7/1/2001
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Media Frenzy — How Shot Blasting Gets Done
By Doug Kaufman
Opinions on which type of blast medium works best for cleaning automotive parts vary among engine builders about as much as the types of media themselves. One rebuilder may say he never uses a particular type of media to blast clean his parts, while another swears by it. Expand that out to the entire rebuilding population, and you’ll soon see that selecting a method of blast cleaning is not necessarily a simple procedure.
"There’s a lot of variation in what people use," says Dave Fister of the National Center of Remanufacturing and Resource Recovery (NCR3) located at the Rochester Institute of Technology (RIT). "For the rebuilders in this example, both are successful, both get good results. Somehow, they’ve accommodated different processes to achieve the same results."
Fister says there is no single right answer when the question, "What is the best cleaning to use?" is asked, but rebuilders need to look at the many options available to them.
NCR3 has compiled a massive amount of cleaning data from hundreds of tests to determine what works, what doesn’t and why. The center will make the data available in a fact sheet to be published later this summer, giving users additional information to consider when they’re making their buying decisions.
Blasting may be used to clean surfaces, remove existing surface finishes (stains, rust or paint) and to prepare the finish for repainting. At first glance, blast media appears to be about the simplest, most straightforward type of cleaning process. An alternative to chemical cleaning, blast cleaning employs mechanical forces to do its work. In the process, a type of media is directed onto the surface of the engine component in order to achieve the results. In other words, the impact of the media against the surface does the work, rather than a solvent or other type material.
Blast It All!
Manufacturers and suppliers may call their products and processes many different things, but in reality, blasting may be roughly grouped into one of two different types.
Centrifugal blasting is a method which uses wheels turning at a high speed to direct the steel or other blasting media onto the component to be cleaned. Also known as "automatic" machines, the centrifugal machines take much of the guesswork out of the process, requiring attention only during the loading and unloading process.
"In terms of what you can do with these machines," says NCR3’s Fister, "depending on the size of the machine, you can clean small parts like alternator housings to entire engine blocks."
Steel shot blasters are used most effectively as a secondary cleaning step to remove rust or ash or for a "finishing" operation to remove flashing, varnish or paint. "In most cases, you would use a shot blaster after another cleaning operation, whether a spray wash or a thermal operation," says Fister. "You want to make sure most of the grease or road grime is off the part, because the media recirculates through the machine. Otherwise, you can end up with ‘clumping’ of the media, and it becomes a real mess."
Different types of shot can be useful for different types of cleaning operations. "For example, zinc shot has an interesting characteristic that allows rebuilders to increase the durability of the parts they rebuild," says Fister. "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.
NCR3’s research shows that 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," Fister explains. "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, says Fister. 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. "Otherwise, you may end up remachining the gasket surface because you gouged the metal."
In short, says Fister, determining which media is appropriate is a balance between what you’re trying to take off versus what you’re trying to preserve.
Although baking soda is used like the other compressed air media, it has the unique characteristic of being a "one-time use" medium. While the others are recirculated through the machine over and over, baking soda is used once and then disposed of.
"The disadvantage of baking soda is obvious — you need to use a lot more media. But the advantage it holds is that because it is not reused, rebuilders can actually use it to remove grease or carbon from the surface," explains Fister. "It may save a prewash step."
Available in different sizes and different grades, baking soda can be used for removing paint, gaskets and baked-on carbon. However, though it is sufficiently mild not to damage many surfaces, it’s a bit too mild to perform heavy rust removal.
With baking soda being the lone exception in this field, blast media recirculates through the cabinets. Anything that is smaller than a certain size (whether it is dust, paint, rust or other surface coatings) is useless and is caught in a collector bin or bag. The remaining media usually cycles through a screen and gets used again.
"In theory, you don’t really have to replace media," says Fister. "You use it until it’s no good anymore. What you’re disposing of is disintegrated media and whatever you’ve cleaned off."
Of course, theory and reality are not always compatible, says Fister, offering as an example a used steel shot blaster that NCR3 purchased. "When we took delivery, we found that the ‘shot’ was actually filled with pieces of bolts, welding rods and broken parts. The extra metal wouldn’t work as a blast medium, but it was far too big to be caught in the bag house."
Fister faced the double whammy of not being able to use the media (as a testing center, the process has to be consistent ) and having to find a recycling center for steel shot and assorted pieces.
Screens and other preventive measures usually keep this from happening to rebuilders, however. "For the typical shop, an occasional bolt or some other foreign part might get caught in the media, but not that often," Fister says.
Good Vibrations
While not as aggressive as blasting media, vibratory media can be used for many of the same operations. Made of many different materials, including ceramic, steel, aluminum and zinc, in very interesting shapes, these products can remove burrs and rust. But because they also use some sort of a liquid solution to clean and lubricate, these media can also be used for grease, dirt and oil removal.
The ultimate in automatic operations, parts are put into a tub-like cabinet and the media is vibrated to flow around the parts. Depending on the system, automatic unloading operations can also be incorporated saving even more operator effort.
"One of the main concerns with this type of system is media lodging, especially when it starts to wear down," explains Fister. "The media size needs to be maintained to keep the small stuff from getting into all sorts of uncomfortable places."
This is good advice for any other type of media-related cleaning operation, as well. "It’s almost universally important to keep the media out of the internal engine components, so there are often ‘post cleaning’ cleaning operations that must be conducted," Fister explains. "Whether it’s a shaking procedure or a spray washer, you’ll probably need to do something to get all the media out."
One of the missions of the NCR3 is to give unbiased test information to end users. Doing so, Fister hopes, will dispel the myths that: a) blasting is always right for every application and b) blasting isn’t right for anything.
"So many people say they won’t do blasting because it doesn’t work – well that’s just not true," Fister says. "To say you don’t want to do the extra cleaning steps often associated with blasting is one thing, but to make a blanket statement that it isn’t effective is wrong."
Sure, there are steps you must take to ensure that the medium you use is correct for the part. And you may have to deal with health and safety concerns of different types of media. But balancing your expectations with the facts is critical.
"One type of blasting may be more expensive initially than another," concludes Fister. "But if you don’t have to worry about other steps because they’ve already been taken care of, the higher cost may be worth it. You must determine on a case-by-case basis which is most important."
For more information on cleaning, go to www.engine-builder.com click on the "For a Closer Look" icon, then click on "Parts Cleaning." Additionally, a directory of shot blasting media and equipment suppliers is available with this feature at www.engine-builder.com.
You may e-mail Doug Kaufman at dkaufman@babcox.com.