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Molding and casting
There are various methods that are employed for making plastics.
However, the most important ones include molding, casting,
laminating, extrusion and calendering. The process of molding is
done in three steps. It begins with melting solid pellets (taken in
the form of pellets or powder form) into a thick syrup. In the
second step, the molten resin is forced into a mould under great
pressure. Lastly, the molded resin is hardened in order that it
retains its shape and form even after removal from the mould.
Thermoplastic resins are allowed to cool
and this process alone is sufficient to harden them. They can be
further molded into new shapes. Thermoplastic resins can be heated
repeatedly and the method of molding them is called injection
molding. Thermosetting plastics are hardened in the moulds by
adding heat and applying pressure, since they cannot be remelted.
The heat and pressure cause chemical reactions to take place, which
harden them. The process of molding is referred to as compression
molding. Blow molding is the process by which bottles and other
hollow products are made.
The process of casting is somewhat similar
to molding, except that pressure is not used. The molten resin is
poured into a mould and chemicals are added to it to harden the
plastics. Casting is a procedure employed to harden and shape both
thermoplastic and thermosetting plastics.
Extrusion
The process of extrusion is used to make pipes, moldings used
in the building industry, sheets, fibers and other products that
have the same shape throughout their length. In this process,
machines are used to form plastics by squeezing molten plastics, in
a continuous stream, through a specially shaped die. A large screw
pushes solid resin molten resin through a heating chamber.
Laminating and calendering
The process of calendering is as simple as applying butter on
bread and that of laminating is like making sandwiches. In the
process of laminating, the “sandwiches are made from sheets of
paper, cloth or metal foil. The sheets are passed between rollers
and then coated with molten resin. The sheets are next piled one on
top of another and mechanically pressed together until the resin has
joined them firmly. The process is employed to make products that
require to be strong, such as tabletops and electrical insulation.
To give paper, cloth, wood and other
materials a thin coating, the process of calendering is used. Here,
rollers spread molten resin over sheets of paper, cloth or wood, to
form a protective finish.
So the next time, you see plastics products
take a minute off to recapture the method of production. Dinnerware
is produced using compression molding and playing cards are
calendered resins. |
