System origin

Foreword
The "cryogenic" process could have been classified in the suction, or pressure, category because it uses a gaseous vector linked to these two propulsion modes to project the product. However, its particularity, which makes of it a special process, lies in the fact it can only project one kind of product, under different forms, CO2 or carbon dioxide. The projection principles remain the same, however the implementations are very different because they are adapted to the product constraints.

Historical background
The process is the result of the own manufacturing of the product to project, that is why its settlement and its first patents take place in the XX century, in the 60's/70's, that is to say after the sandblasting origin. However the manufacturing of the carbon dioxide took place in the 30's, it is in 1945 only the American navy made the first scouring experiences with dry ice.

Evolution
In 1963, Reginals LINDALL registers a patent for his method consisting in "removing meat from the bone using projected carbon dioxide particles".
In 1972, Edwin RICE registers a patent for his method consisting in "removing the non desired parts of an object pulverizing ice particles to high velocity".
In 1977, Calvin FONG registers a patent for "scouring or sandblasting with pellets of a material capable of sublimation".

Projectiles

Features
The projectile is CO2, or carbon dioxide, which, as any pure body, has the distinctive feature of changing of state according to the temperature and pressure conditions. It changes then from the gaseous state to the liquid or solid state, with intermediate transitional states.
In this way we obtain different projectiles which are:
Carbonic dry ice: it is in a solid state (-78°C) for the scouring or cleaning of resistant residues.
Dry ice: it is in a liquid/solid state for eliminating on surface localized pollution.
Supercritical CO2: it is in a single-phase state halfway between liquid and gaseous state for cleaning delicate supports.

Manufacturing
The particles, or carbonic dry ice grains, also called according to their diameters "Pellet" (1.6 to 3.2 mm) or "Nugget" (6.4 to 19 mm) are made by a cylindrical extruder made of a jack chamber and a dry ice chamber.
The carbon dioxide is introduced under liquid state in the dry ice chamber where it gets solidified again forming a "nugget". The nugget is then forced by a jack against a mould with several openings, with such a pressure that the carbon dioxide gets liquid entering the openings. While it flows out in the mould, the liquid changes its phase and becomes solid while the openings maintain the compression on CO2 for creating high density pellets (d=1.54).

Nota:
To make pellets or nuggets it exists other techniques of hammer or rotation-type. The pellets can be made on site or supplied in isolated and sealed containers at a specialized gasman.

Projection

Projection principles
The pellets are conveyed and shot thanks to compressed air through a hose, either by a suction system, from a loading hopper, or by a pressure system, from honeycombed drum distributor located at the end of the storage hopper (see picture opposite).
- in the first case, the final average speed of the particles at the end of he nozzle ranges between 60 and 120 m/sec.
- in the second case, the speed can reach up to 290 m/sec at the end of a special nozzle with a venturi profile.

Impact particularity
This is the only feature of the impact which makes the cryogenic process a special system. Indeed, when the pellets hit the part surface, the dry ice sublimates in carbon dioxide, going from solid state to gaseous state, without going through liquid state. Becoming again a natural inert gas, it disperses in the atmosphere (see picture below). In addition to kinetic energy linked to the projection, the dry ice uses its low temperature to increase the cutting constraint and to break the coating which covers the surface to treat.
"This is a mechanical and thermic shock".
The thermic shock being the key element, it is possible to increase the process by heating up slightly the part. The fall in temperature caused by the impacts remains localized and does not cause any thermic tension on the substratum. The dry ice does not practically have any hardness compared to the other solid abrasives and thus does not bring this chiseling effect of the surface. It is a "not aggressive" product.

Conclusion:
The process is dry, not conductive, not abrasive, not toxic, and does not leave any trace on the treated part. It does not generate any secondary waste linked to the abrasive that is why it is a not polluting technology. The contaminant which has been removed from the surface is the only thing to remove after the treatment. It makes it possible to cleaning the equipments on site, sometimes while working and without any precaution for the environment.

Machines

General points
They are generally compact, autonomous and transportable, constituted with a pellets loading hopper which feeds a suction or pressure projection system. They can also be associated to a confined sleeves cabin. Some machines are fitted with an extruding machine to manufacture the pellets from liquid CO2. They can be automated according to the applications.

Applications
This process will be used anywhere the part to treat requires a rigorous geometric and dimensional respect, each time it will be possible to move or dismantle the part to treat (work in situ), on any delicate support, and anywhere the contaminant is resistant or dangerous. The main activitie areas are: moulding and plastic industries, smelting works, food business industry, chemistry, packaging, car industry, printing, building industry, petrochemistry, nuclear energy, electronics, mechanics, electro-mechanics.