In describing a no-frills version of the plastic injection moulding process to curious onlookers, the transformative mechanism is usually condensed into a few steps. There’s the “injection” of melted plastic pellets. Pressure is applied to the liquid stream as it’s forced to fill specially shaped moulds, and, upon cooling, the finished product is released for final processing. Eminently repeatable and capable of creating countless ranks of identical parts, the technique deserves some details to fill in this rough-edged portrayal, thus adding definition to one of the mainstays of the plastics manufacturing industry.
Rows of identically automated moulding machinery fill manufacturing plants, with dedicated feed systems delivering granular material in the form of a base thermoplastic. The key to absolutely filling the intricate mould located at the terminating end of the machinery is to melt this mass of raw material and to feed it consistently through a hollow barrel. Strategically placed heating elements convert the pellets into a liquid state, a plasticized stream that’s forced forward by a reciprocating screw. The genius of the machinery at this critical juncture lies in keeping the liquefied current consistent so that it can evenly fill the mould.
The modern plastic injection moulding process employs cutting-edge technological solutions to ensure each manufactured component is identical to the next. Sensors control the speed of the motor and the reciprocating screw. The temperature within the machinery is constantly monitored, promoting an evenly distributed thermal curve across the entirety of the process, but it’s the moulds, the hollow tool sections at the end of the cycle that bear the responsibility for what comes next. The liquefied flow of thermosetting plastic has arrived, driven by the ramming force of the screw, but dozens of variables are still on the machining horizon. Enough clamping force has to be applied to the mould to keep the tool halves mated properly. Alignment issues can occur here as the platen wears or fluid dynamics create a weakened final product. But, if all goes well, the sensors guide the amalgamated electro-mechanical parts and the hydraulic hoses, filling the moulds with a buildup of pressurised plastic. The computer-controlled flow then contours to every surface of the mould, every cavity, creating a perfectly engineered component.
The true wonder of the mechanism is how the plastic moulding injection process repeats over and over, making identical parts. But, and this is a noteworthy point, the parts of the machinery are constantly subjected to immense pressure, so the barrel and moulds must be built from hardened alloys.