When component designers are working on a new product, they will always be looking to design in added benefits so that their products will stand out from the crowd. Two-shot moulding and overmoulding are methods of producing single components made up of two different plastics. At its simplest, these methods are used for cosmetic reasons – when manufacturing methods that give these benefits as allow the creation of single plastic parts from two different plastics. This allows the designer to design components that are to be two colours – to incorporate a logo, for example. Two-tone colourways can differentiate products or to aid identification of contents, e.g: a pharmaceutical dosage vial. Two plastics can enhance surface texture and appearance. The outer plastic can cover blemishes such as gate scars, which would otherwise be hidden by a label on a single-shot product. The mechanical properties of each Plastic can be utilised to enhance to useability for the end consumer. So, a base ABS combined with a TPU means the finished item is both strong, and high in petro-chemical resistance. The outer ‘skin’ acts as a shockproof barrier to protect the delicate electronics within, and its ‘grippy’ qualities aid tactility – especially in gloved hands, or in medical devices for the impaired. Added value benefits include the ability to produce components that are both hard and soft or clear and opaque at the same time. hard and soft or clear and opaque at the same time. So, for example: windows, buttons and seals can be created as integrated elements. of an otherwise conventional injection moulded part. As both techniques create seemingly similar parts, the questions that are most we are often asked are; “what’s the difference? Aren’t they both the same thing?” The answer is no.! An overmoulded part is made in two distinct operations. First the base or ‘pre-mould’ is made in a conventional injection moulding machine. Once the batch is complete, these parts are loaded either by hand or robot into a second mould tool (in a second machine) where the second outer material is over-moulded over the base. This outer material bonds to the base by chiefly mechanical means (designed-in lugs or recesses in the base) and to a lesser degree, a chemical bond, due to the shear heat of the second polymer. (the clue was in the name!) Two shot moulding uses a specialised machine that has an extra injection barrel so that the two separate polymers can be injected into specially designed mould simultaneously. This is usually achieved by making the mould with two identical cores on the moving side of the tool and different cavity’s on the fixed side. The part that has been moulded with the 1st shot material stays on its core as the tool opens and the whole tool half rotates 180°. This means that this moulding now becomes part of the 2nd cavity once the tool is closed. The 2nd material now covers this part to create the finished two shot part. This is ejected as the tool opens and the cycle continues. As two shot moulding is made in one cycle, the polymers can be are chemically bonded: the heat from the first shot is retained, the part uncured, when the second shot is injected. This means high repeatability and consistency of parts: one tool, one machine. The chemical bond created by two-shot moulding means IP67 rating can be more easily achieved. So, if we compare that to an overmoulded part – any inconsistency in the time between pre-moulding and over moulding can cause variations in the strength of bond between the two materials. And the possibility of surface contamination or water absorption of the pre-moulds could cause product failure due to water ingress. So the main difference between both manufacturing techniques is process reliability and material bond: in essence, two shot moulding is the fully automated process of overmoulding, and each method has its advantages over the other when applied to certain projects – horses for courses. Considerations such as production time, variation of components, labour intensity and complexity of tooling/machinery required, along with volume requirements and regulated specifications (IP, ATEX, Intrinsic Safety) all need to be taken into account when choosing the most efficient method of production – each project and the product lifecycle needs to be assessed on individual merit.
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