When it comes to parts cleaning by blasting,
the idea of propelling abrasives at parts to clean them of rust,
carbon, paint or other difficult contaminants is a simple one.
But the technology involved with this cost-effective cleaning
procedure has improved, and rebuilders can take advantage of many
options to eliminate out-dated, labor-intensive cleaning practices
in the shop.
Scott Christiansen of the Guyson Corp., Saratoga
Springs, NY, said blast equipment manufacturers are always seeking
to improve quality and productivity for rebuilders. "The
evolution of what was once a simple sandblaster is continuing
in response to the changing requirements of the automotive rebuilder,"
Christiansen said. "Recent developments have extended the
range of dry-blast process capabilities and media selection for
the reconditioning of automotive components."
There are two basic ways of propelling blast
media – air blasting and airless blasting. And there are numerous
types of equipment available that utilize each method. Compressed
air blasting propels shot either by drawing it through a nozzle
by suction or venturi force, or by pressure blasting, where the
air and abrasive are conveyed through a blast hose to the nozzle.
Airless centrifugal shot blasting units can use an electrically-driven
spinning impeller shaft (turbine) with paddles or cups attached,
or a center-fed wheel to propel shot.
Just as there are differences in the methods
of blasting, there are differences as to which equipment to use.
Some in the industry argue that turbine equipment is more energy-efficient
than air-blast media delivery. For example, a small diameter turbine
driven by a 5 hp electric motor can throw 150 lbs. (68 kg) of
blast material per minute, creating a blast pattern about 4"
wide and 24" long (10 cm X 60 cm). By comparison, a 5 hp
compressor will power a single suction gun that delivers about
5 lbs. (2.3 kg) per minute of the same media creating a blast
pattern about the same size as a half-dollar.
When using manual blast equipment, the particle
stream is directed by hand, requiring constant employee attention.
Here, the quality of the results can vary from operator to operator
and part to part. However, the method for propelling shot varies
with the shop’s needs. Although some engine rebuilders prefer
to clean ferrous engine blocks and heads in airless shot blasters,
electrical and some small parts rebuilders prefer using air blasting
units with glass bead, aluminum oxide and zinc oxide media. For
a list of manufacturers and suppliers of blast equipment and media,
refer to Automotive Rebuilder’s January 1997 Automotive and Truck
Purchasing Directory.
Why shot blasting?
Shot blasters can be used for cleaning parts,
or for increasing surface hardness on some parts such as connecting
rods and crankshafts. According to Grant Ebersole, industry manager
of Standard Systems for Pangborn Corp., Hagerstown, MD, blasters
provide a simple method for rebuilders to clean parts quickly.
"A machine becomes necessary when a rebuilder realizes that
production by hand is too time consuming," Ebersole said.
"The implementation of a machine will help allow a shop to
achieve desired production, plus enough extra production to justify
the cost of the machine, as well as improve profit goals."
Guyson’s Christiansen said appearance is another
consideration. "The quest for a unique, clean and OEM-type
appearance has spawned considerable interest in blast equipment
specially adapted to use exotic shot for the production of distinctive
cosmetic finishes," Christiansen said. "Of special interest
for these purposes, and in some rebuilding applications, are a
number of extremely fine metallic media in size ranges under 300
microns (.0118" diameter), sometimes referred to as ‘microshot’.
These media are denser and more fracture resistant than fine non-metallic
shot, so they are compatible with airless blast as well as airblast
delivery systems."
Christiansen said in contrast to the comparatively
gross surface alteration of some treatments with conventional
shot, these low-aggression media modify component appearance only
slightly and can be used to produce subtle conditioning affects
and surface finishes. "But because the degree of texturing
is less with these finer, lighter media than with conventional
shot, they demonstrate less peening effect and may be inappropriate
for preparing surfaces which may need to be coated rather than
simply reconditioned," Christiansen said. "Generally,
applying a coating after reconditioning requires a rougher, more
profiled finish."
Dry-blasting can also provide rebuilders additional
surface finish requirements, such as blast etching, profiling
or texturing. And dry-blast methods allow a rebuilder to remove
unsatisfactory coatings, paint, etc., often leaving the surface
of the component in an improved condition for re-coating.
When choosing a shot blaster for the shop,
Zack Wittke of Serv-Equip, Black River Falls, WI, said it’s important
not to over-buy for your needs. "Start with equipment that
will handle the size of the parts you want to clean on a consistent
basis," Wittke said. "If you do one or two large pieces
per year, it is probably worth the extra money for a larger machine.
"Also do a capacity flow chart,"
he added. "Take the time to determine your cleaning volume
for the next two to three years and size your system accordingly."
David Cox of Kansas Instruments, Council Grove,
KS, said rebuilders planning to purchase shot blasting equipment
need to look at blasting expenses. Cox said besides the initial
cost of the blasting equipment, there are some expenses for this
type of cleaning process. "Electricity usage, shot replenishment
and fixture wear-out are some cost considerations rebuilders should
be aware of," said Cox.
But many manufacturers say these costs are
minimal when compared to other cleaning methods. Information on
typical costs to operate blasting machines is available from various
equipment manufacturers.
Other cost considerations may include precleaning
processes. Dennis Marble of Sunnen Products Co., St. Louis, MO,
said precleaning of engine components (thermal and wet) is a step
some rebuilders take prior to blasting. But once the blasting
process begins, parts must be dry and grease-free to prevent media
adhesion. "Parts must be completely degreased and absolutely
dry before shot blasting," he explained. "If this is
not done, both the parts and the blast media will become contaminated.
Even the slightest amount of grease or moisture will temporarily
trap blast media and prevent complete removal."
Other manufacturers such as Serv-Equip’s Wittke
and Pangborn’s Ebersole say the need to preclean parts has been
reduced with today’s blasting equipment. "I won’t say you
never have to preclean anything, but blasting should take care
of about 95% of your cleaning work," Wittke said. "However,
you have to be careful when blasting sheet metal parts, as too
much time in the machine will start to distort them."
Ebersole concurred. "Parts can be introduced
to the blast machine without pre-cleaning," he said. "But,
the machine parts last longer when excess sand is vibrated off
of molds before they are cleaned."
Marble also cautions rebuilders about residual
magnetism. "While somewhat uncommon, parts that contain residual
magnetism should be demagnetized before blasting or final cleaning,
regardless of the cleaning process used," he explained. "Rods,
crankshafts or blocks which have spun bearings may contain residual
magnetism caused by friction. Always use correct procedures when
removing magnetism from items that have been checked in a magnetic
particle inspection (wet mag) machine." Magnetism can be
checked with a field strength indicator which is available from
shop supply vendors.
Maintenance
Proper maintenance on your equipment is vital
to adding years of life to your blasting equipment. One area that
needs attention is the dust filtration system. "Shot breaks
into smaller and smaller particles as it is used, eventually creating
a fine dust," Marble said. "If the dust is not continuously
removed by a filtration system, it will end up on the parts being
cleaned. This is especially important when blasting parts which
have been cleaned in an oven, as they will be covered with the
ash residue left from the oven cleaning process."
Another important equipment maintenance procedure
is to grease bearings at the intervals recommended by the manufacturer.
Avoid over-greasing, as this can lead to premature bearing failure.
"If hot parts will be blasted, such as after oven cleaning,
be sure to use an appropriate grease designed for this application,"
Marble offered.
Pangborn’s Ebersole said routine maintenance
checks will not only reduce wear on equipment, it also will reduce
cleaning times. "Keep the screens in the separator clean;
these should be cleaned regularly," Ebersole said. "And
conduct monthly examination of wear parts to determine if they
need to be replaced."
Marble explained Sunnen/AmPro’s and some other
blasters have impeller blades which should be turned over as they
become worn. "When the impeller blades become worn about
halfway – usually after about 500 hours of use – turn them over
to expose the unworn side which will increase their usable life,"
said Marble.
Many equipment manufacturers provide equipment
operation and maintenance training. Gene Tarabek, national training
manager for U.S. Filter/ Wheelabrator, Lagrange, CA, said increasing
the awareness and understanding of all of the facets of a shop’s
blast cleaning systems is like renewing a resource. "Our
seminars are one of the best ways for operators and maintenance
personnel to make sure their efforts will always contribute to
cost effective productivity gains, along with the lowest possible
operating and maintenance costs," he said. Tarabek said his
company’s training program, which consists of one-day seminars
on critical maintenance issues conducted at various locations
throughout the country, is a way of sharing information with customers.
Seminars also can be conducted at a customer’s facility.
Choosing abrasives
After determining which equipment best suits
your shop, the next step is to chose which abrasives are best
for specific cleaning applications. According to Ervin Industries,
Ann Arbor, MI, the shot that transmits the greatest amount of
energy to the workpiece with the greatest fatigue life will produce
the most economical and consistent shot peening operation. Shot
with proper hardness, microstructure, chemical analysis, size
and a minimum of physical defects will continue to provide greater
transmitted energy and fatigue life.
Michael Wigart of LS Industries, Wichita, KS,
said choosing the right abrasive is critical to getting the right
finish. "There are many types of abrasive media available
for airless blast equipment," explained Wigart. "One
of the first things we ask our customers is what type of finish
are they looking for. This helps us decide what type of machine
and media to choose."
Although the most common abrasive is steel
shot, there are a number of other media used in blast equipment
including grit, sand, glass beads, plastic, zinc, walnut shells,
coal slag, aluminum, stainless steel, soft shot, ultra blast and
a form of ceramic and plastic mixture.
According to Sunnen’s Marble, while beadblasters
(also referred to as glass bead machines) can use many types of
media, glass beads are the most popular media used in these machines
to clean engine parts. "Glass beads are basically round in
shape and are designed to break into smaller pieces when subjected
to high impact forces," he said. "This allows glass
beads to be used as an effective blast media for most alloys without
being so aggressive that they cause damage to the surface of the
parts being cleaned."
Plastic shot, which can be used in applications
similar to glass beads, which won’t damage metal and has little
chance of causing damage due to shot retention, has a short cycle
life and can be expensive.
Some small parts and electrical rebuilders
favor zinc abrasive. Wire cut zinc has a long cycle life and can
produce a satin finish on aluminum components.
Sand and grit media can also be used in beadblasters,
but these are typically harder than glass beads and have sharp,
irregular edges resulting in a much more aggressive media. Although
inexpensive, these materials can cause etching to critical surfaces
during the blast cycle.
Sand and grit can even become temporarily imbedded
into the part, especially on soft alloys such as aluminum. If
the media comes loose later, severe damage can be the result.
Sand and grit media, while often cheaper to purchase, are not
typically the best choice for critical engine parts.
In airless blasters, engine parts are most
often cleaned with steel shot or stainless steel blast media (often
made from cut stainless wire). According to those we talked to,
steel shot works well in most applications and is relatively inexpensive
per pound. But steel shot doesn’t last as long and produces more
dust than stainless steel shot, so filter systems need to be adequately
sized and require more frequent maintenance. Stainless steel shot
will provide a brighter, more polished finish on both cast iron
and aluminum alloys. And stainless steel shot is non-magnetic,
making it easier to remove from complex castings.
Aluminum abrasive is a media that works well
on softer parts – like aluminum – because it is, itself, a soft
metal. And aluminum abrasive will give a softer, brighter finish
to parts.
LS Industries’ Wigart said aluminum shot, which
comes in the same sizes as steel shot, is used by some rebuilders
for cleaning heads. "It generally takes longer to clean with
aluminum shot as it is softer," he said. "It will leave
an aluminum skid mark on cast iron and other hard materials and
therefore is not used in some applications where contamination
of an internal part would be critical."
Although aluminum shot produces a good finish
on aluminum parts, the dust it generates can be explosive. Explosions
can occur by spontaneous combustion generated when oxygen that
is occupied by the dust comes into contact with moisture.
Gus Enegren, president of Viking Corp., Wichita,
KS, said such explosions normally occur when rebuilders load parts
still wet with solvent into the blast cabinet, adding that there
are also other causes of combustion. "A static discharge
in the dust collector or the impact of abrasive striking parts
in the blast cabinet can also create a spark to ignite dust in
the system," Enegren said. "Careful and meticulous grounding
of the machine frame and the electrical connection can minimize
risks, but they will always be present."
Kansas Instruments’ Cox said proper dust filtration
will help aid in the prevention of combustion. "Aluminum
shot is very light and can be removed by most dust collection
systems," Cox said, adding when operating equipment using
aluminum shot, a dust collection system is required, and a Class-D
fire extinguisher should be available close by.
Pangborn’s Ebersole said there are safety packages
available for the blasting machines so that should an explosion
occur, it will exit the machine at its weakest point. "The
weakest point is in the rear of the machine, opposite from the
operator," Ebersole explained.
Shot size
After the blasting media is chosen, the next
consideration is shot size. Shot size is a critical issue for
rebuilders, as the size of the blast media affects the blast pattern,
as well as the surface finish. A shot size should be chosen that
will not cause damage to sensitive or critical surfaces, but will
still effectively clean the types of parts and alloys that need
to be blasted in a reasonable amount of time. The larger the shot,
the more aggressive it will be.
Every shot machine propels a constant number
of pounds of shot per hour, which depends either on the horsepower
of the centrifugal wheel or the volume of air used per hour in
the air blast machine. Therefore, the number of particles in a
pound is a function of the particle size. For example, a pound
of 660 shot contains about 14,000 particles. A pound of 330 shot
(half the diameter of 660) contains about 110,000 particles, eight
times as many as the 660 shot. So a wheel that throws one pound
would throw 14,000 impacts of 660 shot or 110,000 impacts of the
330 shot.
Using larger media particles for blasting does
not mean those particles will clean faster than smaller ones.
Blasting with larger particles will leave some surface areas untouched
because there are fewer particles per square inch hitting the
surface. But consider that a small shot particle may not have
sufficient energy to peen effectively and provide the compressive
stresses to the depth required. Therefore it is important to select
the smallest size of shot that will produce the needed intensity,
as small shot will saturate the surface more rapidly.
Many rebuilders, Guyson’s Christiansen notes,
will test-blast surfaces to determine the best particle size to
use. "Unless the choice of medium is already known, there
is no substitute for a media selection process consisting of the
demonstration of alternatives, trials and thorough evaluation
of results," he said. "There is well over 100 different
blast media choices of natural and man-made materials available
in a myriad of sizes and screenings with particle diameters anywhere
from 50 microns (.002") to more than 2 mm (.079"). Only
extensive experience and experimentation can guide in blast medium
selection."
Hardness:
Blasting media also come in different hardnesses.
The hardness of most abrasives (excluding steel abrasive) is measured
by the Moh’s Scale which ranges from one (softest) to 15 (diamond).
Steel abrasive shot is measured on a Rockwell "C" scale
and stated in RC increments. The standard hardness range for cast
steel shot and grit used in blasting equipment is 40-50 Rc.
Media hardness plays an important role in selecting
shot for peening. The harder the material, the faster it will
clean and the faster it will break down. The hardness of shot
for shot peening must be approximately equal to the hardness of
the work being peened. Shot that is too soft will not produce
the proper level or depth of residual compressive stresses. The
effect of low hardness shot cannot be compensated for by increasing
peening cycle times.
Abrasive density or the weight by volume in
cubic feet, is another consideration. The more dense the abrasive,
the greater the energy its particles deliver to the part. But
the abrasive’s density may contribute to its weight. Whereas sand
weighs about 100 lbs. per cubic foot, steel grit weighs about
250 lbs. per cubic foot.
The density of steel shot in an airless blast
application is not much of a factor. However, it becomes a factor
in suction blast applications where the blast unit may not have
enough suction to pick up and throw steel abrasives which are
heavier than natural abrasives such as sand.
Another consideration when choosing a blasting
media is the material’s friability, or its tendency to break up
upon impact. Materials with a low friability have a longer life
cycle than a high friability material. For instance, steel shot
has a low friability, and can be reused 200 times or more. Silica
sand, on the other hand, has a high friability factor and cannot
be reused.
LS Industries’ Wigart said disposal of used
shot is yet another issue rebuilders need to address, adding how
abrasives are disposed of depends on what they were used for.
"Steel shot and grit are not hazardous material and can be
processed with other rubbish that is thrown away," Wigart
said. "But if the application, for example, is to remove
a material such as lead-based paint, then it becomes a hazardous
material and should be disposed of according to local, state and
federal Environmental Protection Agency (EPA) guidelines."
Depending on the type of parts cleaned during
the life of the media, shot can also be contaminated with hazardous
materials such as heavy metals. Spent abrasive should be tested
and disposed of accordingly. The burden of proof to determine
if a waste is hazardous or not, rests on the waste producer.
As we see, shop owners have many options when
it comes to using blasting as a cleaning process. And there is
no all-inclusive media, machine or shot size that can be used
for every application. Each type of media and blasting machine
offers its own set of features and benefits which must be matched
to the parts being cleaned. But through matching the right abrasive
and blasting equipment with your shop’s needs, and adhering to
scheduled maintenance routines on the equipment, blasting can
be a viable and cost-effective parts cleaning process for the
rebuilder.
