by Daniel Green
When Penn. State local Benny Tilghman combined an old clothes wardrobe with a high-pressure mix of tap water and beach sand. He changed the face of surface preparation forever. Tilghman swiftly patented the design and soon after blasting cabinets began popping up nationwide.
Dry blasting was popularised as technology evolved. This was followed by experimentation with different media. In 2022 we have no less than a dozen common media types that are used to return substrates back to base. These range from organic materials like corn kernels to actual sand to beaded and ground glass.
In commercial and industrial settings, steel is often heavier duty than in domestic, automotive or aerospace applications. Having that inherent strength and hardness makes it an ideal candidate for material blasting. So aside from distressing denim for killer Insta selfies and etching cemetery headstones, high-pressure media has enormous uses in the steel remediation industry. How? Stay tuned to find out.
This is the process of assessing, repairing/replacing and recommissioning steel components back into service. These can be anything from fixed and mobile plants, structures such as bridges or decorative features like statues. Primary assessment is completed first. But a thorough understanding of potential issues can only occur after removing all traces of surface coatings.
Traditionally, this was done mechanically, that is – by physically breaking chemical bonds. In its most rudimentary form, this consisted of scraping or abrading the coating off. Whilst effective for the period, it wasn’t successful in hard-to-reach places. It was also slow, cumbersome and took vast physical exertion.
As time marched on chemically breaking those bonds was a popular alternative. This consisted of applying solvents, caustics and acids to the base material in liquid or paste form, and allowing the ensuing chemical reaction to do the work. In almost all cases it was faster and easier. However, the remaining chemical residue would often wreak havoc with the new surface coatings.
Until Tilghman arrived on the scene and changed the world.
A blasting cabinet or blasting room is a permanent fixture, often at a blasting facility that is specifically designed and outfitted with surface preparation equipment. While most cabinets can only handle smaller parts up to the size of perhaps a pizza box, a blasting room can usually fit an entire car, truck or more.
This gives a significant advantage over the cabinet in that larger and/or multiple pieces can be abraded simultaneously. This makes it a more commercially viable application than a blast cabinet. However, a blasting room is only effective in remediation when disassembly can occur on-site.
A well-designed blasting room has bulkhead fittings for the operator’s air supply, multiple light sources, frames & clamps for hanging & holding items, spare blasting media and plenty of room to move.
In Situ Blasting
This occurs when disassembly is impossible, extremely difficult and/or overly costly. The blasting environment must be brought to the project, not the other way around. This means an airtight/media-tight capsule must be constructed around the components. Before works can begin, all aspects of a permanent blasting room must be available: power, compressed air, dust extraction & suppression, media, PPE, access, light, waste collection and more.
Types of Media
Given its physical nature and natural abundance, sand was the first choice for media in the early days. Still in use today it is typically used where the base material has 8 Mohs of hardness or more. It is one of the cheapest forms of media.
Aluminium oxide media is both tough and lightweight. It means it acts on surfaces faster than heavier, more cumbersome materials such as steel shot. It also comes in a 99% pure variety for use on surfaces that are sensitive to contamination such as laboratories.
As a by-product of coal-burning power plants, coal slag needs very little remanufacturing for reuse. As a result, it is extremely cost-effective. One further bonus is that it’s low in silica, an advantage even sand cannot claim.
Corn kernels are the greenest of all blasting media. Utilising rejected corn not considered fit for human consumption, this organic material is soft enough to be used on thinner base materials such as automotive or aeronautical applications.
As one of the most recyclable types of blast media, these spherical-shaped soda lime glass beads are best used for cleaning and deburring instead of heavy-duty blasting. They are chemically inert, environmentally friendly and relatively inexpensive.
Glass grounds are similar to glass beads but are the by-product of glass bead blasting and crushed consumer bottles. They are less effective but are also far less costly.
Garnet is one of the most versatile blast media due to its general use, cleanliness and cost-effectiveness. Like glass beads, it is non-toxic and inert, making it an ideal candidate in environmentally sensitive locales like shipyards. It is safe to use on aluminium and fibreglass and is regularly in marine applications.
Silicon carbide is a hard, highly angular material. It has an extremely fast cutting speed allowing for much shorter blast times. This does come at a cost though as the media is more than three times the price of sand.
The bicarbonate of soda is one of the softer materials despite the recipe being more commercial in nature. Designed as a single-use item, it is at its most advantageous when cycling media back through the system would recontaminate the surface. It’s also very inexpensive.
Steel shot is extremely low dust, low waste, high-outcome material that hammers surface coatings. With recyclability rates upwards of 3000 cycles, steel shot is actually extremely cost-effective. Not recommended on delicate materials.
The stainless shot is similar to a mild steel shot except its non-corrosive nature allows for a longer shelf life between blasts. This does come at a cost though because it is more than twelve times the price of sand.
Considered one of the more aggressive media, steel grit will strip anything: paint, epoxy, enamel, rubber and more.
The Walnut shell is considered an anomaly in that its base state is angular and multi-faceted yet it is considered a soft abrasive. When used in conjunction with the appropriate equipment, it is often used to polish the jewellery, clean plastic mouldings, remove graffiti and clean inside electrical components.
It was also cheap.
Plastics like melamine, urea and acrylic
Plastics like melamine, urea and acrylic are best used for delicate operations. Most often used in aircraft or military applications where garnet and sand are too harsh. These three maintain an adequate stripping speed whilst protecting vital background surfaces and components.
As a by-product of volcanic heat, pumice is a light, airy media that is best used where plastics will compromise the natural environment. Its absorbency, grittiness and softness make it distinguished.
Dry ice is almost completely free of waste products other than the emissions of CO² brought about by its exposure to the atmosphere. It is soft yet nonabrasive, non-conductive and non-flammable.
As you can see, understanding steel remediation via blast and paint is more complex than abrading the Harbour Bridge with steel shot. You can find part II of this in-depth topic here.
To Be Concluded…
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