Blast Cleaning Technology
Many engine builders still rely on blasting as an integral part of their shop productivity
By Doug Kaufman
The future, they promised us, would be full of amazing things. We would be commuting in flying cars, we would be using robots to do most of our manual chores and space travel would be fairly routine.
First, the future was 1984 - then it was 2001. Now, in 2005 we do have highly exotic and exciting things in our daily lives to be sure. But when it comes to cleaning engine parts, sometimes the most effective procedure is not a fancy new technology but a tried-and-true method that uses appropriate force to do the job.
Blast cleaning is a fairly straightforward and relatively simple technology: abrasive material is thrown at parts to clean them of rust, carbon, paint or other contaminants. But its reliability is evident, and many engine builders still rely on blasting as an integral part of their shop productivity. While the technology is fairly simple, there are several types of machines available.
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.
Types of Blasting
Depending on the media and method used, abrasive blasting can be used to clean a variety of metal surfaces and parts. There are two basic ways of doing this: 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.
According to the National Center for Remanufacturing and Resource Recovery (NCR3) at Rochester Institute of Technology, which provides technical assistance and applied research to industry and government agencies, blasting can be both an economically and environmentally sound method of cleaning parts in engine rebuilding facilities. However, proper selection of method and media are required if you wish to see the best results.
Why Use Shot Blasting?
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.
In an automated system, an operator loads parts 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.
In both cases, 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.
The NCR3 says 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 the amount of 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.
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.
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.
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.
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 loss of thread interference 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.
The key to getting the best results, according to the NCR3, 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.
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.
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. Zinc shot, for example, has an interesting characteristic that allows rebuilders to increase the durability of the parts they rebuild. Zinc isn't the best at removing a lot of rust, but it does leave a small amount of zinc on the surface of the steel, giving it a bit of "corrosion resistance."
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.
But users of this type of equipment are cautioned that a steady hand is necessary for optimum results. The finish of the metal can be altered with the media, and the air pressure can also make a huge difference. You can go from a fine satin finish to a much rougher finish just by changing the air pressure. But you also risk damaging the metal.
Especially prone to damage are gasket surfaces. In the case of gasket removal, rebuilders should opt for 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.
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.
Another note of caution with using a media blaster is that you may damage the surface of the aluminum 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.
One form of media that does neither of these 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. It is considered a "one-pass" product. This means that it can only be used once and cannot be recycled. However, even though it is one pass, that doesn't mean it's not efficient or affordable for most shops, say experts.
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 cannot be recycled. However, 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.
Since baking soda does not harm the part, NCR3 has found that 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, NCR3 compared cleaning intake valves with soda blasting and with glass bead blasting. 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.
With soda blasting, the process took between 45 seconds to 1 minute total because masking was unnecessary.
On a per item cost basis, this is what NCR3 discovered:
Cost of Glass Bead ($0.47 per pound): $0.38
Cost of Baking Soda ($0.50 per pound): $0.44
Labor cost for Glass Bead ($12/hr. rate): $0.95
Labor cost for Baking Soda ($12/hr. rate): $0.18
Energy cost for Glass Bead blasting: $0.05
Energy cost for Baking Soda: $0.03
Total: Glass bead blasting: $1.38 per valve
Total: Baking soda blasting: $0.65 per valve.
In this case, the differences are dramatic, however if a part is rusted, has scale, heavy oil or grease on it, you'll likely find that another method of cleaning is easier and more cost effective, say the industry experts.
A Need To Know Basis
If you've been in this business a while, you may think you know everything there is to know about cleaning an engine, especially with a technique as simple as blasting. However, the experts we talked to say there are still a lot of misconceptions about techniques and capabilities when it comes to this type of procedure.
Media retention is a valid concern of many rebuilders, but equipment suppliers say that, with some common sense, it does not need to be a major obstacle. First, the part to be cleaned must be dry and free of any oils or grease before blasting commences. According to a leading manufacturer, if you're using a thermal oven first, follow this procedure: if you lift the lid and the parts are still smoking, lower the lid and bake for 5-10 more minutes. Dry parts will ensure the shot falls out during the tumble cycle.
Always remove all of the oil galley and core plugs before cleaning. While this may seem like an unnecessary waste of time, there is no way to properly clean and dry all of the oil galleys and other passages in an engine without removing those plugs.
Blast cleaning can be an effective, efficient part of your shop's cleaning operation. For additional information on suppliers of blast cleaning equipment and media, utilize the interactive Engine Builder Buyers Guide, available online at www.engine-builder.com.