Digital Layer Construction

Transcription

1 A D D i t i v e M a n U F A c t u r i n g Digital Layer Construction Sylvia Monsheimer Additive manufacturing holds untapped potential, particularly for low-volume production. Extrusion and injection molding are not always the best way to mold plastics. An alternative is mold-free production, which combines maximum flexibility with high customer orientation and cost efficiency. With laser sintering, for example, it can produce complex and technically demanding industrial and consumer goods. Figure 1. The principle of additive manufacturing on the example of laser sintering, in which a laser is used to build parts layer by layer based on a computer model Most pathbreaking technologies are based on one simple and persuasive idea. The same is true of methods for mold-free production of parts experts also refer to this as rapid manufacturing, rapid prototyping, additive manufacturing, and additive fabrication. These buzzwords are based on one principle: liquids, powders, strands and films are layered to build three-dimensional structures without the use of a mold. In the laser sintering process, which requires powdered starting material, containers are filled with fine metal, ceramic or plastic powder. A laser located above the powder bed, and precision-guided by CAD software and suit able optics, lights only certain areas of the uppermost particle layer. These areas melt and solidify after cooling. An automatic mechanism then low ers the floor of the powder container by fractions of millimeters, and spreads a fresh layer of parti cles. Again, the laser lights this layer only in certain places (Fig. 1). The result of this method is a spatial component built of ultra-thin layers, whose complexity is limited by almost nothing but the specified electronic construction data. This free-form fabrication uses no casting molds, tools or space consuming production plants. Additive manufacturing (AM), therefore, is the counterpart to conventional methods, in which parts are molded into specified forms, for example, or cut from a massive block. With AM, components are created directly from a digital construction plan. This enables the production of forms that have been long considered impossible by con ventional series production in fact, they can be created fast, flexibly, and with fewer machines. >>> Scanner system Roller Powder delivery system Laser Fabrication powder bed Laser scanning direction Sintered powder particles Laser beam Laser sintering Pre-placed powder bed Object being fabricated Powder delivery piston Fabrication piston Unsintered material in previous layers Source: Materialgeeza/Wikipedia 18 elements32 evonik science newsletter

2 D E S I G n i n G w i t H P o l Y M e r S The FinGripper of Festo AG & Co. KG, Esslingen, which specializes in automation engineering. Produced by selective laser sintering, the FinGripper is light, flexible and adaptable. Like the human hand, it adjusts itself to the shape of the object to be gripped, which allows fast and safe handling of ripe fruit, bulbs and pressure-sensitive foods. The FinGripper is produced by applying layers of polyamide powder 0.1 millimeters thick on top of each other and selectively melting them by laser. The result, after cooling, is a solid component Photo: Festo AG & Co. KG elements32 evonik science newsletter 19

3 Unmistakable: designer Dan Yeffet used selective laser sintering to repro duce his fingerprints in the Detail.MGX light by Materialise N.V. Head quartered in Leuven, Belgium, the company specializes in rapid-proto typing technologies. Photo:.MGX by Materialis Evonik s laser sintering plant in Marl The intake manifold for the Evonik-sponsored Lotus racing car (s. p. 6) was produced by laser sintering from polyamide 12 powder. The geo m- etry of the intake manifold a three-dimen - sional curved, ellip tical tube cannot be produced by conven tional metal processing methods or by injection molding This is why additive manufacturing is particularly common in prototype construction. Prototypes are essential for stressing and testing components, checking their fit and functionality, and if necessary optimizing these properties in a new prototype. When it comes to the processes used to create prototypes, gathered under the term rapid prototyping, the name says it all: first and foremost, prototypes must be built fast, with costs playing a secondary role. Cost-effective batch production But additive manufacturing can do more. The opportunity to analyze and optimize the product in the virtual stage prior to production, select from a wide range of materials, and design according to function means that parts can be not only designed and developed based on the individual needs of the customer but produced at lower cost. Because of the strong expansion in model diversity in the industry, the demand for adaptive tools and equipment has grown enormously. The use of handling robots is a good example: AM-generated 20 elements32 evonik science newsletter

4 D E S I G n i n G w i t H P o l Y M e r S grippers can easily adapt to objects of a wide variety of shapes, and make the gripping process efficient and flexible. Between prototype construction and mass production lies the increasingly important field of low-volume production. Many product requests are for relatively high but limited piece counts. For these products, conventional mass production with its costly molds and large plants is simply too expensive. The special strengths of additive manufacturing technology, therefore, are even more readily apparent when it comes to smaller piece counts (Fig. 2). A few examples of products manufactured in small batches include headlight housing for high-priced cars, steering components for vehicles driven from the right side, and housing for specialty machines. Not least, lightweight construction for airplanes and cars is a key sector for additive manufacturing. Lightweight construction is an undisputed construction principle in the trans portation industry, for example, where it is used to reduce fuel consumption and emissions. In addition to low-volume production, another field of application is individually modified components. Examples include not only medical devices such as hearing aids, implants or surgical instruments, and drill guides for operations, but also helmets and shoes for professional sports and respirator masks. Until AM technology, the high costs of creating a mold to produce a single component made such individual parts as these impossible. Variants are handled exclusively through software solutions from the capture and processing of the individual data, to a single set of construction data for each part. The industry has now developed a whole host of variations of mold-free production: instead of solid particles, there are processes that run in liquid beds. Others work with strands that are stacked to form a part. The individual layer can be formed by spraying or pressing binding material or adhesives. All of these methods have one thing in common: they can execute even the most complex forms in a single operation. And they are flex ible. Without high equipment costs, the part can be modified and optimized by changing the spatial construction data until it meets customer and technical requirements precisely. New functionalities through custom-tailored plastics Figure 2. Additive manufacturing technologies are significantly more economical for low-volume production than injection molding, which is cost-effective only for mass production, owing to the high cost of the mold. The minimum piece count required before injection molding begins offering cost advantages depends, among other things, on the size and complexity of the part to be produced and the mold Cost per unit Injection molding Figure 3. Comparison of the material properties of a standard polyamide and an ultraflexible polyamide specially developed for additive manufacturing Standard grade mance Polymers Business Line, experts have spent roughly ten years developing thermoplastics for additive manufacturing. Converting from low-volume production to additive manufacturing increases the demands on the materials: aircraft construction requires polymers that can withstand extremely high temperatures, and are flame resistant. The sports and shoe industries need soft materials to manufacture components with high flexibility. For example, Evonik developed an ultra-flexible polyamide (PA) that has eight times the flexibility and five times the tensile strength of the standard material (Fig. 3). Another development, namely PEEK powder for laser sintering, stands out for its high melting point of 340 C, which makes it suitable for parts exposed to high temperatures during operation. Optimized polymers like this enable new functionalities, while at the same time creating ways of replacing other materials, such as met als, with plastics. Thermoplastics are ideal for additive manufacturing: they are easy to pulverize, can be selectively melted, and their chemical and physical properties can be customized. In Evonik s High Per for- >>> Additive manufacturing Number of units New flexible material E modulus 1,700 MPa MPa (246,500 psi) (14,500 36,200 psi) Elongation at break 15 % >100 % Tensile strength 45 MPa 8 MPa (6,250 psi) (1,160 psi) Notched impact strength 3.5 KJ/m² No break Melting point 186 C 150 C (366 F) (302 F) Common refreshing rate 50 % Not necessary elements32 evonik science newsletter 21

5 Laser sintering enables layers only millimeters thick Produced in one piece by selective laser sintering: the Faltstuhl One_Shot.MGX of Materialise N.V. The chair is on display, among other places, at the Museum of Modern Art in New York Photo:.MGX by Materialise When it comes to polymers, additive manufacturing competes with extrusion and injection molding. One process that is particularly well suited to plastics is selective laser sintering (SLS), which can produce layers 0.15 mm thick. Far thinner layers (down to 0.08 mm) are also possible, although at this level the powder becomes hard to handle because interior forces of attraction prevent the tiny particles from trickling. While chemical flow aids can prevent adhesion, there is the risk that the thermal conductivity of the sintered layer will change at the edges, to the detriment of the process capability. Laser sintering is an historical term and somewhat misleading: it refers to a pressure-free process in which only a short processing time is required for each layer just enough time for the areas that form the part to melt and form a closed melted film. With polymers, a CO 2 laser is used to directly stimulate the polymer chain without the need for an absorber. The speed of the laser is normally between 5 and 10 m/sec. Under these conditions, a component will grow two to three centimeters per hour. But a variety of components can be produced in the same layer almost without losing speed the production area can be packed full of parts. Compared to other additive manufacturing technologies, powder-based laser sintering has the advantage that the powder bed around the component assumes the function of an all-round support special supporting structures that hold projecting forms in position are unnecessary. Because not only the material itself but the production process influences the technical properties of a part, the parameters of AM products differ from those of injection-molded products. Comparative measurements show that the density and elongation of a part produced in layers are lower, and that the elastic modulus and tensile strength show higher values (Fig. 4). New demands through low-volume production In addition to new material properties, the use of laser sintering in low-volume production places even more demands on the laser sintering process. Reproducibility and reliability take on a whole new meaning: the entire quality assurance system must be ensured something that played essentially no role in the creation of prototypes. Since last year, the Direct Manufacturing Research Center (DMRC) at the University of Pa d- erborn has worked on the challenge of making 22 elements32 evonik science newsletter

6 D E S I G n i n G w i t H P o l Y M e r S this transition from prototype production to serial (low-volume) production. The DMRC is the result of the merging of eight companies and research institutes, and receives financial support from the government of North Rhine-Westphalia. The four founding companies Evonik, Boeing, EOS and MTT Technologies will invest a total of 2 million in the DMRC over the five-year contract period. Since the opening of the DMRC in May 2009, the participating experts have focused mainly on practical questions, such as man aging the temperature of the machines being used or the long-term properties of sintered parts. While a good, substantive idea for a new technology can come into being quite suddenly, it may still have difficulty reaching the heads of key players. This is true in the case of additive manufacturing. Many universities teach the conditions for freedom of design and function-driven design inadequately, if at all. While engineers receive intensive training in conventional processes, they usually learn very little about freedom of design, which opens the door for them to additive manufacturing. Indeed, additive manufacturing is based on the idea of fundamentally different design, since the designer must think in terms of complete functionalities and not decoupled component parts. Standards as pathbreakers Lack of training at the universities is not the only hurdle. In the past, development, modification and use of mold-free production processes was quite unsystematic. This is why there are currently no standards to promote widespread use of the process and regulate evaluation of existing products. As a result, part testing yields different values for elasticity module, tensile strength and elongation, for example, depending on the set of param eters used (Fig. 5). Both the ISO and ASTM, however, are working on developing the first standards for laser sintering. ISO is already in force for the production of test bodies. VDI is also drafting guidelines. Despite the hurdles and unanswered questions that still exist, the importance of AM will grow appreciably over the next ten years. The advantages outweigh these problems: developers can produce functional hollow structures in small batches, and the structures can be precisely modified to changing stress requirements. The components can be customized with specific porosities or surfaces, and ultra-lightweight components are also possible. Here, the airline industry is one of the pioneers. There are already about 30 SLSsintered components installed in the Boeing 787. Figure 4. Comparison of the technical properties of a part produced by laser sintering and one produced by injection molding Test method Laser sintered Injection molded test bar test bar Density g/cm³ DIN E modulus MPa DIN ,700 1,400 Tensile strength MPa DIN Elongation % DIN >50 Figure 5. Depending on the parameter set, tests on the same part can yield differing values and make evaluation more difficult. Standards should improve this situation Test method Parameter Parameter Parameter set 1 set 2 set 1, upright built Density g/cm³ Modulus of elasticity MPa DIN ,872 1,920 1,921 Tensile strength MPa DIN Elongation % DIN ,2 8,4 7 According to estimates by Airbus Industries, an aircraft produced entirely through additive manufacturing would be 30 percent lighter and 60 percent more cost-effective than current machines. Resources and energy efficiency combined with economical production are the central challenges of the future for Germany, if it intends to remain competitive in a fast-changing world. In the past, it was often the work of people that think outside the box and therefore helped breathe life into a new technology. They also play a role in additive manufacturing. One thing is certain: sooner or later, additive manufacturing technologies will become key players in placing advanced industrial production on a cost- and resource-efficient footing. l SYLVIA MONSHEIMER Born in 1965 Sylvia Monsheimer is responsible for global market development of additive manufacturing for Evonik s High Performance Polymers Business Line. Previously, she managed the business line s Strategic Innovation Projects department. Monsheim came to Evonik in 1989 after receiving her degree in construction engineering. Since then, she has held a wide variety of positions in application engineering for high performance polymers. Her more than ten-year focus on powder development and application engineering for selective laser sintering and other tool-free processes is a hallmark of her work at High Performance Polymers , sylvia.monsheimer@evonik.com elements32 evonik science newsletter 23