|
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
|
|
