Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
limestone. A finely ground mineral may be mixed with a polymer such as
polypropylene, which is then formed and moulded by normal methods. Other
minerals used include silicates such as kaolin, talc and quartz. Glass is also
used as a filler in the form of solid spheres, flakes or hollow microspheres.
These mineral fillers will contribute to an increase in stiffness of the polymer
(see Section 6.4.1).
Natural products such as wood flour can be used to provide low-cost
fillers and there are certain special fillers that are introduced to confer a fireretardance
to the material. Alumina trihydrate is in the latter category, since
above 200°C it decomposes to produce alumina and about 35% by weight of
water.
Fibre-reinforced plastics
Either thermosetting resins or thermoplastics may be used as matrices, and
their selection is determined by their ease of fabrication for a particular
product.
There are numerous manufacturing routes, which we can group into two
types of process, namely closed mould processes and open mould processes.
Examples of closed mould processes include:
(a) injection moulding, in which molten polymer, mixed with short fibres, is
injected into a split mould, where it solidifies or cures,
(b) resin injection into the mould cavity which contains fibres in cloth form,
and
(c) cold press moulding, where the fibres are impregnated with resin before
pressing between matched dies.
Open mould processes include:
(a) Filament winding, where the fibres are fed through a bath of resin before
winding on to a mandrel to form a tubular component, which is removed
from the mandrel after the resin is cured.
(b) Hand lay-up, in which fabrics made from the fibres are placed on the
mould and impregnated with resin, building up layers until the required
thickness is achieved. The component cures without the application of
heat or pressure.
(c) Autoclave curing of pre-pregs. Unidirectional sheets of fibres may be
pre-impregnated with resin and partially cured to form a ‘pre-preg’. Prepreg
sheets may be stacked on the mould surfaces in the desired sequence
of orientations and final curing is completed in an autoclave, with the
assembly being consolidated by means of pressure applied from an inflated
flexible bag which in placed upon the sheets.