Today, CO2 blasting is being effectively used in a wide array of applications
from heavy slag removal to delicate semiconductor and circuit board
cleaning. Imagine a process that can be used on-line without damaging
equipment or requiring a machine "teardown". Unlike conventional
toxic chemicals, high pressure water blasting and abrasive grit blasting,
CO2 blasting uses dry ice particles in a high velocity air flow to remove
contaminates from surfaces without the added costs and inconvenience
of secondary waste treatment and disposal.
In 1945, stories exist of the U.S. Navy experimenting with Dry Ice
as a blast media for various degreasing applications. In May 1963, Reginald
Lindall received a patent for a "method of removing meat from bone"
using "jetted" Carbon dioxide particles. In November 1972,
Edwin Rice received a patent for a "method for the removal of unwanted
portions of an article by spraying with high velocity Dry Ice particles".
Similarly, in August 1977, Calvin Fong received a patent on "Sandblasting
with pellets of material capable of sublimation".
The work and success of these early pioneers led to the formation of
several companies in the early 1980's that pursued the development of
Dry Ice Blasting Technology. In 1986, Cold Jet®, Inc. was founded
in the State of Ohio by Mr. Newell Crane.
Dry Ice pelletizers and blast machines entered the industrial markets
in the late 1980's. At this time the blast machines were physically
large, expensive, and required high air pressure for operation (pressures
greater than 200 psi). As the CO2 blast technology advanced, the blast
machine's size and cost dropped. The latest nozzle technology has made
blasting effective at shop air pressures (80 psi).
CO2 is a natural media which serves many life sustaining purposes.
It is a key element involved in the carbon cycle; it is the only source
of carbon for the carbohydrates produced by agriculture; it stimulates
plant growth; and it helps to moderate the temperature of the earth
overall. Animal respiration is believed to add 28 million tons of Carbon
Dioxide per day into the atmosphere. By contrast, the U.S. CO2 industry
can supply only 25,000 tons per day and 95% of this amount is from by-product
sources, or less than 0.04% of the other sources combined.
With a low temperature of -109° F, Dry Ice solid has an inherent
thermal energy ready to be tapped. At atmospheric pressure, solid CO2
sublimates directly to vapor without a liquid phase. This unique property
means that the blast media simply disappears, leaving only the original
contaminant to be disposed of. In addition, cleaning in water sensitive
areas is now practical.
The grade of carbon dioxide used in blasting is the same as that used
in the food and beverage industry and has been specifically approved
by the FDA, the EPA and the USDA. Carbon dioxide is a non-poisonous,
liquefied gas which is both inexpensive and easily stored at work sites.
Of equal importance, it is non-conductive and non-flammable.
CO2 is a natural by-product of several industrial manufacturing processes
such as fermentation and petrol-chemical refining. The CO2 given off
by the above production processes is captured and stored without losses
until needed. When the CO2 is returned to the atmosphere during the
blasting process, no new CO2 is produced. Instead, only the original
CO2 by-product is released.
Listed in Table 1 are the physical properties and conversion factors
for CO2 in its various forms:
Table 1. Carbon Dioxide (CO2 ) properties.
Another technique is to manufacture hard pellets of Dry Ice in a pelletizer
then immediately blast with the pellets or store the pellets in an insulated
container until the pellets are required. These pellets are generally
on the order of 0.080" to 0.120" in diameter, and 0.100"
to 0.400" in length.
In this method, Dry Ice is manufactured by flashing pressurized liquid
CO2 into snow, followed by compression of the snow into solid form.
The snow is either directly nuggetized into pellets (mechanical compression)
or is extruded into solid pellet form through a die under hydraulic
pressure. The latter process allows for more efficient conversion from
the liquid phase to the solid phase. Generally, it is desirable to have
pellets which are well compacted, to minimize entrapment of gaseous
CO2 and/or air which may affect product quality.
As can be seen in Table 1, the yield achieved when flashing liquid
carbon dioxide into snow increases as the temperature of the liquid
CO2 decreases, so it is important to pre-chill the incoming liquid CO2
via heat exchangers with the outgoing CO2 vapor. Figure 1 is a block
diagram showing a basic pelletization process.
Figure 1. Pelletization Process.
Several manufacturers make Dry Ice pelletizers which
may prove beneficial to have on-site for customers with high pellet
demand. Facilities required for such an arrangement are generally
as follows: a refrigerated liquid CO2 tank, a pelletizer, and liquid
CO2 lines to reach the equipment. Some manufacturers make combined
Dry Ice pelletizer/blast machines which manufacture Dry Ice and blast
all in one operation. Facilities required for such an arrangement
are: An air compressor (typically either 120 psi at 250 SCFM or 350
psi at 250 SCFM), a liquid CO2 tank, a pelletizer/blast machine, compressed
air hose and liquid CO2 lines to reach the equipment, blast hose from
the machine to the blasting operation, and the appropriate nozzle(s)
for the application. This equipment is best suited to high volume,
continuous blasting applications where the cost savings of manufacturing
pellets on-site justifies the capital expenditure for the system.
How Does Dry Ice Blasting Work?
The Basic Process
Dry Ice particle blasting is similar to sand blasting, plastic bead
blasting, or soda blasting where a media is accelerated in a pressurized
air stream (or other inert gas) to impact the surface to be cleaned
or prepared. With Dry Ice blasting, the media that impacts the surface
is solid carbon dioxide (CO2) particles. One unique aspect of using
Dry Ice particles as a blast media is that the particles sublimate
(vaporize) upon impact with the surface. The combined impact energy
dissipation and extremely rapid heat transfer between the pellet and
the surface cause instantaneous sublimation of the solid CO2 into
gas. The gas expands to nearly eight hundred times the volume of the
pellet in a few milliseconds in what is effectively a "micro-explosion"
at the point of impact. Because of the CO2 vaporizing, the Dry Ice
blasting process does not generate any secondary waste. All that remains
to be collected is the contaminate being removed.
As with other blast media, the kinetic energy associated
with Dry Ice blasting is a function of the particle mass density and
impact velocity. Since CO2 particles have a relatively low hardness,
the process relies on high particle velocities to achieve the needed
impact energy. The high particle velocities are the result of supersonic
propellant or airstream velocities.
Unlike other blast media, the CO2 particles have a very
low temperature of -109° F. This inherent low temperature gives
the Dry Ice blasting process unique thermodynamically induced surface
mechanisms that affect the coating or contaminate in greater or lesser
degrees, depending on coating type. Because of the temperature differential
between the Dry Ice particles and the surface being treated, a phenomenon
known as "fracking" or thermal shock can occur. As a material's
temperature decreases, it becomes embrittled, enabling the particle
impact to break-up the coating. Refer to Figures 2 and 3.
FIGURE 2. Thermal shock induces micro-cracking in the
surface coating
FIGURE 3. CO2 gas expansion and pellet kinetic effects
break away and remove coating particles.
Also, the thermal gradient or differential between two
dissimilar materials with different thermal expansion coefficients
can serve to break the bond between the two materials. This thermal
shock is most evident when blasting a non-metallic coating or contaminate
bonded to a metallic substrate.
Quite often companies examining this process are concerned
with the effect the thermal shock will have on the parent metal. Studies
have shown that the temperature decrease occurs on the surface only,
there is no chance of thermal stress occurring in the substrate metal.
To illustrate this principle, an experiment was performed where thermocouples
were imbedded into a steel substrate at varying depths (flush with
the surface to 2 mm deep). Refer to Figure 4.
FIGURE 4. Thermocouple Distance From Plate Surface.
A CO2 blast jet was constantly traversed across the
test specimen for 30 seconds (a relatively long dwell time for this
process) and the thermal couples recorded the changing temperatures
at the various depths. As shown in Figure 5, the surface mounted thermocouple
shows a temperature drop each time the blast jet impinged directly
upon the thermocouple (50° C in about 5 seconds). In contrast,
the thermocouples imbedded at various depths in the substrate recorded
a slow gradual drop in temperature corresponding to the overall test
plate temperature drop. The thermocouple 2mm deep only dropped 10°
C after 30 seconds. This curve illustrates that the "Thermal
Shock" occurs only at the surface where the coating or contaminate
is bonded to the substrate (Reference 1) and has no detrimental effect
to the substrate.
FIGURE 5. Temperature Response Of Thermal Couples Placed
At Various Depths In The Substrate
Another approach to looking at thermal stress is by
studying the use of Dry Ice blasting in the molded rubber industry.
Here, hot steel molds operating at 300+ °F are blasted with -109
°F Dry Ice particles. The temperature difference between the hot
mold and cold Dry Ice will not cause cracking. There are two reasons
for this phenomenon. First, as seen above, the temperature gradient
occurs at the surface. Secondly, the thermal stresses involved are
much less than those encountered during normal heat treatment.
The thermal stress due to a temperature differential
can be estimated using equation 1 where sy is stress (psi), DT is
temperature gradient (°F), a is coefficient of expansion and g
is Poisson's Ratio.
Equation 1 
The corresponding parameter values are
and the thermal stress (psi) is 
where the temperature differential will be 135 °F
(Based on Figure 5). This temperature gradient leads to a low tensile
stress of 30,240 psi. Even if the mold temperature was brought down
to the temperature of the ice (an unrealistic extreme), the temperature
gradient would be -109 °F - 350 °F which gives 459 °F.
The corresponding tensile stress is 102,800 psi. This calculated stress
is below the yield point of steel in the hardened condition. Again,
these thermal stresses would be far less than those encountered during
normal heat treatment where the temperature differentials would exceed
500 °F (Reference 2).
Even at high impact velocities and direct "head-on"
impact angles, the kinetic effect of solid CO2 particles is minimal
when compared to other media (grit, sand, PMB, etc.). This is due
to the relative lack of hardness of the particles and the almost instantaneous
phase change to a gas on impact which effectively provides an almost
nonexistent coefficient of restitution in the impact equation. Because
CO2 blasting is considered non-abrasive and relies on the thermal
effects discussed above, the process may be applied to a wide range
of materials without damage. Soft metals such as brass and aluminum
cladding can be CO2 blasted for the removal of coatings or contaminates
without creating surface stresses (pinging), pitting, or roughness
(Reference 3).
Blast Machine Types
There are two general classes of blast machines as characterized
by the method of transporting pellets to the nozzle: two-hose and
single-hose systems. In either type of system, proper selection of
blast hose is important because of the low temperatures involved and
the need to preserve particle integrity as the particles travel through
the hose.
In the two-hose system, Dry Ice particles are delivered
and metered by various mechanical means to the inlet end of a hose
and are drawn through the hose to the nozzle by means of vacuum produced
by an ejector-type nozzle. Inside the nozzle, a stream of compressed
air (supplied by the second hose) is sent through a primary nozzle
and expands as a high velocity jet confined inside a mixing tube.
When flow areas are properly sized, this type of nozzle produces vacuum
on the cavity around the primary jet and can therefore draw particles
up through the Ice hose and into the mixing tube where they are accelerated
as the jet mixes with the entrained air/particle mixture. The exhaust
Mach number from this type of nozzle, in general, slightly supersonic.
Advantages of this type of system are relative simplicity and lower
material cost, along with an overall compact feeder system. One primary
disadvantage is that the associated nozzle technology is generally
not adaptable to a wide range of conditions (i.e. tight turns in a
cavity, thin-wide blast swaths, etc.). Also, the aggression level
and strip rate of the two-hose system is less than comparable single-hose
blast machines.
In a single-hose system, particles are fed into the
compressed air line by one of several types of airlock mechanisms.
Reciprocating and rotary airlocks are both currently used in the industry.
The stream of pellets and compressed air is then fed directly into
a single hose followed by a nozzle where both air and pellets accelerate
to high velocities. The exhaust Mach number from this type of nozzle
is generally in the 1.7 to 3.0 range, depending on design and blast
pressure. Advantages of this type of system are wide nozzle adaptability
and the highest available blast aggression levels. Disadvantages include
relatively higher material cost due to the complex airlock mechanism.
Blast machines are also differentiated into Dry Ice
Block Shaver blasters and Dry Ice Pellet blasters. The Block Shaver
machines take standard 60 LB Dry Ice blocks and use rotating blades
to shave a thin layer of ice off the block. This thin sheet of Dry
Ice shatters under its own weight into sugar grain sized particles.
These particles then fall into a funnel for collection. A two-hose
delivery system is used to transfer the particles at the bottom of
the funnel to the surface to be cleaned. The low mass of these particles
combined with the inefficient two-hose system limits the block shavers
to light duty cleaning. Because the shaved ice machines deliver a
particle blast with high flux density (Number of particles striking
a square area of surface per second), they are effective on thin moderately
hard coatings such as an air dried oil based paint. The disadvantage
of the ice shaver is the particle size and flux density is fixed,
as well as, the particle velocity.
In contrast, Pellet Blast machines have a hopper that
is filled with pre-manufactured CO2 pellets. The hopper uses mechanical
agitation to move the pellets to the bottom of the hopper and into
the feeder system. As stated earlier, the pellets are extruded through
a die plate under great pressure. This creates an extremely dense
pellet for maximum impact energy. The pellets are available in several
sizes, ranging from 0.040" to 0.120" in diameter. With a
single-hose delivery system, the final pellet size and blast flux
density exiting the nozzle is governed by the type of blast hose (hose
diameter and interior wall roughness) and nozzle used. Because of
its design, the single hose pellet blast units are capable of "dialing-in"
the correct blast type needed for a wide range of individual coatings
or contaminate removal.
For example, soft coatings such as rubber, silicone,
foams and waxes, release agents, food ingredients, etc. need large
pellets with low flux density for maximum strip rate and efficiency.
These coatings require maximum thermal energy (i.e. pellets with large
mass) and large spacing between pellets (i.e. low flux density) for
optimum cleaning performance. In contrast, hard coatings such as paints,
varnish, baked on sugars, carbon build-up, etc. require smaller particle
size with high flux density and high particle velocity.
Blast machines are further differentiated into all-pneumatic
and electro-pneumatic types. All-pneumatic machines have particle
feed mechanism and controls operated pneumatically. This may include
the use of air motors. The advantage of such a machine is the availability
of compressed air at the blast locations, especially outdoors. One
disadvantage is that the operation of the machine may be susceptible
to disruption due to moisture or contamination in the compressed air
supply. In addition, these machines are more prone to freeze-ups and
are better suited for light duty spot cleaning applications. Also,
if the machine is powered by an air motor, it will have a continuous
exhaust of oily air. This same air motor can be easily flooded with
water if the air system is not adequately dried.
Electro-pneumatic machines are truly "Environmentally
Friendly" because there is no oily exhaust and these machines
are more tolerant of moisture and contaminants in the air supply.
The electro-pneumatic machines rarely freeze-up which makes them ideal
for automated line applications where around-the-clock blasting is
required. Also, these machines provide pulse free blasting for uniform
cleaning and efficient use of the Dry Ice. There is, however, a slight
inconvenience factor associated with supplying both electrical power
and compressed air to the machine at each blast location.
One of the most challenging technologies associated
with either type of blast machine is the achievement of smooth, continuous
pellet feed. One surprising property of Dry Ice is that it is not
smooth or slippery like water ice nor smooth-flowing like sand or
glass bead. Instead it is somewhat resistant to flow. Because of this,
Dry Ice blast machines tend to have various agitators, augers and
other devices in the hopper to improve pellet flow. Generally, the
poorer the quality of dry ice, containing; for example, water ice
build-up or a large percentage of CO2 "fines" or snow, the
more difficult it is to flow through a system. An additional property
of Dry Ice is that it is extremely cold and will draw moisture out
of the surrounding air in the form of frost. The machine, therefore,
must be tolerant of repeated freeze-thaw cycles and the associated
mois ture accumulation that will take place over time.
Generally the difference between a high quality blast
machine and a mediocre one lies in the units ability to do a cleaning
job quickly, cost-effectively, and in the reliability of smooth and
continuous pellet flow under real-world conditions.
Nozzle Technology
The nozzle is where the Dry Ice particles are accelerated to
the highest velocity possible in order to create an effective blast
stream. Figure 6 shows schematics of the two types of nozzles used
for Dry Ice blasting. The science of two-hose ejector nozzles compared
to single-hose convergent-divergent supersonic nozzles operating under
the same conditions (i.e., air volume, pressure, temperature, CO2
particle mass...etc.), shows significantly higher efficiency capability
for the described single-hose type nozzles. This difference in capability
directly relates to the two-hose ejector nozzle's overall supplied
energy being used not only to accelerate the CO2 particles, but also
to create the vacuum pulling the secondary pellet flow through the
secondary hose. Then more energy is drained to mix this low velocity
particle flow with the high velocity jet flow in order to accelerate
the particle through the two-hose nozzle. In simple terms, the net
resultant energy available for pellet acceleration is inherently lower
for two-hose systems because much of the available energy is lost
simply in combining the CO2 particle flow with the air-jet flow.
Figure 6. CO2 Particle Blast Nozzle Types
Since the size of the Dry Ice particles effect the cleaning
performance, a blast system should have the flexibility to "Dial-In"
the correct particle size. This can be done a couple of different
ways. First, the size of the pellet being produced by the pelletizer
may be varied. Once the pellet is in the blast machine hopper, the
size of the pellet reaching the surface to be cleaned can be varied
several ways. The diameter and type of blast hose used will either
keep the pellet intact or break the pellet up into smaller particles.
Also, the nozzle may be intentionally mis-expanded to produce partially
destructive shock waves in the nozzle. Both techniques are used independently
or together to optimize the particle size, blast stream velocity,
and flux density for any cleaning job.
When sand or any similar media with very small diameter
is used in blasting, the size of the nozzle throat is very large compared
to the blast media. In Dry Ice blasting, however, the nozzle throat
may only be slightly larger than the dry ice particle being accelerated.
Table 2 is a chart indicating the approximate size of a round nozzle
throat for four
different levels of blast pressure at a constant airflow
of 200 Standard Cubic Feet per Minute (SCFM) and typical flow rate
available for blasting operations. At higher pressures, the Dry Ice
particle size needs to be smaller to correspond with the smaller throat
size. The high pressure blast stream is described as high velocity
small particles with high flux density. Again, this particle blast
profile is suited best for removing hard coatings such as paint. The
chart shows a larger nozzle throat diameter corresponding to low pressure
operations. As stated above, large pellets impacting the surface with
low flux density is ideal for cleaning soft coatings.
Table 2. Pressure To Nozzle Throat Diameter Relationship
| Blast Pressure (psi) |
Throat Diameter (in) |
| 80 |
0.360 |
| 120 |
0.302 |
| 250 |
0.216 |
| 300 |
0.198 |
Dry Ice blasting nozzles tend to be long as a result
of the requirement to accelerate particles to as high a velocity as
possible. Therefore, a very long nozzle with a small throat tends
to have high scrubbing surface area per unit airflow. This explains
the higher efficiency of Dry Ice low pressure nozzles compared to
high pressure nozzles. A minimum cost blast system for industrial
use has a design point at 80 psi, a typical pressure for a plant air
system.
Benefits of CO2 Blast technology
Cost Reduction
The natural sublimation of Dry Ice particles eliminates the cost
of collecting the cleaning media for disposal. In addition, containment
and collection costs associated with water/grit blasting procedures
are also eliminated.
Because CO2 blast systems provide on-line maintenance
capabilities for production equipment (cleaning "on-line"),
timely and expensive detooling procedures are kept to a minimum. Dedicated
cleaning cycles are no longer required as preventative maintenance
schedules can be adopted which allow for equipment cleaning during
production periods. As a result, throughput is increased without adding
labor or production equipment.
Extension of Equipment's Useful Life
Unlike sand, walnut shells, plastic beads and other abrasive
grit media, dry ice particles are non-abrasive. Cleaning with Dry
Ice will not wear tooling, texture surfaces, open tolerances, or damage
bearings or machinery. In addition, on-line cleaning eliminates the
danger of molds being damaged during handling from press to cleaning
area and back.
A Dry Process
Unlike steam or water blasting, CO2 blast systems will not damage
electrical wiring, controls, or switches. Also, any possible rust
formation after cleaning is far less with Dry Ice blasting when compared
to steam or water blasting. When used in the Food Industry, Dry Ice
blasting reduces the potential for bacteria growth inherent to conventional
water blasting.
Environmental Safety
Carbon dioxide is a non-toxic element which meets EPA, FDA, and
USDA industry guidelines. By replacing toxic chemical processes with
CO2 blast systems, employee exposure and corporate liability stemming
from the use of dangerous chemical cleaning agents can be materially
reduced or eliminated completely. Since CO2 gas is heavier than air
(CO2 gas displaces oxygen), care must be taken if blasting in enclosed
areas or down in a pit.
Current CO2 Blast applications
Molded Products
Dry Ice blasting cleans unwanted release agents ("parting agent")
and/or residual material build up from the product contact surfaces.
That is, the build-up of release agents or residual product from the
hot mold is easily removed. Dry Ice blasting allows the tools or molds
to be cleaned while the mold is hot and still in the press. This reduces
the "press down time due to cleaning" by 80% to 95%. Since
the process is non-abrasive, the CO2 blast cleaning will not wear
the tools or open critical tool tolerances. Furthermore, "micro
vents" are typically cleaned by Dry Ice blasting. This eliminates
hand drilling of plugged vents needed for optimum gas escape.
