 |
These are
3mm dry ice pellets rapidly subliming on the
lid of a Clean Surface blast unit.
Dry ice, which is pure solid carbon dioxide,
is made by decompressing liquid CO2 to
create CO2 snow. The snow is then compacted
and extruded through a die plate to form
solid CO2 pellets.
Dry ice is unstable above minus 78.6 °C, but
instead of melting into CO2 liquid it
sublimes directly into CO2 gas. It is this
sublimation process that creates the
cleaning effect when dry ice is used as a
blasting medium.
During blasting the pellets are accelerated
to speeds between 200 and 300 m/s with
compressed air. They break up as they travel
through the blaster system and arrive at the
work surface as fast moving pinhead sized
particles. The particles embed themselves in
the pores of any surface deposits and very
quickly sublime into a much larger volume of
CO2 gas. The expansion factor varies with
temperature, but gas of at least 500 times
the volume of the solid particle is
generated within the surface deposit, which
blows it apart and breaks its bond with the
substrate material. |
