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{{Short description|3D printing technique}}
'''Selective Laser Sintering''' ('''SLS'''®, a registered trademark of [http://www.3dsystems.com 3D Systems, Inc.]) is an additive [[rapid manufacturing]] technique that uses a high power [[laser]] (for example, a [[carbon dioxide laser]]) to fuse small particles of plastic, metal, or ceramic powders into a mass respresenting a desired 3-dimensional object. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (e.g. from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.▼
{{For|metal 3D printing|Selective laser melting}}
{{missing information|terminology: [[electron-beam melting]], [[selective heat sintering]]|date=November 2020}}
[[Image:3dprinter.jpg|right|250px|thumb|An SLS machine being used at the [[Centro de Pesquisas Renato Archer]] in [[Brazil]].]]
'''Selective laser sintering''' ('''SLS''') is an [[additive manufacturing]] (AM) technique that uses a [[laser]] as the power and heat source to [[Sintering|sinter]] powdered material (typically [[nylon]] or [[polyamide]]), aiming the laser [[Automation|automatically]] at points in space defined by a [[3D modeling|3D model]], binding the material together to create a solid structure.<ref>{{Cite journal |last1=Lekurwale |first1=Srushti |last2=Karanwad |first2=Tukaram |last3=Banerjee |first3=Subham |date=2022-06-01 |title=Selective laser sintering (SLS) of 3D printlets using a 3D printer {{sic|comprised |hide=y|of}} IR/red-diode laser |journal=Annals of 3D Printed Medicine |language=en |volume=6 |pages=100054 |doi=10.1016/j.stlm.2022.100054 |s2cid=247040011 |issn=2666-9641|doi-access=free }}</ref><ref>{{Cite journal |last1=Awad |first1=Atheer |last2=Fina |first2=Fabrizio |last3=Goyanes |first3=Alvaro |last4=Gaisford |first4=Simon |last5=Basit |first5=Abdul W. |date=2021-07-01 |title=Advances in powder bed fusion 3D printing in drug delivery and healthcare |url=https://www.sciencedirect.com/science/article/pii/S0169409X21001538 |journal=Advanced Drug Delivery Reviews |language=en |volume=174 |pages=406–424 |doi=10.1016/j.addr.2021.04.025 |pmid=33951489 |s2cid=233869672 |issn=0169-409X}}</ref><ref>{{Cite journal |last1=Charoo |first1=Naseem A. |last2=Barakh Ali |first2=Sogra F. |last3=Mohamed |first3=Eman M. |last4=Kuttolamadom |first4=Mathew A. |last5=Ozkan |first5=Tanil |last6=Khan |first6=Mansoor A. |last7=Rahman |first7=Ziyaur |date=2020-06-02 |title=Selective laser sintering 3D printing – an overview of the technology and pharmaceutical applications |url=https://doi.org/10.1080/03639045.2020.1764027 |journal=Drug Development and Industrial Pharmacy |volume=46 |issue=6 |pages=869–877 |doi=10.1080/03639045.2020.1764027 |issn=0363-9045 |pmid=32364418|s2cid=218490148 |url-access=subscription }}</ref> It is similar to [[selective laser melting]]; the two are instantiations of the same concept but differ in technical details. SLS (as well as the other mentioned AM techniques) is a relatively new technology that so far has mainly been used for [[rapid prototyping]] and for [[Job production|low-volume production]] of component parts. Production roles are expanding as the [[commercialization]] of AM technology improves.
==History==
Selective laser sintering (SLS) was developed and patented by Dr. [[Carl R. Deckard|Carl Deckard]] and academic adviser, Dr. [[Joe Beaman]] at the [[University of Texas at Austin]] in the mid-1980s, under sponsorship of [[DARPA]].<ref>Deckard, C., "Method and apparatus for producing parts by selective sintering", {{US patent|4863538}}, filed October 17, 1986, published September 5, 1989.</ref> Deckard and Beaman were involved in the resulting start up company Desk Top Manufacturing (DTM) Corp, established to design and build the SLS machines. In 2001, 3D Systems, the biggest competitor to DTM Corp. and SLS technology, acquired DTM Corp..<ref>Lou, Alex and Grosvenor, Carol "[http://www.me.utexas.edu/news/2012/0712_sls_history.php#ch4 Selective Laser Sintering, Birth of an Industry]", ''The University of Texas'', December 07, 2012. Retrieved on March 22, 2013.</ref> The most recent patent regarding Deckard's SLS technology was issued January 28, 1997 and expired January 28, 2014.<ref>[https://patents.google.com/patent/US5597589?oq=US5597589 US5597589]</ref>
A similar process was patented without being commercialized by R. F. Housholder in 1979.<ref>Housholder, R., "Molding Process", {{US patent|4247508}}, filed December 3, 1979, published January 27, 1981.</ref>
As SLS requires the use of high-powered lasers it is often too expensive, not to mention possibly too dangerous, to use in the home. The associated expense and potential danger of SLS printing due to lack of commercially available laser systems with [[Laser safety|Class-1 safety]] enclosures means that the home market for SLS printing is not as large as the market for other additive manufacturing technologies, such as [[Fused filament fabrication|Fused Deposition Modeling]] (FDM).
[[Category:Production and manufacturing]]▼
==Technology==
▲
[[Image:SLS_schematic.svg|center|780px|thumb|
Selective laser sintering process<br/>
'''1''' Laser '''2''' Scanner system '''3''' Powder delivery system '''4''' Powder delivery piston '''5''' Roller '''6''' Fabrication piston '''7''' Fabrication powder bed '''8''' Object being fabricated (see inset)
'''A''' Laser scanning direction '''B''' Sintered powder particles (brown state) '''C''' Laser beam '''D''' Laser sintering '''E''' Pre-placed powder bed (green state) '''F''' Unsintered material in previous layers
]]
Because finished part density depends on peak laser power, rather than laser duration, a SLS machine typically uses a [[pulsed laser]]. The SLS machine preheats the bulk powder material in the powder bed somewhat below its melting point, to make it easier for the laser to raise the temperature of the selected regions the rest of the way to the melting point.<ref name="Yarlagadda">{{cite book|author1=Prasad K. D. V. Yarlagadda|author2=S. Narayanan|title=GCMM 2004: 1st International Conference on Manufacturing and Management|url=https://books.google.com/books?id=v4Tm1of3UEcC&pg=PA73|access-date=18 June 2011|date=February 2005|publisher=Alpha Science Int'l |isbn=978-81-7319-677-5|pages=73–}}
</ref>
In contrast with SLA and FDM, which most often require special support structures to fabricate overhanging designs, SLS does not need extra material or special considerations for support structures because the part being constructed is surrounded by unsintered powder at all times. This allows for the construction of previously impossible geometries. Since the machine's chamber is always filled with powder material, "[[Nesting (process)|''Nesting'']]" can be used to save time and cost by printing multiple parts at once. While SLA and FDM prints can contain parts that are hollow yet fully enclosed, SLS requires hollow enclosures to have an opening for the unsintered powder within to be drained.
Affordable home SLS printers have become possible as relevant patents have started to expire, but the heating process still has stringent requirements, with a power consumption of up to 5 kW and temperatures having to be controlled within 2 °C for the three stages of preheating, melting and storing before removal. [http://3dprintingindustry.com/2015/04/23/coming-soon-a-5000-sls-3d-printer-from-sinterit/] {{Webarchive|url=https://web.archive.org/web/20150428151915/http://3dprintingindustry.com/2015/04/23/coming-soon-a-5000-sls-3d-printer-from-sinterit/ |date=2015-04-28 }}
== Materials ==
The quality of printed structures depends on the various factors include powder properties such as particle size and shape, density, roughness, and porosity.<ref>{{Cite journal|last1=Leturia|first1=M.|last2=Benali|first2=M.|last3=Lagarde|first3=S.|last4=Ronga|first4=I.|last5=Saleh|first5=K.|date=2014-02-01|title=Characterization of flow properties of cohesive powders: A comparative study of traditional and new testing methods|url=http://www.sciencedirect.com/science/article/pii/S0032591013007572|journal=Powder Technology|language=en|volume=253|pages=406–423|doi=10.1016/j.powtec.2013.11.045|issn=0032-5910|url-access=subscription}}</ref> Furthermore, the particle distribution and their thermal properties have a significant effect on the flowability of the powder.<ref>{{Cite journal|last1=Leu|first1=Ming C.|last2=Pattnaik|first2=Shashwatashish|last3=Hilmas|first3=Gregory E.|date=March 2012|title=Investigation of laser sintering for freeform fabrication of zirconium diboride parts|url=http://dx.doi.org/10.1080/17452759.2012.666119|journal=Virtual and Physical Prototyping|volume=7|issue=1|pages=25–36|doi=10.1080/17452759.2012.666119|s2cid=137566316|issn=1745-2759|url-access=subscription}}</ref>
Commercially-available materials used in SLS come in powder form and include, but are not limited to, polymers such as [[polyamide]]s (PA), [[polystyrene]]s (PS), [[thermoplastic elastomer]]s (TPE), and [[polyaryletherketone]]s (PAEK).<ref>{{Cite web|url=https://www.eos.info/material-p|title=High-end Plastic Materials for Additive Manufacturing|website=www.eos.info|access-date=2019-02-19}}</ref> Polyamides are the most commonly used SLS materials due to their ideal sintering behavior as a [[Semi-crystalline polymer|semi-crystalline]] [[thermoplastic]], resulting in parts with desirable mechanical properties.<ref name=":0">{{Cite journal|last1=Kloos|first1=Stephanie|last2=Dechet|first2=Maximilian A.|last3=Peukert|first3=Wolfgang|last4=Schmidt|first4=Jochen|date=July 2018|title=Production of spherical semi-crystalline polycarbonate microparticles for Additive Manufacturing by liquid-liquid phase separation|journal=Powder Technology|volume=335|pages=275–284|doi=10.1016/j.powtec.2018.05.005|s2cid=103342613|issn=0032-5910|url=https://zenodo.org/record/4633734}}</ref> [[Polycarbonate]] (PC) is a material of high interest for SLS due to its high toughness, thermal stability, and flame resistance; however, such [[Amorphous solid|amorphous]] polymers processed by SLS tend to result in parts with diminished mechanical properties, dimensional accuracy and thus are limited to applications where these are of low importance.<ref name=":0" /> Metal materials are not commonly used in SLS since the development of [[selective laser melting]].
=== Powder production ===
Powder particles are typically produced by [[cryogenic grinding]] in a [[ball mill]] at temperatures well below the [[Glass transition|glass transition temperature]] of the material, which can be reached by running the grinding process with added cryogenic materials such as [[dry ice]] (dry grinding), or mixtures of [[liquid nitrogen]] and [[Solvent|organic solvents]] (wet grinding).<ref name=":1" /> The process can result in spherical or irregular shaped particles as low as five [[Micrometre|microns]] in diameter.<ref name=":1">{{Cite journal|last1=Schmidt|first1=Jochen|last2=Plata|first2=Miguel|last3=Tröger|first3=Sulay|last4=Peukert|first4=Wolfgang|date=September 2012|title=Production of polymer particles below 5μm by wet grinding|journal=Powder Technology|volume=228|pages=84–90|doi=10.1016/j.powtec.2012.04.064|issn=0032-5910}}</ref> Powder particle size distributions are typically [[Normal distribution|gaussian]] and range from 15 to 100 microns in diameter, although this can be customized to suit different layer thicknesses in the SLS process.<ref name=":2">{{Cite journal|last1=Yang|first1=Qiuping|last2=Li|first2=Huizhi|last3=Zhai|first3=Yubo|last4=Li|first4=Xiaofeng|last5=Zhang|first5=Peizhi|date=2018-08-13|title=The synthesis of epoxy resin coated Al2O3 composites for selective laser sintering 3D printing|journal=Rapid Prototyping Journal|volume=24|issue=6|pages=1059–1066|doi=10.1108/rpj-09-2017-0189|s2cid=139324761|issn=1355-2546}}</ref> Chemical [[Binder (material)|binder]] coatings can be applied to the powder surfaces post-process;<ref name=":3">{{Cite journal|last1=Kruth|first1=J-P.|last2=Mercelis|first2=P.|last3=Van Vaerenbergh|first3=J.|last4=Froyen|first4=L.|last5=Rombouts|first5=M.|date=February 2005|title=Binding mechanisms in selective laser sintering and selective laser melting|journal=Rapid Prototyping Journal|volume=11|issue=1|pages=26–36|doi=10.1108/13552540510573365|s2cid=53130687 |issn=1355-2546|url=https://lirias.kuleuven.be/handle/123456789/157828|url-access=subscription}}</ref> these coatings aid in the sintering process and are especially helpful to form composite material parts such as with [[Aluminium oxide|alumina]] particles coated with [[Thermosetting polymer|thermoset]] [[epoxy]] [[resin]].<ref name=":2" />
=== Sintering mechanisms ===
[[File:Necking.png|thumb|Diagram showing formation of neck in two sintered powder particles. Original shapes are shown in red.]]
Sintering in SLS primarily occurs in the liquid state when the powder particles forms a micro-melt layer at the surface, resulting in a reduction in viscosity and the formation of a concave radial bridge between particles, known as necking,<ref name=":3" /> due to the material's response to lower its surface energy. In the case of coated powders, the purpose of the laser is to melt the surface coating which will act as a binder. Solid state sintering is also a contributing factor, albeit with a much reduced influence, and occurs at temperatures below the melting temperature of the material. The principal driving force behind the process is again the material's response to lower its free energy state resulting in [[diffusion]] of molecules across particles.
== Applications ==
SLS technology is in wide use at many industries around the world due to its ability to easily make complex geometries with little to no added manufacturing effort. Its most common application is in [[prototype]] parts early in the [[design cycle]] such as for [[investment casting]] patterns, automotive hardware, and [[wind tunnel]] models. SLS is also increasingly being used in [[limited-run manufacturing]] to produce end-use parts for aerospace, military,<ref>{{cite journal |last1=Islam |first1=Muhammed Kamrul |last2=Hazell |first2=Paul J. |last3=Escobedo |first3=Juan P. |last4=Wang |first4=Hongxu |title=Biomimetic armour design strategies for additive manufacturing: A review |journal=Materials & Design |date=July 2021 |volume=205 |pages=109730 |doi=10.1016/j.matdes.2021.109730|doi-access=free }}</ref> medical, [[3D printed medication|pharmaceutical]],<ref>{{Cite journal|last1=Trenfield|first1=Sarah J.|last2=Awad|first2=Atheer|last3=Goyanes|first3=Alvaro|last4=Gaisford|first4=Simon|last5=Basit|first5=Abdul W.|date=May 2018|title=3D Printing Pharmaceuticals: Drug Development to Frontline Care|url=https://doi.org/10.1016/j.tips.2018.02.006|journal=Trends in Pharmacological Sciences|volume=39|issue=5|pages=440–451|doi=10.1016/j.tips.2018.02.006|pmid=29534837|s2cid=3845926 |issn=0165-6147}}</ref> and electronics hardware. On a shop floor, SLS can be used for rapid manufacturing of tooling, [[Jig (tool)|jigs]], and [[Fixture (tool)|fixtures]].<ref>{{Cite web|url=https://www.3dsystems.com/on-demand-manufacturing/selective-laser-sintering/applications|title=Selective Laser Sintering Applications Overview {{!}} Quickparts|website=www.3dsystems.com|access-date=2019-02-25|archive-date=2019-04-08|archive-url=https://web.archive.org/web/20190408005111/https://www.3dsystems.com/on-demand-manufacturing/selective-laser-sintering/applications|url-status=dead}}</ref>
== Advantages ==
* The sintered powder bed is fully self-supporting, allowing for:
** High overhanging angles (0 to 45 degrees from the horizontal plane)
** Complex geometries embedded deep into parts, such as [[conformal cooling channel]]s
** Batch production of multiple parts produced in 3D arrays, a process called nesting
* Parts possess high strength and stiffness
* Good chemical resistance
* Various finishing possibilities (e.g., metallization, stove enameling, vibratory grinding, tub coloring, bonding, powder, coating, flocking)
* Bio compatible according to EN ISO 10993-1<ref>{{Cite book|title=Biological evaluation of medical devices - Part 1 : Evaluation and testing within a risk management process (ISO 10993-1:2009)|date=2009|publisher=International Organization for Standardization (ISO)|oclc=839985896}}</ref> and USP/level VI/121 °C
* Complex parts with interior components can be built without trapping the material inside and altering the surface from support removal.
* Fastest additive manufacturing process for printing functional, durable, prototypes or end user parts
* Wide variety of materials with characteristics of strength, durability, and functionality
* Due to the reliable mechanical properties, parts can often substitute typical injection molding plastics
== Disadvantages ==
* Parts have porous surfaces; these can be sealed by several different post-processing methods such as cyanoacrylate coatings,<ref>{{Cite web|url=https://www.anubis3d.com/technology/selective-laser-sintering/|title = Selective Laser Sintering (SLS) Mississauga | SLS Sintering}}</ref> or by [[hot isostatic pressing]].
==See also==
* [[3D printing]]
* [[Digital fabricator]]
* [[Direct digital manufacturing]]
* [[Electron-beam melting]]
* [[Fab lab]]
* [[Fused filament fabrication]]
* [[Instant manufacturing]], also known as ''direct manufacturing'' or ''on-demand manufacturing''
* [[Rapid manufacturing]]
* [[Rapid prototyping]]
* [[RepRap Project]]
* [[Selective heat sintering]]
* [[Solid freeform fabrication]]
* [[Stereolithography]]
* [[Von Neumann universal constructor]]
==References==
{{Reflist}}
==External links==
* [http://laseroflove.files.wordpress.com/2009/10/dmls_history.pdf DMLS – DEVELOPMENT HISTORY AND STATE OF THE ART]
* [http://www.me.utexas.edu/news/2012/0712_sls_history.php Selective Laser Sintering, Birth of an Industry]
* [https://www.additive.blog/knowledge-base/3d-printers/laser-sintering-melting-sls-slm-dmls-dmp-ebm-shs/ Laser sintering, melting and others – SLS, SLM, DMLS, DMP, EBM, SHS]
{{3d printing}}
{{Lasers}}
{{Authority control}}
[[Category:American inventions]]
[[Category:3D printing processes]]
[[Category:Laser applications]]
[[Category:Metalworking]]
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