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17/02/2012
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The entirely justified excitement about 3D printing and Additive Layer Manufacture (ALM) has somehow resulted in the idea that these techniques are almost entirely free of manufacturing constraints. This is based on the fact that, by building a part layer by layer, product designers don't need to worry about things like draft angles and undercuts, so any geometry is possible. All of this is true, but the conclusion that it means the end of manufacturing constraints ignores the most important one of all. Cost.
Most production constraints are related to cost from finding the most efficient way to get plastic into and out of a mould to reducing the amount of time it takes to cut and fold a piece of sheet metal. Without these constraints, we'd just machine products out of solid blocks and they would be insanely expensive. Most ALM parts are not subjected to cost constraints; they are often aimed at medical, aerospace, motorsport or luxury markets, which don't need to worry too much about money.
Apply some production and cost efficiency methods, however, and the idea that the industrial design of ALM parts is constraint-free soon begins to wobble.
There are very simple issues like build chamber size, for example. If you are planning to use an ALM machine to make a production batch, you need to take the size of the build platform and chamber into account to make the most efficient use of the process. Can the parts be nested to maximise density, or can they be designed to be built flat and then folded up?
Secondly, will the geometry of adjacent parts result in any thermal distortion? SLS parts, for example, ideally have consistent wall sections, like mouldings, to prevent distortion during the cooling process. Putting a thin section next to a thick one can have the same result between components.
The issues become far more complex when you consider the relatively new area of Direct Metal Laser Sintering (DMLS). DMLS is in the process of becoming a commercial technology and industry experts are still working on issues like repeatability and understanding what builds easily and what doesn't.
The fundamental difficulty with DMLS, unlike SLS, is that the geometry is not supported by the powder bed that surrounds it. This means that any downward facing surfaces, below certain angles, need to be supported by structures during the build process, more like SLA parts.
These then need to be removed by machining or wire cutting. This, and several similar limitations, means that efficient DMLS parts need to be subject to some fairly serious design rules. These affect product design issues like wall thickness; the overall shape of the part; geometry that can be used to reduce weight; how to create holes; and where the base of the part is, to aid removing it from the build platform by wire cutting. Far from being constraint-free, these rules create a remarkably limited range of design options for efficient DMLS parts.
Crucible is currently working on these design rules as part of a Technology Strategy Board funded project with Exeter University, Simpleware, EOS, 3TRPD, Delcam and Plunkett Associates. The project is titled SAVING and is aimed at examining ways to make ALM techniques more energy efficient. It is expected that the first draft of the new design guidelines will be available by the end of Q2 of this year. Please call 01235 833785 if you would like to know more.
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Author
by Mike Ayre, Crucible Industrial Design Ltd
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