2.2.6 Slurry blasting
During this application - that operates like the already described injection
blasting in a pressure blasting pot – a mixture consisting of abrasive
and water, together with inhibitor additive, if necessary, is filled in the
blasting pot instead of the dry abrasive.
2.2.7 Ice Blasting
This application mainly uses carbon dioxide ice. It is suitable for removing
paints, dirt, grease and similar coating on surfaces.
Carbon dioxide ice is carbon dioxide (CO2) in solidified form. It is odourless
and not toxic.
Fluid carbon dioxide being under high pressure is decompressed in special
machines. This process generates the carbon dioxide ice with a temperature
of – 79°C. Then, fine granular material is made of this carbon
dioxide ice; this granular material can be used as abrasive.
You can get the carbon dioxide ice abrasive in special cooling boxes.
When using carbon dioxide ice special compressed-air and injection blasting
units are necessary to use. Within these units the granular material is
blasted onto the surface to be cleaned. The soiling contracts and embrittles
because of high supercooling during the contact time of the abrasive and
the surface of the workpiece. This so-called “thermo tension” detaches
the layer form the basic material. The coating will then completely be
removed with the help of the speed of the subsequently impinging granular
parts. This is called the ”thermal effect”.
Immediately after the impact the granular material dissolves completely
into gas and goes back into the atmosphere.
2.2.8 Water- jetting cleaning and hydro-jetting
Two systems have already proven to be very good, which differ from the
power required to generate kinetic energy.
Water-jetting cleaning
In these devices, water is taken via a pump to the ejection nozzle, and,
if desired, it is also heated there. The following values are common:
Operating pressures 25 – 170 b adjustable
Temperatures 30 – 155° C adjustable
Water quantities up to 2,000 l/h
In general, cleaning and inhibitor admixtures (admixtures to achieve a certain
reaction) are fed.
It can be advantageous to spray on the cleaner at higher concentrations
at first and to use the high-pressure jet only after sufficient exposure
time.
These devices are used for cleaning surfaces and degreasing, but not much
for derusting and roughening.
The main application concentrates on removing oily, fatty and wet impurities
as well as of water-soluble contaminations at numerous objects, in particular
of machine and steel construction, in the field of car mechanics and railway
engineering; it is also used for preparing hot-galvanized surfaces. If coating
shall be maintained wholly or partially temperature and cleaning agent have
to be taken into account accordingly.
Hydro-jetting
The following values are common:
Operating pressures 250 – 2,000 b
Flow volumes 1,000 – 6,000 b
These systems can be used both for cutting, parting as well as for cleaning
and roughening surfaces.
The high mass fraction is particularly advantageous resulting in a hard
jet so that soft or elastic contaminations can be blasted considerably quicker
than with other techniques. The difficult manual handling of the nozzles
lances due to the high reactive power is a big disadvantage.
The high water quantities avoid, of course, the generation of dust. The
disposal at building sites is disadvantageous because toxic substances can
be transported through the water into the environment or the sewerage system.
At both methods, small amounts of abrasive can be added by injection to
increase performance.
2.2.9 Wet blasting for fine surface treatment
Nowadays modern methods on fine surface treatment like e.g. liquid honing,
smooth blasting, polishing blasting, etc. base only partially on the further
development of the well-known wet blasting systems operating on the basis
of a water-abrasive mixture.
The application of abrasives of good quality, at very fine and also precisely
calibrated granularity is of utmost importance to be able to reach the required
fineness of surface treatment.
This method aims at reducing the originally existing surface irregularities
and to achieve the required low roughness height. When selecting the grain
size to be used for this method not only the desired surface roughness will
be decisive, but the existing roughness height of the surface of the workpiece
not yet being treated has to be taken into account as well.
In addition, the inclination angle under which the jet impinges on the surface
of the workpiece influences essentially the blasting result. The abrasive
grains for blasting have to have, on the one hand, enough kinetic energy,
but, on the other hand, the horizontal component of the impact speed must
be high enough to get such an efficiently chip removing effect.
2.2.10 Combination of different blasting methods
The combination of two different blasting methods presented here consists
of compressed-air blasting in connection with hydro-jetting and it is called “µ-jet©
method”.
- The abrasive/mixture of compressed air is led – as usual – from
a pressure blasting pot to a jet nozzle (dry jet nozzle) and is pre-accelerated.
Behind this jet nozzle a hydro-jet is added in a secondary chamber.
- The abrasive/mixture of compressed air is post-accelerated with the hydro-jet
in the second nozzle (output nozzle) for the second time.
The high pressure of the hydro-jet (up to 2,700 b) enables to reach very
high exit velocities of the abrasive.
This is also because of the fact that an abrasive grain for blasting can
be caught and accelerated more easily by a denser carrying medium like water
than by a comparably less dense carrying medium like air.
The kinetic energy of the abrasive increases to the square with the same
mass at increasing speed. In case of doubling the speed quadruple of the
kinetic energy will be reached, and in case of tripling even nine times of
the kinetic energy will be achieved.
Due to the high kinetic energy of the abrasive and the thus resulting high
abrasive performance this combined method is applied in the field of industrial
corrosion control and in ship building.
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