9Creating cost-effective assemblies often means lower STRATEGIES FOR DESIGNING MULTIPART ASSEMBLIES best practices are shrink the bill of materials (BOM) and make products easy to assemble. If that sounds easier said than done, read on. 32 June 2017 D E S I G N ProtoLabs. Two basic tenets of design for manufacturing and assembly This design tip offers several ways to at alternative manufacturing technologies according to USbased make your next multipart design less like a root canal and more like a day on the beach. Many of these tips are geared around injection-moulded parts, but some apply to machined and 3D-printed parts as well. These include: symmetrical parts, self-mating parts, family moulds, hardware components, monoform parts, assembly considerations, overmoulding and insert moulding, liquid silicone rubber parts, alternatives processes. Searching for symmetry It might be hard to wrap your mind around at first, but many assemblies traditionally constructed of two unique components can often part counts, flexible yet simple product designs, and a look be made from a pair of identical parts. Consider the two halves of a housing for a prototype gear housing assembly. Conventional wisdom might say to put bolt holes in one half, threaded bosses in the other, then use half a dozen or so fasteners and washers to bolt it all together. But this requires machining two separate injection moulds (or machining two unique parts, if a metal prototype is needed). By making the two halves symmetrical, distributing the holes and threads in each part equally and opposite one another, a single mould can be used, cutting tooling costs in half. Self-mating parts But why not get rid of the screws altogether? Replace the clip mechanisms where possible with a living hinge along one edge of the power drill housing and you now have a single-piece design. The bill of materials (BOM) has been reduced by 13 pieces, assembly time has been cut to zero, and there’s just one mould to buy. You’re a hero. You can also, by the way, 3D print living hinges with SLS nylon. Family time Another way to achieve hero status is the use of family moulds, which are ideal when producing a series of parts with similar shapes and sizes. The two mating parts mentioned above are good candidates, as is any group of parts that make up a complete assembly. Obviously, all must be made of the same material, and the combined part extents must fit within the roughly 175 sq. in. (1,129 sq. cm.) working mould cavity volume. Hardware harmony Everyone’s heard the adage about reinventing the wheel. Designing and manufacturing your own nuts, screws, shafts, pins, keys, and dozens of other off-the-shelf components should be avoided at all costs. Better to redesign your product around standard hardware items, especially if production volumes are expected to rise at some point. Moving to monoform The properties that make plastics such as polypropylene (PP) and styrene butadiene (SB) excellent candidates for living hinges also makes them suitable for other flexible components, providing clever part designers with opportunities to simplify assemblies and shrink BOMs—in some cases, a once-complex assembly can be reduced to a single piece. For example, a series of metal shock absorbers in a piece of sporting goods equipment might be replaced with a plastic one, and moulded together with the mating component. Instead of bolt-on metal leaf springs in a small lot of radio-controlled cars, mould them out of nylon as part of the chassis. Nor do designers need to adhere to traditional shapes—a coil spring can easily take a curvy zig-zag shape, or that of an S, C, or sideways W. Just be sure the mechanical, chemical, and electrical properties of the chosen plastic are appropriate for the environment in which they’ll be used.
EN-June2017-eMag2
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