How Plastic Injection Moulding Works

Plastic injection molding might seem like a complex industrial process used to make sophisticated products, but it’s actually quite commonplace. You can find it in everything from Lego bricks to plastic tableware. The reason it’s so popular is that it produces high volumes of the same product quickly and efficiently.

Injection molding is the most commonly used manufacturing technique for producing plastic parts. It’s also used for a wide variety of other materials, including metals, glass, and rubber. Plastic injection moulding produces high-quality products in large quantities for a fraction of the cost of other manufacturing techniques.

The process works by injecting molten plastic into a mold. When the material cools, it solidifies into the shape of the mold. Injection molding is very quick, with a cycle time of typically less than a minute. This is much faster than other types of production, such as CNC machining. This reduces costs and increases profitability.

To start the process, pelletised raw plastic is fed through a hopper into a heated barrel with a reciprocating screw. The screw mixes and homogenises the thermal and viscous distributions of the material and then, if required, heats it by shear action to a molten state. Once the molten plastic has reached its required temperature, it is injected into the mold by the screw moving from its metering position to a transfer position.

During the injection phase, the screw plunges like a giant plunger into the mold to fill it. Depending on the type of plastic and the thickness of the molded part, it can take between milliseconds and several minutes for the plastic to fully cool and solidify. While the plastic is cooling, a stream of liquid coolant flows in and around the mold to speed up the process and prevent damage.

When the molded part has completely cooled, it is ejected from the machine and the cycle is complete. Compared to other manufacturing processes, such as CNC machining, plastic injection molding produces very little material waste. For example, if you start with 100g of plastic, then on average only 20g will be discarded during the injection molding process.

Injection molding is highly versatile and suitable for a range of materials, including thermoplastics (such as ABS, polyethylene, and polypropylene), elastomers (including TPU) and thermosets (such as phenolic). Each type of material has its own unique properties which can be exploited through the injection moulding process. The appearance, strength, and quality of a molded part depends on achieving a consistent blend of the raw materials used. This requires the correct mix of virgin pellets, pelletized post-consumer material, scrap regrind, and colourant or performance/property additives. This mixture is then fed into the injection machines.

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