Modern lightweight design: achieving stiff yet strong structures

Enabling technologies: artificial intelligence-based design and a Direct Metal Laser-Sintering (DMLS) manufacturing process

Authors:

· Dr. Christof M. Stotko, EOS Electro Optical Systems GmbH

· Dr. Siavash H. Mahdavi, Managing Director, WITHIN Technologies Ltd.

Global markets are facing ever shortening product life cycles, with product variety on the rise and an increasing demand for customized products. Traditional manufacturing methods based on economies of scale can no longer meet these challenges as their credo means selling high volumes of identical products. Both product development and manufacturing therefore have to shift their paradigms – moving away from static towards additive and flexible methods. If we go one step further and lightweight requirements are the top priority, an adequate design and manufacturing concept together with lightweight materials such as titananium can achieve this. The following article impressively illustrates that parts based on an artificial intelligence-based bionic design concept can be consequently manufactured with an additive layer manufacturing process such as laser-sintering, enabling the creation of products and parts that are impossible to achieve with traditional technologies. So wouldn’t it be great to be able to vary the stiffness of an object continuously throughout the object and to tailor the exact stiffness of each and every region of the object? With a unique optimised lattice structure design which will then be manufactured using an additive manufacturing method such as laser-sintering, the stiffness of products can be tailored in every region to suit its application requirements.

Design-driven manufacturing: an introduction to laser-sintering

Imagine highly complex parts which can be manufactured without additional tooling efforts and which can still offer the required material properties for given application requirements. This is exactly what EOS can offer with their laser-sintering technology. Laser-sintering is an additive layer manufacturing technology. Any three-dimensional geometry can be built effectively and flexibly, making laborious milling path programming obsolete. As a prerequisite, 3D CAD geometry data have to be made available. During production, the 3D CAD model is sliced into layers. The laser-sintering technology then builds the required geometry layer by layer. During the production process, the energy of a laser solidifies powder-based materials, for example plastic, metal or foundry sand and allows for the production of several different parts in one single build job.

As such, laser-sintering frees designers from the restrictions of conventional manufacturing technologies. This opens up opportunities from product development to the point of new business models. Under this concept laser-sintering drives the design and not vice versa. As a consequence, laser-sintering is revolutionizing manufacturing processes. Any geometry can be created in every phase of the product life cycle. Within this concept, everything that can be CAD-designed can be produced as well. As such, the technology enables a different approach towards product development and manufacturing. For example, this additive manufacturing process enables parts with complex geometries to be built into one another, something which could not be done with traditional processes. Manufacturing costs can be reduced and the overall freedom of design can be increased. As an example, state-of-the-art applications in tooling (conformal cooling channels), for lightweight structures and bionic design show that this freedom of design creates real performance and part-property benefits. Laser-sintering is currently being applied a lot in rapid prototyping, but it is also entering the field of rapid manufacturing applications more and more.

EOS, the worldwide market leader in laser-sintering systems and materials, has been offering solutions for applications in product development and manufacturing since 1989. EOS gained market leadership by assisting numerous industries in Europe, North America and Asia in making use of laser-sintering. Today, the name EOS is synonymous with e-Manufacturing – the fast, flexible and cost-effective production directly from electronic data.

Lightweight design: creating stiff structures with laser-sintering

Apart from shorter product cycles and product individualization, light weight is a factor which is also gaining increasing relevance. In general, a high performance/weight ratio helps to reduce the weight of industrial products and components.  The demand for lighter designs is growing for a broad spectrum of products. As such, the need for stiff structures of particularly light weight arise from functional requirements (e.g. for air- and spacecraft, robots, ships and boats, sporting goods), from considerations of user comfort (e.g. in automobiles or rail vehicles), as well as from economical and environmental constraints. Again, this requires a new way of thinking across a number of areas, including product design, manufacturing processes and materials to meet the high and ever increasing part properties the market is demanding.

To meet these requirements, UK-based WITHIN Technologies Ltd. has developed an algorithmic method of deriving an object’s internal structure based partly on evolution, and partly on deterministic structural engineering principles. This structure is analogous to the fibrous interior of bone, both lightweight and strong, and to be manufactured by an additive layer manufacturing (ALM) method such as laser-sintering. A series of linear structural members acting in either tension or compression traverse the volume of the object to be made and meet at node points, much like a 3D space frame. Rather than consisting of identical members at fixed angles from one another, their position and orientation are dependent on the forces that the object is to carry.

Straight lines were created by the band-saw - today any straight line needs to be questioned

Today, both a varying stiffness and a light weight are the ultimate goals for many parts and their application needs. Here, WITHIN’s artificial intelligence-based design software and EOS’s laser-sintering process are able to beat traditional design and production processes. Making a product stiff in one area while flexible in other regions can be achieved by manufacturing the product in multiple materials. A stiffer material at one end of the object would then be attached to a more flexible material at the other end. But wouldn’t it be great if the same product could be manufactured in one piece and in one material? And why stop there?

Due to traditional manufacturing methods and design rules, products often have multiple regions that are over-engineered. Their stiffness in those regions is more than required. Products can be designed in such a way as to retain the required stiffness in all regions if the product is redesigned e.g. using WITHIN’s optimisation algorithms. Taking the ultimate goal of extensive weight reduction into consideration, this means using less material in order to save costs and to protect the environment. Having a lighter product also saves on shipping costs, and if the product is part of a moving vehicle, it reduces fuel costs. Making products lighter, however, is not an easy job. It has to be done with care so as not to impinge on the product’s performance. And it has to be done in a such a way that any kind of displacement can be kept under control. Being able to predict, with confidence, the amount by which a product will displace when a force is applied is highly advantageous. Being able to then specify the desired amount of displacement at every point on a product enables a level of product optimisation beyond that of any given competitor. WITHIN, through their suite of technologies, are able to accurately specify the distance by which the edge of a product displaces when a load is applied. This tailoring can be customised, giving every region a different stiffness. Consequently, combining this objective with one of overall weight reduction can result in a product that is lighter in weight and yet maintains its desired structural characteristics.

The direction in which the surface of an object displaces when loaded can also be specified. Shifting the direction of the displacement away from the direction of the load has the effect of redirecting the energy of the load in a direction which is useful for impact absorption. Protective equipment such as helmets and body armour is designed to withstand such impacts. Traditionally, this is achieved by simply absorbing the energy in the direction of the impact. This, however, has its limitations. Firstly, most impact bearing materials are suited only to a very short range of impact velocities and energies. Secondly, absorbing the energy of impacts head-on often requires a lot of padding, which has disadvantages with regard to weight and volume. A suitably optimised lattice can, with very little thickness, redirect the energy of an impact in multiple directions and away from fragile areas. Knowing and indeed designing the relationship between a load and its resulting displacement can, if combined with simple switches, result in the creation of very effective touch sensing.

In order to achieve this, using off-the-shelf software intended for other manufacturing techniques has its limits as it has been optimized according to strict rules and regulations. Such software has been designed and refined for decades to suit more traditional manufacturing techniques. This makes it unsuitable and restrictive when used in conjunction with manufacturing techniques such as laser-sintering, as it still speaks the language of straight lines and bevelled edges. Given the design freedom which is now available, a need arises for a new type of design software that speaks this new language of cost-free complexity and freeform thinking.

Let’s even think one step further. Using the additive layer manufacturing technology in conjunction with lattice optimisation software, intricate and complex lattice designs can be created. The following are examples of lattice designs that have been built. Each has its own volume reduction coefficient which is the amount by which the weight would decrease when compared to a solid of the equivalent volume. Each lattice also has its own use: it can be optimised for impact absorption, stiffness or osteointegration. Similar to the use of more traditional manufacturing techniques, such a design process also takes the build process into consideration. Feature sizes need to correspond as do build angles and orientations. The bulk solid material property of the sintered material also needs to be used as part of the finite element analysis. Once such considerations have been made, the user is faced with unparalleled freedom to design and optimise his component with regard to both weight reduction and enhanced performance.

Lattice, laser-sintered designs that meet weight reduction requirements

1. Spinal Implant

The spinal fusion implant below has a lattice that has been designed in such a way as to retrain the structural integrity of the part, withstanding the loads between the vertebrae whilst also encouraging bone growth into the implant itself. The implant is made from medical grade titanium and sintered using the EOSINT M 270 metal system. After being removed from the building platform and cleaned, the part can be coated in HA to encourage bone growth and then inserted into the spine through the back of the patient. This concept design demonstrates the software’s ability to design complex lattice designs within a small volume that are well suited to the end use of the part. The lattice in this implant was designed with an optimal pore size in mind so as to optimally encourage the growth of bone into its structure, thus fusing the bone and implant together, forming a solid and robust interface. Within this design and manufactured with the laser-sintering technology, a pore size of 800 microns and a total weight-saving of 65 percent can be achieved.

2. Liquid lattice

Fluid that needs to pass through a mass of material can flow through pipes that are usually created by some sort of drilling. When the direction of fluid flow needs to change – i.e. when fluid starts at the top of a component but then needs to flow out from the side of the component - two straight drill holes are constructed which meet at a point inside the component. This has two obvious disadvantages: material waste is created from excavating material from the holes. And the internal junction where the two pipes meet will be at a sharp angle. This adversely affects the flow of fluid within the pipes causing turbulence and friction.

By using an additive layer manufacturing technology such as laser-sintering, a smoothly varying internal piping system can be created that maintains a constant cross-section, at the same time reducing turbulence and friction . And of course, by manufacturing with ALM no material is wasted thereby reducing cost and waste.

So far so good. Now, the solid mass of material that surrounds the pipes can be addressed. This volume can be replaced with an optimised lattice structure that is lighter but is still optimised to withstand any loads that the part may endure. To start with, the part needs to be simulated. The boundary conditions need to be analysed as well as the need to support any over-hanging surfaces (an ALM design rule for metals). Once available, this information is sufficient for WITHIN’s optimisation software to design a light-weight lattice structure to replace the solid volume. The image below shows a more intricate lattice supporting the two pipes. The rest of the lattice, which is much larger here, supports the top surface of the component while also resisting any loads which are present.

The volume of the original solid design was - 622 294mm³ while the new optimized design with an internal lattice structure was - 250 747 mm³. This is equivalent to only 40.2 percent of the original weight of the component. The photos below illustrate the part that was built in one piece on the EOS M 270. The photos are of a part built in stainless steel though a titanium version has also been built.

Ian Halliday of 3T, who was part of the design process of this particular part once neatly summarized the beauty of the design and manufacturing processes when saying: “The liquid lattice is an attempt to combine elegance with engineering in a thought-provoking way. It reduces the mass of the final product, while increasing the efficiency of the fluid mechanics and enabling faultless build using Direct Metal Laser-Sintering (DMLS) based on EOS technology. We loved the result and hope that other people might be inspired to challenge their own perceptions of the possible.”

In conclusion it can be said that the ideal combination of an artificial intelligence-based bionic design and a Direct Metal Laser-Sintering (DMLS) manufacturing process as illustrated in this article is already changing the way we design and manufacture. Bionic design on the one hand decodes natural design principles for industrial product development and as such paves the way towards a transfer of optimal living-nature solutions to technical processes and products. As such, it enables a superior means of engineering design. Laser-sintering, which follows a design-driven manufacturing concept extends this paradigm shift into the manufacturing world. In areas where traditional manufacturing processes are reaching their limits, laser-sintering activates creative imagination and enables the manufacture of products and parts which had previously not been possible. This is no science fiction scenario. It is already state-of-the art for companies such as Festo (Robotics), WITHIN, and Freedom of Creation, and also for a number of applications, for example in the aerospace industry.