Blasting Equipment & Media
Shot blasting technology: air blasting and airless blasting
By Doug Kaufman
In today's society, finding "the next big thing" seems to be all the rage. We no longer want to do things the same way and seem to keep asking "what's the latest and greatest?"
High technology and computerization have made many tedious everyday tasks a breeze, from communication to entertaining to operating your business. But high technology can only go so far, and when it comes to cleaning rust, carbon, paint or other contaminants from engine parts, sometimes low technology is what you need most.
Understand, when we say that blast cleaning isn't exactly high-tech, we are not disparaging its capabilities. Although it is a fairly straightforward and relatively simple technology, in which abrasive material is thrown at parts to clean them, many engine builders still rely on blasting as an integral part of their shop productivity.
Know What You Need
Shot blasting can be used for cleaning parts or for increasing surface hardness on some parts, especially crankshafts or connecting rods. It can also be used to prepare a surface's finish, whether it needs a simple texturing or requires additional coating.
While the technology is fairly simple, there are several different types of machines available. A review of which type of machine is most effective and easiest to use for specific parts, or which type cleans parts at the lowest cost are questions often roundly debated. Your equipment supplier can make recommendations based on your specific needs. Like most other subjects in the engine building business, a "one-size-fits-all" approach is unlikely to give the best results.
There are two basic types of shot blasting technology: air blasting and airless blasting. The typical air blast unit uses compressed air to propel shot, either by drawing it through a nozzle by suction or venturi force, or by pressure blasting, where the abrasive and the air are forced through a blast hose to the nozzle.
Airless blasting units (which may use a center-fed wheel or an electrically driven spinning turbine or impeller shaft with paddles or cups attached) typically use some sort of metal shot media. The blasting operation can range from completely manual to fully automatic.
In an automated system, parts are loaded into a cabinet, either on a conveyor belt or into a basket. Once the cabinet door is shut, the blasting process begins, requiring no additional effort by the operator.
In manual blasting systems, parts are typically cleaned by hand in some type of a cabinet. Cabinet designs may vary somewhat: doors may be on the sides or in the front, but the operator inserts his or her hands into gloves in the cabinet wall and holds a nozzle to blast abrasive material at the part within.
To be effective, blasting requires "line-of-sight," which means if you can't see it it can't readily be cleaned. That's why moving the parts or moving the stream of blasting media is so important.
For any type of dry media blasting, careful degreasing and drying of the part is critical. Blind holes and cross-drilled oil galleys are especially susceptible to media retention, and oil or water can trap the blast media.
Certain types of media are more resistant to lodging in a cavity than others. Glass bead and steel shot (specifically shapes that are more jagged) can become lodged in very tight, hidden areas and require extra attention to get them out. In addition, you may have to mask the area off to keep the media from getting in.
Blasting is not generally used to remove grease or oil from metal parts but is instead often used following a thermal cleaning step. Thermal cleaning, which reduces carbon and gasket materials to dust or ash, needs to be followed by a post-oven media blasting process.
Once you've determined what type of machine is right for your shop, you'll need to decide which type of media to use. Different types of shot can be useful for different types of cleaning operations.
Blasting media come in different hardnesses. The hardness of most abrasives (with the exception of steel abrasive) is measured by the Mohs' Hardness Scale, which ranges from 1 (for the softest material, talc) to 10 (the hardest, diamond).
Steel shot and steel grit are considered hard abrasives and are measured on a Rockwell "C" scale. They are produced in hardness ranging from 42 to 65. Steel grit provides an etch on a steel substrate, whereas steel shot provides a peened surface on a steel substrate.
The key to getting the best results, say experts is to choose the right blast equipment and media for the substrate to be cleaned. For example, if a painted aluminum part needs to be stripped but the surface cannot be damaged, a very soft media (perhaps corn cobs or soft plastic media) with a relatively low blast pressure may be required. On the other hand, if the part needs to have a bright, textured surface for cosmetic purposes, then a harder media should be used.
An alternative to automatic shot blasting is air blasting. The media of choice may include glass beads, crushed glass, walnut or apricot shells (or other agricultural media), plastic media (which can include a huge range of products ranging from urea-type plastics to melamine) or baking soda. Typically much smaller than the steel shot used in automatic blasting units, the air blasting media is usually manually directed onto the part by a compressed air-powered, hand-held gun.
This type of blasting is usually used for oxide removal, paint removal, gasket removal and surface refinishing, with different media giving different finish results. For example, compare glass beads to crushed glass: glass beads will give a "satiny" finish, while crushed glass will give a bit rougher overall finish.
Users of hand-held air blasting equipment should remember that a steady hand is necessary for best results. Not only can the finish of the metal can be altered with the media, the air pressure can also make a huge difference. You can go from a fine satin finish to a much rougher finish just by altering the air pressure slightly. But for that cosmetic advantage, you run the risk of scarring the metal.
Especially in the case of aluminum components, you may damage the surface by moving the metal, affecting tolerances or changing the texture of the surface. Some units throw the shot at a slower speed which reduces the potential for damage to threads, precision surfaces or sensitive castings.
Experts say gasket surfaces are especially prone to damage. In the case of gasket removal, rebuilders are encouraged to use a lower pressure glass bead or use a softer plastic medium or even an agricultural product.
For baked-on carbons, sometimes the only way to remove it is to blast it off with some sort of bead media. Bead media on aluminum can be challenging, however, because you may spend as much time cleaning the media out as you do cleaning. You have to be sure that you don't leave anything behind in the cleaning process.
One form of media that offers many advantages is soda media, essentially made from a form of baking soda. According to soda blasting suppliers, the material used in soda blasting is called sodium bicarbonate. Baking soda is a soft media that is ideal for sensitive substrates where metal removal is not desired or is unacceptable. It can be used with or without water - when used dry, it may create a cloud of dust. When used with water, the hydraulic action of a wet system can improve cleaning of grease and oil.
One of the benefits of soda - its relative softness and the fact that it fractures on impact - may also be the drawback some engine builders are concerned about. Because the media fractures on impact and is no longer effective for cleaning, baking soda is considered a "one-pass" product. This means that it can only be used once and cannot be recycled. But even this potential negative has its upside. Because it is effective on grease and oil and doesn't have to be reused, it can easily blast clean soiled parts without regard for keeping the media clean. This can save time and money on the front-end of the cleaning process, offering efficiency and affordability.
With baking soda, masking is not necessary. It is a completely water-soluble material, so oil galleys, water jackets, bolt hole and other blind areas can be cleaned and then rinsed clean. This can result in time savings for the rebuilding facility.
In a recent experiment, conducted by a leading university research department, different methods to clean intake valves were compared. Soda blasting and glass bead blasting each were used in the experiment. With soda blasting, the process took between 45 seconds to 1 minute total. With glass beads, the process took approximately five minutes per part - up to half of which was spent in masking the valve stems. The concern was that failure to mask the stems would result in pitting from the beads. No masking was necessary with the soda.
In this case, the differences are dramatic, but if a part is rusted, has scale, heavy oil or grease on it, another method of cleaning may be easier and more cost effective, say the industry experts.
A relatively new type of cleaning process uses solid carbon dioxide pellets (known as dry ice blasting) blasted at supersonic speed. The pellets convert directly from a solid blast pellet to a vapor, leaving no other residue than the removed contaminant. According to information on the process, the micro-thermal shock (the pellet hits the surface at -79 degrees C), the kinetic energy of the dry pellets and the air pressure break the bond between the surface and whatever is on it.
Dry ice can be used in machines similar to those using other blast media, according to suppliers.
As effective as blast cleaning is in so many shop operations, explains one supplier of a full-range of blasting equipment, it is important to remember that another type of blasting, the newer-style aqueous blasting, is an option as well. While aqueous-based blast cleaning is not new, it is becoming more prevalent due to the stringent regulations that are ongoing regarding solvent-based solutions. With the evaluation of better water-based chemicals to clean parts/components, this is becoming an effective solution to the solvent-based chemicals and subsequent disposal issues.
New pump designs, cabinet designs, and chemistries have added greatly to the efficiency and safety of aqueous blasting. Some of the newer heat-treated and urethane based coatings, however, will only respond to the higher abrasion of solid media. Some amount of thread damage is also a consideration in the more aggressive blasting solids. Aqueous blasting has become a safe, efficient, cost effective one-step cleaning procedure for most shop operations, but for tough paint and heavy rust the careful use of solids blasting is still the favorite choice.
No matter what type of blasting equipment you use, disintegrated media and contaminants must be separated from the reusable media. A high volume of dust is generated as the abrasive media knocks off dried oil, paint and gasket materials. All shot blasters have some sort of mechanism to remove dust from the interior of the cabinet. Usually, a blower or vacuum air pump transfers the dust out of the cabinet into a bag or cartridge-style dust collector. The heavier media then falls into the storage reservoir for reuse.
Shop owners should take environmental, health and safety factors into consideration when purchasing an abrasive blasting system. Dust and noise are the primary concerns, but well-maintained filtration systems will handle most of the in-use concerns. Operators should be careful when inspecting or replacing the filters or when emptying the dust collectors to wear a filter mask - this will help reduce inhalation of residual dust.
While in use, operators may require hearing protection depending on the type of machine. Some equipment manufacturers add sound deadening materials to their cabinets to help reduce the noise of the media striking the part or the walls. An additional noise factor to consider may be the sound of the blower or vacuum.