angular grains ranging in many sizes including coarse, medium, fine and extra fine grained. The coarser grains can be used to remove heavy rust and provide a high degree of profile, good for coating attachment and bonding. The finer grains can be used for cleaning surfaces and for smoothing surfaces. The media is used on many applications including steel, buildings, railroads and bridges. Coal slag is a fast-cutting media with a hardness of 6-7 on the Moh’s hardness scale. The media is non-recyclable.

Copper Slag Abrasives

Copper slag abrasives are used with water jetting equipment to produce a fast cutting removal of surface contaminants. These abrasives provide a good surface profile when used with coarser grains. The finer grained copper slags will provide removal of lighter rust and mill scale. The material is used applications including steel, offshore oil rigs, power plants and tanks.

Copper slag abrasive material

These abrasives are formed from the smelting process using the by-product, iron silicate. The media comes in cubical form with sharp edges, ranging 6-7 on the Moh’s scale. Copper slag produces low dust and contains less than .1% silica.

Sponge Blasting

Sponge blasting systems are a class of abrasive blasting that uses (1) grit-impregnated foam and (2) nonabrasive blasting media using foam without grit. These systems incorporate various grades of water-based urethane-foam cleaning media. Firms use the nonabrasive media grades to clean delicate substrates. The abrasive media grades are used to remove surface contaminants, paints, protective coatings, and rust from a variety of surfaces. In addition, the abrasive grades can be used to roughen concrete and metallic surfaces. A variety of grit types are used in abrasive media including aluminum oxide, steel, plastic, or garnet.

Sponge Media

The foam-cleaning medium is absorptive and can be used either dry or wet with various cleaning agents and surfactants to capture, absorb, and remove a variety of surface contaminants such as oils and greases. The capability of using the foam-cleaning medium in a wet form provides for dust control without excessive dampening of the surface being cleaned. The equipment consists of three transportable modules, which include the feed unit, the classifier unit, and the wash unit. The feed unit is pneumatically powered for propelling the foam-cleaning medium. The unit is portable and produced in several sizes. A hopper, mounted at the top of the unit, holds the foam medium.

Foam-cleaning unit

The medium is fed into a metering chamber that mixes the foam-cleaning medium with compressed airBy varying the feed-unit air pressure and type of cleaning medium used, sponge blasting can remove a range of coatings from soot on wallpaper to high-performance protective coatings on steel and concrete surfaces.

The classifier unit removes large debris and powdery residues from the foam medium after each use. The used medium is collected and placed into an electrically powered sifter. The vibrating sifter classifies the used medium with a stack of progressively finer screens. Coarse contaminants, such as paint flakes and rust particles, are collected on the coarse screens. The reusable foam medium is collected on the corresponding screen size. The dust and finer particles fall through the sifter and are collected for disposal. After classifying, the reclaimed foam medium can be reused immediately in the feed unit. The abrasive medium can be recycled approximately six times and the nonabrasive medium can be recycled approximately 12 times.

This system removes paint, surface coatings, and surface contaminants from a variety of surfaces. Waste streams produced from this system include: coarse contaminants, such as paint flakes and rust particles; dust and finer particles; and the concentrated residue from the bottom of the wash unit. Sponge blasting systems are compatible in most situations where other types of blasting media have been used.

As with any blasting operations, airborne dust is a safety and health concern. The key advantage to sponge blasting is the low/reduced generation of dust. Proper precautions should be taken to ensure that inhalation of dust and particulate matter is avoided. Additional protective measures should be taken when stripping lead chromate- or zinc chromate-based paints, as these compounds may be hazardous. Inhalation of lead and zinc compounds can irritate the respiratory tract, and some compounds are known to be carcinogenic. Proper personal protective equipment should be used.

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Abrasive Blasting And Cleaning Services

SHOTBLASTING: While many people regard any blast cleaning process as shotblasting, strictly speaking shotblasting refers to blasting with small steel balls or pellets, as distinct from other types of blast cleaning materials.

ABRASIVE BLAST CLEANING: Abrasive blast cleaning is the general term given to a range of blast cleaning processes designed to remove old paint and coatings, debris, oil, grime, rust and other contaminants from a variety of metal surfaces and components. Blast cleaning is conducted by propelling abrasive materials at high speed through a jet nozzle using pressurised air onto the surface of the component, leaving it clean, dry, profiled and ready for applying new coatings. Blast cleaning can be carried out manually, or by using automated blast cabinets and the pressure can be varied to suit the type of metal or treatment required. Specialised materials or blast media in a variety of sizes or grades are used for treating different types of metals, surfaces and components, depending on the finish required.

At BGP we use a range of ferrous and non-ferrous blast media in our blast cleaning operations. See the list below for a description of some of the most commonly used materials in the blast cleaning process.

CHILLED IRON: Chilled iron grit is an angular abrasive material suitable for general blast cleaning applications such as the removal of paint and oxides from carbon steels and ferrous castings. Blasting with chilled iron is a fairly aggressive cleaning process which makes it unsuitable for use with softer metals such as aluminium. It should not be used on machinery that is exposed to moisture, or items which are prone to corrosion such as engine parts or stainless steel.

Applications: Chilled iron grit is ideal for the removal of paint and contaminants from carbon steels and ferrous castings. It is commonly used for de-rusting, deburring, roughening and surface preparation prior to coating or painting.

STEEL BALL: Steel ball shotblasting is one of the most widely used methods for cleaning and stripping metal surfaces and components. The process involves firing small steel balls (1-6mm diameter) at high speed against the surface of the metal or component. The finish is determined by the size of the steel shot. Larger shot has a more aggressive cleaning action and produces a rougher finish. Smaller steel shot creates a smoother, more polished surface.

Applications: Larger steel shot is ideal for removing rust, scale and other contaminants from heavy steel, malleable iron and grey iron castings. Smaller steel shot is suitable for cleaning small to medium sized ferrous and non-ferrous castings and machined parts.

STEEL GRIT: Steel grit blasting is used for aggressive cleaning projects such as stripping contaminants from steel and other industrial metals. The cleaning action of steel grit produces an etched surface providing excellent adhesive properties for a variety of paints and coatings.

Applications: Steel grit blasting is suitable for steel and foundry metals and is also used for aircraft and aero-space components.

ALUMINIUM OXIDE: Aluminium oxide is a sharp, abrasive blasting material used in sand blast finishing. It is harder than most common dry abrasive blast media and will cut even the hardest metals and surfaces.

Applications: Aluminium oxide is ideal for removing rust from cast iron parts like manifolds and water pumps.

GARNET: Garnet is a hard, heavy abrasive used to prepare surfaces on non-ferrous metals. It is an inert natural mineral which cuts faster than many other blast abrasives while producing lower dust emissions. The deep cleaning action of garnet ensures the removal of rust, debris and other contaminants, producing a clean substrate and a uniform surface profile.

Applications: Garnet abrasives are suitable for the treatment of stainless steel, aluminium and other non-ferrous metals.

GLASS BEAD: Glass bead or dry bead blasting uses spherical beads for cleaning metal parts without damaging the surface. This media offers a gentle cleaning process creating a softer, more cosmetic finish than angular abrasives. Glass bead abrasives provide a silica-free option for blast cleaning, peening, honing, descaling and light deburring.

Applications: Glass bead cleaning is suitable for soft metals such as aluminum and brass. Ideal for pistons, engine blocks and light rust removal. A good choice for the restoration of car parts, motorcycles and other components where a gentle cleaning action is required.

PLASTIC: Plastic Abrasives such as Urea, Acrylic. Polyester and Melamine deliver a highly effective stripping rate, removing coatings and contaminants without damaging the base metal. They are ideal for paint stripping, cleaning, deflashing and deburring operations on aluminium and other soft metals.

Applications: Plastic abrasives are widely used for restoring components in the aerospace and automotive industries.

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Abrasive Blasting Media Introduction

Abrasive blasting is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface, or remove surface contaminants. A pressurized fluid, typically air, or a centrifugal wheel is used to propel the media. The first abrasive blasting process was patented by Benjamin Chew Tilghman on October 18, 1870.

There are several variants of the process, such as bead blasting, sandblasting, and sodablasting.

Abrasive Blasting Media

In the early 1900s, it was assumed that sharp-edged grains provided the best performance, but this was later demonstrated to not be correct.

Mineral: Silica sand is the most commonly used type of mineral abrasive. It tends to break up quickly, creating large quantities of dust, exposing the operator to the potential development of silicosis, a debilitating lung disease. To counter this hazard, silica sand for blasting is often coated with resins to control the dust. Using silica as an abrasive is not allowed in Germany, Britain, Sweden or Belgium for this reason.

Another common mineral abrasive is garnet. Garnet is more expensive than silica sand, but if used correctly, will offer equivalent production rates while producing less dust and no safety hazards from ingesting the dust. Magnesium sulphate (kieserite) is often used as an alternative to baking soda.

Agricultural: Typically, crushed nut shells or fruit kernels. These soft abrasives are used to avoid damaging the underlying material such when cleaning brick or stone, removing graffiti, or the removal of coatings from printed circuit boards being repaired.

Synthetic: This category includes corn/wheat starch, sodium bicarbonate, and dry ice. These “soft” abrasives are also used to avoid damaging the underlying material such when cleaning brick or stone, removing graffiti, or the removal of coatings from printed circuit boards being repaired. Sodablasting uses baking soda (sodium bicarbonate) which is extremely friable, the micro fragmentation on impact exploding away surface materials without damage to the substrate.

Additional synthetic abrasives include process byproducts (e.g., copper slag, nickel slag and coal slag), engineered abrasives (e.g., aluminum oxide, silicon carbide aka carborundum, glass beads, ceramic shot/grit) and recycled products (e.g., plastic abrasive, glass grit).

Metallic: Steel shot, steel grit, stainless steel shot, cut wire, copper shot, aluminum shot, zinc shot.

Many coarser media used in sandblasting often result in energy being given off as sparks or light on impact. The colours and size of the spark or glow varies significantly, with heavy bright orange sparks from steel shot blasting, to a faint blue glow (often invisible in sunlight or brightly lit work areas) from garnet abrasive.

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Abrasive Blasting Media Types

Abrasive blasting is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface, or remove surface contaminants. A pressurized fluid, typically air, or a centrifugal wheel is used to propel the media. The first abrasive blasting process was patented by Benjamin Chew Tilghman on October 18, 1870.

There are several variants of the process, such as bead blasting, sandblasting, and sodablasting.

Types

Wet abrasive blasting

Common features include: the ability to use extremely fine or coarse media with densities ranging from plastic to steel, the ability to use hot water and soap to allow simultaneous degreasing and blasting, elimination of dust – so silicacious materials can be used without worry, hazardous material or waste can be removed without danger – e.g. removal of asbestos, radioactive, or other poisonous products from components and structures leading to effective decontamination.

The process is available in all conventional formats including hand cabinets, walk-in booths, automated production machinery and total loss portable blasting units.

Process speeds can be as fast as conventional dry sand blasting when using the equivalent size and type of media. However the presence of water between the media and the substrate being processed creates a lubricating cushion that can protect both the media and the surface from excess damage. This has the dual advantage of lowering media breakdown rates and preventing impregnation of foreign materials into the surface. Hence surfaces after wet blasting are extremely clean, there is no embedded secondary contamination from the media or from previous blasting processes, and there is no static cling of dust to the blasted surface. Subsequent coating or bonding operations are always better after wet blasting than dry blasting because of the cleanliness levels achieved. The lack of surface recontamination also allows the use of single equipment for multiple blasting operations – e.g. stainless steel and carbon (mild) steel items can be processed in the same equipment with the same media without problems.

Bead blasting

Bead blasting is the process of removing surface deposits by applying fine glass beads at a high pressure without damaging the surface.

It is used to clean calcium deposits from pool tiles or any other surfaces, and removes embedded fungus and brighten grout color. It is also used in auto body work to remove paint.

Wheel blasting

In wheel blasting, a wheel uses centrifugal force to propel the abrasive against an object. It is typically categorized as an airless blasting operation because there is no propellant (gas or liquid) used. A wheel machine is a high-power, high-efficiency blasting operation with recyclable abrasive (typically steel or stainless steel shot, cut wire, grit or similar sized pellets). Specialized wheel blast machines propel plastic abrasive in a cryogenic chamber, and is usually used for deflashing plastic and rubber components. The size of the wheel blast machine, and the number and power of the wheels vary considerably depending on the parts to be blasted as well as on the expected result and efficiency. The first blast wheel was patented by Wheelabrator in 1932.[2]

Hydro-blasting

Hydro-blasting, commonly known as water blasting, is commonly used because it usually requires only one operator. In hydro-blasting, a highly pressured stream of water is used to remove old paint, chemicals, or buildup without damaging the original surface. This method is ideal for cleaning internal and external surfaces because the operator is generally able to send the stream of water into places that are difficult to reach using other methods. Another benefit of hydro-blasting is the ability to recapture and reuse the water, reducing waste and the impact on the environment.

Micro-abrasive blasting

Micro-abrasive blasting is dry abrasive blasting process that uses small nozzles (typically 0.25 mm to 1.5 mm diameter) to deliver a fine stream of abrasive accurately to a small part or a small area on a larger part. Generally the area to be blasted is from about 1 mm2 to only a few cm2 at most. Also known as pencil blasting, the fine jet of abrasive is accurate enough to write directly on glass and delicate enough to cut a pattern in an eggshell. The abrasive media particle sizes range from 10 micrometres up to about 150 micrometres. Higher pressures are often required.

The most common micro-abrasive blasting systems are commercial bench-mounted units consisting of a power supply and mixer, exhaust hood, nozzle and gas supply. The nozzle can be hand-held or fixture mounted for automatic operation. Either the nozzle or part can be moved in automatic operation.

Automated blasting

A fully automated blasting system usually includes contained surface preparation and coating applications.

Dry ice blasting

In this type of blasting air and dry ice are used and with the help of a huge mass and air pressure the parent material is cleaned without destroying the properties of the parent material.

Bristle Blasting

Bristle Blasting is, unlike other blasting methods, a process not needing any blast media. The surface is treated by a brush-like rotary power tool made of dynamically tuned high-carbon steel wire bristles. Repeated contact of the rotating sharp bristle tips with the surface results in localized impact, rebound, and the formation of craters, thereby simultaneously cleaning and coarsening the surface.

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