200
7
7.2.7 Demoulding Temperature
Composite moulding (product) and trim must be taken out of the
mould at room temperature or at a prescribed demoulding temperature. If these building products are removed from the mould at
a higher temperature than the demoulding temperature, warpages
may occur in the product due to the generation of residual stresses.
Residual stresses are caused by the thermal nonequilibrium between
the reinforcement and the matrix at a high temperature, even
though curing is complete. The reason for thermal nonequilibrium
is the difference in linear thermal expansion between the reinforcement and the matrix. This problem is observed most often in carbon fibre-reinforced composites because the conductivity of carbon
is much higher than the conductivity of the matrix. Therefore, the
coefficient of linear thermal expansion of carbon fibre is more than
the matrix.
7.2.8 Post-fabrication Finishing Operations
In many composite products, there is a need to carry out postfabrication finishing operations, such as surface painting, metalizing, and determining how the assembly fits. All of these operations
demand care in handling so that there is no damage to the surface.
Many a time, surface defects are not visible, but they remain in the
form of microcracks or crazing. The microcracks or crazing occur
in the matrix, and later on, with the application of stress, it propagates into the bulk composite, causing failure.
7.3 Future Composite Manufacturing
Technologies
A major breakthrough in composite manufacturing technologies is
not likely to occur in the foreseeable future. Most likely, there will
be a series of improvements to existing manufacturing technologies, and manufacturing concepts already generated will be proven.
New developments in open mould composite manufacturing are
expected to occur in the foreseeable future and are described here.
7.3.1 Automatic Filament Winding
This is a mature manufacturing technique that has been in existence
for a long time. Improvements in automation, speed, variable thickness, pad-up insertion, quality consistancy, flexibility in fibre orientation, control of resin and void content, and shapes other than
cylinders will be seen. A combination of robotic and traditional
filament winding systems (with seven to ten axes) is already available in crude form. If this combined system is perfected, it will be
able to wind complex non-axisymmetric shapes, such as ‘T’ and
elbow shapes. One of the most critical requirements for successful
Chapter 7 · Processability in Open Mould Processing of Polymeric Composites
7
7.2.7 Demoulding Temperature
Composite moulding (product) and trim must be taken out of the
mould at room temperature or at a prescribed demoulding temperature. If these building products are removed from the mould at
a higher temperature than the demoulding temperature, warpages
may occur in the product due to the generation of residual stresses.
Residual stresses are caused by the thermal nonequilibrium between
the reinforcement and the matrix at a high temperature, even
though curing is complete. The reason for thermal nonequilibrium
is the difference in linear thermal expansion between the reinforcement and the matrix. This problem is observed most often in carbon fibre-reinforced composites because the conductivity of carbon
is much higher than the conductivity of the matrix. Therefore, the
coefficient of linear thermal expansion of carbon fibre is more than
the matrix.
7.2.8 Post-fabrication Finishing Operations
In many composite products, there is a need to carry out postfabrication finishing operations, such as surface painting, metalizing, and determining how the assembly fits. All of these operations
demand care in handling so that there is no damage to the surface.
Many a time, surface defects are not visible, but they remain in the
form of microcracks or crazing. The microcracks or crazing occur
in the matrix, and later on, with the application of stress, it propagates into the bulk composite, causing failure.
7.3 Future Composite Manufacturing
Technologies
A major breakthrough in composite manufacturing technologies is
not likely to occur in the foreseeable future. Most likely, there will
be a series of improvements to existing manufacturing technologies, and manufacturing concepts already generated will be proven.
New developments in open mould composite manufacturing are
expected to occur in the foreseeable future and are described here.
7.3.1 Automatic Filament Winding
This is a mature manufacturing technique that has been in existence
for a long time. Improvements in automation, speed, variable thickness, pad-up insertion, quality consistancy, flexibility in fibre orientation, control of resin and void content, and shapes other than
cylinders will be seen. A combination of robotic and traditional
filament winding systems (with seven to ten axes) is already available in crude form. If this combined system is perfected, it will be
able to wind complex non-axisymmetric shapes, such as ‘T’ and
elbow shapes. One of the most critical requirements for successful
Chapter 7 · Processability in Open Mould Processing of Polymeric Composites