Air blasting machines operate much like paint-spray systems, but with the reverse effect of removing material from surfaces. Compressed air, instead of conveying paint, carries an abrasive of the finisher’s choice, at a velocity of choice, to a target of choice at an angle of choice. Because the finisher can select from a long list of abrasives in many mesh sizes, the final result becomes a matter of preference—air blasting systems can perform almost any cleaning or profiling job with great precision.
In air blasting equipment, one of two basic approaches—commonly referred to as suction and pressure (Fig. 1)—“pull” or “push” abrasives (known as media) to desired working speeds. Suction systems rely on compressed air from a supply line to create a venturi effect within a blast gun that draws abrasives through a feed line from a storage hopper operating at atmospheric pressure. When triggered, the blast gun releases compressed air and media to the work surface. The advantages of suction systems include lower capital costs and simplified piping, particularly in applications requiring continuous operation and/or multiple blast outlets.
In the case of pressure systems, the media-storage vessel operates at the same higher-than-atmospheric pressure as the air supply line (normally between 10 and 120 psi above atmospheric pressure). When actuated, the system releases abrasives from the storage vessel into a blast hose where the difference between system pressure and atmospheric pressure (sometimes more than 100 psi) drives the abrasive particles during their entire trip to a tapered blast nozzle that adds more speed.
This continuous “push” pays significant dividends in terms of energy use. On some jobs, pressure systems reduce compressed-air consumption by 75%. Moreover, pressure systems control abrasive flow with greater precision at both low and high operating pressures. As a result, they can deploy a broader range of media than suction systems and perform a much wider variety of tasks.
Putting either of these systems to work in a production environment requires a ventilated enclosure to contain the air blasting process.
Prepping Large Parts
To remove rust, dirt, and old coatings from heavy machinery, such as rail cars and bulldozers, one or several operators air blasts their surfaces with an abrasive sized and selected to optimize results. Operators work within an enclosure, known as a blast room (Fig. 2), while wearing protective gear equipped with a breathing-air supply. Because of the relatively long distance between the blasting system and the workpiece in this type of operation, blast rooms exclusively rely on pressure blasting systems.
More sophisticated blast rooms integrate media-reclamation hardware consisting of powered recovery floors that route durable abrasives through a recycling system back into the work process, thereby lowering costs for media replacement and labor. Within large blast rooms, cranes, ground rails, overhead trolleys, work carts, and turntables represent just a few of the tools used to facilitate part handling. Likewise, the addition of catwalks and lifts improves operator access. Indeed, large blast rooms require a significant capital investment. However, when compared to alternative methods, such as hand sanding and scraping, they often present as a bargain. As a more economical alternative, finishers of large parts can slash capital costs and enjoy the savings of media reclamation by installing blast-and-recovery systems within a pre-existing enclosure.
Blast rooms continue to gain appeal because of growing environmental awareness and the development of new types of blast media. Plastic media serve as an example. First used by the U.S. Military as an alternative to stripping fragile aircraft skins with toxic chemicals containing phenols or methylene chloride—both of which are banned from landfills by the U.S. EPA and often cost more than $20 per gallon to incinerate—plastics now play an important role in stripping paint and powder coatings from sensitive substrates such as aluminum and certain composite materials (Fig 3).
Furthermore, the category of “plastic media” now includes a variety of types designed to work in different ways. For example, polyester delivers a very soft touch, one normally reserved for salvaging delicate and expensive composite parts. At the other end of the plastics spectrum is melamine formaldehyde, which excels in removing highly adhesive powder coatings from hard metals. Options lie in between, and the use of plastic media is not confined to blast rooms. Plastics represent only a small portion of air-blasting’s media arsenal. These versatile abrasives provide just one example of the many options air blasting offers within the realm of media alone.
Cleaning & Prepping in Cabinets
The term “beader” may sound familiar to those of us over 50 with experience on a shop floor. In the old days, it referred to the glass beads loaded within a blast cabinet, the jack-of-all-trades machine that fixed flaws and expedited otherwise tedious tasks. While the meaning of “beader” may have faded into history, industry’s reliance on blast cabinets to handle tough jobs has not.
Unlike blast rooms, which contain both the operator and a workpiece, blast cabinets position the operator outside of the blasting enclosure. Basic cabinets consist of a sealed shell with an operator station—including a viewing window and a pair of arm-length, sealed gloves—that enables the operator to manipulate a workpiece as well as a blast nozzle (pressure systems) or gun (suction systems) within the cabinet. With high-production cabinets, a dust collector captures debris from the process after a media reclaimer separates degraded particles from abrasives still fit for work. Besides saving operator time by automatically returning healthy media to the blasting system, reclaimers improve the uniformity of results and reduce media replacement costs.
Much like blast rooms, cabinets operate over a broad pressure band and handle a wide variety of media. More sophisticated models even include reclaimers with an air-flow adjustment for recovering abrasives of various densities. In addition, some of today’s standard cabinets can handle workpieces of up to 6 ft. in length.
Moreover, parts with indents, ridges, and rounded surfaces present no problem. In short, a quality blast cabinet enables an operator to perform more finishing tasks, quicker than any other manual method. Plus, as anyone familiar with finishing operations knows, manual methods remain a must, particularly in short-run operations where part sizes and shapes vary (see sidebar: “Manual Blasting: Still a Must”).
Blast cabinets with optional features address this reality. The addition of manual or powered turntables, for instance, helps the operator handle heavy parts. With the inclusion of a dolly and tracks, the operator can roll parts weighing as much as 1,500 pounds into the cabinet and then rotate the part for improved access. Linking an optional timer to the blast system and a powered turntable in a cabinet with pre-positioned moving nozzles or guns makes it possible to batch process parts with minimal operator involvement. As a next step toward automation, the movements of nozzle/gun oscillators can be coordinated with the powered turntable.
Manual Blasting: Still a Must
“Motor-heads” will no doubt recognize the engine part shown in the opening figure of this article. To help readers not obsessed with squeezing additional horsepower out of their automobile engines, the part, called a “header,” boosts performance by reducing back pressure in exhaust systems. Headers come in all sorts of shapes and sizes to fit under the hoods and on the exhaust ports of innumerable cars. Many are custom made and fabricated with mild steel, which rusts in a hurry when heated by engine exhaust (often over 1,000°F) flowing through the header pipes. Engine vibrations conspire with high temperatures to accelerate deterioration of the header walls: as rust shakes off, heat and oxygen dig a little deeper. Chrome plating provides some defense on show cars, but deteriorates rapidly in the heat of hard driving.
A major breakthrough occurred with the introduction of coatings consisting of metals and ceramics. Beyond stopping rust, these coatings improve engine performance. Successfully applying the metal-and-ceramic mix to either new or used headers, however, requires multi-step factory processing. Some headers arrive at a plant heavily rusted, others coated with chrome. For the metal/ceramic coating to succeed, surfaces require a consistent profile. With the many header shapes, sizes, and surface conditions rolling in for restoration, manual blasting (pressure cabinets dispensing aluminum oxide) presents the only practical choice.
To assure smooth flow of light media, such as plastics, bicarbonate of soda, and very fine abrasives, cabinets often include an aerated media regulator or a differential pressure-control system, as well as a vibrating screen within the media reclaimer. The addition of a magnetic separator to the reclaimer removes ferrous debris to help protect delicate surfaces.
When cleaning involves removing oil or other substances that contaminate blast media, single-pass pressure systems compatible with rooms or cabinets provide a fast method for cleaning tools, molds, and machine parts. Normally, these systems use an inexpensive, non-toxic abrasive, such as bicarbonate, which is discarded after one cycle.
Processing Awkward Parts
For “problem” parts, a modified blast cabinet, as opposed to a custom-engineered system, often provides the most economical solution. By maximizing the use of standard components, modified cabinet designs reduce capital costs for air-blasting equipment by up to 70% versus a blast room or an automated blast system.
Typical modifications include: connecting two cabinets with an expander to handle long parts (Fig. 4); adding baffled entrance and exit vestibules to contain blasting as long, flat sheets or pieces of pipe move through the cabinet; strengthening components to handle multi-ton parts; developing fixtures to mask workpieces; and creating shuttles that not only move parts in and out of the blast enclosure but also center them within the work envelope. Extra nozzles/guns are often added and their movements coordinated to develop custom blast envelopes. With the addition of programmable controls, processing steps for specific parts can be stored and easily recalled.
Automating Prep Work with Blasting
Automated blast systems excel in critical operations such as shot peening, where processing requires tight control and, in many cases, thorough documentation. Because human lives can depend on peening to spec in the case of aircraft components, for example, these systems include fault sensors to shut down blasting if air pressure or shot flow drifts off target, as well as record-keeping capabilities to generate histories.
Certain types of prep work demand similar precision and repeatability. During the rebuilding of truck engines, for instance, the cleaning process must quickly remove carbon deposits from pistons while keeping substrate erosion to a minimum. The deployment of multiple blast guns mounted on independent oscillators (Fig. 5) provides a cost-effective solution. Programmable controls enable systems like this to accommodate part changeovers from one piston size to another in a matter of minutes.
Air blasting also helps in prepping hard-to-reach surfaces. With a 60° angle nozzle attached to a rotating lance (Fig. 6), holes become easily accessible. Even in seemingly simple operations, such as removing rust from steel billets before forging, automated air blasting often demonstrates a competitive edge.
Despite being a mature technology, air blasting remains the best choice—and sometimes the only choice—for a wide range of dirty jobs. The good news for metal finishers: innovations in air blasting controls continue to make the process more precise, more efficient, and a bigger contributor to the bottom line.
Robert B. Heaton is the product support manager/testing laboratory for Empire Abrasive Equipment Co. in Langhorne, Pa. Before taking charge of Empire's test lab 12 years ago, Heaton functioned as the company’s chief engineer on shot peening systems. During more than a decade in this position, he worked closely with U.S. aircraft component manufacturers and airlines in the development of peening specifications, operator training programs, and automated shot peening systems.