Page 296 - Plastics Engineering

Processing of Plastics 279

housings, combs, syringes, paint brush handles, crash helmets, gearwheels,
typewriters, fascia panels, reflectors, telephones, brief cases - the list is endless.
The original injection moulding machines were based on the pressure die
casting technique for metals. The first machine is reported to have been patented
in the United States in 1872, specifically for use with Celluloid. This was an
important invention but probably before its time because in the following years
very few developments in injection moulding processes were reported and it
was not until the 1920s, in Germany, that a renewed interest was taken in
the process. The first German machines were very simple pieces of equipment
and relied totally on manual operation. Levers were used to clamp the mould
and inject the melted plastic with the result that the pressures which could
be attained were not very high. Subsequent improvements led to the use of
pneumatic cylinders for clamping the injection which not only lifted some of
the burden off the operator but also meant that higher pressures could be used.
The next major development in injection moulding, i.e. the introduction of
hydraulically operated machines, did not occur until the late 1930s when a wide
range of thermoplastics started to become available. However, these machines
still tended to be hybrids based on die casting technology and the design of
injection moulding machines for plastics was not taken really seriously until the
1950s when a new generation of equipment was developed. These machines
catered more closely for the particular properties of polymer melts and modern
machines are of the same basic design although of course the control systems
are very much more sophisticated nowadays.
In principle, injection moulding is a simple process. A thermoplastic, in the
form of granules or powder, passes from a feed hopper into the barrel where
it is heated so that it becomes soft. It is then forced through a nozzle into a
relatively cold mould which is clamped tightly closed. When the plastic has had
sufficient time to become solid the mould opens, the article is ejected and the
cycle is repeated. The major advantages of the process include its versatility
in moulding a wide range of products, the ease with which automation can
be introduced, the possibility of high production rates and the manufacture of
articles with close tolerances. The basic injection moulding concept can also
be adapted for use with thermosetting materials.

4.3.2 Details of the Process
The earliest injection moulding machines were of the plunger type as illustrated
in Fig. 4.30 and there are still many of these machines in use today. A pre-
determined quantity of moulding material drops from the feed hopper into the
barrel. The plunger then conveys the material along the barrel where it is heated
by conduction from the external heaters. The material is thus plasticised under
pressure so that it may be forced through the nozzle into the mould cavity.
In order to split up the mass of material in the barrel and improve the heat
transfer, a torpedo is fitted in the barrel as shown.