Blast cleaning
Blasting is a simple two-body interaction where a high speed projectile impacts a target. (Figure 2)
Figure 2: Blasting.
In a given application, the only choices are the blasting parameters and the properties of the projectiles that have the following classification:
Figure
3: Classification of projectiles.
Solid media have a range of abrasivity, hardness, angularity and size. Traditional solid media tend to have high abrasivity as the process was used mainly for abrasive erosion. When abrasivity decreases, erosion rate also decreases, as shown in Figure 4.
Figure 4: Erosion by abrasive blasting.
In applications where erosion is to be controlled, solid media of low
abrasivity such as plastic media, starch media, glass beads, etc., are
used. For solid media of low abrasivity, the impact action is mainly
displacement. One aspect of solid media blasting is the generation of dust
and secondary solid waste from spent media. Therefore, abrasive blasting
is not a cleaning process.
Water
blast
is non-abrasive therefore its applications relate mainly to cleaning. For
effective cleaning, normally cleaning chemicals, as well as physical
agitation means such as brushes, are added to aid the process. The
displacement action is not effective as water does not retain its shape
and integrity and therefore deforms around obstacles. Water is, however, a
very effective carrier for blast debris and soluble contamination.
Phase
change media start
as a solid and ends up in a different phase: Dry ice sublimes to gaseous
carbon dioxide and ice melts into water which further evaporates (a double
phase change). Ice is further unique in that as a blast media it possesses
the combined characteristics of both solid and liquid blasts. Both forms
of ice are not abrasive, therefore are not effective in erosion
applications. Impact interaction is chiefly displacement and interfacial
friction. Being a phase change material, ice does not generate dust on
impact and does not require a large volume to do useful work.
Blast cleaning
Blast cleaning can be a two-body (like simple impact phenomena above) or a three-body interaction in which undesirable matter (contamination) is removed. Contamination can be either foreign or native. Only foreign contamination will be discussed here. Foreign contamination is undesirable deposited matter such as dust, dirt, grease, paint, etc. Figure 5 illustrates the three-body interaction phenomena in which a projectile impacts a target comprising a substrate and foreign contamination.
Figure 5: Removal of foreign contamination.
As
there is generally a clearly defined interface between the deposited
matter and the substrate, the removal of foreign contamination can readily
be verified, quantified and agreed upon. An ideal cleaning process removes
all contamination without changing the substrate material. It is obvious
from this discussion that a non-abrasive blast media that does not
degenerate into dust (foreign contamination) would be an ideal blast
cleaning media. Both Dry ice and ice qualify. Ice has the advantage of
melting to water to carry away blast debris and soluble contamination. Dry
ice has an advantage in applications that cannot tolerate water.
Cleaning process requirement
In
general, a cleaning job requires:
-
“Bulk Removal” where major contamination is first removed. Typically this is a physical removal by displacement, i.e., momentum transfer. This step is therefore best achieved with a material that transfers its momentum efficiently to the contamination. Solids have better momentum transfer than liquids, as they do not flow around strongly adherent contamination.
-
“Detail Cleaning” where some form of mechanical agitation such as scrubbing or polishing is provided to remove minute quantities of the remaining contamination from the surface. By definition, scrubbing means two solids moving relative to each other under applied pressure. Water, as a blast cleaning agent, does not offer this effect.
-
“Final Rinse” where the loosened contamination is rinsed away. Water is a universal solvent that is superior for this action. This demonstrates that fundamentally a complete cleaning process must involve solid-solid interactions as well as a liquid-solid interaction. This is the reason abrasive erosion blasting does not provide a clean surface. This also explains why water blast cannot provide a clean surface without the use of cleaning chemicals.
This leads to the natural conclusion that ice particles which change state from solid to liquid at normal working temperatures are ideally suited for cleaning applications where stringent cleanliness is required and at the same time the use of cleaning chemicals are not desirable. In reality, many hybrid processes are in use today. Brushes are used in combination with water to provide physical agitation and to rinse away contamination. Here special attention must be paid to recontamination by brush bristles. Ice particles, on the other hand, act as new bristles for every scrubbing action.