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5
2. Processing Technologies Driven by Selection Criteria
5 Compatibility with the Form of Resin—Solid resins in the
form of novolacs can be processed only by using compression moulding technology. Other processing technologies
are not suitable to process novolacs due to their solid form.
Other resins, such as phenolic resin, which releases
byproducts, cannot be processed using a compression
moulding process because there will be no outlet for the
byproducts. Rather, the byproducts will get entrapped in
the moulding, causing a defective composite.
5 Flow of Matrix—The processing technology must ensure
the flow of matrix into the entire moulding cavity. Flow of
matrix is also dependent on the design of the mould and
the reinforcement used. All of the moulding processes
support a liquid form of the resin during processing.
5 Compatibility of Mould Design with the Matrix—Process
compatibility depends on both the mould design and
material used for the mould. For high-temperature curing
resins, only metallic moulds are used. For room-temperature curing, resins (such as polyester and epoxy resins),
plaster of Paris moulds, wooden moulds, etc. are used in
addition to metallic (steel and aluminium) moulds due to
their low cost.
5 Production Rate—Resins that have a short processing cycle
are used to achieve high production rates. The curing of
polyester and epoxy resins may be quickened by slightly
increasing the content of the catalyst and the hardener,
respectively. However, this content cannot be increased
indiscriminately because excessive exotherm may damage
the composite. A continuous section of composites having
a regular cross section can be processed through pultrusion. Similarly, a cylindrical product, such as a high-pressure vessel, may be processed using a filament winding
method. These examples prove that the processability of
composites is process-dependent too. The selection of
process ensures the viability of the composite product.
There might be two or more processing methods that can
be used for a composite product, but an engineering
analysis in terms of performance, tooling cost, processing
cost, raw materials, etc. would decide upon a single process
only. In other words, the product can be processed using
one specific method only.
5 Cost of Production—Resins are selected to reduce the cost
of production. Depending upon the service environment
and expected service life of the composite product, a resin
is chosen to reduce the cost of production of composites.
For example, polyester resin has three grades which are (i)
general-purpose polyester resin that is very low in cost
coupled with low mechanical properties, high shrinkage,
and low transparency; (ii) isophthalic polyester resin that has
a moderate cost coupled with good mechanical properties,
low shrinkage, and high transparency; and (iii) vinyl ester
Point to Ponder…
Novolac is a solid form of resin
which melts at application of
heat and, therefore, behaves as
thermoplastic initially. Its melting helps in wetting of reinforcement. On further heating at high
temperature, it solidified with
cross-linked network of thermosetting resin. Novolacs are usually used as prepregging resins.
Chapter 5 · Processability of Thermosetting Composites
5
2. Processing Technologies Driven by Selection Criteria
5 Compatibility with the Form of Resin—Solid resins in the
form of novolacs can be processed only by using compression moulding technology. Other processing technologies
are not suitable to process novolacs due to their solid form.
Other resins, such as phenolic resin, which releases
byproducts, cannot be processed using a compression
moulding process because there will be no outlet for the
byproducts. Rather, the byproducts will get entrapped in
the moulding, causing a defective composite.
5 Flow of Matrix—The processing technology must ensure
the flow of matrix into the entire moulding cavity. Flow of
matrix is also dependent on the design of the mould and
the reinforcement used. All of the moulding processes
support a liquid form of the resin during processing.
5 Compatibility of Mould Design with the Matrix—Process
compatibility depends on both the mould design and
material used for the mould. For high-temperature curing
resins, only metallic moulds are used. For room-temperature curing, resins (such as polyester and epoxy resins),
plaster of Paris moulds, wooden moulds, etc. are used in
addition to metallic (steel and aluminium) moulds due to
their low cost.
5 Production Rate—Resins that have a short processing cycle
are used to achieve high production rates. The curing of
polyester and epoxy resins may be quickened by slightly
increasing the content of the catalyst and the hardener,
respectively. However, this content cannot be increased
indiscriminately because excessive exotherm may damage
the composite. A continuous section of composites having
a regular cross section can be processed through pultrusion. Similarly, a cylindrical product, such as a high-pressure vessel, may be processed using a filament winding
method. These examples prove that the processability of
composites is process-dependent too. The selection of
process ensures the viability of the composite product.
There might be two or more processing methods that can
be used for a composite product, but an engineering
analysis in terms of performance, tooling cost, processing
cost, raw materials, etc. would decide upon a single process
only. In other words, the product can be processed using
one specific method only.
5 Cost of Production—Resins are selected to reduce the cost
of production. Depending upon the service environment
and expected service life of the composite product, a resin
is chosen to reduce the cost of production of composites.
For example, polyester resin has three grades which are (i)
general-purpose polyester resin that is very low in cost
coupled with low mechanical properties, high shrinkage,
and low transparency; (ii) isophthalic polyester resin that has
a moderate cost coupled with good mechanical properties,
low shrinkage, and high transparency; and (iii) vinyl ester
Point to Ponder…
Novolac is a solid form of resin
which melts at application of
heat and, therefore, behaves as
thermoplastic initially. Its melting helps in wetting of reinforcement. On further heating at high
temperature, it solidified with
cross-linked network of thermosetting resin. Novolacs are usually used as prepregging resins.
Chapter 5 · Processability of Thermosetting Composites