Page 26 - Plastics Engineering

General Properties of Plastics 9

strengths and stiffnesses can be achieved with other fibres such as aramid
(Kevlar) or carbon fibres but these are expensive. The latest developments
also include the use of hybrid systems to get a good balance of properties at an
acceptable price. For example, the impact properties of carbon-fibre composites
can be improved by the addition of glass fibres and the stiffness of gfrp can
be increased by the addition of carbon fibres.
Another recent development is the availability of reinforced plastics in a
form very convenient for moulding. One example is polyester dough and
sheet moulding compounds (DMC and SMC respectively). DMC, as the
name suggests, has a dough-like consistency and consists of short glass fibres
(15-20%) and fillers (up to 40%) in a polyester resin. The specific gravity is
in the range 1.7-2.1. SMC consists of a polyester resin impregnated with glass
fibres (20-3096). It is supplied as a sheet would into a roll with a protective
polythene film on each side of the sheet. The specific gravity is similar to that
of DMC and both materials are usually formed using heat and pressure in a
closed mould (see Chapter 4 on moulding of gfrp).

1.3.4 Structural Foam
The concept of structural foams offers an unusual but exciting opportunity for
designers. Many plastics can be foamed by the introduction of a blowing agent
so that when moulded the material structure consists of a cellular rigid foam
core with a solid tough skin. This type of structure is of course very efficient
in material terms and offers an excellent strength-to-weight ratio.
The foam effect is achieved by the dispersion of inert gas throughout the
molten resin directly before moulding. Introduction of the gas is usually carried
out by pre-blending the resin with a chemical blowing agent which releases
gas when heated, or by direct injection of the gas (usually nitrogen). When
the compressed gashesin mixture is rapidly injected into the mould cavity, the
gas expands explosively and forces the material into all parts of the mould. An
internal cellular structure is thus formed within a solid skin.
Polycarbonate, polypropylene and modified PPO are popular materials for
structural foam moulding. One of the main application areas is housings for
business equipment and domestic appliances because the number of component
parts can be kept to the absolute minimum due to integral moulding of wall
panels, support brackets, etc. Other components include vehicle body panels
and furniture.
Structural foam mouldings may also include fibres to enhance further the
mechanical properties of the material. Typical performance data for foamed
polypropylene relative to other materials is given in Table 1.1.

1.3.5 Elastomers
Conventional rubbers are members of the polymer family in that they consist
of long chain-like molecules. These chains are coiled and twisted in a random