209
8
5 Film stacking is carried out at a temperature lower than the
melting temperature, and the resin softens and flows under
pressure, lowering chances of thermal degradation.
5 Sandwich construction also may be used in this process.
The disadvantage of film stacking is that it requires more preparation time for placing the stacks correctly.
8.1.1.2 Commingled Fibres and Hybrid Fabrics
Another method of fabricating the thermoplastic composites using
matched die moulding process is the moulding of commingled
fibres or hybrid fabrics. In comingled fibres or hybrid fabrics, continuous fibres are spun using crystalline or semicrystalline thermoplastic resins. These fibres are interwoven with the reinforcement
fibres at the individual fibre level. This causes the construction of a
hybrid fibre bundle or fabric. This fabric can be heated above the
softening or melting temperature of the thermoplastic resin so that
the thermoplastic fibres melt, flow, and coat the reinforcement
fibres [3–5]. This results in a thermoplastic composite. The potential advantages of this process are:
5 The volume faction of the fibre can be precisely controlled.
5 High-fibre content can be achieved.
8.1.1.3 Powder Impregnation Process
In yet another method, called the powder impregnation process,
fibres are first passed through a chamber of charged and fluidized
powder resin followed by passing them through an oven to make
the polymer permanently adhere on the fibres. This technology can
be used for the coating of rovings and tows, but good impregnation
is difficult to achieve for reinforcement fabrics. The other method of
impregnation involves fibre tows passing first through a fluidized
bed of thermoplastic resin and then through the oven. The coated
powder on the fibre is melted on the fibre surface in continuous
operation, thereby forming a sheath of thermoplastic over the tow.
The advantage of this processing technique is that a controlled
deposition of the matrix over the reinforcement is achieved, providing good control over volume fractions of fibre and matrix. A disadvantage of this process is that coated powder (if not melted properly)
gets separated from fibres while handling them [6–8].
8.1.1.4 In Situ Polymerization
In situ polymerization is another logical method of reducing the
viscosity of the thermoplastic resin. This process utilizes monomers
or solvated prepolymers of the thermoplastic to coat the fibres and
then to polymerize the thermoplastic in situ. Only a few thermoplastic matrices (such as PEI and TPU) can be processed by this
method. An advantage of this processing technique is that most difficult thermoplastics (PEI, PES, etc.) can be handled easily for
moulding products. A disadvantage of this process is that the processing time is long even when compared to thermosetting resinbased composites [9, 10].
8.1 · Closed Mould Processing Technologies
8
5 Film stacking is carried out at a temperature lower than the
melting temperature, and the resin softens and flows under
pressure, lowering chances of thermal degradation.
5 Sandwich construction also may be used in this process.
The disadvantage of film stacking is that it requires more preparation time for placing the stacks correctly.
8.1.1.2 Commingled Fibres and Hybrid Fabrics
Another method of fabricating the thermoplastic composites using
matched die moulding process is the moulding of commingled
fibres or hybrid fabrics. In comingled fibres or hybrid fabrics, continuous fibres are spun using crystalline or semicrystalline thermoplastic resins. These fibres are interwoven with the reinforcement
fibres at the individual fibre level. This causes the construction of a
hybrid fibre bundle or fabric. This fabric can be heated above the
softening or melting temperature of the thermoplastic resin so that
the thermoplastic fibres melt, flow, and coat the reinforcement
fibres [3–5]. This results in a thermoplastic composite. The potential advantages of this process are:
5 The volume faction of the fibre can be precisely controlled.
5 High-fibre content can be achieved.
8.1.1.3 Powder Impregnation Process
In yet another method, called the powder impregnation process,
fibres are first passed through a chamber of charged and fluidized
powder resin followed by passing them through an oven to make
the polymer permanently adhere on the fibres. This technology can
be used for the coating of rovings and tows, but good impregnation
is difficult to achieve for reinforcement fabrics. The other method of
impregnation involves fibre tows passing first through a fluidized
bed of thermoplastic resin and then through the oven. The coated
powder on the fibre is melted on the fibre surface in continuous
operation, thereby forming a sheath of thermoplastic over the tow.
The advantage of this processing technique is that a controlled
deposition of the matrix over the reinforcement is achieved, providing good control over volume fractions of fibre and matrix. A disadvantage of this process is that coated powder (if not melted properly)
gets separated from fibres while handling them [6–8].
8.1.1.4 In Situ Polymerization
In situ polymerization is another logical method of reducing the
viscosity of the thermoplastic resin. This process utilizes monomers
or solvated prepolymers of the thermoplastic to coat the fibres and
then to polymerize the thermoplastic in situ. Only a few thermoplastic matrices (such as PEI and TPU) can be processed by this
method. An advantage of this processing technique is that most difficult thermoplastics (PEI, PES, etc.) can be handled easily for
moulding products. A disadvantage of this process is that the processing time is long even when compared to thermosetting resinbased composites [9, 10].
8.1 · Closed Mould Processing Technologies