dioxide blasting is an alternative process to chemical cleaning and stripping.
The obvious advantage of CO2 blasting over chemical stripping
is that the inert media (CO2) dissipates. There are two basic
types of CO2 blasting systems: pellet blasting for heavy cleaning,
and snow blasting for precision cleaning.
pellets are uniform in shape and the effectiveness of the pellets as
a blast medium is similar to abrasive blasting media. However, the pellets
do not abrade the substrate; therefore, CO2 pellet blasting
is technically not an abrasive operation. This process can be used for
cleaning, degreasing, some de-painting applications, surface preparation,
and de-flashing (flashing is the excess material formed on the edges
of molded parts).
The process starts with liquid CO2 stored under pressure
(~850 psig). The liquid CO2 is fed to a pelletizer, which converts the
liquid into solid CO2 snow (dry ice flakes), and then compresses
the dry ice flakes into pellets at about -110o F. The pellets
are metered into a compressed air stream and applied to a surface by
manual or automated cleaning equipment with specially designed blasting
nozzles. The CO2 pellets are projected onto the target surface
at high speed. As the dry ice pellets strike the surface, they induce
an extreme difference in temperature (thermal shock) between the coating
or contaminant and the underlying substrate, weakening the chemical
and physical bonds between the surface materials and the substrate.
Immediately after impact, the pellets begin to sublimate (vaporize directly
from the solid phase to a gas), releasing CO2 gas at a very
high velocity along the surface to be cleaned. The high velocity is
caused by the extreme density difference between the gas and solid phases.
This kinetic energy dislodges the contaminants (coating systems, contaminants,
flash, etc.), resulting in a clean surface. Variables that affect process
optimization include the following: pellet density, mass flow, pellet
velocity, and propellant stream temperature.
CO2 pellet blasting is effective in removing some paints,
sealants, carbon and corrosion deposits, grease, oil and adhesives,
as well as solder and flux from printed circuit board assemblies. This
process also provides excellent surface preparation prior to application
of coatings or adhesive and is suitable for most metals and some composite
materials. However, thin materials may be adversely affected. Blasting
efficiency is approximately equal to that of other blasting operations
and can approach 1 ft2/minute after optimization. CO2
blasting can be done at various velocities: subsonic, sonic, and even
supersonic. Therefore, equipment noise levels are high (between 95 and
130 dB). This operation always requires hearing protection.
Waste cleanup and disposal are minimized because only the coating or
contaminant residue remains after blasting. There is no liquid waste
because CO2 pellets disintegrate. They pass from liquid to
gaseous state, leaving no spent media residue. With regard to toxic
air control, small quantities of coating particles are emitted to the
air. A standard air filtration system should be provided.
Higher pellet velocities and a more durable pellet are required to effect
paint removal on military coatings. This "more aggressive" process showed
the potential to cause peening, warping, and an increased cold working.
This was especially evident on sheet aluminum less than 0.060 inches
thick. The paint removal rate was still deemed too slow for economical
use. The more durable pellets were achieved using liquid nitrogen injection
to cool the blasting air, which transports the pellets to the blast
In contrast to
CO2 pellet blasting, CO2 snow blasting is a low
impact process. Applications for this process are primarily in the precision
cleaning domain. A typical precision cleaning operation must clean small
contaminant particles that attach to surfaces and/or surface layers
of adsorbed moisture or soil due to electrostatic attraction. These
particles are so small that they have a large
fraction of their surface area attached to the surface layers. CO2
snow blasting is most effective in breaking the adhesive forces and
dislodging particles from the substrate surface. Small flakes of dry
ice transfer their kinetic energy to sub-micron particulate contaminants,
then sublimate, lifting the particulate matter from the substrate surface
as the adhesive bonds are broken. This process is often used as a final
cleaning process for sub-micron particulate and light soils removal.
CO2 snow is generated from liquid CO2, and is
discharged directly from the nozzle of the blasting device. The liquid
CO2 is partially vaporized as it passes through the nozzle,
while the rest of the stream solidifies as pressure is reduced. The
"snow", fine solid particles, is propelled by the fraction of CO2
that vaporizes. No compressed air or other inert gas is needed to propel
Most media cannot be used in precision cleaning because they are too
aggressive or they contaminate the component with media residue. CO2
snow, however, is ideal for this application, since it is relatively
gentle in application, leaves no media residue, and is highly purified,
introducing no new contaminants. CO2 snow blasting is often
done in a clean room or cabinet purged with nitrogen to provide a dry
atmosphere, minimizing moisture buildup on the component.
Carbon dioxide blasting
operations generate less hazardous waste than chemical stripping since
solvents are not used. The decrease in hazardous waste helps facilities
meet the requirements of waste reduction under RCRA, 40 CFR 262, Appendix,
and may also help facilities reduce their generator status and reduce
their regulatory burden (e.g., recordkeeping, reporting, inspections,
transportation, accumulation time, emergency prevention and preparedness,
emergency response) under RCRA, 40 CFR 262. In addition, the decrease
in the amount of solvents on site decreases the possibility that a facility
will meet any of the reporting thresholds of SARA Title III for solvents
(40 CFR 300, 355, 370, and 372; and EO 12856).
The compliance benefits listed here are only meant to be used as a general
guideline and are not meant to be strictly interpreted. Actual compliance
benefits will vary depending on the factors involved, e.g. the amount
of workload involved.
a completely oxidized compound is a non-reactive gas, and thus is compatible
with most metals and non-metals.
Dry ice processes are cold and can cause thermal fracture of a component.
In addition, prolonged use on a component in one spot will cause condensation
and ice buildup. However, this is rarely a problem, because CO2
blasting is a fast-acting, non-stationary process. Particulate and organic
contamination is either quickly removed or unable to be removed by continued
blasting at a single point. Therefore, the component temperature does
not change much, since contact time is short. Nevertheless, should component
temperature drop below the dew point of the surrounding atmosphere, moisture
will accumulate on the component. Heating the component in some manner
so that its temperature remains above the surrounding atmosphere’s dew
point after blasting can mitigate this problem. If components cannot take
heat, then blasting can be performed in an enclosed space purged with
a dry gas to lower or eliminate the dew point problem.
not support combustion and it is non-toxic; however, it is an asphyxiant.
CO2 will displace air since its density is greater than that
of air, causing it to accumulate at the lowest level of enclosed spaces.
When blasting with CO2 pellets, additional ventilation should
be provided for enclosed spaces. Personal protective equipment (PPE) is
also required when blasting.
Due to the ergonomics involved, robotics should be considered for full
time (continuous) depainting operations.
Static energy can build up if grounding is not provided. CO2
blasting should not be conducted in a flammable or explosive atmosphere.
High pressure gases should be handled with great care. Always chain or
secure high pressure cylinders to a stationary support such as a column,
prior to their use.
Consult your local industrial health specialist, your local health and
safety personnel, and the appropriate MSDS for CO2 prior to
implementing this technology.
- Significant reduction
in the amount of hazardous waste and hazardous air emissions generated
compared to chemical stripping.
- Time required
for cleaning/stripping processes is reduced by 80-90%.
- Leaves no residue
on the component surface.
- Effective in
- Introduces no
blasting is not always a one-pass operation; an effective blasting operation
usually requires multiple passes to achieve the desired effect.
- Requires operator
- Can have high
- Fixed position
blasting operation can damage the component’s surface.
- Generates solid
waste containing coating chips that are potentially hazardous; media
does not add to the volume of solid waste.
- Rebounding pellets
may carry coating debris and contaminate workers and work area.
- Some soils (in
cleaning operations) may redeposit on substrate.
system fatigues workers quickly because of cold temperature, weight,
and thrust of the blast nozzle. Automation (robotics) are required for
full aircraft stripping operations.
- Potential hazard
from compressed air or high velocity CO2 pellets.
- Carbon dioxide
(CO2) blasting is not an effective paint removal process
for aircraft. A production rate of 219 hours per aircraft (27 shifts)
is not acceptable for the Air Force. The Air Force developed a liquid
nitrogen injection system to enhance the depaint operation which improved
strip rate. However cost, reliability and complexity of the operation
renders it unsuitable for production operation.
Pellet Blasting: Units come in several different configurations. The
blasting unit alone can be:
- Purchased- $25,000
to $50,000 plus pellet maker, compressor and CO2 storage
tank. For non-pelletizing units, and some up front cost savings, pellets
can be bought in many local markets for the blasting only type units.
This is more economical if process use is expected
to be limited.
- Rented - $1,500
to $4,000 per month.
- Units that combine
pelletizing and blasting are also available, but generally are not economical
unless the blasting operation is performed
24 hours/day, seven days/week. The cost of these units ranges up to
$180,000 plus compressor and CO2 storage tank. Units with
liquid nitrogen injection are estimated to cost $265,000 each.
- Pellet blasting
jobs can be done on a contract basis for a cost between $200 to $300
per hour including labor, pellets, and equipment (not including travel
time or travel expenses).
- Pellet cost:
- Made by a
stand-alone pelletizer that can be purchased for a cost between $50,000
to $130,000 (cost to make pellets from delivered liquid carbon dioxide
is about $0.10-0.15/lb), or
directly from a manufacturer for a cost between $0.10/lb and $0.50/lb
delivered, depending on the purity and the distance from the manufacturer
(pelletizer purchase is reported to be economical only if blasting
is done more than 40 hours/week).
Snow Blasting: Units are much lower in cost and operation, as compared
to CO2 pellet blasting, and again there are several different
configurations to choose from:
- All manual units
cost about $2,000.
units (can also be used in assembly applications) cost between $3,000
- For highest
quality precision cleaning with substantial volume requirements, CO2
purifiers are also available. Units that can purify commercial grade
liquid CO2 start at a cost of about $5,000.
Some of the following
data was obtained from US Air Force for paint stripping of a typical Fighter
Aircraft using CO2pellet blasting for depainting.
- Aircraft skin
area: 3,100 ft.2
- Paint removal
and cleaning area: 2,200 ft.2
The following table
highlights an analysis of CO2 pellet blasting with nitrogen
injection and robotics compared to both traditional chemical stripping
and plastic media blasting.
with N2 with Robotics
Days for Stripping Process
Cost Per Aircraft
per sq ft
per sq ft
Cost per Year
life cycle cost
cycle cost per sq ft - 10% discount rate
cycle cost per sq ft - 5% discount rate
cycle cost per sq ft - 0% discount rate
- Annual Savings
for CO2 Blasting vs. Chemical: -$2,367,236.39
Capital Cost for Diversion Equipment/Process: $50,000
- Annual Savings
for CO2 Blasting vs. PMB: -$236,611.87
- Capital Cost
for Diversion Equipment/Process: $4,140,000
- Payback Period
for Investment in Equipment/Process vs. Chemical: 2 years
- Payback Period
for Investment in Equipment/Process vs. PMB: 15 years
- The life cycle
cost per square foot from the table above indicates PMB is the more
cost effective solution when the time-value-of-money is considered.
Click here to View an Active Spreadsheet for this Economic Analysis
and Enter Your Own Values. To return from the Active Spreadsheet, click
the reverse arrow in the Tool Bar.
The Air Force Corrosion
Program Office does not approve of this process for paint removal and
will not provide technical guidance for this process. Any implementation
of this process for paint removal in the Air Force would require approval
of the engineering authority for specific Weapon System Managers or Equipment
Materials Engineering Section
420 2nd Street, Suite 100
Robins AFB, GA 31098
Phone: (478) 926-4489
FAX: (478) 926-1743
Air Force Corrosion Prevention and Control Office
AFRL/MLS-OLR (Bldg. 165)
325 Richard Ray Boulevard
Robins AFB, GA 31098-1640
Phone: (478) 926-3284
9119 Milliken Ave.
Rancho Cucamonga, CA 91730
Phone: (800) 445-6131
FAX: (909) 980-5696
Service: Manufacturer of carbon dioxide pelletizers and blasting equipment
Cold Jet Inc.
455 Wards Corner Road
Loveland, OH 45140
Phone: (800) 337-9423
or (513) 831-3211
FAX: (513) 831-1209
Contact: Mr. Jerry Raschau
Service: Manufacturer of carbon dioxide pelletizing and blasting equipment
Va-Tran Systems, Inc.
677 Anita Street
Chula Vista, CA 91911-4661
Phone: (619) 423-4555 x 102
FAX: (619) 423-4604
Contact: Mr. Jeff Sloan
Service: Manufacturer of the “Sno-Gun” carbon dioxide pelletizing and
This is not meant
to be a complete list, as there may be other suppliers of this type of
EPA SAGE 2.0 "Solvent Alternative Guide."
Cold Jet® product literature and video.
Va-Tran Systems, Inc. product literature.
Hill, E. A., "Carbon Dioxide Snow Examination and Experimentation," Precision
Cleaning, p. 36-39, February 1994.
Sloan, J., "Dry Ice Snow Surface Cleaning of Electronics, Optics and Metal
MICROCONTAMINATION 93 Conference Proceedings, p. 671-676, 1993.
Randy Ivey, Robins AFB, 2/00.