The capabilities on offer
Product miniaturisation is placing increasing demands on the component manufacturers who supply the medical device sector. One such manufacturer, A K Industries Ltd (AKI) (Hereford, UK) is using a microprocessor controlled, six-tonne Babyplast (www.babyplast.net) injection moulding machine to produce miniature components. These are made with intricate surface detail in a range of engineering plastics such as filled polymers and elastomers for a variety of applications. The Italian-made, microprocessor controlled machine, distributed in the UK by STV Ltd (Milton Keynes, UK), produces high volumes to close tolerances in shot weights from 0.01–15g, says AKI. The product range includes screws, needles, valves, pilot jets and nozzles, locking levers, lenses, luer plugs and diaphragm plates (see Figures 1 and 2).
The machine can be adapted to two-shot moulding as well as insert moulding to incorporate metal components such as magnets, fasteners, mounts, ballast weights or pressed metal contacts and terminals. This versatility opens up a whole new spectrum of design and manufacturing possibilities for tiny components.
Advantages of two-shot moulding
The two-shot injection moulding process offers the ability to produce complex mouldings from two different polymers at the same time during one machine cycle. Separate, but compatible, materials or colours are chemically bonded or mechanically interweaved during the forming process when the second material is injected into the mould, which avoids the need for further assembly or postprocessing. Alternatively, incompatible materials can be mechanically connected by two-shot moulding without them fusing to meet specific product design and assembly needs such as articulation or movement. Some of the benefits of the new moulding technology over conventional injection moulding machines with screw and barrel are described below.
Shot-to-shot consistency. With conventional injection moulding machines that employ a screw and check ring assembly, the smallest screw diameter available is 14 mm, which needs 2–3 mm of screw stroke to shut off the check ring. This gives a variation in material volume of up to 0.5 cm3 and poor shot-to-shot repeatability at low shot volumes. In contrast, there is no check ring in the newer design, which means that all the stroke of the metering unit results in material transfer into the mould cavity to yield tighter tolerances and greater shot-to-shot consistency.
Less degradation. Degradation is reduced because of the shorter residence time of the heated material in the machine by virtue of the smaller volume of material it holds. This prevents material deterioration and discoloration such as the heat induced yellowing of clear plastics. In addition, wastage of expensive high performance polymers is reduced on purging.
Quick tool changeovers. These are quicker to perform because the machine’s platens and tie bars act as the bolster for the mould tool. To manufacture each time requires just a tool insert, which also saves money in tooling costs.
Low energy consumption.The machines use 2.9 KW installed power consumption.
Allen Green, Joint Managing Director of AKI, comments, “We are ideally placed to judge the features and significant benefits of the Babyplast because our company currently runs 31 injection moulding machines up to 450 tonnes clamping force in our manufacturing hall.” Flexibility and versatility are essential attributes in suppliers’ ability to provide design and manufacturing solutions for a variety of moulded components. He continues, “In practice the management of demand in the present climate of fluctuations may be better achieved by using several Babyplast machines rather than one large-tonnage moulding machine running a multi-impression component for a customer.”
Both companies are members of Bradford University’s (Bradford, UK) UK Micromoulding Interest Group, which is headed by Professor Phil Coates (www.ukmig.com).