Composites - An Overview
Press moulding is a mass production
technique of processing Fibreglass Reinforced Plastics. It
can be used to manufacture complex parts to close tolerances
with a good finish on both sides. It incorporates matched
metal moulds between which the material is cured at elevated
temperatures and high pressures, usually applied by hydraulic
press. Although equipment and mould costs are higher than in
contact moulding, the added expense can be justified by the
larger number of parts which can be produced in a shorter
length of time, by the weight saving due to the higher
strength of the composite thus produced and by being able to
achive a smooth and consistent finish.
When glass is drawn into fine strands
its strength increases considerably over that of bulk glass.
There are two types of glass which are used for Fibreglass
manufacture. These are "E"glass, which is a low
alkali-containing borosilicate glass, and"A" glass,
a high alkali content material with a composition similar to
that of window glass. From the composition and general
properties of these two glasses, it can be seen that
"E" glass is the stronger and this is the material
usually specified for reinforcement purposes,unless the
operating stresses are relatively low.
Fibreglass reinforcement is supplied in a number of forms,
are designed for use by the fabricator of reinforced plastics
components. These include random reinforcement materials
known as chopped strand mats and needled mats, bi-directional
materials such as woven rovings and glass fabrics, and
rovings which are used for chopping into random lengths or as
high strength directional reinforcements.
Resin is an essential component of
Fibreglass Reinforced Plastics. The most popular is polyester
resin, which can be cured to a solid either by catalyst and
heat, or catalyst and accelerators can be used at room
temperature. The ability of polyester resin to cure at room
temperature into a hard material is one of the main reasons
for the growth of the reinforced plastics industry. It was
this which led to the development of the room temperature
contact moulding methods which permit production of extremely
large integral units. Epoxide resin is also used for
applications where its greater cost is offset by desired
properties. Both polyester and epoxide resins, by suitable
choice of the basic resin and the method of cure, can be made
to give a very wide range of physical, mechanical, chemical
and electrical properties.
particular properties are required, it is recommended that
the resin manufacturers are approached for their advice on
the best system to use.
resins are included because they have excellent chemical
resistance. They can be used in conjunction with Fibreglass
materials, but it is more usual to incorporate Fibreglass
reinforced polyester resins as the backing material for a
surface of Fibreglass reinforced furane.
chloride and polypropylene are extensively used in chemical
plant. Their use is frequently limited by their low
mechanical strength rather than their chemical resistance.
There is a growing use of these materials in conjuction with
Fibreglass polyester laminates where the latter do not have
the chemical resistance required but can contribute strength.
The P.V.C. or polypropylene is used as an inner skin which is
backed and armoured by a Fibreglass/polyester laminate.
Fibreglass Reinforced Plastics are a
family of materials, and not only does the type of Fibreglass
reinforcement and resin used have an influence of the
combined material, but their relative quantities also have a
varying the properties of resin and Fibreglass reinforcement
and controlling the orientation an extremely wide range of
mechanical properties can be achieved. The strength and
modulus of a laminate in any direction is, in fact,
proportional to both the total amount of glass fibre that is
present (by volume) and the proportion present in that
particular direction. The Fibreglass provides the majority of
the mechanical properties but the resin and Fibreglass are
interdependent and it is not, therefore, possible to consider
either of them separately. The quality of the adhesion of the
resin to the glass fibre at the resin/glass interface is thus
an extremely important factor in Xtra Reinforced Plastics Pvt.Ltd .
Strength is not the only factor which
is important to the designer, but also the physical factors
of thermal expansion, thermal conductively and heat
resistance. The co-efficient of thermal expansion of glass is
itself relatively low-less than most metals. However, when
combined with resins which have a high co-efficient of
expansion, the overall result is usually a co-efficient of
expansion greater than that of metals. By selecting the
correct glass content, it is possible to match the
co-efficient of expansion of steel and this is sometimes done
where it is necessary for both to expand at the same rate.
Thermoplastics have high co-efficients of expansion and glass
fibres can be a very important factor in stabilising them.
plastics materials have relatively low thermal conductivity.
Compared to metals, the conductivity of most plastics
materials is only 1/500th of metals. This means that
reinforced plastics incorporated in chemical plant has an
inherent insulating effect. Their low thermal conductivity
also makes possible at higher temperatures than would be
expected. Where the heat is only applied from one side the
steep temperature gradient across the laminate ensures that
the outer portions of the component are well within the
limiting temperature and, therefore, not seriously weakened.
maximum service temperatures possible for these materials
shows that temperature resistance is a problem with
reinforced plastics components, although the presence of the
Fibreglass helps considerably in stabilising and improving
heat resistance. Where higher heat resistance is required
than shown, it is sometimes possible to utilise phenolic or
silicone resins to give improved performance.
The property of reinforced plastics of
most interest to the chemical and plant engineer is their
resistance to chemicals. This is one of the most difficult
properties to define accurately. It is strongly recommended
that either the resin manufacturers are consulted or
practical trials are carried out to ensure that the material
selected is satisfctory for the intended purpose. In most
cases, at least one resin would be capable of withstanding
completely each of the specified conditions at normal
temperatures. In most cases elevated temperatures together
with strong oxidants are the main factors influencing
chemical resistance. It should ;be noted that 200'F is, in
fact, outside the recommended operating temperature for both
polyvinyl chloride and polyethylene and it is at these high
temperatures that the thermosetting materials, particularly
epoxide and furane, really come into their own. These figures
illustrate why the combination of P.V.C. or polypropylene as
the inner corrosion resistant skin backed by structural
Fibreglass reinforced polyester is ;becoming more widespread,
as the resistance to strong oxidants and strong acids of
P.V.C. and polypropylene is considerably better than that of
Xtra Reinforced Plastics Pvt.Ltd Home
Xtra Reinforced Plastics Pvt.Ltd :Related