An Overview of Plastic Injection Moulding

31 May 2016

Three distinct stages define plastic injection moulding cycles. It begins with a reservoir of sourced material, a hopper filled with pellets of plastic. From here, the molten polymer liquefies and flows into a long barrel, which is where we pick up our overview.


The Injection Unit

The raw plastic is typically sourced from a thermoplastic family, but thermosetting polymers are almost as popular. The dissolved polymer is driven inexorably along the sealed barrel by the injection mechanism and into the mould. Pressure and heat are the twin shaping forces in this simplified portrayal of what’s, in fact, an intricate process.


Filling the Mould with Details (And Plastic)

Old manufacturing methods produced crude negatives, the patterns that formed the interiors of the mould, but contemporary moulds vary greatly in complexity, all the way up to geometrically detailed patterns. Additionally, the mould isn’t a passive cavity. Hot plastic is flowing down the barrel, and it’s pressurised. Special clamping systems use hydraulic power to hold the plates together until the pressure abates, at which point the now solid material has filled every cavity inside the mould.


A Cross-section of the Plastic Injection Moulding Process

A virtual cutaway of the machine clarifies everything. The feed hopper is a gravity-feed device, one that promptly delivers the plastic into the long barrel. The injection mechanism achieves smooth linear motion by adopting a screw-like profile. On advancing own the tube, the soft plastic hs nowhere else to go, which means its final destination must be the mould located at the opposite end of the barrel.


Curing and Cooling

A timed sequence of events guarantees process viability. The heating element melts the plastic, the molten polymer feeds into the hollow tube, and the ramming assembly screws forward, driving the plastic into the hydraulically clamped mould halves. Next, the cooling cycle, a phase dictated by the characteristics of the formulated polymer, ensures a solid product takes form only after the liquid fully contours to the shape of the mould.

Of course, this is a broad strokes description, one bereft of details. Still, we’ve covered the principle work stages, from feed hopper all the way through to the mould. In essence, our overview has taught us where intricately engineered plastic products are born. Once cooled and solidified, the product is now ready to be ejected and packaged. Alternatively, additional processing work enters the cycle, a stage designed to further refine the product.

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