What is Plastic Injection Molding?
Injection
molding is a molding procedure whereby a heat-softened plastic material
is forced from a cylinder into a relatively cool cavity giving the
article the desired shape. Injection molding is a manufacturing
technique for making parts from plastic material. Molten plastic is
injected at high pressure into a mold, which is the inverse of the
desired shape. The mold is made by a mold maker from metal, usually
either steel or aluminum, and precision-machined to form the features
of the desired part. Injection molding is very widely used for
manufacturing a variety of parts, from the smallest component to entire
body panels of cars.
The process of plastic injection molding
An injection molding machine consists of three basic parts, the
mold plus the clamping and injection units. The clamping unit is what
holds the mold under pressure during the injection and cooling.
Basically, it holds the two halves of the injection mold together.
During
the injection phase, plastic material, usually in the form of pellets,
are loaded into a hopper on top of the injection unit. The pellets feed
into the cylinder where they are heated until they reach molten form
(think of how a hot glue gun works here). Within the heating cylinder
there is a motorized screw that mixes the molten pellets and forces
them to end of the cylinder. Once enough material has accumulated in
front of the screw, the injection process begins. The molten plastic is
inserted into the mold through a sprue, while the screw controls the
pressure and speed.
The dwelling phase consists of a pause in the
injection process. The molten plastic has been injected into the mold
and the pressure is applied to make sure all of the mold cavities are
filled.
Then the plastic is allowed to cool to its solid form
within the mold. The clamping unit is then opened, which separates the
two halves of the mold. An ejecting rod and plate eject the finished
piece from the mold.
Extrusion
A machine used to extrude materials is very similar to the
injection-molding machine explained above. A motor turns a thread,
which feeds granules of plastic through a heater. The granules melt
into a liquid, which is forced through a die, forming a long 'tube
like' shape. The extrusion is then cooled and forms a solid shape. The
shape of the die determines the shape of the tube.
Advantages of Injection Molding
- High tolerances are repeatable
- Wide range of materials can be used
- Low labor costs
- Minimal scrap losses
- Little need to finish parts after molding
Disadvantages of Injection Molding
- Expensive equipment investment
- Running costs may be high
- Parts must be designed with specific molding consideration.
Overview
Injection molding is a relatively new way to manufacture parts. It
is a fast process and is used to produce large numbers of identical
items from high precision engineering components to disposable consumer
goods. Injection molding is often used in mass-production and
prototyping. It produces such small products as bottle tops, sink
plugs, children's toys, containers, model kits, disposable razors and
parts of cameras. The process can even mould such large items as dingy
hulls and kit car body shell parts.
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