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The plastic dream factory

The plastic dream factory

Martin Slade, of UPG, takes a look at modern plastic injection moulding and the way the industry has enabled a virtual design revolution in almost every manufactured product that surrounds us, from the lights on cars to laptops, and from medical devices to telephone handsets.

The versatility of the injection moulding process along with the wide range of materials and additives available allows the manufacture of products with almost any desired properties. This has given design engineers the freedom to create and innovate: if a designer can imagine it then injection moulders can, more than likely, make it. It's like a big plastic dream factory.

Injection moulding is the most common method for producing high volumes of plastic products and components at relatively low cost. The use of plastic not only offers a robust solution but also reduces weight over traditional materials.

The process enables the production of plastic components that incorporate multiple materials with almost any desired features such as holes, threads, and hinges in a single operation. The items produced range in size and weight, from a fraction of a gram through to several kilos. They can be simple or complex, thick or thin, flexible or rigid, solid or foamed, reinforced or filled.

Another key benefit to designers is that an almost endless choice of decorative effects can be applied to products manufactured by injection moulding to produce any desired appearance and feel. They can be textured, highly polished, hot stamped, plated, coloured or transparent. The versatility of the process has been enhanced by the development of new materials, fillers and additives and there are now literally hundreds of combinations to produce whatever is required.

Injection moulding machines range in size and capacity from the relatively small to the very large, but they all basically involve two mechanisms: an injection mechanism and a clamping mechanism. Essentially molten plastic is injected at high pressure into a mould tool that is clamped to counteract the injection pressure. When the plastic has set the mould opens and the part is ejected.

The most common type of injection system is made up of a reciprocating screw inside an externally heated injection barrel. Resin material is fed into the barrel from a hopper and this material is then forced forwards by the rotation of the reciprocating screw, usually driven by a hydraulic motor. The resin plasticises, or melts, as the turning screw drags it towards the nozzle end of the barrel. This drag flow of the material causes the polymer molecules to slide over one another creating frictional heat that causes it to melt. External heating of the barrel brings the material to the required temperature before it is injected into the mould.

Depending on the material being processed the reciprocating screw can typically have three zones. Initially there is the 'feed zone' where the shaft diameter of the screw is constant, it then widens in the 'melt zone' closer to the nozzle where the material is compressed and platicised before reaching the 'metering zone'.

Within the metering zone the screw has a one way valve which only allows the material to flow in one direction towards the nozzle. This creates a chamber of material in front of the screw. When there is enough for the injection shot the screw stops turning and is pushed forward to force the material under high pressure through the nozzle and into the mould. The injection pressure is very high, often up to 30,000psi, or higher, and huge forces are required to clamp the mould, the temperature of which is also carefully controlled.

Today' plastic injection moulding machines are highly sophisticated, computer controlled and very often feature robotic automation systems to rapidly and efficiently handle the plastic components when the mould opens. Stringent quality control procedures at UPG have been developed and implemented in pursuit of a zero defect methodology. This includes high end systems and controls using tools such as Poka Yoke and other Six Sigma disciplines.

UPG manufactures the lenses, housings and associated parts for lights in the automotive sector, as well as many other components used in car manufacturing and it's essential that quality control is of a very high standard. The philosophy at UPG is to implement control systems at the front end to ensure the company produces consistently high quality products.

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