The process cycle
for injection molding is very short, typically between 2 seconds and 2 minutes,
and consists of the following four stages:
Clamping - Prior to the injection of the material into the mold, the two halves
of the mold must first be securely closed by the clamping unit. Each half of
the mold is attached to the injection molding machine and one half is allowed
to slide. The hydraulically powered clamping unit pushes the mold halves
together and exerts sufficient force to keep the mold securely closed while the
material is injected. The time required to close and clamp the mold is
dependent upon the machine - larger machines (those with greater clamping
forces) will require more time. This time can be estimated from the dry cycle
time of the machine.
Injection - The raw plastic material, usually in the form of pellets, is fed
into the injection molding machine, and advanced towards the mold by the
injection unit. During this process, the material is melted by heat and
pressure. The molten plastic is then injected into the mold very quickly and
the buildup of pressure packs and holds the material. The amount of material
that is injected is referred to as the shot. The injection time is difficult to
calculate accurately due to the complex and changing flow of the molten plastic
into the mold. However, the injection time can be estimated by the shot volume,
injection pressure, and injection power.
Cooling - The molten plastic that is inside the mold begins to cool as soon as
it makes contact with the interior mold surfaces. As the plastic cools, it will
solidify into the shape of the desired part. However, during cooling some
shrinkage of the part may occur. The packing of material in the injection stage
allows additional material to flow into the mold and reduce the amount of
visible shrinkage. The mold can not be opened until the required cooling time
has elapsed. The cooling time can be estimated from several thermodynamic
properties of the plastic and the maximum wall thickness of the part.
Ejection - After sufficient time has passed, the cooled part may be ejected
from the mold by the ejection system, which is attached to the rear half of the
mold. When the mold is opened, a mechanism is used to push the part out of the
mold. Force must be applied to eject the part because during cooling the part
shrinks and adheres to the mold. In order to facilitate the ejection of the
part, a mold release agent can be sprayed onto the surfaces of the mold cavity
prior to injection of the material. The time that is required to open the mold
and eject the part can be estimated from the dry cycle time of the machine and
should include time for the part to fall free of the mold. Once the part is
ejected, the mold can be clamped shut for the next shot to be injected.
After the injection molding cycle, some post processing is typically required.
During cooling, the material in the channels of the mold will solidify attached
to the part. This excess material, along with any flash that has occurred, must
be trimmed from the part, typically by using cutters. For some types of
material, such as thermoplastics, the scrap material that results from this
trimming can be recycled by being placed into a plastic grinder, also called
regrind machines or granulators, which regrinds the scrap material into
pellets. Due to some degradation of the material properties, the regrind must
be mixed with raw material in the proper regrind ratio to be reused in the
injection molding process.
Injection molding is the most commonly used manufacturing process for the
fabrication of plastic parts. A wide variety of products are manufactured using
injection molding, which vary greatly in their size, complexity, and
application. The injection molding process requires the use of an injection
molding machine, raw plastic material, and a mold. The plastic is melted in the
injection molding machine and then injected into the mold, where it cools and
solidifies into the final part. The steps in this process are described in
greater detail in the next section.
Injection molding is used to produce thin-walled plastic parts for a wide
variety of applications, one of the most common being plastic housings. Plastic
housing is a thin-walled enclosure, often requiring many ribs and bosses on the
interior. These housings are used in a variety of products including household
appliances, consumer electronics, power tools, and as automotive dashboards. Other
common thin-walled products include different types of open containers, such as
buckets. Injection molding is also used to produce several everyday items such
as toothbrushes or small plastic toys. Many medical devices, including valves
and syringes, are manufactured using injection molding as well.