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Design for direct metal laser sintering: no rules or new rules?

01 September 2014

Mike Ayre looks at the design limitations of additive manufacturing (AM) processes and introduces the new Design Guidelines for Direct Metal Laser Sintering (DMLS) produced as part of the Technology Strategy Board’s SAVING project.

The excitement surrounding AM processes is undeniably justified, but the idea that these techniques are almost entirely free of production constraints is something of a myth. The myth is based on the fact thatbuilding a part layer by layer means that product designers don’t need to worry about things like draft angles and undercuts, so any geometry is possible.

Additive manufacturing, 3D printing, rapid prototyping - call them what you like, the processes offer tremendous opportunities to designers, engineers and artists. But as the AM bandwagon gathers pace, the perception that these processes are free from any production constraints has remained largely unchallenged. The reality, however, is that production AM methods do have design constraints. This is particularly true of one of the most exciting of the new processes - DMLS.

What design limitations?
Because DMLS parts are built with molten metal, geometry that cannot support itself during the build process, such as downward facing horizontal surfaces, needs to be supported by structures built into the part. These then have to be removed post production, involving additional processing and wasted material. To minimise cost and waste, designers need to consider the orientation and geometry of the part at the outset to avoid unnecessary production time and post processing.

The completed parts are removed from the build platform using wire erosion – a time consuming and energy intensive process. This means that the way in which parts are laid out on the build platform, and the number of parts that can made in one build, is critical to the economics of a project. Designers should therefore consider the size of the build platform and the way in which parts can be ‘nested’ as part of the design process.

These are just two of the considerations that need to be made when designing parts for production using DMLS. The operative word here is production – very few limits exist when AM processes are used for prototyping, and it is the assumption that one is simply an extension of the other that has led to the myth that design rules do not apply to AM processes.

As one of my main contributions to the Technology Strategy Board’s SAVING project, I examined the DMLS process in detail and developed a set of design guidelines that are aimed at promoting ‘best practice’ in terms of energy efficient manufacture. If followed, the guidelines will minimise the need for post production processes and expense, and result in parts that are quicker to build, less prone to stresses (another side effect of supports), cheaper and less wasteful.

The SAVING project
The SAVING project was a consortium formed from several UK based companies and research bodies. Using funding from the Technology Strategy Board, over a three year period, we investigated how processes like DMLS can be applied to reduce energy use. The research focussed on two main issues – improving the efficiency of the production process, and investigating how DMLS parts can be used to help save energy, for example, by reducing weight.

The guidelines I produced also include a number of example projects to illustrate the best practice principles of designing for DMLS, including a redesigned version of the ubiquitous airline seat buckle. This project focussed on designing a part that could save energy as a result of its use, but was also as efficient to build as possible using the DMLS process.

The airline buckle project
The aerospace industry is under tremendous pressure to reduce its carbon footprint. Ambitious targets have been set for the industry; new aircraft entering service need to emit 50 percent less carbon dioxide than they did 20 years ago. Carbon dioxide emissions are directly related to the amount of fuel burned, so Crucible set out to help reduce the weight of an aircraft by designing an airline buckle that was considerably lighter than current versions.

My objective was to design a buckle that was at least as functional as its conventional counterpart; design it to be as efficient as possible to make using DMLS; and to ensure 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 exactly the same as the standard product. I then optimised the design to suit the DMLS process, with each part orientated to use the minimum of support structures during the build process. This was eventually reduced to just one support structure.

The design team at Crucible produced several versions which were subjected to FEA testing and then built from titanium, which is particularly suitable for the DMLS process. Conventional airline seat 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 weight saving of 87g.

While this may not seem much of a saving, when you put 853 of them in a 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. Using the work of Helm and Lambrect, this could lead to a 3,300,000 litres fuel saving over the life of the aircraft and some 0.74 million tonnes reduction in CO2 emissions. In addition to these savings, the DMLS buckle design also illustrated the best ways to orient a part during DMLS to take advantage of the process and minimise the supports needed. 

New chapters for the rule book
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.

Mike Ayre is managing director of Crucible Industrial Design

Download the Design Guidelines here.

More information on the SAVING project can be found here

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