Plastic
injection molding is the most common method of manufacturing plastic parts because it
typically creates the final part design in a single step, without the need for any secondary
operations such as trimming or finishing.
In the plastic injection molding process, a machine injects a molten polymer
under high pressure into a mold that contains the cavity corresponding to the part
design. The polymer flows from the injection molding machine to the cavity through a
feed
system.
After the
cavity
fills,
the
machine
continues
to pack melted plastic material that solidifies after it cools down. Finally, the
machine ejects the solid part.
The cycle time required for the production of a single part is typically seconds, or at
most, minutes. A short cycle time lets you reach a high volume of production.
The following
schematic
shows a typical injection molding machine.
| Components of Injection
Molding Machine |
Function |
| 1 |
Injection Unit |
Receives the plastic pellets in a hopper (2) and
transfers them into a heated barrel containing a rotating and
reciprocating screw (3), and a nozzle (4). |
| 2 |
Hopper |
Contains and dispenses the plastic pellets for the
injection molding process. |
| 3 |
Screw |
Transports the pellets through the barrel,
heating
them
to
create a homogenous melt, a process termed
plastication. |
| 4 |
Nozzle |
Once the injection unit (1) produces an adequate
amount of melted plastic, the reciprocating action of the screw (3)
injects the melted plastic through the nozzle into the mold unit (5).
|
| 5 |
Mold Unit |
Holds the cavity (6) inside the molding plates
together with the feed system of runners (7), cooling channels (8),
ejector pins, and the moving and stationary plates (external
boundaries). |
| 6 |
Cavity |
The part cavity is filled with molten polymer that
cools, and after injection forms the manufactured part. |
| 7 |
Feed
System |
Provides the flow path for the melted polymer into
the cavity. The feed system
comprises
runners, sprues, and gates. |
| 8 |
Cooling Channels |
Cooling
channels
remove
heat from the mold.
Individual
cooling
channels are usually connected in series with one inlet and one outlet.
You can use water or other coolants such as ethylene glycol in the
cooling circuit. |
| 9 |
Clamping Unit |
Uses a motor drive and tie bars to open and close the
mold, and prevent the mold from opening during the injection
process. |
| 10 |
Control Unit |
Monitors and controls the processing parameters like
temperature, pressure, screw position, and speed. |
| 11 |
Hydraulic Unit |
Adjusts and maintains the proper hydraulic pressure
to control the mechanical pressure. |