Design for direct metal laser sintering: anything goes?

Additive Manufacturing (AM), 3D printing, rapid prototyping; call it what you like but AM processes offer tremendous opportunities to designers, engineers and artists.

But let us get one thing straight. AM processes do not eliminate design constraints, they just have different ones to conventional processes. While we don't need to worry about draft angles anymore, AM techniques have their own limitations particularly if we take cost and waste into account. This is particularly true of one of the latest and most exciting of the new processes: Direct Metal Laser Sintering (DMLS).

Like many other AM processes, DMLS uses data generated from 'slicing' through a CAD model to create a part one layer at a time. Starting with a large metal base, layers of powder (often titanium or steel) are laid down with a blade and then fused with a high powered laser. As the metal base is lowered, and more layers of powder are added and fused, the part is formed. The finished component is then removed, usually by wire cutting, and put through final machining and/or finishing processes.

Like all layer-based production methods it offers tremendous design freedom as geometry can be 'nested' and many conventional design rules ignored. However, the process has its own design limitations.

What design limitations?
As the part is being built with molten metal geometry that cannot support itself during the process (downward facing horizontal surfaces being the best example) needs to be supported by angled struts built into the part. These then need to be removed resulting in additional machining, laser time and wasted material. There are several other types of geometry that require supports, and sometimes they are unavoidable.

However, good design practice can minimise the need for them and result in parts that are quicker to build, less prone to stresses (another side effect of supports), have a lower cost and produce less wasteful. Identifying and developing these design guidelines has been one of main inputs made by Crucible Industrial Design to the SAVING project.

What is the SAVING project?
The SAVING project is a consortium formed from several companies and research bodies using funding from the Technology Strategy Board (TSB). SAVING has been looking at how processes like DMLS can be used to reduce energy use. The research has focussed on two main issues – improving the efficiency of the production process (including the design of the parts), and looking for ways to use DMLS parts in ways that will save energy, for example, by reducing weight.

Crucible Industrial Design has been working on the design guidelines and these are these are now available on the SAVING website. Aimed at providing an accessible guide to best practice, the guidelines focus on ways to design parts that need the minimum amount of support structures and are simple to finish. The guidelines also include a number of example projects to illustrate best practice and the basic principles of designing for DMLS. One of these projects is a redesigned version of the ubiquitous airline seat buckle, designed by Crucible Industrial Design.

The airline buckle project
The objectives set by Crucible for the airline buckle project was to design a buckle that took advantage of the design freedom offered by DMLS; ensure that it was at least as functional as its conventional counterpart; design it to use the minimum number of support structures; and make sure that the final part weighed considerably less than a conventional airline buckle.

These objectives meant that the buckle had to be based on conventional engineering design with the pivots, latches and other mechanisms being exactly the same as the standard product. Crucible then optimised the design to suit the DMLS process with each part orientated to use the minimum of support structures during the build process.

Eventually, this was reduced to just one support structure. Several versions were designed by the design team at Crucible, subjected to FEA testing and built. The final model looks different from a conventional buckle is just as strong and considerably lighter. It also incorporates a number of design features that demonstrate 'best practice' design for DMLS to minimise energy and material wastage. Crucible designed the new buckle to be made from titanium, which is particularly well suited to the DMLS process and is easier to work with than steel.

What are the benefits?
Conventional airline seat belt buckles weigh between 155g (Steel) and 120g (Aluminium). When made from titanium using DMLS, the weight is reduced to 68g without compromising strength. This is a maximum potential weight saving of 87g.

This may not seem much of a saving, but when you put 853 of them in an A380 configured for economy passengers, it certainly mounts up. Exchanging the traditional steel buckle for an additive manufactured titanium buckle would lead to a total weight saving of 74kg. This could lead to a 3,300,000 litres fuel saving over the life of the aircraft and 740tonnes less CO2 emissions.

In addition to these savings, Crucible's buckle design also illustrates the best ways to orient a part during DMLS to take advantage of the process and minimise the supports needed.

Conclusion
AM techniques offer some exciting creative options for designers and engineers. The popular idea that these techniques throw out the book on design limits is, however, a myth – they simply replace some of the chapters.

Crucible's work on design rules for DMLS points the way to the efficient use of the process, and shows that parts can be made that are capable of saving considerable amounts of energy as a result of their use.

The SAVING partners
• Exeter University: has world leading material and computing research group and brings academic experts specialised in material structure, AM and computational optimisation to the project.
• 3T RPD: a leading additive manufacturer with 50% of the UK's SLS capacity and 40% of UK's DMLS capacity. It was the first UK AM producer to gain ISO 13845 (the medical industry standard) and contributes to the Project's expertise on product development.
• Crucible Industrial Design: a specialist in new industrial design and manufacturing solutions for high-value markets. It specialises in designing for AM to reduce product cost and add value.
• EOS UK Ltd: supplier of 3T RPD with SLS and DMLS machines and is part of EOS Group which is one of the largest AM equipment providers in the world.
• Plunkett Associates Ltd: an AM specialist and consultant and has extensive networks within the UK, Europe and USA which are used for dissemination of information about the technology.
• Simpleware: a hi-tech 3D CAD, FEA and CFD software provider and provides the project with 3D image based software platforms which have the latest design optimisation technology.
• Delcam: a global leading CAD/CAM developer and its CAD/CAM tools interface with new design and process optimisation technologies.

Author
Mike Ayre

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