Author: Thump Local

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Dry Ice blasting is safe alternative to traditional cleaning methods is used for mold mitigation, fire restoration, graffiti removal and much more. A non-abrasive, non-conductive, non-poisonous cleaning process that uses a compressed air system with soft dry ice pellets that vaporizes on impact causing no secondary waste but removing over 99.9% of the contaminant. This process uses frozen Carbon Dioxide “dry ice” pellets that turn into gas upon impact causing the contaminant to break into small chips and fall to the ground. Since no abrasive chemicals are used this process is ideal to use in settings where there are young, elderly or sick people. The most common uses for dry ice blasting are graffiti removal, smoke damage, mold remediation and high grade adhesive removal. Please visit www.coldjet.com to find out other uses.

CO2 blasting employs 3 core factors:

1. pellet kinetic energy
2. thermal shock effect
3. thermal-kinetic effect

Dry Ice Blasting optimizes blast performance by combining these forces and adjusting:

• compressed air pressure
• blast nozzle type (velocity distribution)
• CO2 pellet size and density
• pellet mass rate
• flux density (particles per unit area per second)
• Pellet Kinetic Energy

Pellet Kinetic Force

The Dry Ice Blasting process incorporates high velocity (supersonic) nozzles for surface preparation and coating removal applications.

Since kinetic impact force is a product of the pellet mass and velocity over time, the Dry Ice Blasting achieves the greatest impact force possible from a solid CO2 pellet by propelling the pellets to the highest velocities attainable in the blasting industry.

Even at high impact velocities and direct head-on impact angles, the kinetic effect of solid CO2 pellets is minimal when compared to other media (grit, sand, PMB).

This is due to the relative softness of a solid CO2, which is not as dense and hard, as other projectile media. Also, the pellet changes phase from a solid to a gas almost instantaneously upon impact, which effectively provides an almost nonexistent coefficient of restitution in the impact equation.

Very little impact energy is transferred into the coating or substrate, so the Dry Ice Blasting blasting process is considered to be nonabrasive.

Thermal Shock Effect

Instantaneous sublimation (phase change from solid to gas) of CO2 pellet upon impact absorbs maximum heat from the very thin top layer of surface coating or contaminant. Maximum heat is absorbed due to latent heat of sublimation.

The very rapid transfer of heat into the pellet from the coating top layer creates an extremely large temperature differential between successive micro-layers within the coating. This sharp thermal gradient produces localized high shear stresses between the micro-layers. The shear stresses produced are also dependent upon the coating thermal conductivity and thermal coefficient of expansion / contraction, as well as the thermal mass of the underlying substrate.

The high shear produced over a very brief expanse of time causes rapid micro-crack propagation between the layers leading to contamination and/or coating final bond failure at the surface of the substrate.

Thermal-Kinetic Effect

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 800 times the volume of the pellet in a few milliseconds in what is effectively a “Micro-explosion” at the point of impact.

The “Micro-explosion,” as the pellet changes to gas, is further enhanced for lifting thermally-fractured coating particles from the substrate. This is because of the pellet’s lack of rebound energy, which tends to distribute its mass along the surface during the impact. The CO2 gas expands outward along the surface and its resulting “explosion shock front” effectively provides an area of high pressure focused between the surface and the thermally fractured coating particles. This results in a very efficient lifting force to carry the particles away from the surface.
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