The technique of glass making too is based
on similar lines. People learned to make the first glass containers
about two thousand years ago. Molten glass was collected on the ends
of hollow iron pipes and then expanded by blowing through the pipes.
Slowly, people learnt to blow molten glass into moulds. Glass bottle
making machines were introduced in the thirties.
The first plastics blow molding machine
was designed in the early forties; the first plastic bottles were
manufactured using polythene.
In the early seventies, environmentalists
began arguing on the grounds that glass and plastic bottles added to
pollution. This led to the setting up of numerous recycling centers
where people could return bottles for reuse in other bottles. Most
of the recycled plastic is used to manufacture lower quality plastic
than those used to make bottles.
How bottles are made
The entire process of bottle making is almost fully automated.
An automated feeder separates a stream of molten glass into
individual gobs. These are then dropped through tubes in a moving
track. The gob is shaped into what looks like a short bottle with
thick walls and is called a parison. The parison is
transferred to a final mould made of iron, which moves up and clamps
around the glass. Air is blown into the glass till it acquires the
final shape of the mould. This procedure involving expansion is
called blowing. The bottle is then released from the mould
Bottle Making (Molding
Annealing is done by reheating the glass
and gradually cooling it. Such a process removes the stresses and
strains in the glass after shaping. This is an important step and if
not done may cause the glass to shatter as a result of the build up
of tension caused by uneven cooling. After the bottles have cooled
to room temperature, they are inspected and finally packaged.
Plastic bottles may be made from
polyethylene, polypropylene or polyvinyl chloride. Large cold drink
bottles are made of polyethylene terephalate (PET). These
bottles are designed in such a way that the gases used to carbonate
the soft drinks are unable to escape.
There are three different methods used for
processing plastic bottles – extrusion blow
molding (in which the parison is tube shaped), injection
blow molding (in which the parison is prepared by
injecting molten plastic through a small hole) and injection stretch
blow molding (in which the plastic is blown into the mould
while it is simultaneously being stretched by a metal rod).