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Blasting Technology: This Cost-Effective Cleaning Procedure Has Improved

By Ed Sunkin

When it comes to parts cleaning by blasting, the idea of propelling abrasives at parts to clean them of rust, carbon, paint or other difficult contaminants is a simple one. But the technology involved with this cost-effective cleaning procedure has improved, and rebuilders can take advantage of many options to eliminate out-dated, labor-intensive cleaning practices in the shop.

Scott Christiansen of the Guyson Corp., Saratoga Springs, NY, said blast equipment manufacturers are always seeking to improve quality and productivity for rebuilders. "The evolution of what was once a simple sandblaster is continuing in response to the changing requirements of the automotive rebuilder," Christiansen said. "Recent developments have extended the range of dry-blast process capabilities and media selection for the reconditioning of automotive components."

There are two basic ways of propelling blast media - air blasting and airless blasting. And there are numerous types of equipment available that utilize each method. Compressed air blasting propels shot either by drawing it through a nozzle by suction or venturi force, or by pressure blasting, where the air and abrasive are conveyed through a blast hose to the nozzle. Airless centrifugal shot blasting units can use an electrically-driven spinning impeller shaft (turbine) with paddles or cups attached, or a center-fed wheel to propel shot.

Just as there are differences in the methods of blasting, there are differences as to which equipment to use. Some in the industry argue that turbine equipment is more energy-efficient than air-blast media delivery. For example, a small diameter turbine driven by a 5 hp electric motor can throw 150 lbs. (68 kg) of blast material per minute, creating a blast pattern about 4" wide and 24" long (10 cm X 60 cm). By comparison, a 5 hp compressor will power a single suction gun that delivers about 5 lbs. (2.3 kg) per minute of the same media creating a blast pattern about the same size as a half-dollar.

When using manual blast equipment, the particle stream is directed by hand, requiring constant employee attention. Here, the quality of the results can vary from operator to operator and part to part. However, the method for propelling shot varies with the shop's needs. Although some engine rebuilders prefer to clean ferrous engine blocks and heads in airless shot blasters, electrical and some small parts rebuilders prefer using air blasting units with glass bead, aluminum oxide and zinc oxide media. For a list of manufacturers and suppliers of blast equipment and media, refer to Automotive Rebuilder's January 1997 Automotive and Truck Purchasing Directory.

Why shot blasting?

Shot blasters can be used for cleaning parts, or for increasing surface hardness on some parts such as connecting rods and crankshafts. According to Grant Ebersole, industry manager of Standard Systems for Pangborn Corp., Hagerstown, MD, blasters provide a simple method for rebuilders to clean parts quickly. "A machine becomes necessary when a rebuilder realizes that production by hand is too time consuming," Ebersole said. "The implementation of a machine will help allow a shop to achieve desired production, plus enough extra production to justify the cost of the machine, as well as improve profit goals."

Guyson's Christiansen said appearance is another consideration. "The quest for a unique, clean and OEM-type appearance has spawned considerable interest in blast equipment specially adapted to use exotic shot for the production of distinctive cosmetic finishes," Christiansen said. "Of special interest for these purposes, and in some rebuilding applications, are a number of extremely fine metallic media in size ranges under 300 microns (.0118" diameter), sometimes referred to as 'microshot'. These media are denser and more fracture resistant than fine non-metallic shot, so they are compatible with airless blast as well as airblast delivery systems."

Christiansen said in contrast to the comparatively gross surface alteration of some treatments with conventional shot, these low-aggression media modify component appearance only slightly and can be used to produce subtle conditioning affects and surface finishes. "But because the degree of texturing is less with these finer, lighter media than with conventional shot, they demonstrate less peening effect and may be inappropriate for preparing surfaces which may need to be coated rather than simply reconditioned," Christiansen said. "Generally, applying a coating after reconditioning requires a rougher, more profiled finish."

Dry-blasting can also provide rebuilders additional surface finish requirements, such as blast etching, profiling or texturing. And dry-blast methods allow a rebuilder to remove unsatisfactory coatings, paint, etc., often leaving the surface of the component in an improved condition for re-coating.

When choosing a shot blaster for the shop, Zack Wittke of Serv-Equip, Black River Falls, WI, said it's important not to over-buy for your needs. "Start with equipment that will handle the size of the parts you want to clean on a consistent basis," Wittke said. "If you do one or two large pieces per year, it is probably worth the extra money for a larger machine.

"Also do a capacity flow chart," he added. "Take the time to determine your cleaning volume for the next two to three years and size your system accordingly."

David Cox of Kansas Instruments, Council Grove, KS, said rebuilders planning to purchase shot blasting equipment need to look at blasting expenses. Cox said besides the initial cost of the blasting equipment, there are some expenses for this type of cleaning process. "Electricity usage, shot replenishment and fixture wear-out are some cost considerations rebuilders should be aware of," said Cox.

But many manufacturers say these costs are minimal when compared to other cleaning methods. Information on typical costs to operate blasting machines is available from various equipment manufacturers.

Other cost considerations may include precleaning processes. Dennis Marble of Sunnen Products Co., St. Louis, MO, said precleaning of engine components (thermal and wet) is a step some rebuilders take prior to blasting. But once the blasting process begins, parts must be dry and grease-free to prevent media adhesion. "Parts must be completely degreased and absolutely dry before shot blasting," he explained. "If this is not done, both the parts and the blast media will become contaminated. Even the slightest amount of grease or moisture will temporarily trap blast media and prevent complete removal."

Other manufacturers such as Serv-Equip's Wittke and Pangborn's Ebersole say the need to preclean parts has been reduced with today's blasting equipment. "I won't say you never have to preclean anything, but blasting should take care of about 95% of your cleaning work," Wittke said. "However, you have to be careful when blasting sheet metal parts, as too much time in the machine will start to distort them."

Ebersole concurred. "Parts can be introduced to the blast machine without pre-cleaning," he said. "But, the machine parts last longer when excess sand is vibrated off of molds before they are cleaned."

Marble also cautions rebuilders about residual magnetism. "While somewhat uncommon, parts that contain residual magnetism should be demagnetized before blasting or final cleaning, regardless of the cleaning process used," he explained. "Rods, crankshafts or blocks which have spun bearings may contain residual magnetism caused by friction. Always use correct procedures when removing magnetism from items that have been checked in a magnetic particle inspection (wet mag) machine." Magnetism can be checked with a field strength indicator which is available from shop supply vendors.

Maintenance

Proper maintenance on your equipment is vital to adding years of life to your blasting equipment. One area that needs attention is the dust filtration system. "Shot breaks into smaller and smaller particles as it is used, eventually creating a fine dust," Marble said. "If the dust is not continuously removed by a filtration system, it will end up on the parts being cleaned. This is especially important when blasting parts which have been cleaned in an oven, as they will be covered with the ash residue left from the oven cleaning process."

Another important equipment maintenance procedure is to grease bearings at the intervals recommended by the manufacturer. Avoid over-greasing, as this can lead to premature bearing failure. "If hot parts will be blasted, such as after oven cleaning, be sure to use an appropriate grease designed for this application," Marble offered.

Pangborn's Ebersole said routine maintenance checks will not only reduce wear on equipment, it also will reduce cleaning times. "Keep the screens in the separator clean; these should be cleaned regularly," Ebersole said. "And conduct monthly examination of wear parts to determine if they need to be replaced."

Marble explained Sunnen/AmPro's and some other blasters have impeller blades which should be turned over as they become worn. "When the impeller blades become worn about halfway - usually after about 500 hours of use - turn them over to expose the unworn side which will increase their usable life," said Marble.

Many equipment manufacturers provide equipment operation and maintenance training. Gene Tarabek, national training manager for U.S. Filter/ Wheelabrator, Lagrange, CA, said increasing the awareness and understanding of all of the facets of a shop's blast cleaning systems is like renewing a resource. "Our seminars are one of the best ways for operators and maintenance personnel to make sure their efforts will always contribute to cost effective productivity gains, along with the lowest possible operating and maintenance costs," he said. Tarabek said his company's training program, which consists of one-day seminars on critical maintenance issues conducted at various locations throughout the country, is a way of sharing information with customers. Seminars also can be conducted at a customer's facility.

Choosing abrasives

After determining which equipment best suits your shop, the next step is to chose which abrasives are best for specific cleaning applications. According to Ervin Industries, Ann Arbor, MI, the shot that transmits the greatest amount of energy to the workpiece with the greatest fatigue life will produce the most economical and consistent shot peening operation. Shot with proper hardness, microstructure, chemical analysis, size and a minimum of physical defects will continue to provide greater transmitted energy and fatigue life.

Michael Wigart of LS Industries, Wichita, KS, said choosing the right abrasive is critical to getting the right finish. "There are many types of abrasive media available for airless blast equipment," explained Wigart. "One of the first things we ask our customers is what type of finish are they looking for. This helps us decide what type of machine and media to choose."

Although the most common abrasive is steel shot, there are a number of other media used in blast equipment including grit, sand, glass beads, plastic, zinc, walnut shells, coal slag, aluminum, stainless steel, soft shot, ultra blast and a form of ceramic and plastic mixture.

According to Sunnen's Marble, while beadblasters (also referred to as glass bead machines) can use many types of media, glass beads are the most popular media used in these machines to clean engine parts. "Glass beads are basically round in shape and are designed to break into smaller pieces when subjected to high impact forces," he said. "This allows glass beads to be used as an effective blast media for most alloys without being so aggressive that they cause damage to the surface of the parts being cleaned."

Plastic shot, which can be used in applications similar to glass beads, which won't damage metal and has little chance of causing damage due to shot retention, has a short cycle life and can be expensive.

Some small parts and electrical rebuilders favor zinc abrasive. Wire cut zinc has a long cycle life and can produce a satin finish on aluminum components.

Sand and grit media can also be used in beadblasters, but these are typically harder than glass beads and have sharp, irregular edges resulting in a much more aggressive media. Although inexpensive, these materials can cause etching to critical surfaces during the blast cycle.

Sand and grit can even become temporarily imbedded into the part, especially on soft alloys such as aluminum. If the media comes loose later, severe damage can be the result. Sand and grit media, while often cheaper to purchase, are not typically the best choice for critical engine parts.

In airless blasters, engine parts are most often cleaned with steel shot or stainless steel blast media (often made from cut stainless wire). According to those we talked to, steel shot works well in most applications and is relatively inexpensive per pound. But steel shot doesn't last as long and produces more dust than stainless steel shot, so filter systems need to be adequately sized and require more frequent maintenance. Stainless steel shot will provide a brighter, more polished finish on both cast iron and aluminum alloys. And stainless steel shot is non-magnetic, making it easier to remove from complex castings.

Aluminum abrasive is a media that works well on softer parts - like aluminum - because it is, itself, a soft metal. And aluminum abrasive will give a softer, brighter finish to parts.

LS Industries' Wigart said aluminum shot, which comes in the same sizes as steel shot, is used by some rebuilders for cleaning heads. "It generally takes longer to clean with aluminum shot as it is softer," he said. "It will leave an aluminum skid mark on cast iron and other hard materials and therefore is not used in some applications where contamination of an internal part would be critical."

Although aluminum shot produces a good finish on aluminum parts, the dust it generates can be explosive. Explosions can occur by spontaneous combustion generated when oxygen that is occupied by the dust comes into contact with moisture.

Gus Enegren, president of Viking Corp., Wichita, KS, said such explosions normally occur when rebuilders load parts still wet with solvent into the blast cabinet, adding that there are also other causes of combustion. "A static discharge in the dust collector or the impact of abrasive striking parts in the blast cabinet can also create a spark to ignite dust in the system," Enegren said. "Careful and meticulous grounding of the machine frame and the electrical connection can minimize risks, but they will always be present."

Kansas Instruments' Cox said proper dust filtration will help aid in the prevention of combustion. "Aluminum shot is very light and can be removed by most dust collection systems," Cox said, adding when operating equipment using aluminum shot, a dust collection system is required, and a Class-D fire extinguisher should be available close by.

Pangborn's Ebersole said there are safety packages available for the blasting machines so that should an explosion occur, it will exit the machine at its weakest point. "The weakest point is in the rear of the machine, opposite from the operator," Ebersole explained.

Shot size

After the blasting media is chosen, the next consideration is shot size. Shot size is a critical issue for rebuilders, as the size of the blast media affects the blast pattern, as well as the surface finish. A shot size should be chosen that will not cause damage to sensitive or critical surfaces, but will still effectively clean the types of parts and alloys that need to be blasted in a reasonable amount of time. The larger the shot, the more aggressive it will be.

Every shot machine propels a constant number of pounds of shot per hour, which depends either on the horsepower of the centrifugal wheel or the volume of air used per hour in the air blast machine. Therefore, the number of particles in a pound is a function of the particle size. For example, a pound of 660 shot contains about 14,000 particles. A pound of 330 shot (half the diameter of 660) contains about 110,000 particles, eight times as many as the 660 shot. So a wheel that throws one pound would throw 14,000 impacts of 660 shot or 110,000 impacts of the 330 shot.

Using larger media particles for blasting does not mean those particles will clean faster than smaller ones. Blasting with larger particles will leave some surface areas untouched because there are fewer particles per square inch hitting the surface. But consider that a small shot particle may not have sufficient energy to peen effectively and provide the compressive stresses to the depth required. Therefore it is important to select the smallest size of shot that will produce the needed intensity, as small shot will saturate the surface more rapidly.

Many rebuilders, Guyson's Christiansen notes, will test-blast surfaces to determine the best particle size to use. "Unless the choice of medium is already known, there is no substitute for a media selection process consisting of the demonstration of alternatives, trials and thorough evaluation of results," he said. "There is well over 100 different blast media choices of natural and man-made materials available in a myriad of sizes and screenings with particle diameters anywhere from 50 microns (.002") to more than 2 mm (.079"). Only extensive experience and experimentation can guide in blast medium selection."

Hardness:

Blasting media also come in different hardnesses. The hardness of most abrasives (excluding steel abrasive) is measured by the Moh's Scale which ranges from one (softest) to 15 (diamond). Steel abrasive shot is measured on a Rockwell "C" scale and stated in RC increments. The standard hardness range for cast steel shot and grit used in blasting equipment is 40-50 Rc.

Media hardness plays an important role in selecting shot for peening. The harder the material, the faster it will clean and the faster it will break down. The hardness of shot for shot peening must be approximately equal to the hardness of the work being peened. Shot that is too soft will not produce the proper level or depth of residual compressive stresses. The effect of low hardness shot cannot be compensated for by increasing peening cycle times.

Abrasive density or the weight by volume in cubic feet, is another consideration. The more dense the abrasive, the greater the energy its particles deliver to the part. But the abrasive's density may contribute to its weight. Whereas sand weighs about 100 lbs. per cubic foot, steel grit weighs about 250 lbs. per cubic foot.

The density of steel shot in an airless blast application is not much of a factor. However, it becomes a factor in suction blast applications where the blast unit may not have enough suction to pick up and throw steel abrasives which are heavier than natural abrasives such as sand.

Another consideration when choosing a blasting media is the material's friability, or its tendency to break up upon impact. Materials with a low friability have a longer life cycle than a high friability material. For instance, steel shot has a low friability, and can be reused 200 times or more. Silica sand, on the other hand, has a high friability factor and cannot be reused.

LS Industries' Wigart said disposal of used shot is yet another issue rebuilders need to address, adding how abrasives are disposed of depends on what they were used for. "Steel shot and grit are not hazardous material and can be processed with other rubbish that is thrown away," Wigart said. "But if the application, for example, is to remove a material such as lead-based paint, then it becomes a hazardous material and should be disposed of according to local, state and federal Environmental Protection Agency (EPA) guidelines."

Depending on the type of parts cleaned during the life of the media, shot can also be contaminated with hazardous materials such as heavy metals. Spent abrasive should be tested and disposed of accordingly. The burden of proof to determine if a waste is hazardous or not, rests on the waste producer.

As we see, shop owners have many options when it comes to using blasting as a cleaning process. And there is no all-inclusive media, machine or shot size that can be used for every application. Each type of media and blasting machine offers its own set of features and benefits which must be matched to the parts being cleaned. But through matching the right abrasive and blasting equipment with your shop's needs, and adhering to scheduled maintenance routines on the equipment, blasting can be a viable and cost-effective parts cleaning process for the rebuilder.

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