Agriculture Reference
In-Depth Information
An injection-moulding machine basically consists of a hopper connected to a barrel
with a short screw inside. The barrel contains zonal heaters and also a detachable
nozzle in front (screw type). The mould consists of a single cavity or multi-cavity,
with an internal cooling arrangement with water or other liquids in circulation to
cool the hot moulded parts. Two solid platens are mounted vertically on four strong
steel bars with one platen stationary and the other movable. One half of the mould
is mounted on the stationary platen and the other half on the movable platen, with
the option of closing and opening it. The two halves of the mould must be precision-
mounted and synchronised because a misaligned mounted mould causes serious
damage. Moulds are generally of steel or aluminium, precision-made from computer-
aided design/computer-aided machining and are very expensive. With constant use,
one must expect a mould to have a lifespan and this is measured in the number of
mouldings. For example, a mould-maker might say that a particular mould will last
30,000, 50,000 or even 100,000 mouldings. Hence, a customer quote for a part(s)
by a producer will also include this consideration. A hydraulic or electric operating
system for movement and a high-tech electronics system for other factors with a
control panel will more or less complete the basics of an injection-moulding machine.
11.3 Polymer Resins for Injection Moulding
Selecting the correct polymer resins for each end application can be quite a daunting
task, requiring in-depth knowledge of the vast range of the injection-moulding
resins available. Appropriate selection of the correct type and grade of resin hinges
on thorough understanding of the application of the final product. All application
characteristics need to be worked out carefully in addition to knowing the capabilities
and limitations of the machine(s) to be used for injection before a determination is
made. In large processing plants, the design engineers will decide and specify these
selections but, for small processors, coordination and consulting the resin suppliers
will be of great help. Also, for very intricate and special jobs, one may opt to use the
services of a prototyping company where an actual product can be made, adjusted
and finalised before a large production run. There are literally thousands of polymer
resins available to moulders, which include standard ones, engineered resins and
modified resins for special applications.
Polymer resins ready for moulding will generally be in the form of powders or pellets,
with pellets being the first choice. They can be in their natural colour or coloured
and packed in small- or large-volume packs. Each supply of material carries a data-
processing guide of the material with any safety factors to be observed (if any) and
also an indication of the melt flow index (which is very important to moulders).
Thermoforming and thermosetting polymer materials can be injection-moulded but
the emphasis is on thermoforming polymers.
 
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