Basic Technology of Plastics - Introduction to Polymeric Composites with Rice Hulls - page 23

cracks, warping and brittleness due to the action of UV rays, moisture and extreme
temperatures. Fading of colour is another aspect. These features are significantly absent
if these polymeric composites are used for indoor applications. PCRH have definite
advantages because a high content of silica in the composite will nullify these effects.
Addition of additives will also help to a very great extent and also provide enhanced
mechanical properties. The location and climate of a region can affect the mechanical
properties of polymeric composites significantly. The effects of weathering can very
often be countered with the use of pigments such as titanium dioxide or carbon
black, which absorb UV radiation, making it more difficult to penetrate the surfaces
of polymeric composites. Again, the presence of silica in a polymer composite will
form a barrier against radiation and moisture absorption.

2.4 Moulding Process for Plastics

The range of plastic raw materials is immense and they are available to processors as
resins, powders, granules, and pellets. The main focus of this topic is the processing
of composites, but some of the main processing methods are mentioned below for
the benefit of the reader. The manufacture of composites is discussed in detail in later
chapters. Some of the main moulding processes are presented below:

• Extrusion is a process in which plastic materials are moulded into profiled lengths

using an extrusion system comprising a hopper, a barrel with 3-4 heating zones
and a head with a nozzle that determines the profile shape. This will be connected
to a haul-off system with cooling arrangements and a final length-cutting station
or winding system. Products made from this process are: pipes, profiles, cables,
tubes and boards. This is one of the processes in which PCRH will be made into
profiled extruded lengths, and will be discussed in detail in later chapters.

• Injection moulding is a process in which a heated mass of plastic material is injected

into a closed mould through an opening of one-half of the mould. These moulds
can have a single or multiple cavities with a cooling system, and the movement
of one-half of the mould is operated by electric or hydraulic means. The products
made by this process are, in general, solid, and can be bottle caps, handles, trays,
toys, or engineering items. This will be the second process under discussion with
regard to the processing of PCRH.

• Compression moulding is one of the oldest forms of processing plastic resins.

In general, thermosetting materials are used but thermoforming resins can also be
moulded. The plastic material is introduced into a mould cavity placed horizontally,
and the top-half of the mould is moved vertically using hydraulic or electrical power