March 3, 2024
Selective Laser Sintering (SLS) is an additive manufacturing process that uses a powerful laser to selectively sinter and fuse nylon powder to create intricate geometries. Like other 3D printing technologies, it builds a component layer by layer.
Invented in the 1980s by Dr Carl Deckard when he was a student at the University of Texas, and his mentor Dr Joe Beaman at the same institution, the first batch of SLS 3D printers hit the market in the early 1990s. From the start, SLS gained popularity for rapid prototyping of functional parts. Over the years as the technology, machinery, and materials evolved, SLS end-use parts came to be known for their strength, functionality, and strict tolerances.
Today, they are used in a broad range of industries such as healthcare, automotive, aerospace, and consumer goods.
Traditionally, SLS printers have been enormous, taking up space no less than 10 square meters and mostly bought by industrial companies. They were cost-intensive due to the high-quality laser and sophisticated hardware. Industrial SLS printers require trained professionals for operation and maintenance. However, with the advent of the desktop versions a few years back, the machines became smaller, automated and more affordable for small businesses and prosumers. The prices of SLS machines range from $6,000 to $650,000.
Pre-printing: Before printing can commence, the CAD file is converted into a printable format like OBJ or STL. At the pre-printing stage, the build chamber, filled to the mark with powder, is heated to a temperature just below the sintering point. This temperature is maintained throughout the print time. Parameters like layer thickness, laser power, and resolution that influence the final build quality need to be adjusted in the settings beforehand.
Printing: The process begins with the powder recoater uniformly spreading a thin layer of the material on the print bed. It is followed by the laser heating up certain areas of the bed so that the powdered material sinters and coalesces to form the first solid layer of the model. The print bed then goes down by a layer – typically between 80-120 micron [µ]; 1µ= .001 mm. And the process continues till the part is complete. Once printing is complete, the objects are given time to cool down and later extracted from the printer.
Post-processing: Since SLS parts are mostly made from nylon powder, they come out rough and grainy. Depending on their end-use applications, they will require post-processing. The first step in this phase is removing the unsintered powder they are encased in.
The cleaning up can be done with compressed air or manually with rice or copper fibre brushes. Some manufacturers prefer media blasting, using sand or glass beads at high pressure to remove excess powder and improve the surface of the objects. However, it is laborious and might result in inconsistent finishes. Vibro polishing is a similar process but a gentle way of polishing parts and delivers uniform results.
Following this, the parts could be subjected to dyeing, spray painting/lacquering, and vapour soothing.
Dyeing works when parts with complex features needs a single homogenous colour.
Spray painting is for multiple colours on intricate geometries. Lacquer coating can improve resistance to wear and tear, water tightness, surface hardness, and impart a neat look to the part.
Vapour soothing involves giving the parts a bath in chemical solvents to fill up minute cavities on the surface and form a smooth, shiny shell around the parts.
While it is established that SLS is capable of rapid production of engineering-grade parts with excellent mechanical properties and fine resolution, customers need to define their performance requirements. This is because plastics used in additive manufacturing have different chemical, mechanical, thermal and optical characteristics that determine how the parts will perform.
The most popular SLS material is Nylon (PA) 12 synthetic powder derived from petroleum. Known for durability, chemical resistance, high tensile strength, and thermal stability, it can pull off complex designs and offer high environmental stability.
Nylon (PA) 11 powder has similar properties to PA 12 but comes from organic sources like castor oil and has lower environmental impact.
Nylon is also reinforced with glass (for greater stiffness and thermal stability), carbon (for stability, lightweight and improved performance), and aluminium (for stiffness, thermal conductivity, and metallic shine).
Elastomers (TPU, TPE, TPC) too are used for SLS printing. Prized for their durability, surface finish, and tear resistance, thermoplastic elastomers have rubber-like flexibility and functionality.
SLS uses several bio-compatible polymers such as Polyetheretherketone (PEEK) and Polyetherketoneketone (PEKK).
Pros: Let’s look at the advantages the SLS technology has to offer
Freedom from support structures: Engineers, innovators, and entrepreneurs consider SLS ideal for fabricating complex geometries as the objects do not need support elements during printing. The unused powder from the printing process provides adequate strength and support to the part.
Speed and production maximisation: SLS is arguably the fastest additive manufacturing technology, which is a blessing for prototype manufacturing. The lasers scan the cross-section of the model at a fast speed with brilliant accuracy. The printer also saves time and effort in other ways. By virtue of packing in complex design of an object in a single print run, it eliminates the need for printing multiple parts of the same object. SLS not only ensures optimal use of the build space but allows design modifications of the product while printing is in progress. This makes the product-development process iterative. The technology is best suited for on-demand manufacturing for medium to low production runs.
Isotropic properties: It prints isotropic parts with reliable strength in all directions (along the x, y, and z axes), making them ideal for functional parts and prototypes.
Sustainability: Recovery of unsintered powder is important in SLS as it can be reused for subsequent printing, thus reducing waste and material costs. There is less material usage also because of no support structures. SLS also uses biodegradable material, which can be consumed by bacteria present in industrial composting plants.
Cons: The technology has its shortcomings such as:
High shrinkage: There are chances of significant shrinkage as the object cools down after printing. The big and flat portions of the part are particularly vulnerable to distortion due to major temperature difference within the object.
Rough and porous surface: This is both a blessing and a problem. Blessing because such a surface is excellent if one is looking for dyeing and spray painting the part to enhance the look. Problematic because the absence of a smooth surface leads to the quality of the final product getting diminished. Prior to post-processing, the parts are grainy and lack water tightness.
Post-processing: SLS products needs a thorough clean-up before being used. This can be a costly and time-consuming affair. Media blasting is not suitable for parts with fine details and intricate features.
Industrial applications
The technology has use cases in a wide spectrum of industries:
Automotive: Additive manufacturing, especially SLS, has changed the auto industry in unforeseen ways. Jaguar and Porsche Classic use SLS-printed parts for their vintage models. SLS is routinely used in Ford’s 3D printing lab. To test the waters, SLS parts have also been used in the interiors of a fully electric race car built by the students of the Munich University of Applied Sciences for the Formula Student Germany racing series. Sauber Motorsport AG, the company operating the Alfa Romeo Sauber F1 Team, preferred SLS and SLA parts for a scale test model of a Formula 1 race car. Brose, a German automotive supplier, uses SLS to produce end-use parts.
Aerospace: SLS can hugely impact the aviation sector with prototype fabrication and customized and small batch production of lightweight parts and. Several airlines have adopted the technology to build aircraft cabin components such as air ducts and engine compartment.
Healthcare: The technology has gained increased acceptance in clinical and research-based healthcare activities. When it comes to joint-replacement implants in load-bearing region, metal 3D printing through SLS is a reliable option. Medical grade PEEK in powder form is used for SLS for low-volume production and specialist designs where it is difficult to achieve rapid 3d prototyping using metal and traditional techniques.
Consumer goods: SLS is extensively used for designing perfume bottles, mobile phone accessories, toys, mixer grinders, bicycles, and the exteriors of washing machines and dishwashers. It is also deemed ideal for prototyping and functional prototyping of consumer electronic devices such as GPS products.
The global SLS market is likely to expand at a considerable rate between 2023 and 2030. Experts say that industrial usage of SLS has increased in the recent years due to the growing use of metal-based printers in aerospace and medical equipment industries. The automotive and consumer goods sectors have also done their bit to popularise SLS. Advancements in SLS-compatible plastic materials and a rise in demand for lightweight but strong parts across industries are other important growth factors.
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