Most people have never heard of injection molding, however the products
that are produced through this process are as common as a toothbrush, a compact
disk, or even drinking glasses.
Injection Molding is one of the principal processes used in converting
plastics into useful products. An injection molding machine can be large or small
but work through the same general process. The machine heats a thermoplastic
material that in its pre-processing state is pelletized or granular, to a liquid-like
“flowable” state. It then injects the melted plastic into a mold that is used to create
the desired shape of the product. The plastic in the mold is then allowed to cool
and removed form the mold by an ejector system.
Different raw plastics materials can be used in the injection molding
process. Most commonly injection molding machines provide processing for
phenolics, melamine, silicone, elastomers and polyester. These materials can be
numerous colors and can be used for making the various products depending on
what characteristics like elasticity. flexibility and hardness are needed.
The injection molding machine has two main components the injection unit
and the clamp unit. The injection unit melts and injects the materials. There are
usually about eight main parts that make up the injection unit, and they are the
barrel, the nozzle, the screw and non return valve, heater bands, a motor to rotate
the screw, and a hydraulic cylinder to move the screw forward and backward.
Control systems are used for temperature regulation, and the timing of the screw
rotation and injection strokes.
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The screw consits of three main sections the metering zone, the transition
zone and the feed zone. The feed zone makes up about 1/2 the total length of the
screw. It has deep flights and is where the pellets first enter the screw. The
transition zone is about 1/4 the length of the screw and has flights that are closer
together to compress the pellets and aid in the melting process. The metering zone
makes up the last portion of the screw mechanism and is where any final melting
of the pellets occurs before the pellets pass through the non return valve and
nozzle into the mold.
The Clamping unit opens and closes the mold and ejects the parts. The two
most common methods to generate clamping forces are direct hydraulic clamps
and toggle clamps that are both actuated by hydraulic cylinders. In addition to
those parts most injection molding machines also have a hydraulic pump and
resivoir.
Safety is a very important aspect of injection molding so most injection
molding machines are equipped with safety devices. These devices can be mold
closing safety systems, purge guards, and rear door systems. Mold closing
systems are required to have a mechanical drop bar, electrical interlocks and
hydraulic interlocks which all help protect the operator from injury.
The actual injection molding process consists of 5 basic steps:
1. The mold closes.
2. The screw moves forward and forces the melted material into the mold
3. the screw maintains pressure through the nozzle while the plastic is
cooled.
4. The timers stop injection pressure and the screw turns to draw fresh
material
5. The mold opens and ejector pins remove the molded part.
All these steps can also be grouped in a time cycle being: fill time, pack
time, cooling time, and dead time.
There are advantages and there are disadvantages to injection molding.
Some of the advantages are that the surface finish can be controlled to make any
texture, metal inserts can be used to make things like screwdrivers, and with
thermo plastics the waste material can be ground and used again. There are also
disadvantages like the high cost of the molds and that the molds are to expensive
for just short product runs.
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Through advances in injection molding plastics are becoming more popular
and are being used to produce a wider variety of products.
