EFFECTIVE USE OF STEEL SHOT AND GRIT FOR BLAST CLEANING

Effective Use of Steel Shot and Grit for Blast Cleaning

By E.A. Borch, Ervin Industries, Inc.

Unified Technologies is proud to offer a full range of top-quality continuous-duty automatic centrifugal wheel blast machines. Our shot blast equipment will improve your results, reduce labor costs, and save you money. Get the facts and discover why our reliable and efficient equipment is unstoppable!

Essential for an effective and cost efficient blast cleaning operation is a solid, basic understanding of the characteristics of cast steel blast cleaning abrasives, their selection and use, as well as knowledge of the blast cleaning equipment, its maintenance, and key process control features.

Metalworking industries are the principal users of cast steel shot and grit: Steel mills, ferrous and nonferrous foundries, forge shops, and metal fabricators. Blast cleaning with steel abrasives is a vital and critical operation at various stages of primary metal production. The basic functions performed by blast cleaning fall into these categories:

  1. Remove surface contamination, providing a completely clean surface that aids in inspection for process defects.
  2. Surface preparation: provide a surface profile (etch, matte finish, or anchor pattern) preparatory to further processing such as painting, coating, bonding, etc.

The Blast Cleaning Process

Blast cleaning with cast steel shot and grit can best be described as an impact cleaning operation, in which the workpiece surface is subjected to successive bombardment by a high velocity blast stream containing millions of hardened, effective-size cast steel abrasive particles. The effect of this powerful blast stream impacting upon the work is two-fold:

  1. Contaminant is broken, pulverized, and removed, exposing the clean, virgin metal surface.
  2. Simultaneously, the impact-force of the individual steel particles will impart to the clean workpiece surface a finish profile, the appearance and texture of which is determined by the user’s choice of steel abrasive size, hardness and shape (shot or grit).

Mechanics of Blast Cleaning

Properly operated, not only will the blast cleaning process be effective in satisfying the users’ quality needs and goals, but will also assure optimum productivity and the lowest possible operating costs.

Surveys conducted by Ervin Industries’ Blast Cleaning Task Force have revealed that seven out of ten users of the process often fail to observe proper operating practices. Result: Substandard finish quality, productivity 33% to 50% lower than it should be, and oper¬ating costs 50% and more higher than they should be. There are three key operating variables that account for 90% of the reasons why users’ fail to achieve both effective cleaning and acceptable operating costs.

The purpose of this article is to make users aware of the three variables and describe the simple, easy-to-use control measures needed to protect against the devastating problems they cause.

Given that it is the impact-force of the individual steel particles that performs the dual function of contaminant removal and profiling, it is necessary to understand how the impact-force is first generated, then harnessed and controlled, to assure effective and cost-efficient results. Certainly, it can be seen that the process must impose a mighty big challenge for these mighty small steel abrasive particles:

S-660 size – 1/16 inch

S-70 size – approx 1/ 100 inch

Impact energy of the steel abrasive is governed by its mass and velocity in accordance with the equation for kinetic energy: KE = 1/2 MV squared.
The key to understanding the effect on the “mass” factor via size choice, is that the mass of a sphere varies as the cube of its diameter.

Doubling shot size increases the mass, or impact-power per pellet eight times! Conversely, doubling shot size reduces the pellets per pound to one-eighth.

Velocity is derived from either airless blast equipment where steel shot or grit is hurled by centrifugal force from a bladed wheel (Fig. 1), or by air-blast equipment in which the abrasive is contained and metered into a compressed airstream via conveying hoses and nozzles to impact on the workpiece. Velocity in centrifugal blast units is governed by wheel diameter and RPM. Standard 19-1/2″ diameter wheels, at 2250 RPM, develop an abrasive velocity of approximately 245 FPS.


Fig. I. A bladed wheel hurls the abrasive in centrifugal blast units.

Field experience, over the years, has found the velocity of 245 fps to be effective for the vast majority of blast cleaning applications. Where standard wheels are in use, the velocity factor can be consid¬ered a constant. Thus, the impact-force delivered to the workpiece will change only if the mass factor (the abrasive size) is altered. The relationship of abrasive size to both impact-power and coverage is shown in Fig. 2

Impact-Force: How Much?

Looking at the other side of the coin, what are the character¬istics of the contaminant to be removed? How much impact-force is required?

Before the advent of metallic abrasives, blast cleaning was done using sand as the media, i.e, sandblasting. Even with lightweight sand, the impact was sufficient to remove the contaminant and produce an etch finish. Size for size, at the same velocity, steel abrasive has 2-1/2 times more impact-force than sand, and when steel shot or grit is larger than sand, its impact-force would be many, many times greater, thereby cleaning faster and better.

Consider, for example, oxide scale. Typically, it is hard and brittle. If a small piece is pried or chipped off, then struck lightly with a hammer, it will go to powder. It doesn’t require a sledgeham¬mer blow (which would also do a great job of destroying the work¬piece).

The challenge for removing most oxide scale is not its tough¬ness and high resistance to fracture. It is primarily the manner in which it is attached to the workpiece at the interface between the virgin metal and the first layer of oxide scale. Each fraction of each square inch must be impacted by the abrasive. Only by hurling many millions per minute of the mini-ballpeen hammers (shot) or mini¬ chisels (grit) at the workpiece can the job be done effectively and cost-efficiently.

An Effective Work-Mix

An effective, cost-efficient work-mix contains a properly balanced distribution of large, medium, and small particles. The larger pellets, with maximum impact-force, must be large enough to perform the major task of loosening thick, tightly adhering contami¬nants, and still provide an acceptable finish profile. The small parti¬cles provide the coverage necessary for fast removal of the lighter contaminants, and to scour and clean rust, etc., in minute pits and crevices that large shot or grit cannot reach.

Selection of the new, original size shot or grit to be used, auto¬matically determines how large the largest particles in the work-mix wilI be. What determines how small the smallest particles should be? First, all contaminants (oxide scale, sand, spent abrasive, etc.) must be kept out of the work-mix. (As little as 2% sand in the work mix can double wear on blast wheel components.) The separator and dust collector system, its condition and its operation, determine what is removed – and what remains in the system!

How small can the abrasive particles be and still aid in clean¬ing? Perhaps this is best answered by pointing out that shot as small as S70 is effective in removing tenacious oxide scale from hot-rolled stainless steel strip.

Development of a Work-Mix

Given reasonably good assistance via proper operating practices, Mother Nature will develop a well-balanced work-mix. While sand as a blast cleaning media could not withstand the punishment of even one impact against the workpiece, steel shot and grit will withstand many hundreds of punishing impacts before finally succumbing.

The first effect of ilie impact punishment is to work-harden the surface of the steel particle, which leads to the flaking/spalling action shown in Fig. 3. This “onion-peel” effect can cause a pellet to lose enough diameter to become one or two sizes smaller than when new.


Fig. 3. Flaking/Spalling of Steel Shot

During the flaking/spalling phase, severe internal damage is inflicted on the pellets, evidenced by voids and ruptures, which ultimately work their way to the surface and cause fracture-failure. Fractured particles, amazingly, upon further repeated impacts, are cold-worked, or forged, into spheres of smaller diameter, which ultimately re-fracture into even smaller particles.

It is this combination of events that produces the work-mix. This mixture of new, nearly new, onion-peeled, and fractured particles make up the needed balance of proper impact and coverage. The size distribution of tile work-mix will undergo constant change, as spent fines are exhausted from the system and new abrasive additions are made (frequently enough to avoid having the work-mix swing from too fine to too coarse).

Noting the tremendous differences in impact by shot size in Fig. 2, it is obvious that controlling the size distribution in the blast stream work-mix must be a high-priority challenge for the user’s cleaning room team.

Out-of-Balance Work-Mixes

A work-mix with a preponderance of fines has insufficient impact-force to be effective (too few large pellets to break up thick contaminant). Conversely, a work-mix with a preponderance of large pellets has a low pellet count resulting in a wide, open pattern that requires much more blast time to do the job.

Out-of-balance work-mixes requiring extended blast time, or reduced line speed, and/or re-blast, have serious adverse effects on product finish, productivity, and operating costs.

Troubleshooting Check-List

Work-Mix Too Coarse: Causes

  • (a) Large and infrequent additions of new material.
  • (b) Excessive air through separator, pulling out medium and small abrasive particles.
  • (c) Excessive carry-out with work, requiring heavy replenishment.

Work-Mix Too Fine: Causes

  • (a) Poor distribution over shed plate.
  • (b) Insufficient air through separator.
  • (c) Excessive time lapse between additions.
  • (d) Large, sporadic additions of recycled abrasive from floor spillage or system leakage.

Controlling the Work-mix

  1. Add new abrasive every operating shift. Maintain feed hopper at or above 3/4 level at all times.
  2. Do not allow abrasive spillage or leakage to accumulate; return to system daily.
  3. Check work-mix size distribution weekly. Recommended: Use the Ervin Spot-Check Gauge (Fig. 4), which requires less than five minutes to use, providing instant feedback indicating whether or not the work-mix is in proper balance.


Fig. 4. Ervin Spot-Check Gauge

The Blast Stream … ls It On Target?

Misdirection of the blast stream, with some abrasive missing the work, and impacting instead on equipment wear parts, results in these problems:

  1. Incomplete contaminant removal
  2. Excessive parts wear
  3. Excess machine downtime
  4. Excess abrasive usage
  5. Lower productivity due to extended blast time or re-blast

When asked—”When did you last check your blast pattern, and how frequently is it done?”—most users did not know, never having seen it done. Other thought it unnecessary, because “the setting of the wheel clock dial is where it’s always been.” Ervin Task Force surveys have found as many as 7 out of 10 with off-target blast streams—fig. 5. As little as a 10% shift in blast pattern away from proper aim can translate into 25% loss in cleaning efficiency.


Fig. 5. Poorly aimed blast stream

What must be recognized is that the inevitable factor of wear and tear on blast wheel components will eventually cause a shift in the location and concentration of the blast pattern. Exceptional wear tolerance has been built into blast equipment, but when wear goes beyond that tolerance, components can no longer perform properly, and the blast pattern strays from target.

Consider a 40 HP wheel: Every minute, 1,000 pounds of abra¬sive passes through the impeller, out the control cage opening, and is then hurled off the blades. Impeller: When wear on the leading edge of the impeller segments exceeds 1/8″, the abrasive will hit the back of the blade rather than being delivered to the throwing face. The hot-spot and overall blast pattern becomes badly diffused. Bad aim! Control Cage: When wear of the beveled edge exceeds 1/2″ (in some cases only 1/4″, the blast pattern is lengthened, often to the point some abrasive misses the work. Blades: When blades become deeply grooved, channeling of the abrasive occurs, and because it is not flowing across the full width of the blade, the pat¬tern is distorted. Tramp Metal: When wedged between the impeller and control cage, tramp metal can cause the cage and the blast pattern to shift.

Checking the Blast Pattern

Consider where the workpiece surface to be cleaned is in rela¬tion to the blast wheel. The object is to place and secure a sheet metal target-plate at the location of the workpiece, then, after blast¬ing for 10, 20, or 30 seconds, check to see the area and location of the blast pattern. The hot-spot (hot to the touch, and usually about 3″ x 10″) should be located approximately 8″ in advance of the centerline of the wheel. This is the area of concentrated, maximum intensity.

While inspection of degree of wear of the components should be routine (every eight (8) hours of operation), determining the exact degree of wear vs. the critical wear tolerance is difficult at best. Too often judgment is erroneous. A weekly blast pattern check is a foolproof procedure that tells you, now, whether the blast pattern is on target.

Insufficient Abrasive Flow

Ammeter: This sensitive device registers the amperage load on the motor driving the blast wheel. It is the only practical way of deter¬mining how much abrasive is thrown by the wheel during the blast cycle. Peak efficiency from the blast equipment is attained only when the wheel throws the rated maximum quantity of abrasive (Fig. 6).


Fig. 6. The ammeter at full load Fig. 7. Low ammeter reading

When blast cycle time, or line speed, is based on the rated maximum flow, but the ammeter shows less than full load amps (Fig. 7), incomplete contaminant removal occurs. When, in order to improve cleaning, blast cycle time is increased, line speed is reduced, or work needs re-blasting, productivity is slashed and costs skyrocket.

Low Amps — Causes:

  1. Failure to recognize that the ammeter is a sensitive device in a rough working environment makes it subject to failure and inaccuracy. What to do: Calibrate all ammeters on a regular basis. Be sure to keep them in good working order.
  2. Failure to establish and post the target full-load amps right above the ammeter so operators are fully aware of what the amp reading must be. What to do: Post full-load amp reading above the ammeter and make sure it is clearly visible at all times.
  3. Hopper level too low. What to do: Abrasive additions to be added each operating shift; maintain feed hopper at least 3/4 full, always.
  4. Excess wear of the impeller, which can lead to flooding the wheel spout as the impeller loses its ability to keep up with the abrasive flow. What to do: Inspect impeller wear daily. When leading edges are worn more than 1/8″, change the impeller.
  5. Belt slippage: On the drive from motor to wheel-shaft – power transmission will fall below normal. On elevator belts – elevator unable to deliver required quantity of abrasive. What to do: Check belts regularly.
  6. Foreign material in the system, impeding abrasive flow through scalp screens, elevator boots, spouts. What to do: Keep all trash out of the blast cleaning equipment.

Recognizing Problems

The key to effective and cost-efficient blast cleaning lies in being able to recognize problems as and when they occur. This can be done with the fast, easy system checks described. Education is important in understanding the tools being used and their function, and it also makes the operational data more meaningful to the worker.

A solid, basic understanding of the principles involved in blast cleaning, coupled with discipline in following a performance evaluation program (SPC) will make the blast cleaning department an effective and cost-efficient part of any operation.

The basics of wheel blasting

The fundamentals of wheel blasting for part cleaning and paint prep

THE FABRICATOR AUGUST 2013
AUGUST 1, 2013
BY:
TIM HESTON

Blast cleaning can be an extremely effective way to prepare a part for coating. But if the machine isn’t set up, maintained, or monitored properly, the operation can go awry.


Figure 1: In blasting, it’s the media—be it angular grit or round shot (a magnified view depicted here)—that does the work. Image courtesy of Rosler Metal Finishing USA.

A fabricator can spend serious time manually preparing workpieces to obtain an optimal finish. If the paint or coat coming off the line doesn’t adhere properly, often it’s because someone skipped a surface preparation step, or didn’t prepare the entire surface completely. A company can ensure a finishing team knows the best techniques for all this, or, if volumes are sufficient, buy or send the part out for blasting. The latter seems much, much simpler.

But blast cleaning really is anything but simple. Consider the common wheel blast system (see Figures 1 and 2). It could be an inline system, where a monorail or conveyor carries parts in a continuous flow; or a batch system, where batches of parts are placed in a blast cabinet. In both systems, a blast wheel rotates and throws shot or angular grit toward the workpiece, cleaning off rust and mill scale and prepping the surface for paint. That’s the ideal, when all variables are adjusted properly for the job at hand.

If they aren’t adjusted and monitored as they should be, the operation can go awry. At best, a worker may find that paint isn’t adhering to a batch of parts and trace it back to a problem in the blasting system—say, an improper abrasive mix. At worst, a blasting cabinet’s liner can deteriorate to the point where the machine self-destructs.

It comes back to the fundamentals, the “basic basics” of blast system operation and maintenance. The heart of the system beats inside the wheel housing (see Figure 3). Inside this unit, an impeller pumps the blast media through what’s known as the control cage located in the center of a rotating wheel, and onto the blades that throw the media toward the workpiece in an arc or wave.

This is how the machine produces a blast pattern on the part. The pattern usually is several inches wide and can be several feet long, depending on the machine and the wheel’s distance from the workpiece. The farther the wheel is from the part surface, the larger and less concentrated (that is, fewer media impacts per square inch) the blast pattern becomes. And as sources explained, it’s the abrasive media that really does the work. Four, eight, or more wheels strategically placed in a system can ensure the shot or grit impacts all workpiece surfaces that need to be cleaned and paint-prepped.

Some variables aren’t under the operator’s control. A blast machine must be designed or purchased for the application—for a specific part or part family, or, at a job shop, to handle the majority of parts requiring paint prep. If a machine doesn’t have enough wheels in the right place for a certain part, there’s little a fabricator can do but send the part out to a company that does have such a system, or prep the part manually.

But other problems are under the operator’s control. As with manual paint prep, piece parts must be degreased and cleaned. And when parts reach the machine itself, the operator needs to ensure all variables are correct for the job at hand. These include media type, size, concentration, and mix, as well as basic preventive maintenance (PM).

Media Type and Speed

The basics start at the end of the line: the parts coming out of the paint or powder coating booth. What are their coating coverage requirements, and what surface finish does the powder or paint manufacturer specify to achieve the best results? What’s the average peak-to-valley depth of the abraded surface (measured in mils, with 1 mil equivalent to 25 microns, or 0.001 in.)?

“If it’s a high-pitch peak count per inch, then you’ll use a grit media, because [compared to round shot] the angular grit can get you many more impacts in a given surface area,” said Rick Roth, blasting product manager for Rosler Metal Finishing USA, Battle Creek, Mich.

Blasting imparts kinetic energy onto the sheet metal surface, and too little or too much energy can lead to problems. The amount of energy depends on the size of the blasting media particle as well as its speed.

Figure 2: A blasting system preps part surfaces for coating quickly and completely—if, of course, it’s properly set up and maintained. Photo courtesy of Wheelabrator Group

Industry charts might show that to obtain a 3-mil surface profile a blast machine needs to be loaded with a certain media size. As Roth explained, most standard tables, from the Society of Automotive Engineers and elsewhere, are calculated with the assumption that the media is moving between 245 and 250 feet per second. (The machine and wheel characteristics determine the speed of the abrasive media. For a speed approximation, multiply the wheel diameter by its RPM and divide by 180.) But if large shot were to hit the surface of a 10-gauge sheet at that speed, the particles would warp the surface.

“In blasting, you build up compressive stresses,” said Peter Mosier, applications and sales manager at Wheelabrator Group, Burlington, Ont. “If you do that to only one side of the part and not the other, you’re going to cause the part to bend or warp.”

Round shot can have a particularly effective peening action when it hits the surface, which is why the media is used in shot peening, a far more controlled process that’s designed to induce compressive stresses on a workpiece.

“If you’re using round shot, you may intend to clean the surface, but you will also be peening it at the same time,” said Steve Seabrook, applications engineer at Wheelabrator. “If you’re cleaning 0.5-inch steel plate, a little bit of peening on the surface doesn’t matter. But if you’re cleaning light-gauge parts, you may want to use small-sized grit, which helps avoid putting excessive compressive stresses onto the part. You can control [these stresses] in a lot of different ways, depending on the kind of media you’re using, the workpiece exposure time in the blast zone, as well as the velocity and volume of the media being thrown.”

Material Mix

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.

Expensive Defects

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.[1]

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.[2] 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.

Wheel 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.[4]

Blast cabinet

A blast cabinet is essentially a closed loop system that allows the operator to blast the part and recycle the abrasive.[6] 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.

Blast room

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.[7]

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[8]
  • 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.[9]

Media

In the early 1900s, it was assumed that sharp-edged grains provided the best performance, but this was later demonstrated not to be correct.[10]

Mineral: Silica sand can be used as a type of mineral abrasive. It tends to break up quickly, creating large quantities of dust, exposing the operator to the potential development of silicosis, a debilitating lung disease. To counter this hazard, silica sand for blasting is often coated with resins to control the dust. Using silica as an abrasive is not allowed in Germany, United Kingdom, Sweden, or Belgium for this reason.[11] Silica is a common abrasive in countries where it is not banned.[12]

Another common mineral abrasive is garnet. Garnet is more expensive than silica sand, but if used correctly, will offer equivalent production rates while producing less dust and no safety hazards from ingesting the dust. Magnesium sulphate, or kieserite, is often used as an alternative to baking soda.

Agricultural: Typically, crushed nut shells or fruit kernels. These soft abrasives are used to avoid damaging the underlying material such when cleaning brick or stone, removing graffiti, or the removal of coatings from printed circuit boards being repaired.

Synthetic: This category includes corn starch, wheat starch, sodium bicarbonate, and dry ice. These “soft” abrasives are also used to avoid damaging the underlying material such when cleaning brick or stone, removing graffiti, or the removal of coatings from printed circuit boards being repaired. Sodablasting uses baking soda (sodium bicarbonate) which is extremely friable, the micro fragmentation on impact exploding away surface materials without damage to the substrate.

Additional synthetic abrasives include process byproducts (e.g., copper slag, nickel slag, and coal slag), engineered abrasives (e.g., aluminum oxide, silicon carbide or carborundum, glass beads, ceramic shot/grit), and recycled products (e.g., plastic abrasive, glass grit).

Metallic: Steel shot, steel grit, stainless steel shot, cut wire, copper shot, aluminum shot, zinc shot.

Many coarser media used in sandblasting often result in energy being given off as sparks or light on impact. The colours and size of the spark or glow varies significantly, with heavy bright orange sparks from steel shot blasting, to a faint blue glow (often invisible in sunlight or brightly lit work areas) from garnet abrasive.