Blast Cleaning Technology Handbook

by:
Pages: S2-S7; October, 2000

Blast cleaning, in its purest sense, relies primarily on mechanical forces (rather than chemical) to achieve cleaning performance. To blast clean, some form of media needs to be directed and "blasted" via some method of delivery, toward a surface.

The mechanisms of delivery and types of media are wide and diverse and the possible combinations are many. With this versatility, blast cleaning, if specified properly, can be used to clean in many applications. (See "How is Blast Cleaning Used?" below.)

The purpose of this technology handbook is to serve as a resource and direct manufacturers to additional sources of information, equipment, media, and systems. It is written for those considering using blast cleaning as a cleaning system, and those who are not familiar with blast cleaning as an alternative way of cleaning.

Process Choices

Blast cleaning can be used for applications from simple oil or particulate removal to deburring and removing cured paints, rust, or oxide. Methods like carbon dioxide (CO2) or ice blasting sometimes incur some form of brief chemical effect immediately on impact. However, the bulk of the work is done as a function of the kinetic energy delivered by the impacting media. Kinetic energy is defined as 1/2mv2, where m = mass and v = velocity of the media.

Blasting systems can be fine-tuned with regard to this equation to give different results. This tuning may be necessary due to material sensitivities or the need for more energy to remove difficult contaminants. This involves changing the delivered energy by changing the mass of the media or the velocity at which it is delivered. An increase or decrease in either will result in a directly proportional increase or decrease in kinetic energy. It is up to the user to decide which is most efficient for the process.

There are several methods and equipment options available for delivering blast media -- again, these depend on the application. Different applications require different media and there are many media choices from which to select. In all cases, some form of contaminant and abrasive containment and/or treatment system should be employed to not only reduce safety risks, but to reduce media recycling and reclaiming costs. For example, in applications involving CO2 or baking soda blasting, media is either vaporized immediately or can be sent to the sewer, depending on the contaminants.

Blast cleaning is line-of-sight in nature. As such, only those surfaces that can be impacted by the blast spray may be effectively cleaned. This can present a challenge when cleaning blind holes and spaces. Blast cleaning may be used in many cases as a pre- or post-treatment for liquid cleaning methods. Liquid cleaning methods also are used prior to blast cleaning to remove oils and greases, which can foul the media and make it more difficult to recycle. Suppliers of blast media systems can assist manufacturers in determining what type will work best for different applications.

What Type of System is Best?

That answer depends on the type of application. Developing the "best" system depends on specifying the right mix of equipment, delivery system, and media.

One of the first decisions to be made is the type of cabinet, room, or system that is best for your blast cleaning application. For manufacturing production environments, blast cabinets, blast rooms, and automated systems typically are employed.

Blast cabinets are designed for small- to medium-size parts. The work is done in an enclosed cabinet. The operator is isolated from the process except for glove-protected hands. Blast rooms can be utilized to accommodate larger parts, but the worker must wear protective gear. Automated systems may be designed for cleaning parts of specific sizes and types and require minimal operator exposure intervention.

These are key questions to consider:

  • Does the blasting media or the contaminants being removed need to be contained? Are they harmful to workers or the environment?
  • Will a lack of containment cause excess dust in the facility?
  • Can the work be conducted out of doors? What threats does the weather pose?
  • Will noise pollution be an issue?
  • Do we have space for a cabinet or hood?
  • What sizes are the parts being cleaned and will they fit in an enclosure?
  • Will enclosure assist in desired recycling of blast media?
  • What volume of parts needs to be blasted? Will this equipment be used in a production mode or for occasional use only?
  • What velocities are required?
  • What is the expected life-cycle of the equipment?
  • What safety features are required?

    Equipment manufacturers sell a broad range of cabinet sizes and capacities. Light-duty cabinets are for infrequent use and handle small parts. Non-production cabinets allow for more frequent use but may lack a dust collection filter area. Cabinets used for daily production needs should be sized and equipped to handle filtration requirements along with the workload.

    Delivery

    There are numerous delivery mechanisms for blast cleaning. Choice depends on both the media being used and the finish desired from the blasting process.

    Wheel blasting operates by using a high-rpm bladed wheel to deliver media to the surface being blasted. The media is delivered to the wheel where it is then accelerated centrifugally toward the surface. Wheel blasting can operate with almost any type of media but excels in efficiency over other techniques when using heavy media and shot.

    Air blasting can be accomplished via two methods: suction and pressure.

    Suction systems work on the premise that by passing air over an orifice (eduction), a vacuum is created that will draw media into the air stream. Pressure systems operate by feeding media directly into a pressurized air stream such that it then accelerates toward the surface to be blasted. Both systems carry a number of options that can be utilized to obtain a custom fit for almost any application. Air blasting is typically best suited for lighter-weight media that will respond well to air acceleration.

    Jim Klomparens, Abrasive Products, Grand Haven, MI says that one of the most critical parts of an air-based blasting system is the compressed air. If the compressor is not large enough to supply the required cubic feet per minute (cfm), the blasting equipment will not function properly. The cfm requirement is determined by the inside diameter of the blast nozzle and should be available from the manufacturer of the equipment.

    The air compressor should also have a cfm rating and will be stated as X cfm at Y psi (for example: 12 cfm at 80 psi). The importance of the compressor cannot be overemphasized. When properly matched with the blast equipment, it assures a consistent flow of abrasive and air. This consistency allows controlled finish results by increasing or decreasing the velocity at which the mixture impacts the parts. In essence, the air compressor will determine, for the most part, how well your blast equipment will work, says Klomparens.

    Wet blasting propels blast media in liquid suspension (slurry) onto the surface to be cleaned. The media used in these applications is generally small in diameter, as it must be suspended in the liquid. In some cases, thickeners like xanthan gum are used to allow heavier media to be adequately suspended.

    Media

    The media used to accomplish blast cleaning is far ranging. Abrasives are granular or powdered materials that will clean, cut, abrade, gouge or otherwise change the condition or appearance of the targeted surface. These abrasives may be a natural substance or one manufactured for blasting.

    A short list of choices includes CO2 snow and pellets, ice, baking soda, corncobs, walnut shells, plastics, glass, aluminum oxide, sand (silica), silicon carbide, steel grit, wire cuttings, and metal shot. The abrasive selected will greatly impact the speed and quality of the blast cleaning.

    Factors that influence media selection include:

    • Tenacity of soil to be removed from surface
    • Recycling requirements
    • Environmental and worker compatibility
    • Safety
    • Media cost
    • Media disposability
    • Substrate sensitivity and type, and
    • The level of desired cleanliness.

      It is very important that the correct tool be chosen for the job. Six factors to consider when selecting an abrasive are outlined in "Abrasive Selection Factors," below.

      Media Recycling

      Manufacturers using blast cleaning can realize safety, environmental, and cost benefits of recycling and re-using blast media. Although they vary in style, almost every blast cleaning equipment manufacturer can provide some type of recycling system aimed at minimizing media losses.

      When considering a system, remember the key rule in any cleaning situation: make sure the right equipment is used for the application. Different types of media and delivery systems require different types of reclaim systems in order to operate at maximum cost and performance efficiency. Be sure your vendor knows the difference.

      Safety

      As in all manufacturing processes, safety should be a primary concern. Blast cabinets used in production settings should have safety door interlocks, which shut off the blasting if the door is opened. Other safety considerations are:

    • Operator training
    • Static electricity generated by the friction of high-speed abrasives moving through blast hoses
    • Damage to hearing from noise generated by abrasive blast nozzles
    • Broken air lines and air fittings on air compressors
    • Injuries caused by failure of nozzles, blast hoses or couplings
    • Injuries while loading abrasives
    • Dust, including contaminants, from the cleaning process

      Suppliers of blast cleaning equipment and media offer technical support in specifying an appropriate blast cleaning system for a particular application.

      How is Blast Cleaning Used?

      Here are just some applications using blast cleaning technology.

      Aircraft: Clean and peen jet rotor blades and other precision parts. Clean welds, castings, manifolds, sub-assemblies, and parts before magna-fluxing or after heat-treating. Clean carbon from exhaust stacks to prevent fire and general aircraft maintenance.

      Automotive: Remove corrosion, old paint and grime from bodies, fenders, and wheels. Clean indentations and other areas not accessible to hand sanders. Remove carbon, gum, and varnish deposits from internal engine parts.

      Bicycle Manufacturers: Clean mil scale, flux and all foreign matter in preparation for new coating.

      Cement & Cast Forms Manufacturers: Remove cement from pre-cast forms leaving the exposed aggregate. Clean residue from molds, forms, crushers, rolling stock, and for general plant maintenance.

      Cylinder Repair: Remove old paint and grime from gas cylinders during required inspection and pressure testing periods.

      Contractors: Remove heavy corrosion and old paint from equipment, steel, and stone structures.

      Circuit Boards: Light deburring and cleaning flux and solder splash.

      Deburring: Remove light burrs from miscellaneous materials.

      Electrical & Electronics: Clean transistors bodies and leads, and ceramic seals. Remove excess brazing and solder from wire guides. Clean electric motors, bell housings, end bells, armatures, windings, brush holders, and starter boxes prior to reconditioning.

      Fabricating & Welding Shops: Remove weld slag from aluminum tanks and frames to inspect weld seam. Clean ornamental and hard to reach areas.

      Foundries & Forging Shops: Remove heat-treating scale from casting and forgings. Remove core sand before grinding and for enhancing product appearance from snagging. Clean all residual removal film build-up from molds for better impressions.

      Glass Industry: Clean all residual film build-up from core boxes and molds. Produces a frosted effect.

      Government: Blast cleaning and finishing for maintenance purposes at various agencies, highway department, shipyards, water tanks, aircraft, military, etc.

      Machine Shops: Deburr and finish all machined parts.

      Metalizing And Plating: Clean foreign matter form surface prior to plating.

      Pipe Maintenance & Manufacturing: Remove corrosion, old coatings, weld slag, and scale from exterior and interior prior to coating.

      Protective Coating Companies: Prepare surface properly with a uniform anchor pattern prior to applying coating.

      Plastics: Deflashing plastic parts using a plastic blasting media without affecting tolerances. Cleaning molds.

      Truck, Tank & Trailer Manufacturers: Remove rust, old paint, mill scale, corrosion, etc. Prepare surface for coating.

      Utilities: General maintenance at gas and electricity plants for cleaning turbine blades, pipe, valves, meters, etc.

      -- Econoline Abrasive Products

      Abrasive Selection Factors

    • Type of abrasive: Most natural abrasives, silica, mineral sands and garnet materials are considered as expendable and are good for outdoor blast applications. Manufactured abrasives such as glass bead, aluminum oxide, silicon carbide, and plastic are reusable and used in systems that recycle the material.

    • Physical shape: The shape of the abrasive will define the result and speed of the blast process. Angular, sharp, and irregular shaped abrasives will clean faster and etch the targeted surface. Round and spherical abrasives clean parts without removing any of the base material(s) of the targeted piece.

    • Hardness: The hardness will influence many of the results obtained by blasting. These include the speed at which it cleans, the amount of dust created and the breakdown/reuse factor.

    • The dust factor: Most expendable abrasives inherently produce more dust. A softer abrasive will break into more particles than harder ones. Larger manufactured abrasives will create less dust than smaller mesh sizes.

    • Mesh size: The physical size of a grain of abrasive. Abrasives are passed through a series of screens and then collected off several of these screens giving the abrasive a controlled size. Generally, the smaller the mesh number, the larger the abrasive particles.

    • Consistency: Natural abrasives will not be as consistent as manufactured ones. When a consistent finish is required, purchase a quality abrasive of uniform mesh size that will not break down quickly.

      -- Econoline Abrasive Products

    • To Learn More

      Additional resources are available online on www.PartsCleaningWeb.com

      "Tech Spotlight - Blast Cleaning"
      A focus on blast cleaning equipment
      Pages: 36-37; September, 1999

      "The Power of Crystals: The Kinder, Gentler Strength of Baking Soda"
      by: Paul G. Arle
      By the tablespoonful or by the blast pot full, baking soda is showing up in places you might least expect. In the cleaning industry, manufacturers are discovering the attractive benefits of a low-impact blasting process known as "soda blasting."
      Pages: 14-22; July, 1999

      "Dry Ice Blasting Basics"
      by: David R Linger
      A natural by-product of manufacturing, carbon dioxide is pelletized and put to use as a cleaning agent.
      Pages: 9-14; January, 1998

      "Having A Blast"
      by: James A Boomis
      Fundamentals of water jetting for industrial cleaning and surface preparation.
      Pages: 10-13; September, 1997


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