212
8
tic resin in a solvent. This slurry of thermoplastic and solvent is
used as the matrix to impregnate the reinforcement. Later on—
during compression moulding—the solvent is evaporated to
obtain the moulded composite product. The advantages of this
process are the following.
5 A very high molecular weight of amorphous thermoplastics
can be easily dissolved and processed.
5 This process allows the processing of thin films of thermoplastic with micro and/or nano filler.
The disadvantages of this process are the following.
5 The complete recovery of solvent is very difficult, making
the process costly.
5 Use of a solvent makes the process hazardous.
5 The complete evaporation of the solvent is difficult.
The disadvantage of compression moulding is that the initial investment cost is very high, which means that normally a large number of
mouldings are required to be produced in order to reach a breakeven
point and ensure viability of the process. With press- moulded prepregs, there is a limit on the size of mouldings due to the high cost
and difficulty manufacturing the large metal moulds and hydraulic
presses. In recent years, the development of SMC- type compounds,
which are moulded at lower pressures (called LPMC for low-pressure
moulding compound), has allowed large parts to be produced economically. However, SMC is essentially a medium- to high-volume
process. The production rate is also low as this uses a batch process.
8.1.3 Autoclaving
5 Processing Technology—Autoclaving is an advanced process for
composite fabrication. Although it is a costly process, it yields
good composites of high quality for aerospace applications.
Mostly, autoclaving is used when byproducts are to be removed
during the processing. In autoclaving, isostatic pressure on the
outer surface of the moulding, the vacuum inside the moulding, and the temperature can be applied simultaneously, as
shown in . Fig. 8.5. This process needs prepregs, which are
resin pre-impregnated fabric, so that the volume fractions of
the resin and the reinforcement are well controlled. The
prepregs are placed in a matched die mould or an open mould,
depending upon the composite requirements.
The autoclaving process requires consumables such as a
breather cloth, bleeder cloth, release film, sealing putty, and
bagging film to complete the process.
5 A breather cloth is used for applying a uniform vacuum all
over the moulding.
5 A bleeder cloth is used for the absorption of residual
byproducts.
5 Release film is applied between the bleeder cloth and the
moulding to prevent them from sticking together.
Chapter 8 · Processability in Closed Mould Processing of Polymeric Composites
8
tic resin in a solvent. This slurry of thermoplastic and solvent is
used as the matrix to impregnate the reinforcement. Later on—
during compression moulding—the solvent is evaporated to
obtain the moulded composite product. The advantages of this
process are the following.
5 A very high molecular weight of amorphous thermoplastics
can be easily dissolved and processed.
5 This process allows the processing of thin films of thermoplastic with micro and/or nano filler.
The disadvantages of this process are the following.
5 The complete recovery of solvent is very difficult, making
the process costly.
5 Use of a solvent makes the process hazardous.
5 The complete evaporation of the solvent is difficult.
The disadvantage of compression moulding is that the initial investment cost is very high, which means that normally a large number of
mouldings are required to be produced in order to reach a breakeven
point and ensure viability of the process. With press- moulded prepregs, there is a limit on the size of mouldings due to the high cost
and difficulty manufacturing the large metal moulds and hydraulic
presses. In recent years, the development of SMC- type compounds,
which are moulded at lower pressures (called LPMC for low-pressure
moulding compound), has allowed large parts to be produced economically. However, SMC is essentially a medium- to high-volume
process. The production rate is also low as this uses a batch process.
8.1.3 Autoclaving
5 Processing Technology—Autoclaving is an advanced process for
composite fabrication. Although it is a costly process, it yields
good composites of high quality for aerospace applications.
Mostly, autoclaving is used when byproducts are to be removed
during the processing. In autoclaving, isostatic pressure on the
outer surface of the moulding, the vacuum inside the moulding, and the temperature can be applied simultaneously, as
shown in . Fig. 8.5. This process needs prepregs, which are
resin pre-impregnated fabric, so that the volume fractions of
the resin and the reinforcement are well controlled. The
prepregs are placed in a matched die mould or an open mould,
depending upon the composite requirements.
The autoclaving process requires consumables such as a
breather cloth, bleeder cloth, release film, sealing putty, and
bagging film to complete the process.
5 A breather cloth is used for applying a uniform vacuum all
over the moulding.
5 A bleeder cloth is used for the absorption of residual
byproducts.
5 Release film is applied between the bleeder cloth and the
moulding to prevent them from sticking together.
Chapter 8 · Processability in Closed Mould Processing of Polymeric Composites