Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 92

Manufacturers of conventional strand pelletising systems have been supplying the
plastic industry with newer models for many but the advent of bio-plastic composites
brings the need for new processing technology. Manufacturers of underwater pelletising
systems (who have production been in this business for a long time) claim that
using underwater pelletising systems allow higher bio-filled composites from 40%
(conventional) to 65% (underwater systems). Due to the development of pelletising
systems, many newer models are appearing on the market and a producer of polymer
composites will have the option of using these systems or even opt for a custom-made
system to suit a particular polymer composite formulation. Also, a significant feature
of the newer models is that one system can produce different sizes of pellets on the
same run.
PCRH are rapidly becoming more important in the plastic injection moulding industry
because these moulders are getting more comfortable with using them while realising
the importance of reduced resin cost. A separate chapter on this subject is presented
later.
7.7 Processing of Polymeric Composites by Extrusion
The largest volume of polymeric composite products, whether they be WPC or
PCRH, are produced by extrusion. Other emerging processing methods are injection
moulding and compression moulding, in which the composites must be resin pellets
for easy moulding, unlike in extrusion, where the material components can be mixed,
compounded and processed in a single in-line processing operating system. More than
75% of polymer composite products are made by extrusion, so this chapter will deal
with the basics of extrusion technology.
Extrusion is a process by which a thoroughly softened plastic mass is forced through
an orifice or die of pre-determined shape and size, which is called the extrudate . To
extrude a thermoplastic mass, first it must be softened by heat so that it can flow
freely and be shaped through the die. This softening by heat is called plasticisation or
thermal softening . The machine that carries out this process is called an extruder and
may consist of one-, two- or multiple screws that rotate inside the extruder barrel to
mix the plastic mass thoroughly and then push the molten melt through the orifice
or profiled die to produce extrudates that are usually cooled by water. Extrusion is
a continuous process and after the cooling station a haul-off system with cutting
equipment decides the length of an extrudate.
A shape or profile is obtained by a die, but the size of the extrudate is dependent upon
the size of the extruder, which is generally defined by the screw diameter (e.g., 25, 60
and 120 mm). The important aspect of a screw is its L/D and its configuration. Different