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Injection Molding from a Molders Point of View

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To a molder, an injection molding process is simply an indispensable and most popular technique for manufacturing plastic parts. This is due to the enormous variety of ways in which this material can be molded, even when they are complex, as well as being a fast and efficient process.

One of the most important advantages is that the molded parts require very little finishing work because this process allows manufacturing an infinity of articles of a single piece, with textures, colors and other variables defined directly from the injection in the mold.


The Process of Plastic Injection: Step by Step

Power Unit

The process starts in a hopper that is filled with plastic granules through a dispenser. This is the raw material of any product, which is fed into the barrel that conducts the polymer through the injection unit.

Hydraulic Unit

For the molten material to advance through the barrel of the injector unit, the spindle is driven by a hydraulic system enabled by an electric motor, which causes an axial movement of the barrel and its blades in an endless flow.

Injection Unit

The polymer is melted with the heat generated by various resistor bands that are placed around the barrel. The fluid is injected into the mold through the nozzle, exerting enough pressure to fill and solidify inside the mold.

Molding Unit

It consists of a hydraulic or mechanical press integrated by two mold plates, which cause the hermetic connection of both parts of the mold to form the cavity of the piece and resist the strong pressure that is applied when the polymer is injected into the mold.

One of the two parts of the mold remains fixed, which is the one that is stuck to the injection unit of the polymer, while the other part that remains in motion during the molding cycle and is known as the extracting or closing part.

This same unit is opened again when the injected piece solidifies when it is cooled with the help of cooling fluid and finally it is expelled by the bolts of the extraction side, to start the cycle again, which is carried out continuously.


The mold is the most important part of the injection machine because it is where the piece of plastic will take its shape and finish. It is an interchangeable piece that is screwed into the press through a mold holder. It consists of two equal parts that are hermetically joined.

Each of the parts has a cavity that will be filled with the fluid of the hot polymer, to take the form and replicate the corresponding piece. The material is pressed by the injector unit to fill the mold cavity 100% before cooling.


The parts of the mold are:

Channels: They are the conduits through which the molten polymer passes into the cavity due to the pressure applied by the injection unit.

Cavity: It is the space where the injected polymer takes the form of the piece.

Respirators: These are ducts through which air is released from the cavity, as molten plastic fluid enters.

Cooling system: These are the ducts through which the refrigerant circulates to regulate the temperature of the mold. The cooling is crucial since this depends on the piece for distortion or the final surface is expected.

Punch bolts: When opening the mold, these bolts located in eject the molded part out of the cavity.


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