170
6
5 Thermal and Hygrothermal Stability—The thermal stability of
synthetic fibres (such as glass, carbon, and Kevlar) is good,
and they can be used under any thermal processing. Natural
fibres are thermally less stable, and care must be taken for
their processing, as mentioned previously. Moisture affects all
fibres, and the ingress of moisture in Kevlar deteriorates its
mechanical properties. In glass fibres, moisture causes
depletion of the coupling agent. In carbon fibres, moisture
affects the polymeric sizing of the polyimide or the epoxy over
it. Natural fibres absorb moisture; thus, their mechanical
properties come down.
5 Post-Processing Operations—The role of reinforcement and its
machinability is important for post-fabrication operations.
Kevlar reinforced composites are difficult to machine, and they
require water jet or laser cutting operations for smooth cutting
edges. Kevlar composites having a fibril structure will cause
deformation from fibrillization of the fibres and give a rough
surface on mechanical machining by standard cutting tools
(hacksaw, milling cutter, turning tools, etc.) The processability
of composites does not end with striking the correct combination of raw materials and processing methods to just get the
product moulded. Both the designer and the process engineer
must be prudent enough to plan in the processing of the
composite products for post-fabrication finishing. Thus, the
raw materials, moulding surfaces, and processing technologies
need to be chosen in such a manner that they ensure a minimum of post-fabrication operations for moulded composite
products.
5 UV Resistance—UV radiations do not affect natural fibres and
most of the synthetic reinforcing fibres—except polyolefin
fibres, such as PP and UHMWPE.
6.7.2 Processing Technology Driven by Selection
Criteria
Primarily, there are only three processes (i.e. injection, compression, and pressure bag moulding), which are used for fabrication of
thermoplastic composites. Of late, a thermoplastic prepreg-based
filament winding process also has been developed.
5 Compatibility with the Form of Reinforcement—Short fibres,
staples, whiskers, and particles only can be used as the
reinforcement in an injection moulding process. For
example, the Motorola Ultra smartphone has a rear housing
made of a Kevlar short fibre-reinforced PC/PET blend, as
shown in . Fig. 6.4. In a compression moulding process,
reinforcement in any form may be used. Roving and tow
preregs are used in filament winding technology, but
thermoplastic filament winding is tedious because it involves
many processing steps and stringent control of processing
parameters. The pressure bag method also employs fabric
prepregs.
Chapter 6 · Processability of Thermoplastic Composites
6
5 Thermal and Hygrothermal Stability—The thermal stability of
synthetic fibres (such as glass, carbon, and Kevlar) is good,
and they can be used under any thermal processing. Natural
fibres are thermally less stable, and care must be taken for
their processing, as mentioned previously. Moisture affects all
fibres, and the ingress of moisture in Kevlar deteriorates its
mechanical properties. In glass fibres, moisture causes
depletion of the coupling agent. In carbon fibres, moisture
affects the polymeric sizing of the polyimide or the epoxy over
it. Natural fibres absorb moisture; thus, their mechanical
properties come down.
5 Post-Processing Operations—The role of reinforcement and its
machinability is important for post-fabrication operations.
Kevlar reinforced composites are difficult to machine, and they
require water jet or laser cutting operations for smooth cutting
edges. Kevlar composites having a fibril structure will cause
deformation from fibrillization of the fibres and give a rough
surface on mechanical machining by standard cutting tools
(hacksaw, milling cutter, turning tools, etc.) The processability
of composites does not end with striking the correct combination of raw materials and processing methods to just get the
product moulded. Both the designer and the process engineer
must be prudent enough to plan in the processing of the
composite products for post-fabrication finishing. Thus, the
raw materials, moulding surfaces, and processing technologies
need to be chosen in such a manner that they ensure a minimum of post-fabrication operations for moulded composite
products.
5 UV Resistance—UV radiations do not affect natural fibres and
most of the synthetic reinforcing fibres—except polyolefin
fibres, such as PP and UHMWPE.
6.7.2 Processing Technology Driven by Selection
Criteria
Primarily, there are only three processes (i.e. injection, compression, and pressure bag moulding), which are used for fabrication of
thermoplastic composites. Of late, a thermoplastic prepreg-based
filament winding process also has been developed.
5 Compatibility with the Form of Reinforcement—Short fibres,
staples, whiskers, and particles only can be used as the
reinforcement in an injection moulding process. For
example, the Motorola Ultra smartphone has a rear housing
made of a Kevlar short fibre-reinforced PC/PET blend, as
shown in . Fig. 6.4. In a compression moulding process,
reinforcement in any form may be used. Roving and tow
preregs are used in filament winding technology, but
thermoplastic filament winding is tedious because it involves
many processing steps and stringent control of processing
parameters. The pressure bag method also employs fabric
prepregs.
Chapter 6 · Processability of Thermoplastic Composites