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138
ALUMINIUM IN COMMERCIAL VEHICULES CHAPTER X DECORATION AND FINISHING 138 | 139
3.1. Brushing
Brushing is a rather seldomly
used method for the decoration
of trucks and trailers. It can
mostly be seen on tankers for
the transport of fluid goods.
Like polishing, brushing is based
on abrasion effects between the
brush surface and the aluminium
surface. Due to the brush
being the harder part, aluminium
is removed from the surface
by an abrasive effect. Brushing
is done with rotary brushing
tools or machines. Normally no
additional brushing compounds
or chemicals will be used.
Like for every surface treatment of
aluminium, the part to be brushed
has to be cleaned and degreased
properly before applying the
brushing process. The cleaning is
done to remove any dust, dirt, oil,
emulsion or other residues from
the rolling process prior to brushing
and to prevent particles from
being squeezed into the surface
during brushing. To secure a uniform
surface appearance, it is of
great advantage to use an automatic
process with several brushes
in one single station, which are
simultaneously controlled.
3.2. Polishing / Buffing
Polishing or buffing is a quite
common method in the North
American market to provide a
decorative surface finish. 3 main
methods can be applied:
• Use of mirror-finished aluminium
sheets and plates fabricated
in the rolling mill
• Polishing / buffing of mill finish
sheets to the desired surface
appearance
• Manual polishing
The use of mirror-finished plates
or the use of already buffed or
polished sheets has the advantage,
that the work on site is
reduced to the manual polishing
of weld seams or places which
have been damaged during fabrication.
Great care must be
taken when handling or working
with these sheets, as every little
trace of a mechanical defect
caused by fabrication must be
manually polished.
Mirror finished plates are fabricated
in the rolling mill by the
use of a special rolling routine
with work rolls which have nearly
no surface roughness. This
makes it a very demanding
process and great care must be
taken to secure a reliable and
constant quality of the sheets.
Buffing or polishing of large
plates is done on automatic lines,
where the surface is polished
with rotary polishers across the
whole width of the sheet at the
same time. The rotary polishers
have special pads on their surface,
which polish the aluminium
surface under the help of polishing
compounds. The polishing
compounds works as a slight
abrasive and removes the top
layer of the aluminium surface in
the range of the surface roughness
produced by the rolling mill.
As the result of polishing is very
much depending on the type of
alloy and temper, the surface
hardness, the type of polishing
paste and the machine setting
(like rotational speed, pressure
and type of pad), this is a method
of “trial and error” to find the
right setting per specification.
3. Mechanical finishing
139
EUROPEAN ALUMINIUM ASSOCIATION
In any case, before polishing, the
aluminium plates should be
degreased and cleaned to remove
any kind of dust and dirt to prevent
abrasive particles from being
ground into the aluminium surface.
The same rules apply for manual
polishing. This process is difficult
to apply and a large and extensive
experience is necessary to
reach a satisfactory and reproducible
result. After removal of
surface dirt or oil, the manual
process starts with a rotary polisher
and the use of quick- cutting
abrasive paste. The pad
should be a wool compounding
type. Speed of the polisher must
be limited to prevent burning of
the surface. The polisher should
be moved back and forth, up and
down to ensure a uniform abrasion
of the surface. As the pad
will suddenly turn black (caused
by the polishing residue), great
care must be taken to regularly
clean or change the pad. After
the rough first polish, the type of
paste should be changed to one
with lower abrasion. Before
applying the final polishing step,
it is useful to clean the surface
again to remove the black
residue, which might be trapped
into the surface. The final result
of manual polishing should be a
mirror-like, uniform, swirl-mark,
black speck- and bright sparklefree
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