This is the third in our series of excerpts from "Better Be Running! Tools to Drive Design Success" by Dr. Ronald Hollis, President, CEO, and Co-founder of Quickparts.com (Atlanta, GA). Written for business managers, the book focuses on manufacturing processes, tooling choices, and production strategies that can help companies bring products to market faster. To order the book, go to www.betterberunning.com .
Selective Laser Sintering (SLS) creates solid 3D objects by fusing or sintering particles of powdered material with a hot CO2 laser. A thin layer of powdered thermoplastic material is rolled onto a heated build platform. The laser beam directs cross-sectioned CAD data onto the surface of the powder bed. The heat laser then traces each layer, melting plastic particles to the previous plastic layer. After each cross section is scanned, the powder bed is lowered by one layer thickness, then a new layer of material is applied on top. The process is repeated until the part is complete.
Borderline magic, SLS turns powder into parts in a matter of hours, typically building at a rate of one cubic inch per hour. While SLS began as a way to build prototype parts early in the design cycle, it is now being used in low-volume manufacturing to produce strong, fully functional parts with an accuracy of 0.005 inch (five thousandths).
Originated by DTM Corporation, SLS was acquired by 3D Systems, Inc., in 2001. 3D Systems, based in Rockhill, South Carolina, now manufactures and sells SLS systems and materials worldwide.
Another major player in SLS equipment is EOS (Electro Optical System) GmbH of Munich, Germany. While technically better, EOS leads SLS sales only in Europe. 3D Systems has benefited from replicating many features of the EOS system.
Step by Step with SLS
First, an operator converts your CAD file to a Standard Tessellation Language (STL) file, then the SLS system software processes the file and orients the part for optimum build. Next, the SLS software slices the STL file into electronic layers and sends it as instructions to direct the operation.
Thermoplastic powder is spread by a roller over the surface of a build cylinder. The piston in the cylinder moves down one layer thickness to accommodate each new layer of powder. The powder delivery system is similar in function to the build cylinder. Here, a piston moves upward incrementally to supply a measured quantity of powder for each layer.
A laser beam is then traced over the surface of this compacted powder to selectively melt and bond it to form a thin layer of the object. The fabrication chamber is maintained at a temperature just below the melting point of the powder so that heat from the laser raises the temperature slightly to cause sintering. A nitrogen atmosphere inside the fabrication chamber prevents the material from burning. The process is repeated until the entire object is fabricated.
SLS Applications – Shift Happens
Layered manufacturing for low volumes has made it possible to experience a manufacturing "sea change" in our generation. Seemingly, all of a sudden, low-volume production of 100 to 30,000 parts is available as a reasonable manufacturing strategy for product development. With the enhancements in the technologies and materials, the use of layered manufacturing makes it possible to skip rapid prototyping and go directly to end-use, functional parts.
SLS is a dynamically growing industry because the world wants mass customization now. Designers want parts faster. Managers want to save money by utilizing a process that allows them to make parts without the tooling time and expertise. SLS means that you can shave weeks off the traditional product development cycle so that the process yields greater flexibility in the parts you produce. The technology of SLS provides a basis for producing functional parts that withstand heat and environmental conditions, making it the clear choice for high-heat and chemically-resistant applications.
An intake manifold is an excellent application for SLS. The heat-resistant SLS part can effectively be airflow-tested in an engine's exhaust system. The part will be durable enough to be mounted on the engine and used in its real environment. Door handles are another good application because SLS withstands a lot of force. Despite repeated use in the real world, these parts won't break. While laser sintering technology and materials have not changed much in 15 years, EOS GmbH is making some exciting headway with experiments in composite materials. To really move the technology to the next level, materials have to become much more functional and practical. Unfortunately, a real part "look and feel" is still missing from the process.
Resolution – The Nitty Gritty
Because the SLS laser beam spot size is relatively large, standard resolution for SLS is typically plus or minus 0.005 to 0.006 inch (five to six thousandths) for the first inch, and plus or minus 0.003 inch (three thousandths) for each additional inch, with a layer thickness of 0.004 inch (four thousandths).
With the additive process, the z height (vertical axis) standard tolerances of plus or minus 0.01 inch (one ten-thousandth) will impact the first inch, and 0.003 inch for every inch thereafter. Orientation defines how these will impact the part.
Unlike Stereolithography (SL), with SLS there is no option for higher or super-high resolution. The spot size of the laser beam reduces the precision of the SLS system, which cannot produce fine features. Being a heat-based system, the SLS material expands and contracts with changes in temperature, causing warpage and tolerance issues at a microscopic level.
Benefits – The Buzz is Real
The chief benefits of SLS are tougher, heat-resistant, functional parts that embrace complex geometry. Thermoplastics used in SLS are also easily bondable and machinable, being less brittle than SL materials.
While SL encourages optimization of a 2D build platform, SLS features a full 3D build envelope. Parts can be "nested" electronically, so that boxes can be built inside boxes inside boxes. A fully utilized 3D build cube and its ability to utilize powdered materials make for fast production throughput. Since SLS does not require support structures, post-processing is already at a minimum.
A key advantage of SLS over SL revolves around material properties. Evolving into greater flexibility with new composites, SLS companies are now experimenting with a wide range of materials that approximate the properties of thermoplastics, composites, nylon, glass-filled nylon, metal composites, and ceramics.
To get the best results with SLS, remember that it favors thick, prismatic parts and does well with complex geometries if they are not too thin. Fine features, thin walls, and organic details are a no-no with this somewhat brutish process designed to make thick parts that last. SLS does not need post-curing but it does have a long cool-down period. A design with thin walls can be problematic, resulting in distortion and longer cooling times.
A major automotive company used SLS to compress cylinder head development time from 16 weeks to 4 weeks and reduce cost from $75,000 down to $12,000. A military jet project containing over 80 SLS parts makes full use of rapid technologies. Race car manufacturers, including Formula 1 teams, have used SLS for several years to make diverse parts such as housings and aerodynamic components. A high-end luxury car maker eliminated the need for an expensive injection molding tool by using SLS to produce parts for a window lifter assembly. Many case studies report the use of SLS for medical purposes such as hearing aids and prosthetics. Consumer product and electronics design is the largest industry sector using SLS, making up one-quarter of the application pie.
The future is bright for SLS. Because part durability, it's a natural foundation for layered manufacturing and the future of custom plastic manufacturing. Reminiscent of the cartoon world of The Jetsons, this technology will soon be able to reproduce replacement parts directly from the machine that needs them. In fact, space colonization how-to is based on using machines that can "build themselves" once they are shipped to the moon.
About the Author
Ronald L. Hollis is the President, CEO and Co-founder of Quickparts.com, Inc. He has provided the leadership and execution to build Quickparts into one of the fastest growing companies in the U.S. This was accomplished through innovation and accepting the risks to change the way you buy custom parts with instant online quoting. Dr. Hollis and Quickparts are also the recipient of many awards including Ernst & Young Entrepreneur of the Year finalist; Deloitte Fast 500; Inc 500; and 2004 Catalyst Innovator of the Year. He is a graduate of the MIT Birthing of Giants program and served in leadership positions for YEO and YPO. He earned a BSME, as well as a MSE and Ph.D. with a focus in technology-based business from the University of Alabama. Dr. Hollis is passionate about building businesses that apply technologies to solve problems and drive efficiency.