Choosing to use injection moulding
When the time comes to designing a new product, or redesigning an existing one, it’s always important to select the best manufacturing method.
As evidenced by the proliferation in use of plastic parts in products in almost every walk of life in the last 50 years, injection moulding has become the most popular method of producing precision parts in large volumes.
Injection moulding allows great design flexibility; modern control methods ensure that parts can be made with high precision; and the latest materials can replace parts that are currently made from metal. The technique is also cost-efficient for large production volumes.
In principle, injection moulding is straightforward: a precise volume of molten plastic is injected under pressure into a mould tool, which may have multiple cavities. The parts are then cooled and ejected from the mould. The faster this can be achieved, while maintaining part accuracy, the more efficient the production process.
However, injection moulding is not appropriate for all products, so you should consider its benefits carefully, perhaps with expert help, before committing to this method of component manufacture.
For instance, the cost of designing and engineering the mould tool can require a significant investment of both time and money. If the final parts you make are of high value and produced in sufficient volume, then you will soon reap this investment back: this is especially true with multi-cavity tools, which are capable of producing tens or even hundreds of parts in a single operation.
Injection moulding is not generally appropriate for small production runs, or for low-value parts. Here, it is best to stick with traditional methods such as metal casting or sometimes a newer technique like 3D printing if small runs of highly ‘tailored’ components are required. There is also a size restriction for injection moulded parts, due to a physical upper limit on the size of moulds and machines. However, it’s worth noting that parts as large as wheelie bins and car bumpers are made using injection moulding.
The latest injection moulding machinery is extremely efficient, producing many identical parts, to close tolerances, in short cycle times. Another bonus is that plastic parts rarely require post-production treatment, unlike many metal parts, for instance. Scrap levels are also low: any excess material produced during part production can in some applications be re-granulated and returned to the start of the process, to be re-melted and made into finished parts.
But the main advantage of injection moulding is probably its design flexibility and low cost: plastic parts can be designed to perform multiple functions, something that typically requires several component parts if made in metal. Automotive companies routinely look to redesign metal components in plastic, partly to cut weight, but also to reduce costs. One plastic component, for instance, might replace several metal components, which also reduces assembly and associated costs.
At first glance, it may seem like a retrograde step to replace a metal part with one made of plastic. However, the latest engineering plastics, combined with fibre reinforcement, clever design and the precision of injection moulding, are easily capable of withstanding conditions in, say, a car engine or a deep fat fryer. If the production volume for these parts is high enough, injection moulding will be the most viable way of making them, every time.