| Fabrication
The fabrication of articles from latex concentrate and
from dry rubber (that is rubber which is in the form of
bales of either natural or synthetic rubber) is completely
different, although both involve shaping and vulcanization
and some of the shaping processes are common to both materials,
but in general the latex and dry rubber industries are totally
distinct. This section refers entirely to dry rubber processing.
Latex fabrication is discussed elsewhere.
Fabrication of most rubber products demands three main
processes:.
- Mixing
- Shaping (and building, if a composite article is being
made)
- Vulcanization.
Mixing
The mixing process is usually performed in heavy internal
mixers, capable of processing 200 kg batch weight in two
minutes. This process has two functions: firstly, to soften
the rubber (this is often known as mastication) and, secondly,
to admix the rubber with the compounding ingredients, which
may include fillers, vulcanizing agents, protective agents
and blends with other rubbers (which are usually synthetic
rubbers). This technique is known as compounding.
After mixing, the compounded rubber is plastic and is now
ready to be shaped. This is done in a variety of ways and
is frequently combined with vulcanization in which the rubber
undergoes a chemical reaction at a high temperature, becoming
converted from the plastic state into a strong, highly elastic
material.
Shaping
The most common process is by moulding under pressure in
a heated mould in which vulcanization also occurs. There
are three important variants of the process: compression,
transfer and injection moulding. Other important shaping
techniques include extrusion and calendering. Bonding of
rubber to metal (or thermoplastic/glass fabric, etc) normally
takes place as part of the moulding process. The metal component
will require pretreatment to clean it and the application
of a bonding agent.
Compression Moulding
A blank of appropriate weight is shaped in a steel mould
within a heated press. The majority of rubber articles are
made by this process. It is
- Cheap
- Rapid
- Capable of long or short runs
- Gives a fair degree of precision if care is taken
High rates of production are possible in multi-daylight
presses in which down-time is reduced to a minimum by using
alternately each of two separate sets of moulds, mechanically
handled.
Transfer Moulding
A slug of rubber compound is put into the upper cavity
of the transfer mould. When placed in the press the rubber
is squeezed through a small aperture to fill the second
cavity, having the required shape, in the rubber is vulcanized.
This method is used for precision work which justifies the
high mould cost and relatively slow throughput. Short runs
are feasible and the method is particularly useful for components
having metal inserts (such as engineering components).
Injection Moulding
This is a semi-continuous process in which rubber is extruded
from a heated barrel of a screw or ram machine through a
nozzle. The work of extrusion produces a further rise of
temperature, before the rubber is forced into a heated mould
where the rubber is vulcanized. Because the rubber reaches
the mould at a high temperature, vulcanization times are
short and thick articles are homogeneously vulcanized. The
high capital cost is justified by the use of the machine
for long runs of articles of good quality, particularly
those which are difficult to mould by compression moulding.
Extrusion
During extrusion compounded rubber is passed from a short
screw extruder through a die of appropriate shape. Vulcanization
is a separate process, and can be performed in a variety
of ways: (i) batchwise in a steam or air autoclave, or (ii)
continuously in steam or hot air, or (iii) in a bath containing
a eutectic mixture of molten metal salts or in a fluidized
bed. Simple or complex sections, either solid or sponge,
may be extruded.
Extrusion is used for the manufacture of hose, weatherstrip
seals for automobiles and cables. Parts of tyres are also
extruded, notably the sidewalls.
Calendering
In the calendering process, rubber is passed through a
three- or four-roll calender either to produce a sheet of
controlled thickness or to force the rubber into close contact
with a textile or metal cord. Calendering is frequently
followed by a building process, eg. sheet may be built up
into a rubber roller or the rubberized textile may be built
up into a carcass for a tyre or conveyor belt. Vulcanization
is then performed in heated presses or in autoclaves.
Finishing
Most moulded goods will require some form of finishing,
such as the removal of flash.
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