- What are the different forms of abrasive media for sandblasting?
- Which abrasive media to use for paint removal?
- Is garnet sand a good abrasive?
- What is the best blast media for rust removal?
- Which abrasive media is best for polishing?
- What size abrasive media should be used for sandblasting?
You may have heard the old adage in the industry “A coating can only be as good as the preparation of the underlying surface”. Despite all the advances made in manufacturing and development of coatings, this maxim stands the test of time and holds true even today.
Surface preparation for any coating application begins with choosing the right abrasive. With the right abrasive, one can maximize profits by getting the job done faster as well as use as little abrasive media as possible.
Many blasters make a decision on choosing the abrasive based on limited considerations such as the Purchase Price of Media alone, or past practice or in some cases, blindly following others in the industry. We believe that choosing the abrasive should depend on a number of different considerations. The purchase cost of the abrasive is only one of the factors involved and in determining the cost. One can say that the purchase cost of the abrasive is the tip of the iceberg that is visible. The larger part is the operational cost which is quite often not considered. In our opinion, the Right abrasive for the Right application must be considered together with calculating the Total Cost of the application Per Square Metre.
The following factors should be considered in choosing the right abrasive.
- Surface Properties/Characteristics
A hard surface such as steel would generally require a harder abrasive such as garnet, steel shots, aluminium oxide etc whereas a softer surface such as wood or concrete is better blasted with a softer abrasive such as crushed glass that is also inexpensive. It is also worth considering the risk of embedment in the surface. Tougher abrasives reduce the risk of breaking down and getting embedded in the surface and provide a cleaner surface.
- Required Surface Profile
The next step is to determine the surface profile and the quality of finish required.
Cleaning a surface is a very different application to preparing a surface for coating. In a cleaning application, the goal is to simply clean the surface without changing the substrate. Abrasive media such as glass beads, walnut shell, soda bicarbonate or plastic grit can be used for cleaning a surface.
Surface preparation for coating on the other hand requires removal of all contaminants as well as prepping the substrate for the coating to adhere. It is important to check the technical data of the coating that specifies the recommended surface profile.
Coatings adhere poorly to smooth, flat surfaces and required an etched profile to adhere.
An Anchor Pattern is the term used in the industry to refer to the pattern of indentations that are formed when the abrasive media particle impacts the surface. When a surface is very smooth, coatings have a difficult time adhering strongly. A scraper or even a fingernail, for instance, easily removes a coating on glass. On the other hand it is difficult to remove a coating on a rough surface like sandpaper. Steel, when it is abrasive blasted, has a surface that is rough like sandpaper, with a series of tiny peaks and valleys called surface profile. Coatings anchor themselves to the valleys of the profile, and the peaks are like teeth. This is why surface profile created by blasting is sometimes called an “anchor pattern” or “mechanical tooth.”;mThe depth of the crate determines the quantum of the coating the can anchor to the surface and its adhesion. If the crater is too deep, it does not necessarily mean better adhesion as the ridges may poke out through the surface of the coating and can cause rusting and weakness in the coating.
The general thumb rule is that the right abrasive for the job is the finest grade that can provide the requisite depth profile needed to apply the proposed coating.
Sandblasting is also used to Strengthen the Substrate through Peening or increasing the resistance. Industrial parts such as gears, suspension rings etc benefit from an increase in life cycle when they undergo a shot peening process to reinforcement the surface. Steel shot is a popular abrasive for reinforcement of the surface. Shot peening increases the life cycle of parts that are subjected to cyclic mechanical constraints such as gears, suspension springs etc. It is a cold work process used to impart compressive residual stresses on to the surface of a component and it results in adding strength and reducing the stress profile of components.
Sandblasting with glass beads can be used a Polishing Process for certain automotive components, sensitive metal surfaces, aluminium propellers and turbine blades. Glass Beads produce a much smoother and brighter finish than angular abrasives. Glass Beads can be recycled approximately 30 times. These are softer than many other abrasive media like garnet and typically leave a shiny and clean surface.
- Blast Media Properties
Size
Particle sizes are commonly classified by mesh sizes and based on the particle distribution, are given a range eg: 30-60 mesh. This indicates that 95% of the mix will fall through a 30 mesh but not pass through the 60. The mesh size number indicates the number of mesh lines per square inch in a sieve. Some abrasive manufacturers prefer to term their media with their brands and do not readily disclose the mesh size leaving customers in the dark about the actual mesh size.
Typically, the larger the abrasive media particle, the deeper is the profile it will make on the substrate. It must be noted that the larger the particle, the less number of particles that can be comprised in the air flow. In other words, blasting with large particles gives lesser impact than an equal volume of smaller particles. All things being equal, small particles clean faster, provide better coverage, and result in a more uniform profile. Therefore, the most efficient way to blast is to use the smallest particle necessary to achieve the desired profile. This approach will also reduce the need to paint over rogue peaks or fill oversized troughs within the surface profile.
Shape
The shape of the abrasive particle affects the depth of how it cuts into the coating and the underlying substrate. Shapes are classified according to angularity.
There are four elementary shapes in abrasive media:
- Angular
Angular abrasives have a sharp profile, multiple facets and jagged vertices. Angular particles cut through soft coatings and rust, cleaning faster, and producing sharper anchor patterns. Angular abrasives such as crushed glass, copper are suitable for stripping corrosion and rust.
- Sub-angular
Sub-angular abrasives have lesser jagged profile and edges compared to angular abrasives. Rock garnet and plastic urea are sub angular and are suitable for blasting corrosive metal, painted steel etc.
- Sub- rounded
Sub-rounded abrasives are not fully smooth but don’t have as many jagged edges as sub-angular abrasives. Sub-rounded staurolite and walnut shells are used for blasting where a deep profile is not required.
- Rounded
Rounded abrasives are smooth in shape and may be spherical. Rounded abrasives create a dimpled profile, like the surface of a golf ball and create an effect that is known as peening. Rounded abrasives such as glass beads or steel shot are used in removing mill scale, hard brittle coatings, polishing or shot peening.
Hardness
Generally speaking, the harder the particle, the deeper the profile it will impart, except in cases where a high-velocity hard particle shatters, delivering less than optimum force.
Softer abrasives, like organic materials and plastics, are good for removing dirt, oil, grease and paint without creating an anchor pattern in the underlying substrate.
Hardness is measured on the Mohs Hardness Scale and the lower the number, the softer the abrasive particle. The higher the number, the harder the abrasive particle. Diamonds are ranked 10 in harness on the Mohs scale whereas talc is 1.
Density
A dense particle has more mass per unit volume. That means more pressure upon impact, meaning it will dig deeper than a larger particle of the same mass. In sandblasting, particles with higher density impact the surface with higher kinetic energy, resulting in a deeper profile. If you recall, Galileo’s thought experiment of dropping two spheres from the Leaning Tower of Pisa, you will note that the heavier sphere has a larger pressure upon
Upon impact, dense particles also deform lesser. Denser particles transfer more of their kinetic energy into impacts. Softer, less-dense particles will compress when they collide with the object, meaning they absorb the collision and transfer their momentum less efficiently.
Heavier media such as steel shots are therefore used in shot peening whereas softer particles such as crushed glass are used in paint removal and light surface cleaning.
Friability:
Abrasive media that breaks up easily into smaller pieces are said to have higher friability compared to abrasives that don’t break up easily. For example rock garnet is more angular and sharper but has higher friability compared to alluvial garnet which is more rounded but is less brittle.
- Recycling the abrasive
Recycling the abrasive media can help to reduce the abrasive media cost. The abrasive media will deteriorate with each repeated use and typically, is used until the cost benefit outweighs the drop in efficiency.
Abrasive media such as garnet, steel shot that rate higher on the Mohs hardness scale can be screened and separated to recycle. On the other hand, some types of blasting media such as crushed glass are categorized as single-pass media and can only be used once as when blasted, this media disintegrates into particles too small to recycle.
However, sandblasting media can’t be recycled when used in portable blasting applications that have no containment method.
If you’re recycling blast media, properly cleaning it with a blast recovery system before reusing it can be extremely beneficial. It can greatly reduce consumable expenses and improve surface finish.
The following table can be used as a guide to the characteristics and applications for the commonly used abrasive media
Characteristics and applications
- Purchase cost of the abrasive
Abrasive media such as garnet and staurolyte are natural mineral products whereas steel shots, crushed glass, aluminium oxide are manufactured products. Purchase cost varies significantly depending on local availability, freight, competition etc.. It is best to shop around for a quality product and enter into a long term contract to secure the supply with a reputed supplier. While this cost is the easiest component of the total cost of blasting to be compared and many blasters keep it as the only consideration, it is worth remembering that this is only one part of the total cost and usually is not the biggest cost. The labour cost is the biggest cost in most countries. It is also worth remembering that all abrasives do not perform the same.
- Disposal Cost of used media
A cost often overlooked is the disposal cost of the used abrasive media. In light of stringent waste management regulations and heightened awareness of environmental contamination, this issue is now getting increased attention.
The used abrasive blast media may contain materials from the cleaned surface which is mixed with hazardous substances. Surface coatings with paint are often necessary to protect from deterioration in the environment, most notably the marine environment (eg – ships and bridges). These paints usually contain heavy metals which act as anti-fouling and anti-corrosion agents. Abrasive blasting may also be used to remove lead coatings which are extremely hazardous. Used abrasive media is a solid waste, and must be disposed off in accordance with local environmental legislation. Environmental regulations may require that a Toxicity Characteristic Leaching Procedure (TCLP) test be performed to determine if the material is hazardous.
- Proposed coating
Coatings are formulated to perform properly under particular conditions, such as over a specified degree of surface cleanliness and a specified anchor profile, and under certain environmental conditions. If these and other conditions are not met, coatings may not achieve their expected performance. When dry abrasive blasting is the specified method of surface preparation, many conditions must be taken into consideration by the blaster. When contaminants on a surface are painted, they interfere with the mechanical and chemical adhesion of the coating to the substrate so that the coating is likely to fail. On the other hand, when all soils are removed, the coating can achieve complete and continuous contact with the substrate, thus assuring the best possible adhesion. When a coating adheres well, it is likely to be an effective barrier. The coating can minimise or prevent moisture (the electrolyte in the corrosion process) from reaching the substrate.
The following levels of abrasive blasting are specified in ISO 8501 issued by the International Standards Organisation. It is a pictorial standard with description of the cleanliness levels
Check the coating manufacturer’s specifications to determine the level of surface finish required. Independent inspectors may also be appointed to projects to check the surface finish.
- Health and Safety
Dry abrasive blasting can be hazardous and when it was initially developed, many people developed respiratory problems from breathing in the dust or other particulates. Wet abrasive blasting or dustless blasting does have the same hazardous risks as dry blasting.
- Hearing Loss– Since these machines propel particles at high speeds, they need powerful motors to keep them running. Air and water compression units can be excessively loud, and prolonged exposure without hearing protection can lead to semi or permanent hearing loss.
- Respiratory Illness– dry blasting creates a lot of dust. If employees breathe in this dust, it could cause serious lung damage. In particular, silica sand can cause a disease known as silicosis, lung cancer, and breathing problems. Coal slag, garnet sand, copper slag, nickel slag, and glass may also cause lung damage similar to the effects of silica sand. Job sites that use metal particles may create toxic dust that could lead to worse health conditions or death. These materials can contain trace amounts of toxic metals like arsenic, cadmium, or beryllium that become airborne and can be inhaled.
- Skin Irritation– The dust created by abrasive blasting can get into clothing quickly and easily. As workers move around, the grit or sand can rub against their skin, creating rashes and other painful conditions.
- Eye Damage– Some particles used in abrasive blasting are incredibly tiny and can cause damage to the eye. Suitable eye protection must be used in both dry and wet abrasive blasting.
- Skin Abrasion– Blast suits and blast gloves must be worn to protect the body from flying sandblasting particles and accidents.
- Lead Poisoning– Over time, workers can develop heavy metal poisoning if these the surface being blasted contains lead coatings and the particles end up in the bloodstream, either through breathing or injury.
If you are in doubt between choosing a softer vs a harder abrasive media, start with the softer media first. You can always add a harsher abrasive later, but it is significantly more work to undo the work of an abrasive that was too hard and risk damage to the underlying substrate.
We hope the above article is helpful in explaining the different forms of abrasive media and helps you to choose the right abrasive media for sandblasting.