Two-colour moulding--a suitable case for simulation.(Software)

TWO-colour or two-component moulding, once a curiosity confined largely to typewriter keys, has become an important technique for the part designer. Also known as two-shot moulding or over-moulding, the process can be used with differently coloured materials to create mouldings with permanent, and indeed irremovable, markings in the spirit of the original concept.

However, there are many other possibilities. Dissimilar materials can be combined to create an object with the attributes of both. The material combination can be cheap and costly, slippy and grippy, flexible and rigid; the possibilities are limited only by the ingenuity of the designer--and it must be said, of the moulder and toolmaker as well. There is more to go wrong in a two-component moulding and the second shot is uncharacteristic of normal moulding in that it is bounded not just by mould steel but by the first shot as well.

The complications of manufacturing two mouldings in one make the trial and error approach particularly hazardous. It is no surprise to find that players in this field are turning to software simulation to better the odds of success. A recent example was a study of a two-colour control knob, performed for valve manufacturer Danfoss by FlowHow. This replaced a conventional one-shot single-colour knob. The original knob had two major drawbacks. Thick and thin sections caused filling and quality problems, and led to long cycle times. And the applied symbols tended to rub away in service as the knob was continually adjusted by hand.

The change to a two-colour two-shot moulding produced dramatic results. Cycle time was cut from 32 seconds to 11 seconds. Material consumption fell by 11 percent. The total cost of the optimised design was only 20 per cent of the original version, while the aesthetics remained very close to the initial concept. Much of this was due to the role of simulation in predicting and eliminating pitfalls in the new design.

The Danish consultancy FlowHow is also the European distributor and support service for Moldex3D, the injection simulation software developed in Taiwan by CoreTech System. Moldex3D simulates the injection moulding process by both 2 1/2D and 3D methods, and both played a part in engineering the new control knob.

In the new concept, the symbols and graduations on the knob were to be produced by upstanding areas on the first shot that project through the second shot made in a contrasting colour. There were two problems. The second-shot melt could not flow through two symbols; that was corrected by minor changes to the symbols. More difficult was the question of flow through the graduation symbols. These were made up of ...