Iron and Steel - mild steel, tool steels.
Steels rust (except stainless steel) so they need to be protected against
moisture and air.
Coating in oil or grease is effective as long as the coating is not
removed. All of the steel that is delivered to the school is greasy.
Plastic dip coating is very
effective as it can change the appearance as well e.g. the shelves in
fridges and dishwashers are plastic coated. In school the steel item
is heated up to straw colour (blue is too hot), then dipped into a fluidising
tank, where air is blown through the polythene powder to make it fluid
enough for the item to be dipped in. The coated item is then removed
and allowed to cool. Any excess coating is trimmed off with a knife.
Painting is the most common
way of protecting steel from rusting e.g. car bodies, white goods, filing
cabinets etc. The paint process is:- First to abrade the surface to
make it smooth and to give it a key (rough so that the paint will grip).
Second degrease using a solvent to dissolve any oil or grease. Third
apply a primer which grips securely to the surface and will also grip
the next layer of paint. Fourth apply finishing coats, sanding down
between coats if hand brushing.
Blue and oil. This a very traditional
method used on tools and springs. Heat the item up to blue then quench
Lacquer – this a clear
finish like a varnish. The surface is thoroughly polished or patterned,
degreased and then lacquered.
This is used to apply a layer of another metal such as chromium or nickel
which do not rust and can be highly polished.
Galvanising is a form of electroplating
which applies a layer of zinc a dull silver grey finish is usually produced.
The layer of zinc is usually thick to protect steelwork used outside
e.g. fences, water tanks etc.
Aluminium is very resistant to corrosion under most conditions because
it forms a tough but very thin layer of aluminium oxide as soon as it
is cut. Salt water will attack aluminium however so it is replaced by
stainless steel and brass for maritime use. (boats and ships at sea).
Aluminium can be polished or have its surface textured e.g. satin finish.
Aluminium can be painted.
Anodising aluminium is an electrical
process which makes the oxide layer increase its thickness. This thicker
oxide layer can be dyed any colour.
Brasses and bronzes.
These alloys are usually simply polished and lacquered or left unfinished.
They are resistant to serious corrosion in normal use but do become
Plastics are usually polished or protected against damage to keep their manufactured finish e.g. acrylic. To polish plastics or metals it is essential "to go through the grades". This means start with a coarse grade of abrasive paper to remove deep scratches, then go onto successively finer grades removing feinter and feinter scratches. Wet and dry abrasive papers are usual. When the finest grade paper has been used a metal polish can be used to give the final highly polished effect. A buffing wheel can also be used for the final stages where a calico wheel coated with an abrasive soap removes scratches and polishes the surface.
Self finishing - many products are manufactured with their final finish. Injection moulded products are formed in a mould which is either textured or highly polished - this finish is imparted to each product produced by the mould. Most plastics are manufactured with a very good surface finish.