Injection molding is a
process of shaping plastic by melting it and injecting it into a predesigned
mold. The process was first designed in the 1930s and was originally based on
metal die casting designs. It offers many advantages to alternative manufacturing
methods, including minimal losses from scrap (since scrap pieces can be melted
and recycled), and minimal finishing requirements. This process differs from
metal die casting in that molten metals can simply be poured; plastic resins
must be injected with force.
The process uses large injection molding machines, which advance
the resins through six major processes to produce everything from computer
parts to plastic Halloween spiders. Although this machine is a complex piece of
equipment, it consists of two basic elements: the injection unit and the
clamping unit.
The process starts with a
mold, which is clamped under pressure to accommodate the injection and cooling
process. Then, pelletized resins are fed into the machine, followed by the
appropriate colorants. The resins then fall into an injection barrel, where
they are heated to amelting point, and then
injected into the mold through either a screw or ramming device.
Then comes the dwelling phase, in which the molten plastics are
contained within the mold, and hydraulic or mechanical pressure is applied to
make sure all of the cavities within the mold are filled. The plastics are then
allowed to cool within the mold, which is then opened by separating the two
halves of the mold. In the final step, the plastic part is ejected from the
mold with ejecting pins. The completed part may contain extraneous bits called
runners, which are trimmed off and recycled. The entire process is cyclical,
with cycle times ranging from between ten and 100 seconds, depending on the
required cooling time.
The injection moldingprocess requires some
complex calculations. Every different type ofresin has a shrinkage
value that must be factored in, and the mold must compensate for it. If this
value is not precisely determined, the final product will be incorrectly sized
or may contain flaws. Typically, this is compensated for by first filling the
mold with resin, holding it under pressure, and then adding more resin to
compensate for contraction. Other complications may include burned parts
resulting from the melt temperature being set too high, warpage resulting from an uneven surface temperature, or incomplete
filling due to a too slow of an injection stroke.
Injection molds themselves can be surprisingly expensive,
sometimes upward of $100,000 US Dollars (USD). If the desired part quantity is
great enough, however, the mold cost becomes relatively insignificant, and the
resulting plastic parts are very reasonably priced. Some molds are made with
more than one cavity; these multicavity molds cost more than their single
cavity counterparts, but due to increased production efficiency, the cost per
part is minimized.
This method of molding can
be used with a variety of plastic resins. The most popular resins for this type
of molding include polypropylene (PP), polyethylene (PE), and ABS. Each has its own set of advantages and
disadvantages, and they are chosen based on the desired characteristics of the
final part.
