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Injection moulding is a manufacturing process for producing parts by the high-pressure injection of molten plastic material into a mould. It is then cooled prior to the mould opening and ejection or removal by a robot or operator.
The mould or tool has two halves, one containing a ‘male’ core and the other a cavity. When closed, the gap between core and cavity is where the moulding is formed. Limitations of the process relate to the need to remove the finished item once cooled. For example, a hollow ball couldn’t be produced without making it in two halves and then welding or bonding these together.
Initial tooling costs can be relatively high, but tools can have a long service life. Injection moulding machines can also run autonomously and continuously for long periods, making this process best suited to high volume production.
There are a huge variety of polymer grades that can be selected, depending on the application. Colour and additives can also be added; for example, the addition of glass fibre can produce plastic components that are sufficiently strong to allow for the replacement of previously metal parts.
The speed at which components can be manufactured (cycle time) depends on the wall thickness of the plastic in combination with the amount of cooling capacity within the production tool. Components cannot be removed from the tool until the plastic has set to a sufficient level. Thin-walled food containers can be produced with sub-five second cycle times, often using multi-impression tools.
Injection moulding machines are highly accurate and range widely in their physical size, meaning that the components they produce can range from those that weigh just a few grams, to larger items such as car bumpers.
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Structural foam moulding is usually used for parts that require thicker walls than typical. Introducing a small amount of chemical blow agent or gas into the plastic material produces micro-cellular foaming within the walls of the component that significantly reduces both weight and cycle time.
Lower injection pressure also means that larger parts can be produced. These can have widely differing wall sections without creating surface defects. Parts can be stress-free and more rigid and stable than if produced with conventional injection moulding.
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Gas injection moulding is ideal for the production of mouldings with thick sections that require a high cosmetic finish or a reduction in weight. When plastic cools, it usually contracts, which can leave depressions called ‘sinks’ in the outer surface.
With gas injection, plastic is injected into the mould, and then an inert gas (typically nitrogen) is injected into the molten core of the thicker sections. The internal pressure prevents sinking and can also hollow out the component, reducing weight.
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Multi-material or multi-colour moulding involves an injection moulding machine that has two or more independent injection units. This process allows multiple plastic material types or colours to be combined into a single product. Each plastic is injected in turn and allowed to cure between stages. The mould tool is sometimes rotated (see section on rotary platens) or uses internal slides to form the separate details to be filled. Alternatively, a robot transfers the component between distinct stations within the tool.
Typical examples would be products such as toothbrushes or paintbrush handles with a soft touch grip.
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With insert moulding, a non-plastic component is inserted into the mould tool. The tool is closed and plastic is moulded around part of the component. Examples would be adding the handle to a screwdriver or incorporating an electrical terminal into a plastic component.
With overmoulding, a plastic part is inserted into a tool and a second polymer moulded around it. An example would be a car gear knob, where a hard inner core has a softer material moulded around it.
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Cleanrooms are used to control contamination levels. Moulding machines and assembly areas are placed inside this controlled environment. The level of control is dependent on the type of product. Medical and pharmaceutical applications are the most common, but sensitive electronics also require protection.
The most basic controlled environment is a white room, which is an area subject to higher cleanliness levels than usual. Surface contamination of plastic parts is minimised, for example by removing them from a machine with a robot or gloved hand and then placing them into sealed containers.
True cleanrooms require a degree of restricted access, an air filtration system and specialised clothing for personnel. Contamination levels are measured in terms of particle size and number per cubic metre. The higher the level of control, the lower the cleanroom classification. An ISO 1 cleanroom allows only 12 particles per cubic metre of 0.3 μm and smaller.
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Liquid silicone rubber (LSR) is a highly inert material with high levels of biocompatibility, temperature resistance, compression and dielectric properties. It has excellent clarity and can also achieve very vibrant colours.
The liquid silicone rubber injection moulding process is a very precise process (unlike traditional high consistency rubber moulding). It provides excellent dimensional accuracy and flash-free parts, combined with high outputs and low labour content. These properties make it ideally suited for many markets, including medical, healthcare, cookware, aerospace and automotive.
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