An encyclopedia could be written on the various abrasive types and mixes available, but as sources described, one common thread is the importance of the proper media mix. A cleaning application may call for S-280 shot (called such because 85 percent of the shot particles are retained on a 0.0280-in. mesh screen).
The particles, though, break down into smaller particles after use as they cycle repeatedly through the machine. So what starts as S-280 eventually becomes S-230, S-170, S-110, and smaller before being separated out into a hopper.
Having an even balance between different particle sizes is critical, which is why adding media to a machine can be a delicate affair. As the machine discards used, too-small-to-be-effective abrasive, the same amount of new, larger abrasive should be fed into the system.
“The rule of thumb is that you will consume, in pounds per hour, half of what your wheel horsepower is,” said Tyler Cotton, president of Blast Abrade Inc., Elyria, Ohio. “So if you have a single blast wheel that’s 20 horsepower, you’ll consume about 10 pounds of shot per hour. If you have four 20-HP blast wheels, that’s 80 HP altogether, so you’ll consume about 40 pounds of media per hour.”
Many machines add media automatically, but some systems require operators to add media as needed. To do so they monitor an ammeter that measures the amperage load on the wheel. For most efficient usage, that ammeter should be at a fully loaded reading, as specified by the equipment manufacturer.
But say an operator on a prior shift didn’t pay attention to the ammeter and fails to replenish the system with media. So as small particles separate out and exit the system, the abrasive level declines significantly. This puts a serious damper on blasting efficiency, but it also throws the media mix off-kilter.
Figure 3: The wheel turbine is the heart of the blasting system. Photo courtesy of Rosler Metal Finishing USA.
An inexperienced operator on the next shift might notice that the abrasive level is low and so dump a large amount of fresh media into the system, only to find he’s still having trouble. The ammeter shows the wheel is fully loaded, but the mix level remains uneven, with too many large particles. This swings the pendulum the other way, making the blasting action much too aggressive. In this case, he must cycle the media through the machine (with test pieces, to avoid accelerated machine wear) until enough particles wear, re-creating the optimal mix of media sizes in the system.
“Fluctuations in media consistency in your blast system can really throw a monkey wrench in your processes down the line,” Seabrook said, adding that along with monitoring media levels, operators need to keep an eye on parts exiting the system. “Some parts can carry a lot of abrasive media out of the machine.”
Finding the Hot Spot
When setting up a machine, the operator needs to read the blast pattern, often called “checking the hot spot” because the impacted area becomes hot to the touch. Running a painted test piece through the machine (30 seconds of media exposure usually does the trick) can reveal what that blast pattern looks like.
“If you blast for 30 seconds, you feel the area on the part, and it’s not hot, well, it’s hot somewhere,” Cotton said. “The media may be hitting the roof or floors [of the cabinet], so you have to make the adjustments until you get that hot spot on your setup piece.”
The position of the control cage opening determines where the abrasive shot or grit wave will start and, ultimately, the resulting blast pattern. The size of the control cage opening determines how long the pattern will be and, hence, how dense the impacts are on the workpiece (see The leading edge of the control cage opening, which fits in the center of the wheel, determines where the blast wave starts and, ultimately, the resulting blast pattern. Image courtesy of Wheelabrator Group.”
Figure 4 and Figure 5).
The abrasive media’s path between the control cage opening and the workpiece isn’t a straight line. Once the media exits the control cage, the wheel blades rotate and propel the particles in a completely different direction. To account for this, control cages are set to positions analogous to a clock face. A control cage set at, say, 1 o’clock may propel a wave of shot or grit downward, close to the 6 o’clock position, depending on the wheel diameter, RPMs, and other parameters.
But not everyone in a plant may know this. “A classic problem occurs when an inexperienced factory maintenance person looks at the cage opening and thinks that’s where the media comes out, and so aims that opening directly toward the workpiece,” Roth said. “But it takes up to 180 degrees to come off the blades. So when he’s done, the system blasts media right up into the wheel housing, and the shot or grit is batting around everywhere. You can destroy a wheel housing very quickly doing this.”
Why Preventive Maintenance Matters
When components wear, things go awry, and their causes may not be obvious. A.W. Mallory, in his back-to-basics guide Guidelines for Centrifugal Blast Cleaning, describes a situation in which the operator sees the ammeter drop below full load. This means the wheel isn’t throwing enough abrasive and is below its maximum cleaning power—so he should add more media, right?
Not necessarily. If the operator shuts off abrasive flow to the wheel and sees the ammeter jump briefly to full load before falling off to a no-load reading, the wheel actually may have excessive abrasive flow and, like a car engine, be choked or flooded. If the needle just drops after shutting off abrasive flow, then the wheel is indeed being starved of media.
However, lack of media in the hopper still may not be the problem. Both a flooded or starved wheel may be caused by a malfunctioning flow-control valve, worn wheel parts, power loss from motor drive problems, or obstructions in the abrasive recirculating system.
Figure 5: Optimizing the blast pattern for the job at hand is critical for proper blast machine setup. Image courtesy of Rosler Metal Finishing USA.
The last includes an air-wash separator system that removes scales, fines, and tramp metal from used abrasive, and also filters out abrasive particles that are too small to use—again, to maintain the optimal abrasive mix. Spent abrasive and particles from the workpiece are moved via gravity, or rotary-screw or shaker conveyors, from the base of the blast cabinet, up an elevator conveyor system, through a screen mesh, and to the air-wash separator, where the media falls in what should be a uniform curtain. Air flows through the curtain to separate out the waste particulate, which falls into a collection hopper, while dust and fines are blown to a dust collector (see Figure 6 ). The good abrasive flows back into the system for reuse.
“The length of that curtain is critical to maintaining your abrasive mix,” Seabrook said. “If it’s too long, you don’t get the right air-to-shot ratio. If it’s too short, your abrasive curtain is too thick for the air to flow consistently. We recommend that the operator check to ensure he has a full curtain of abrasive [in the separator] at least once a day.”
As Mallory’s guide details, too much airflow through the curtain can remove excessively large particles; too little airflow won’t remove fines; either problem negatively affects the blast media mix. Airflow problems also can come from holes or leaks in the separator housing. If the abrasive curtain is uneven, something may be lodged in the screen mesh above the separator unit, or the baffles or spreader bars may be improperly adjusted.
The air-wash separator can’t work properly if the dust collector isn’t properly maintained. The dust collector needs to be maintained and cartridges changed periodically to achieve a certain differential pressure, specified by the manufacturer. “If you open the blast cabinet and see a puff of dust, you may have problems,” Roth said.
Other high-wear items include internal blast-wheel components, including the impeller, control cage, and blades on the wheel itself. “As those internal components start to wear—especially the impeller and control cage—the blasting media does not flow to the blades properly. This can cause abrasive turbulence inside the wheel, increasing wear and causing the abrasive wave to spread out farther, thereby making it less concentrated than it once was,” Seabrook said. This all makes blasting less efficient.
“If your blades start to wear as well,” Seabrook said, “blast media will go all over the place. If they wear too much, you can actually shatter the components inside the wheel.”
The cabinet liner is another component that needs to be regularly checked and replaced. “A telltale sign of a failed liner is a hole in your cabinet and high-speed shot sailing across the shop,” Seabrook said.
Basic wheel-blast operation and maintenance haven’t changed for decades, but that doesn’t make the process any less critical. In any fab shop, blast cleaning occurs near the end of the manufacturing process. A lot of upstream labor—cutting, bending, welding, grinding—goes into any part entering the blasting system. The later a problem occurs in manufacturing, the more expensive that mistake is.
Abrasive blasting, more commonly known as sandblasting, is the operation of forcibly propelling a stream of abrasive material against a surface under high pressure to smooth a rough surface, roughen a smooth surface, shape a surface or remove surface contaminants. A pressurised fluid, typically compressed air, or a centrifugal wheel is used to propel the blasting material (often called the media). The first abrasive blasting process was patented by Benjamin Chew Tilghman on 18 October 1870.
There are several variants of the process, using various media; some are highly abrasive, whereas others are milder. The most abrasive are shot blasting (with metal shot) and sandblasting (with sand). Moderately abrasive variants include glass bead blasting (with glass beads) and media blasting with ground-up plastic stock or walnut shells and corncobs. Some of these substances can cause anaphylactic shock to both operators and passers by. A mild version is sodablasting (with baking soda). In addition, there are alternatives that are barely abrasive or nonabrasive, such as ice blasting and dry-ice blasting.
In wheel blasting, a spinning wheel propels the abrasive against an object. It is typically categorized as an airless blasting operation because there is no propellant (gas or liquid) used. A wheel machine is a high-power, high-efficiency blasting operation with recyclable abrasive (typically steel or stainless steel shot, cut wire, grit, or similarly sized pellets). Specialized wheel blast machines propel plastic abrasive in a cryogenic chamber, and is usually used for deflashing plastic and rubber components. The size of the wheel blast machine, and the number and power of the wheels vary considerably depending on the parts to be blasted as well as on the expected result and efficiency. The first blast wheel was patented by Wheelabrator in 1932.
A blast cabinet is essentially a closed loop system that allows the operator to blast the part and recycle the abrasive. It usually consists of four components; the containment (cabinet), the abrasive blasting system, the abrasive recycling system and the dust collection. The operator blasts the parts from the outside of the cabinet by placing his arms in gloves attached to glove holes on the cabinet, viewing the part through a view window, turning the blast on and off using a foot pedal or treadle. Automated blast cabinets are also used to process large quantities of the same component and may incorporate multiple blast nozzles and a part conveyance system.
There are three systems typically used in a blast cabinet. Two, siphon and pressure, are dry and one is wet:
- A siphon blast system (suction blast system) uses the compressed air to create vacuum in a chamber (known as the blast gun). The negative pressure pulls abrasive into the blast gun where the compressed air directs the abrasive through a blast nozzle. The abrasive mixture travels through a nozzle that directs the particles toward the surface or workpiece.
Nozzles come in a variety of shapes, sizes, and materials. Tungsten carbide is the liner material most often used for mineral abrasives. Silicon carbide and boron carbide nozzles are more wear resistant and are often used with harder abrasives such as aluminum oxide. Inexpensive abrasive blasting systems and smaller cabinets use ceramic nozzles.
- In a pressure blast system, the abrasive is stored in the pressure vessel then sealed. The vessel is pressurized to the same pressure as the blast hose attached to the bottom of the pressure vessel. The abrasive is metered into the blast hose and conveyed by the compressed gas through the blast nozzle.
- Wet blast cabinets use a system that injects the abrasive/liquid slurry into a compressed gas stream. Wet blasting is typically used when the heat produced by friction in dry blasting would damage the part.
A blast room is a much larger version of a blast cabinet. Blast operators work inside the room to roughen, smooth, or clean surfaces of an item depending on the needs of the finished product. Blast rooms and blast facilities come in many sizes, some of which are big enough to accommodate very large or uniquely shaped objects like rail cars, commercial and military vehicles, construction equipment, and aircraft.
Each application may require the use of many different pieces of equipment, however, there are several key components that can be found in a typical blast room:
- An enclosure or containment system, usually the room itself, designed to remain sealed to prevent blast media from escaping
- A blasting system; wheel blasting and air blasting systems are commonly used
- A blast pot — a pressurized container filled with abrasive blasting media
- A dust collection system which filters the air in the room and prevents particulate matter from escaping
- A material recycling or media reclamation system to collect abrasive blasting media so it can be used again; these can be automated mechanical or pneumatic systems installed in the floor of the blast room, or the blast media can be collected manually by sweeping or shoveling the material back into the blast pot
Additional equipment can be added for convenience and improved usability, such as overhead cranes for maneuvering the workpiece, wall-mounted units with multiple axes that allow the operator to reach all sides of the workpiece, and sound-dampening materials used to reduce noise levels.